# Properties and morphologies of Lyman break galaxies at z similar to 1 in the Chandra Deep Field South, inferred from spectral energy distributions

Z. Chen, C. G. Shu, D. Burgarella, V. Buat, J. -S. Huang, Z. J. Luo

► To cite this version:

Z. Chen, C. G. Shu, D. Burgarella, V. Buat, J. -S. Huang, et al. Properties and morphologies of Lyman break galaxies at z similar to 1 in the Chandra Deep Field South, inferred from spectral energy distributions. Monthly Notices of the Royal Astronomical Society, 2013, 431 (3), pp.2080–2105. 10.1093/mnras/stt230. hal-01434419

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# Properties and morphologies of Lyman break galaxies at $z \sim 1$ in the *Chandra* Deep Field South, inferred from spectral energy distributions

Z. Chen,<sup>1,2,3</sup> C. G. Shu,<sup>1,2*</sup> D. Burgarella,<sup>3</sup> V. Buat,<sup>3</sup> J.-S. Huang<sup>4</sup> and Z. J. Luo<sup>2</sup>

Accepted 2013 February 6. Received 2013 February 6; in original form 2011 March 29

## ABSTRACT

After carefully cross-identifying previously discovered – and selected by the *Galaxy Evolution Explorer (GALEX)* – Lyman break galaxy (LBG) candidates one-to-one with their optical counterparts in the field of the *Chandra* Deep Field South (CDF-S), we re-estimate their photometric redshifts using multiwavelength data from ultraviolet and optical to near-infrared. Considering their re-estimated photometric redshifts and spectral energy distributions, we refine a new updated sample of 383 LBGs at $0.7 \lesssim z \lesssim 1.4$, with two confirmed active galactic nuclei being excluded. There are 260 and 111 LBGs classified as starburst and irregular types, respectively. The ages of the LBGs span from several Myr to 1.5 Gyr with a median of $\sim 50$ Myr. Their dust-corrected star formation rates (SFRs) and stellar masses ($M_*$) are 4–220 $M_{\odot}$ yr$^{-1}$ and from $2.3 \times 10^8$ to $4 \times 10^{11}$ $M_{\odot}$, with median values of $\sim 25$ $M_{\odot}$ yr$^{-1}$ and $\sim 10^{10}$ $M_{\odot}$. The rest-frame far-ultraviolet luminosity function of the LBGs is presented with the best-fitting Schechter parameters of $\alpha = -1.61 \pm 0.40$, $M^* = -20.40 \pm 0.22$ and $\phi^* = (0.89 \pm 0.30) \times 10^{-3}$ Mpc$^{-3}$ dex$^{-1}$. LBGs of irregular types are mainly distributed along the main sequence of star-forming galaxies, while most LBGs of starburst types are located in the starburst region. Together with previous studies, we suggest that the star formation mode for LBGs at $z > 3$ is mainly starburst, and that it evolves to be more significant to the quenching mode after $z \sim 3$. A downsizing effect is clearly found, and we discuss the physical implications and comparisons with previous studies in detail. LBGs with larger SFRs are, on average, more compact. In the rest-frame colour ($U - B$)-$M_*$ diagram, LBGs are distributed in the ‘blue’ cloud. We suggest that LBGs might evolve along the blue cloud from later to earlier types. The *Hubble Space Telescope* images in F606W (V band) and F850LP (z band) are taken from the Galaxy Evolution from Morphology and SEDs (GEMS) survey and the Great Observatories Origins Deep Survey South (GOODS-S) for morphological studies of LBGs. *SEXTRACTOR* and *GALFIT* are applied in order to obtain their morphological parameters. We establish an image gallery of 277 LBGs, commonly detected in both bands, by visually classifying individual LBGs into the following types: chain, spiral, tadpole, bulge and clump. We define a morphological sample of 142 LBGs with reliable results of Sérsic indices and sizes in both bands. We find that LBGs at $z \sim 1$ are dominated by disc-like galaxies, with median sizes of 2.34 and 2.68 kpc in F606W and F850LP, respectively. The correlations between the photometric and morphological properties of LBGs are investigated. Strong correlations between their half-light radii and $M_*$ (i.e. size–stellar mass relations) are found in both bands. We discuss the physical connections between the correlations and the downsizing effect.

**Key words:** galaxies: evolution – galaxies: formation – galaxies: luminosity function, mass function – galaxies: statistics – galaxies: structure.

<footnote n="1"><sup>1</sup>Shanghai Astronomical Observatory, 80 Nandan Rd, Shanghai, 200030, China</footnote>

<footnote n="2"><sup>2</sup>Shanghai Key Laboratory for Astrophysics, Shanghai Normal University, 100 Guilin Road, Shanghai, 200234, China</footnote>

<footnote n="3"><sup>3</sup>Observatoire Astronomique Marseille Provence, Laboratoire d'Astrophysique de Marseille, F-13012 Marseille, France</footnote>

<footnote n="4"><sup>4</sup>Harvard-Smithsonian Center for Astrophysics, MS 65, 60 Garden Street, Cambridge, MA 02138, USA</footnote>

<footnote>*E-mail: cgshu@shao.ac.cn</footnote>
# 1 INTRODUCTION

It is well established that massive galaxies are the results of mergers of small galaxies (White & Frenk 1991; Cole et al. 2000; Somerville, Primack & Faber 2001). Disc and elliptical galaxies form as a result of gas angular momentum by tidal torques and through mergers among discs, respectively (e.g. Mo, Mao & White 1998; Efstathiou 2000; Steinmetz & Navarro 2002; Governato et al. 2007; Croft et al. 2009; Scannapieco et al. 2009). The epoch of major mergers ends at $z \sim 2$ for the brightest and most massive systems (see Conselice et al. 2003, and references therein). Although this hierarchical scenario of galaxy formation and evolution has been very successful in explaining many observational phenomena, in general, there are still some phenomena, such as the role of active galactic nucleus (AGN) feedback, star formation, morphological evolution, etc., that are poorly understood. Details can be found in the recent review by Silk & Mamon (2012).

Currently, there are two regimes in which we understand the global evolution of galaxies well. First, we have the local Universe, where disc and elliptical galaxies have been studied in great detail for several decades (for a detailed review, see Longair 2006; see also de la Fuente Marcos & de la Fuente Marcos 2009; Bell et al. 2010; Boissier et al. 2010; van Dokkum & Conroy 2010; Tempel et al. 2011). In particular, after the Two-degree Field Galaxy Redshift Survey (2dFGRS; Colless et al. 2001) and the Sloan Digital Sky Survey (SDSS; York et al. 2000), which have provided huge multi-colour and spectroscopic galaxy samples with large sky coverages, many important and global statistical results have been obtained and taken as the zero-points to calibrate the paradigm of galaxy formation and evolution as a whole (e.g. Blanton et al. 2003, 2005; Shen et al. 2003; Tremonti et al. 2004; Blanton 2006; Chang et al. 2006; Shao et al. 2007; Salim et al. 2007; Kauffmann et al. 2003a; Kauffmann et al.2003b; Kauffmann, Heckman & Best 2008; Abazajian et al. 2009; Li et al. 2008; Li & White 2009). Together with deep surveys, such as Classifying Objects by Medium-Band Observations – a spectrophotometric 17-filter survey (COMBO 17; Wolf et al. 2004), the Deep Extragalactic Evolutionary Probe 2 (DEEP2; Davis et al. 2003; Madgwick et al. 2003) and the Cosmic Evolution Survey (COSMOS; Scoville et al. 2007), samples of more than $10^4$ galaxies, with either photometric or spectroscopic redshifts from local to $z \sim 1$, have shown the picture of galaxy evolution from the half-age of the Universe to the present day (Bell et al. 2004, 2006; Borch et al. 2006; Conroy et al. 2007; Faber et al. 2007; Gerke et al. 2007; Scoville et al. 2007; Jahnke et al. 2009; Maier et al. 2009; Salim et al. 2009; Yan et al. 2009; Cucciati et al. 2010; Kovacˇz et al. 2010; Cooper et al. 2011; Coppa et al. 2011).

Secondly, we have the regime at high redshift ($z \sim 2.5$), where colour-selected samples of galaxies, pioneered by Steidel, Pettini & Hamilton (1995), have been established, such as Lyman break galaxies (LBGs; Adelberger et al. 1998), submillimetre (submm) galaxies (Smail et al. 2004), distant red galaxies (Franx et al. 2003) and BzK-selected galaxies (Daddi et al. 2004). For instance, more than 1000 LBGs at $z \sim 3$ have been found and confirmed by follow-up spectroscopic observations with further studies (Steidel & Hamilton 1993; Steidel et al. 1996; Pettini et al. 2000; Teplitz et al. 2000; Giavalisco 2002; Steidel et al. 2003). Applying the Lyman break technique to different bands, star-forming galaxies have also been found successfully at different redshift ranges. For instance, Wilkins et al. (2010) have found six candidates for star-forming galaxies at $z \sim 7$, by comparing images of the Y (980 nm) band in the Wide Field Camera 3 (WFC3) with those of the z (850 nm) band in the Advanced Camera for Surveys (ACS) from observations of the Hubble Space

Telescope (HST). Adopting the multiwavelength imaging data of the European Southern Observatory (ESO) Remote Galaxy Survey, Douglas et al. (2009) have established a sample of 257 LBGs at $z \sim 5$. Using B, V, R, i' and z' filters in the SUBARU deep field, Ouchi et al. (2004) have compiled a large sample of 2600 LBGs with $i' \lesssim 27$ at $z \sim 4.5$. Based on far-ultraviolet (FUV) and near-ultraviolet (NUV) data in the Galaxy Evolution Explorer (GALEX) deep fields, Burgarella et al. (2006, 2007) have obtained a very deep sample of LBG candidates at $z \sim 1$, cross-correlated with optical data from Classifying Objects by Medium-Band Observations – a spectrophotometric 17-filter survey (COMBO 17), Nilsson et al. (2011a) have analysed the slitless grism observations from the HST ACS for 15 LBGs at $z \sim 1$, drawn from the sample of Burgarella et al. (2007, hereafter B07). They have concluded that they are young, while massive and dusty. Also, based on GALEX data but cross-correlated with optical data from the Sloan Digital Sky Survey (SDSS), Haberzettl et al. (2009) have obtained a shallower sample of LBGs, compared to the sample of Burgarella et al. (2006) and B07, at $z \sim 1$ with a sky coverage of $1 \text{ deg}^2$.

In recent years, there have been many studies within these two regimes (i.e. $1.5 \lesssim z \lesssim 2.5$, which is called the ‘redshift desert’; Steidel et al. 2004). For instance, Genzel et al. (2006, 2008, 2010, 2011) have studied star-forming galaxies at $z \sim 2$, according to the Spectroscopic Imaging Survey in the Near-Infrared with SINFONI (SINS) at the ESO Very Large Telescope (VLT). They have found that discs appear at $z \sim 2$ when star-forming regions are very clumpy. Huang et al. (2009) have established a sample of ultra-luminous infrared (IR) galaxies (ULIRGs)<fnref n="1" /> at $z \sim 1.9$ by the selection of an Infrared Array Camera (IRAC) colour criterion. They have claimed that ULIRGs at this redshift display variable morphologies in their rest-frame UV band. Conselice et al. (2011) have found that massive galaxies at $1.7 < z < 2.9$ show bimodality in the colour–magnitude space, according to deep near-infrared (NIR) surveys, utilizing the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) on the HST. With the Photodetector Array Camera and Spectrometer (PACS; Poglitsch et al. 2010) onboard the newly launched Herschel Space Telescope (Pilbratt et al. 2010), Nordon et al. (2010) have investigated the SFRs of massive galaxies at $1.5 < z < 2.5$ in the field of GOODS-N. By analysing both their spectral energy distributions (SEDs) from rest-frame UV to mid-IR and the direct 160-µm far-infrared (FIR) observations by PACS, Oteo et al. (2012) have found that spectroscopically selected star-forming Lyα emitting galaxies at $2.0 \lesssim z \lesssim 3.5$ show ages mostly below 100 Myr, with a wide variety of dust attenuations, SFRs and stellar masses. Also, it is suggested that their morphology ranges from bulge-like galaxies to highly clumpy systems. Using the Herschel observations, Magdis et al. (2010b) have investigated the FIR properties of mid-IR-selected ULIRGs at $z \sim 2$, and Bongiovanni et al. (2010) have studied the FIR counterparts of selected Lyα emitters at $z \sim 2.2$. Using the NUV dropout from the GALEX data, Burgarella et al. (2011) have established a sample of LBGs at $z \sim 2$ and have analysed their FIR luminosities. Nilsson et al. (2011b) have studied Lyα emitters at $z \sim 2.3$ in the COSMOS field and have found that their stellar masses are higher than those at higher redshifts. Using a similar method, Haberzettl et al. (2012) have recently established a sample of 73 LBG candidates at $z \sim 2$, with a median stellar mass of $\sim 10^{10} \text{ M}_\odot$, most of which show disc-like structures. Basu-Zych et al. (2011) have studied LBGs at $0.5 \lesssim z \lesssim 2$, based on observations in the Chandra Deep Field South (CDF-S) using

<footnote n="1">¹ ULIRGs are defined by their IR luminosities $10^{12} < L_{\text{IR}} < 10^{13} \text{ L}_\odot$.</footnote>
the UV/Optical Telescope (UVOT) on board Swift (Gehrels et al. 2004), together with multiwavelength data. They have found that their LBGs have stellar masses slightly lower than those at $z \sim 3$ and slightly higher than $z \sim 1$ CDF-S galaxies.

One of the key questions concerning galaxy formation and evolution (i.e. how galaxies evolve from high-$z$ to present day) has arisen because so many distant galaxies have been found. Most current studies focus on photometric properties, such as colour, spectrum features, size, mass and spatial clustering, in order to help us to understand the connections between high-$z$ and local galaxies. For instance, the spectra of some bright LBGs (Pettini et al. 2000; Teplitz et al. 2000) are remarkably similar to those of local starbursts, with their UV luminosities directly relating to the number of short-lived, massive stars. Submm observations reveal a population of FIR-bright galaxies that might be similar to local (ultra)luminous IR galaxies [(U)LIRGs;<fnref n="2" /> Blain 1999]. Based on the correlation length of clustering, Shu, Mao & Mo (2001) have suggested that LBGs and Submillimeter Common-User Bolometer Array (SCUBA) galaxies at $z \sim 3$ correspond to the present-day bulges of disc galaxies (see also Giavalisco, Steidel & Macchetto 1996) and giant elliptical galaxies, respectively. Heckman et al. (2005) and Hoopes et al. (2007) have suggested that local ($z < 0.3$) Lyman break analogues have similar UV surface brightness to high-redshift LBGs, in contrast to Scarpa, Falomo & Lerner (2007).

Although most galaxies at high redshift must somehow be the progenitors of modern galaxies, the connection between specific high-redshift populations and low-redshift counterparts remains a complicated issue (Delgado-Serrano et al. 2010). We still have very little knowledge of when and how the modern Hubble sequence came into place. The direct way to study this issue is to investigate the morphologies of galaxies at different redshifts and to compare with local galaxies. This is not an easy task because it depends greatly on the resolutions and sensitivities of the observing facilities. Some observations have shown that the morphologies of galaxies at $z > 2.5$ appear very irregular, very different from local ellipticals or discs (Giavalisco et al. 1996). Familiar Hubble types are observed with perhaps comoving volume densities at $z \sim 1$ similar to those at $z \sim 0$ (van den Bergh et al. 2000). Galaxies at high-$z$ are characterized observationally by compact structures with high surface brightness in their central regions, and it is harder to detect their outer regions with lower surface brightness. Thus, it is difficult to analyse their morphological properties. As pointed out by Burgarella et al. (2001), only compact star-forming regions can be easily detected in deep HST observations.

In this paper, we study the photometric and morphological properties of LBGs at $z \sim 1$ in detail, adopting broad multiwavelength data from UV and optical to NIR, together with high-quality HST images from the Galaxy Evolution from Morphology and SEDs (GEMS) survey and the Great Observatories Origins Deep Survey (GOODS). The preliminary LBG candidate sample is taken from B07. The structure of this paper is as follows. We give a description of the sample in Section 2, and detailed studies on the photometric properties of LBGs in Section 3. In Section 4, we describe the HST images, obtained from the GEMS survey and from GOODS, and the visual classifications. We study the morphological properties in Section 5. We discuss the correlations between the photometric and morphological properties of LBGs in Section 6, and we conclude in Section 7.

Throughout the paper, we take the concordance cosmology with $\Omega_0 = 0.3$ and $\Omega_\Lambda = 0.7$. The Hubble constant is $H_0 = 100 h^{-1}$ km s$^{-1}$ Mpc$^{-1}$ with $h = 0.7$, if needed. Under this cosmology, the luminosity distance $D_L$ and the angular diameter distance $D_\Lambda$ are 6.607 and 1.652 Gpc, respectively, at $z = 1$, with a size scale of $8.01$ kpc arcsec$^{-1}$ and the age of the Universe 5.75 Gyr. Moreover, magnitudes are in the AB system (Oke & Gunn 1983) and colours are given in the rest frame unless otherwise stated.

# 2 SAMPLE OF LYMAN BREAK GALAXIES

## 2.1 Preliminary sample and photometric data

The preliminary sample of LBG candidates at redshift $z \sim 1$ used in this paper has been described by B07, and belongs to CDF-S. It can be briefly summarized as follows. Based on the deep (76 444 s) NUV image available in GALEX Release 2 (GR2) centred at $\alpha = 03^h32^m30.7$ and $\delta = -27^\circ52'16.9$ (J2000.0), and the deep (44 668 s) image survey (DIS) in FUV also by GALEX (Martin et al. 2005), B07 have cross-identified GALEX sources within a radius of 2 arcsec to their optical counterparts in the field of 0.263 deg$^2$ overlapped with COMBO 17 (Wolf et al. 2004). The LBG candidates are selected among sources with NUV $\lesssim 26.2$, taking into account the criteria FUV - NUV > 2 and the photometric redshift data from COMBO 17. Discarding objects classified in COMBO 17 with the flags 'Star', 'WD', 'QSO(gal)' and 'Strange Object', with the completeness of $\sim 80$ per cent down to NUV $\sim 24.8$, 420 LBG candidates are selected with their photometric redshifts from COMBO 17 between 0.9 and 1.3. Among these, two candidates have quadruple counterparts found in COMBO 17, 40 have pair counterparts and the other 378 have single counterparts individually. Moreover, 62 of these 420 LBG candidates have been detected in the 24-µm image from the Multiband Imaging Photometer for Spitzer (MIPS). Based on SED studies, B07 have obtained a median age of 250 Myr and a median SFR of $\sim 30$ M$_\odot$ yr$^{-1}$ for their LBG candidates. A typical dust attenuation in FUV of $A_{FUV} \sim 2.5$ is estimated for candidates with 24-µm detections, while other candidates without 24-µm detections display less, with $A_{FUV} \sim 1.8$. Details can be found in B07.

Note that the Multiwavelength Survey by Yale-Chile (MUSYC) catalogue (Cardamone et al. 2010; see later in this subsection for details), which is more recent than COMBO 17, can also be adopted to select LBG candidates. If we replace the COMBO 17 catalogue by the MUSYC catalogue and take the same processes as B07, 442 LBG candidates are found, with more than 85 per cent objects common to B07. The difference is not significant for the statistical studies of the LBG population at $z \sim 1$. Moreover, taking into account the updated catalogue of COMBO 17 (Wolf et al. 2008), we find that more than 95 per cent of the LBG candidates in B07 can still be selected as LBG candidates and that the mean offset of the photometric redshifts is only 0.015. Although the MUSYC catalogue covers more bands, the COMBO 17 catalogue displays higher survey sensitivity and spectral resolution. We prefer to keep the selection of B07 for the following studies. It should be pointed out that the sample of B07 is unbiased, is much deeper and is more complete than their previous samples (Burgarella et al. 2006). For more clarity, we plot the distribution of the B07 LBG candidates in the field of the CDF-S in the upper panel of Fig. 1, using diamonds and circles to denote candidates with single and multiple counterparts in COMBO 17, respectively, squares to denote spectral data available from the MUSYC catalogue and from other previous studies, and crosses to denote AGNs (see later in this subsection).

<footnote n="2">² LIRGs are defined by their IR luminosities $10^{11} < L_{IR} < 10^{12} L_{\odot}$.</footnote>
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Upper panel: LBG candidates of B07 in the field of the CDF-S with diamonds and circles denoting candidates with single counterparts and multiple counterparts in COMBO 17, respectively, squares denoting spectral data available from the MUSYC catalogue and other previous studies and crosses denoting AGNs. Lower panel: same as the upper panel but for the refined LBG sample (Section 2.3) although here circles denote LBGs with no spectral data available (see text for details).</figcaption></figure>
The image resolutions of UV and IR observations are too low (e.g. 4.5 arcsec in FUV and 6 arcsec in NUV for *GALEX*) for us to investigate the morphological properties of LBGs at $z \sim 1$. Optical images by the *HST* are adopted for the morphological studies (see Section 5 for details). So, the optical counterparts for individual LBG candidates must be carefully identified. We start to redo the cross-identifications of optical counterparts in the catalogue of COMBO 17 for the LBG candidates, using the same processes proposed by B07. Because the space resolution of *GALEX* is 2.55 arcsec (B07) and the intrinsic astrometric precision of COMBO 17 is about 0.5 arcsec (Wolf et al. 2004), together with an astrometric precision lower than 0.2 arcsec between the catalogues of COMBO 17 and MUSYC (Cardamone et al. 2010, see below), an appropriate radius of 3 arcsec is used here for cross-identification.

In a slight change from B07, two of their LBG candidates with quadruple optical counterparts are found to be one with four counterparts and the other with two counterparts in COMBO 17. One candidate with pair counterparts in B07 is found to have a single counterpart. For the other LBG candidates, we have the same results as B07 for the optical counterparts found in COMBO 17. To summarize, among the 420 LBG candidates we have found that 40 have a pair of counterparts, one has four counterparts and the other 379 (378 in B07) have single optical counterparts in COMBO 17. In total, 463 optical counterparts are found.

It is important to check the contamination of AGNs in our sample of LBG candidates. According to Silverman et al. (2010), based on the optical spectra of X-ray sources, four confirmed AGNs are found among the LBG candidates in B07, with UV IDs 81300, 39096, 68162 and 69870. Their corresponding spectral redshifts are 0.688, 0.652, 0.979 and 0.972, respectively. These four AGNs are classified above as LBG candidates with single counterparts. Note that only three AGNs, which are among the above four, can be found in the *Chandra* 2-Ms CDF-S catalogue (Luo et al. 2008, 2010). This means that the AGN fraction in our preliminary sample of LBGs is ~1 per cent, which is slightly less than those of LBGs at $z \sim 1.5$ (Basu-Zych et al. 2011) and $z \sim 3$ (Steidel et al. 2003; Lehmer et al. 2008). Because we focus on photometric and morphological studies for LBGs, these are excluded in the following part of the paper. So, the number of LBG candidates becomes 416, with 375 of these found to have single optical counterparts in COMBO 17.

Because cross-identifications are carried out within a radius of 3 arcsec centred on the coordinates of individual LBG candidates, it is reasonable to assume that the counterparts of these 375 LBG candidates with only single counterparts are their ‘true’ optical counterparts individually. For candidates with more than one counterpart in COMBO 17, more accurate estimates of their redshifts are needed for further identifications. To study in detail the photometric properties of LBGs at $z \sim 1$, FUV, NUV and more wavelength observations must be considered to re-estimate their photometric redshifts, rather than simply taking the results from the optical catalogue of COMBO 17 only (see Section 2.2).

The MUSYC *BVR*-selected catalogue covers the field of the present study, with multiwavelength data available from optical to NIR, together with several tens of spectroscopic data from other observations (Cardamone et al. 2010). The main features of the MUSYC *BVR*-selected catalogue are as follows. In the catalogue, *UBVRI* data are from the ESO archive, collected and calibrated as part of the Garching-Bonn Deep Survey (GaBoDS; Hildebrandt et al. 2006), and *z*-band data are from the Mosaic-II camera on the Cerro Tololo Inter-American Observatory (CTIO) 4-m Blanco telescope (Gawiser et al. 2006; Taylor et al. 2009). The *H*-band data are obtained from Son of ISAAC (SofI), the infrared spectrograph

and imaging camera on the ESO 3.6-m New Technology Telescope (NTT; Moy et al. 2003) and the *JK*-band data are from the Infrared Side Port Imager (ISPI), also on the CTIO 4-m Blanco telescope (Taylor et al. 2009). The IRAC data in four channels (3.6, 4.5, 5.8 and 8.0 µm) are from the *Spitzer* IRAC/MUSYC public Legacy in the ECDF-S (SIMPLE) project (Damen et al. 2011). The photometric redshifts of individual objects in MUSYC are also listed. Note that the magnitude limit of the MUSYC *BVR*-selected catalogue is $R \lesssim 25.3$, slightly shallower than COMBO 17, which is $R \lesssim 26$ (Wolf et al. 2004; Faber et al. 2007). Details of the MUSYC *BVR*-selected catalogue can be found in Cardamone et al. (2010).

With the aim of extending the observational wavelength coverage of COMBO 17 for the photometric redshift re-estimates of the LBG candidates, we perform cross-identifications for the optical counterparts of LBG candidates found in COMBO 17 with those in the MUSYC *BVR*-selected catalogue. The astrometric precision of less than 0.2 arcsec between the two catalogues enables us to perform cross-identifications without any multicounterpart problems from the COMBO 17 to MUSYC *BVR*-selected catalogues. Because the magnitude limit is slightly shallower than that of COMBO 17, fewer counterparts are found in the MUSYC *BVR*-selected catalogue. Among the 375 LBG candidates with single counterparts found in COMBO 17, 351 candidates are found with single counterparts and the remaining 24 candidates are found without any counterparts in the MUSYC *BVR*-selected catalogue. For the 40 LBG candidates with pair counterparts in COMBO 17, two and 36 candidates are found with pairs and single counterparts available, respectively, while the remaining two candidates are found with no counterparts. For the one candidate with four counterparts in COMBO 17, only one counterpart is found in the MUSYC *BVR*-selected catalogue. In total, 392 counterparts are found in the MUSYC *BVR*-selected catalogue among 459 counterparts in COMBO 17 for 416 LBG candidates in B07. There are 70 and 66 of these 459 and 392 counterparts in the COMBO 17 and MUSYC *BVR*-selected catalogues, respectively, with spectral data available from the MUSYC *BVR*-selected catalogue, the *K*-selected catalogue of Taylor et al. (2009) and the recent work carried out by Nilsson et al. (2011a).

Detailed results of the cross-identifications are summarized in Table 1. The first and second columns in Table 1 denote the number of counterparts of an LBG candidate and the corresponding

<table><caption>**Table 1.** The cross-identification results of optical counterparts in the COMBO 17 catalogue and the corresponding results in the MUSYC *BVR*-selected catalogue for 416 LBG candidates (see text for details). The first and second columns denote the number of counterparts for an LBG candidate and the numbers of LBG candidates found in COMBO 17, respectively. The third column denotes the results of cross-identifications from the COMBO 17 to MUSYC *BVR*-selected catalogues. The fourth column shows the number of spectroscopical data available.</caption><thead><tr><th>Counterpart(s)</th><th>COMBO</th><th>MUSYC</th><th>z<sub>spe</sub></th></tr></thead><tbody><tr><td>Single</td><td>375</td><td>351</td><td>60</td></tr><tr><td>Pair</td><td>40</td><td>36 single<br>2 pair</td><td>9<br>1</td></tr><tr><td>Four</td><td>1</td><td>1 single</td><td>—</td></tr><tr><td>Total</td><td>459</td><td>392</td><td>70</td></tr></tbody></table>
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Comparison of photometric redshifts between $z_{COMBO}$ from COMBO 17 and $z_{MUSYC}$ from MUSYC for the optical counterparts of LBG candidates commonly available in the two catalogues. The solid diagonal line indicates where these would agree.</figcaption></figure>

numbers of LBG candidates found in COMBO 17, respectively. The third column denotes the results of cross-identifications from the COMBO 17 to MUSYC BVR-selected catalogues. The fourth column shows the number of spectroscopical data available.

## 2.2 Photometric redshifts of the preliminary sample

As in the previous subsection, the preliminary sample of LBGs is selected in UV, together with photometric redshifts from their optical counterparts in COMBO 17. Most of the counterparts are also available in the MUSYC BVR-selected catalogue with NIR and IR data. The MUSYC BVR-selected catalogue covers observed SEDs with more bands, while the COMBO 17 catalogue shows higher sensitivity and spectral coverage in general. Both provide good estimates of photometric redshifts globally. It is interesting to compare in Fig. 2 the photometric redshifts of the 392 counterparts available in both the COMBO 17 and MUSYC BVR-selected catalogues. Although the global accuracies of photometric redshifts for both catalogues are high, it can be seen from the figure that many counterparts of the LBG candidates show significantly different photometric redshifts between the two catalogues.

It must be pointed out that FUV and NUV data, which are adopted to select LBG candidates at $z \sim 1$, are not taken into account for redshift estimates in either the COMBO 17 or MUSYC BVR-selected catalogue. So, it is worth re-estimating the photometric redshifts for the optical counterparts of LBG candidates individually, combining FUV and NUV data from B07 and data from MUSYC/COMBO 17 together.

We re-estimate the photometric redshifts of individual counterparts for LBG candidates by fitting their observed SEDs to templates using the code HYPERZ<fnref n="3" /> (Bolzonella, Miralles & Pelló 2000), taking the extinction law as Calzetti et al. (2000). The templates adopted

<table><caption>**Table 2.** The SpTs (with the star formation timescales $\tau$ if available) and their corresponding SpT numbers of different templates adopted for SED fittings, with BC03 denoting templates from Bruzual & Charlot (2003), CWW from Coleman et al. (1980) and Arp220 and Arp220-modifying from Huang et al. (2009).</caption><thead><tr><th>Spectral type (SpT)</th><th>Templates</th><th>τ (Gyr)</th></tr></thead><tbody><tr><td>E (1)</td><td>BC03</td><td>1</td></tr><tr><td>E (2)</td><td>CWW</td><td>–</td></tr><tr><td>S0 (3)</td><td>BC03</td><td>2</td></tr><tr><td>Sa (4)</td><td>BC03</td><td>3</td></tr><tr><td>Sb (5)</td><td>BC03</td><td>5</td></tr><tr><td>Sbc (6)</td><td>CWW</td><td>–</td></tr><tr><td>Sc (7)</td><td>BC03</td><td>15</td></tr><tr><td>Scd (8)</td><td>CWW</td><td>–</td></tr><tr><td>Sd (9)</td><td>BC03</td><td>30</td></tr><tr><td>Irr (10)</td><td>BC03</td><td>∞<sup>a</sup></td></tr><tr><td>Irr (11)</td><td>CWW</td><td>–</td></tr><tr><td>Dusty (12)</td><td>CWW</td><td>–</td></tr><tr><td>Burst (13)</td><td>BC03</td><td>0<sup>a</sup></td></tr><tr><td>Arp220-modifying (14)</td><td>Arp220-rebuilt</td><td>–</td></tr><tr><td>Arp220 (15)</td><td>Arp220</td><td>–</td></tr></tbody></table>

are those of Bruzual & Charlot (2003, hereafter BC03), with the assumptions of an exponentially decaying star formation history and star formation time-scales $\tau$ for different types (Bolzonella et al. 2000), the mean SEDs of local galaxies (Coleman, Wu & Weedman 1980, hereafter CWW) and the observed starburst templates of Arp220 types (Huang et al. 2009). Table 2 lists the spectral type (SpT; with the star formation time-scales $\tau$ if available) and the corresponding SpT number of the different templates.

Considering the sensitivity of the survey (Wolf et al. 2004, 2008), broad-band data from COMBO 17 are preferred in the following studies, although intermediate-band data would have provided a more accurate estimate of the photometric redshifts. Moreover, the calibration update of the COMBO 17 (Wolf et al. 2008) has been taken into account. For the 392 counterparts of LBG candidates available in the both COMBO 17 and MUSYC BVR-selected catalogues (see Table 1), we take the data of 13 bands (i.e. FUV, NUV, U, B, V, R, I, z, J, H, K, 3.6 µm and 4.5 µm – the latter two from IRAC) to fit their SEDs. For the remaining 67 counterparts available only in the COMBO 17 catalogue, the data of seven bands are taken: FUV, NUV, U, B, V, R and I. Because dust emission feathers are not taken into account for the BC03 templates, we have ignored the 5.8- and 8.0-µm data of IRAC for the specific case of galaxies at $z \sim 1$.

As can be seen in Table 1, there are 70 of the 459 optical counterparts of LBG candidates with known spectroscopic data. We compare their re-estimated photometric redshifts with their spectroscopic redshifts in Fig. 3. Two counterparts with UV IDs 67958 and 59121 in B07 (IDs 56523 and 42593 in MUSYC) are labelled in the figure with asterisks because of their unreliable spectroscopic redshifts resulting from very bad spectroscopic observations. To quantitatively clarify, we list the uncertainties of our re-estimated photometric redshifts for LBG candidates as a function of their apparent magnitudes in the R band in Table 3. The corresponding uncertainties for the specific case of LBG candidates in the COMBO 17 and MUSYC BVR-selected catalogues are also listed in the table. Four AGNs and two counterparts with bad quality spectral

<footnote>a Note that $\tau = \infty$ and 0 correspond to a constant SFR and a single burst, respectively.</footnote>

<footnote n="3">³ http://webast.astobs.mip.fr/hyperz/</footnote>
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Comparison of the re-estimated photometric redshifts $z_p$ and the spectroscopic redshifts $z_{spe}$ for 70 LBG counterparts with available $z_{spe}$. The solid diagonal line indicates where they would agree. Unreliable $z_{spe}$ with UV IDs of 67958 and 59121 (IDs 56523 and 42593 in MUSYC) are labelled with asterisks because of their bad quality spectroscopic observations.</figcaption></figure>

<table><caption>**Table 3.** Photometric redshift accuracies as a function of apparent magnitudes in the R band for LBG optical counterparts. The first column lists the magnitude ranges. Columns 2–4 are the averaged offsets of $z_p$ to $z_{spe}$ and their corresponding standard deviations $\delta z$, for the present paper, the COMBO 17 (C) and the MUSYC (M) catalogues, respectively (see text for details).</caption><thead><tr><th></th><th>Δz, δz</th><th>Δz, δz (C)</th><th>Δz, δz (M)</th></tr></thead><tbody><tr><td>R &lt; 22</td><td>-0.021, 0.061</td><td>0.014, 0.021</td><td>0.002, 0.014</td></tr><tr><td>22 &lt; R &lt; 23</td><td>-0.020, 0.063</td><td>-0.014, 0.041</td><td>-0.004, 0.024</td></tr><tr><td>23 &lt; R &lt; 24</td><td>0.002, 0.061</td><td>0.015, 0.089</td><td>0.027, 0.074</td></tr><tr><td>24 &lt; R</td><td>-0.067, 0.062</td><td>0.082, 0.097</td><td>0.017, 0.045</td></tr><tr><td>Total</td><td>-0.017, 0.063</td><td>0.009, 0.072</td><td>0.010, 0.050</td></tr></tbody></table>

observations are excluded from the table. Here, $\Delta z$ and $\delta z$ are defined, respectively, as the average of $(z_p - z_{spe})/(1 + z_{spe})$ – that is, the mean offset of $z_p$ to $z_{spe}$ in a given magnitude range – and its corresponding standard deviation. The first column in the table lists

<figure><img src="image_6.png" /><figcaption>**Figure 4.** The reduced $\chi^2$ distribution of SED fittings.</figcaption></figure>

the magnitude ranges in the R band. Columns 2–4 are the results of $\Delta z$ and $\delta z$ for the present paper, the COMBO 17 catalogue (C) and the MUSYC (M) catalogue, respectively.

It can be seen from Fig. 3 that our re-estimated photometric redshifts of counterparts agree well with their corresponding spectral data, if available, except for two with unreliable spectroscopic redshifts. It can also be seen from Table 3 that the average offset overall is -0.017 with a deviation of 0.063 for re-estimated photometric redshifts, comparable to those from the COMBO 17 and MUSYC catalogues. For counterparts with $R > 23$, which contains half of the spectral data available, the accuracies of our re-estimated photometric redshifts are better than those from the COMBO 17 catalogue and slightly worse than those from the MUSYC catalogue. To further demonstrate the goodness of the SED fittings for LBG counterparts, the distribution of reduced $\chi^2$ (per deg) is shown in Fig. 4. It can be seen that most SED fittings are good ($\chi^2 \lesssim 4$) with a median value of 1.8 for $\chi^2$. Together with Fig. 3 and Table 3, it can be concluded that our photometric redshift re-estimates are fairly good. Note that there are 15 counterparts with large $\chi^2$ (>10), because most of them are very bright with small magnitude error bars, especially in NIR bands. We discuss the reliability of the SED fittings in Section 2.3.

<table><caption>**Table 4.** Best-fitting Schechter parameters of FUV LFs for galaxies at different redshift $z$ including the results of the present paper for LBGs at $z \sim 1$. The references are: 1, Wyder et al. (2005); 2, Arnouts et al. (2005); 3, Oesch et al. (2010); 4, Reddy & Steidel (2009); 5, Bouwens et al. (2007); 6, Bouwens et al. (2008).</caption><thead><tr><th>z</th><th>α</th><th>φ*(10<sup>-3</sup> Mpc<sup>-3</sup> dex<sup>-1</sup>)</th><th>M*</th><th>Refs</th></tr></thead><tbody><tr><td>0–0.1</td><td>-1.16 ± 0.07</td><td>5.50 ± 0.14</td><td>-18.23 ± 0.11</td><td>1</td></tr><tr><td>0.4–0.6</td><td>-1.55 ± 0.21</td><td>1.69 ± 0.88</td><td>-19.49 ± 0.25</td><td>2</td></tr><tr><td>0.8–1.2</td><td>-1.63 ± 0.45</td><td>1.14 ± 0.76</td><td>-20.11 ± 0.45</td><td>2</td></tr><tr><td>0.7–1.4</td><td>-1.61 ± 0.40</td><td>0.89 ± 0.30</td><td>-20.40 ± 0.22</td><td>This work</td></tr><tr><td>1.7–2.1</td><td>-1.60 ± 0.51</td><td>2.19 ± 0.83</td><td>-20.16 ± 0.52</td><td>3</td></tr><tr><td>2.7–3.4</td><td>-1.73 ± 0.13</td><td>1.71 ± 0.53</td><td>-20.97 ± 0.14</td><td>4</td></tr><tr><td>3.8</td><td>-1.73 ± 0.05</td><td>1.30 ± 0.20</td><td>-20.98 ± 0.10</td><td>5</td></tr><tr><td>5.0</td><td>-1.69 ± 0.09</td><td>0.9<sup>+0.3</sup><sub>-0.2</sub></td><td>-20.69 ± 0.13</td><td>5</td></tr><tr><td>7.3</td><td>-1.74</td><td>1.1<sup>+1.7</sup><sub>-0.7</sub></td><td>-19.8 ± 0.4</td><td>6</td></tr></tbody></table>
<figure><img src="image_7.png" /><figcaption>**Figure 5.** SED fittings shown as solid lines for two LBG candidates that have 24-µm detections (red LBGs) with $ID_{UV}$, $z_p$, $z_{spe}$, arrows and dashed lines denoting their IDs in UV from B07, the re-estimated photometric redshifts, the spectroscopic redshifts, the upper limits of the flux detections and the extrapolations of the SED fittings, respectively. The upper and lower panels are for candidates with and without spectroscopic redshifts, respectively (see text for details).</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 6.** Same as Fig. 5 but for two LBG candidates without 24-µm detections (blue LBGs; see text for details).</figcaption></figure>

As examples, Figs 5 and 6 show the individual SED fittings of two LBG candidates without/with 24-µm detections from B07 (i.e. blue/red LBG candidates; see Section 2.3), respectively, available in both the COMBO 17 and MUSYC catalogues. Their corresponding IDs in UV from B07 are labelled in the figures. In each figure, one LBG candidate with (upper panel) and the other without (lower panel) spectroscopic redshifts are selected for clear comparison

<figure><img src="image_9.png" /><figcaption>**Figure 7.** Comparisons of photometric redshifts (replaced by $z_{spe}$ if available) between our results re-estimated with those in COMBO 17 and available in the MUSYC catalogues, respectively, with the solid diagonal indicating where they would agree.</figcaption></figure>

with their reduced $\chi^2$ (not small); the re-estimated photometric $z_p$ and spectroscopic redshifts $z_{spe}$ are labelled. They are classified as three starburst (SB) galaxies and an irregular (Irregular) galaxy. The arrows in the figures are the upper limits of the flux detections. The dashed lines are the extrapolations of the template SEDs, because the 5.8- and 8-µm data of IRAC are not taken into account during SED fittings. Together with Table 3 and Figs 3 and 4, this further shows that our results for the SED fittings of the LBG candidates are good.

It is interesting to compare the photometric redshifts (replaced by $z_{spe}$ if available) of our re-estimated results with those in the COMBO 17 and MUSYC catalogues, as shown in Fig. 7. It can clearly be seen that there are significant differences between our results and those of COMBO 17. Because we use a broader coverage in wavelength, especially for UV and NIR data, than COMBO 17 to estimate photometric redshifts, the quality of our re-estimated $z_p$ for the specific case of LBG candidates at $z \sim 1$ should be better (see also Table 3). Moreover, the agreement between our results and those of MUSYC is better, mainly because NIR data have been included in MUSYC. Note that there are still some outliers of LBG candidates between our re-estimated photometric redshifts and those from the MUSYC catalogue (see the lower panel of Fig. 7). After checking carefully, two sources (MUSYC IDs of 56523 and 42593) with spectral redshifts $z_{spe} > 2$ in the MUSYC catalogue have very bad quality spectroscopic observations (see also Fig. 3). For the other outliers with photometric redshifts higher than 1.6 or lower than 0.5 in the MUSYC catalogue, these are either very bright or very faint in the NIR bands compared to their optical magnitudes. This leads to an overestimation or an underestimation of their photometric redshifts if FUV and NUV are not considered, as in the MUSYC catalogue. We conclude that the consideration of UV data does play an important role in the determination of photometric redshifts for LBG candidates.
## 2.3 Sample refinement

The processes of selecting the true optical counterpart of an LBG candidate are as follows. There are 70 counterparts with spectroscopic redshifts available from previous studies. We replace the photometric redshifts for 68 of these 70 counterparts by their observed spectroscopic redshifts, except for two with very bad spectroscopic observations (see Sections 2.1 and 2.2). The counterparts of 375 LBG candidates with only single counterparts are assumed to be their corresponding counterparts, as discussed above. For a candidate with a pair of counterparts or with four counterparts, the re-estimated photometric/spectroscopic redshifts and the reduced $\chi^2$ during SED fittings are considered for its true counterpart selection. If only one counterpart with photometric redshifts/spectroscopic is within the range $0.9 \lesssim z_p \lesssim 1.3$, we assume that this is the true optical counterpart of the candidate, because the selection criterion of FUV - NUV > 2 strongly suggests that it biases to locate at $z \sim 1$. If more than two counterparts of a candidate with re-estimated photometric redshifts or none are within the redshift range, the counterpart with smaller $\chi^2$ is selected as the true counterpart.

Thus, we have finished the determination of the true optical counterparts of all 416 LBG candidates selected by FUV - NUV > 2 from the preliminary sample of LBGs of B07, with four AGNs excluded. The distribution of their re-estimated photometric (and spectroscopic, if available) redshifts $z_p$ is plotted as the solid histogram in Fig. 8. For comparison, the distributions of the original photometric redshifts from COMBO 17 and the results of 394 candidates from MUSYC are also plotted as dashed and dotted histograms in the figure. It can be seen that the re-estimated photometric (and spectroscopic, if available) redshifts of all LBG candidates are between 0.5 and 1.6, because of the selection criterion FUV - NUV > 2, as expected. Almost two-thirds of the LBG candidates (268 of 416) are within the redshift range of $0.9 \lesssim z_p \lesssim 1.3$, as in B07. The numbers of candidates are 339 and 387 within the redshift ranges of $0.8 \lesssim z_p \lesssim 1.4$ and $0.7 \lesssim z_p \lesssim 1.4$, respectively.

Although the goodness of the SED fitting is parametrized by $\chi^2$, we need to check the reliability of individual LBG candidates

one-by-one, especially for 15 objects with $\chi^2 > 10$. We find that 399 of 401 LBG candidates with $\chi^2 < 10$ display reliable SED fittings, except for two candidates (UV IDs of 64312 and 75583) with their resulting $z_p$ of 0.883 and 0.802, respectively, which show very uncertain photometric data. A candidate of UV ID 47588 with its resulting $z_p = 0.930$ and $\chi^2 > 10$ displays a very strange SED with a shape jump from FUV, NUV to U (FUV - NUV ~ 4 and NUV - U ~ -2.5). Because the error bars of its photometric data are ~0.25, it is hard to find a template to fit it reliably. Moreover, a candidate with UV ID 54502, $z_p = 0.906$ and $\chi^2 = 11.2$ shows its SED with a bump between the I and H bands, and then drops very steeply from the H band to 4.5 µm. Together with its magnitudes in 5.8 and 8.0 µm, which are much brighter than its magnitude in 4.5 µm, we suggest that its SED fitting is unconvincing. For the remaining 13 candidates with $\chi^2 > 10$, we find that their SED fittings are reasonable, together with the consideration of the extrapolations of the SED fittings to 5.8 and 8.0 µm. Their large $\chi^2$ values are a result of small photometric error bars in individual bands. In total, four candidates with unreliable SED fittings are discarded in the following.

For our further study in the following, we define 383 LBG candidates with reliable SED fittings at $0.7 \lesssim z_p \lesssim 1.4$ for our LBG sample at $z \sim 1$ for the following three reasons: (i) the Lyman limit (i.e. 912 Å) moves to 1550 Å at redshift 0.7, slightly away from the effective FUV wavelength, 1530 Å; (ii) the Lyman limit leaves NUV if the redshift is larger than 1.4; (iii) the uncertainty $\delta z$ of the re-estimated photometric redshifts obtained above is about 0.1 at $z \sim 1$. To clarify and for ease of comparison, we plot the distribution of LBGs in the CDF-S field in the lower panel of Fig. 1 using circles and squares, respectively, to denote LBGs without and with spectral data available from the MUSYC catalogue and other previous studies. Fig. 9 shows their redshift distribution and the corresponding distribution of 142 LBGs for morphological studies (see Section 5 for details) as the solid and dashed histograms, respectively.

Note that two AGNs would be included in the LBG sample if we were to consider only the colour-selection criterion and the redshift range. Comparing with Steidel et al. (2003) and Lehmer

<figure><img src="image_10.png" /><figcaption>**Figure 8.** The distributions of the re-estimated (solid histogram) and the original photometric redshifts from COMBO 17 (dashed histogram) for LBG candidates, with the dotted histogram denoting results from MUSYC.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Figure 9.** The redshift distributions of all LBGs and LBGs for morphological studies (see Section 5) shown as solid and dashed histograms, respectively.</figcaption></figure>
et al. (2008), the AGN contamination (~0.5 per cent) is smaller for LBGs at $z \sim 1$ than for LBGs at $z \sim 3$ (~3 per cent). This might imply that the connection between star formation activities and AGNs is stronger at higher redshifts for colour-selected samples. Moreover, in this paper, we take the same criterion concerning the presence/absence of 24-µm detections to classify red/blue LBGs, as suggested by B07. For our LBG sample at $z \sim 1$, there are 324 (85 per cent) and 59 (15 per cent) blue and red LBGs, respectively, among the total of 383 LBGs.

# 3 PHOTOMETRIC PROPERTIES OF LYMAN BREAK GALAXIES

## 3.1 Spectral types

To investigate the SpT of a galaxy using its SED fitting, we have to bear in mind that its resulting $z_p$ and SpT during the SED fitting degenerate with each other. To check the reliability, we consider the results of the second solutions of photometric redshifts $z_{p2}$ for individual LBGs. As expected, the reduced residuals of SED fittings for $z_{p2}$, called $\chi_2^2$, of the individual LBGs are always larger than $\chi^2$. More than 70 per cent of the LBGs show $\chi_2^2$ larger than 7.8. The median value of $\chi_2^2$ for LBGs is 16.7, while that of $\chi^2$ is 1.8. There are more than 80 and 70 per cent of LBGs with $\chi^2$ smaller than 4 and 3, respectively, while about 10 and 5 per cent of them show $\chi_2^2$ smaller than 4 and 3, respectively. This means that the contamination of the degeneracy between $z_p$ and SpT for LBGs is less than 5 per cent at the $1\sigma$ level. We further check the SpTs of LBGs with $\chi_2^2 < 3$ when the second solutions are adopted. It is found that more than half of them still display the same types as obtained for the first solutions. We conclude that the SpTs of LBGs obtained through SED fittings are reliable and can be used the further studies below.

The distribution of SpTs for LBGs is shown in Fig. 10 as the solid histogram, with the corresponding templates (i.e. SpT numbers) listed in Table 2. It can clearly be seen from the figure that the LBGs

in our sample are dominated by starburst galaxies, with 249 and 15 LBGs classified as BC03 and Arp 220 types, respectively. There are 97 and 10 LBGs classified as CWW- and BC03-type irregulars, respectively. Moreover, 10 LBGs and one LBG are classified as CWW-type Scd galaxies and a CWW-type Sbc galaxy, respectively. The SpT distribution is consistent with the selection of LBGs that must be star-forming galaxies at $z \sim 1$. This is also consistent with the fact that most LBGs in morphological studies are late-type galaxies with their Sérsic indices smaller than 2, as in Section 5.

Note that only one LBG in our sample (UV ID 74754) is classified as a BC03-type elliptical galaxy at $z_p = 0.834$, with the age of $\sim 1$ Gyr. The reduced $\chi^2$ of its SED fitting is 7.68, relatively larger than most LBGs but still acceptable. Its SFR and stellar mass are $22.9 \ M_{\odot} \ yr^{-1}$ and $1.4 \times 10^{11} \ M_{\odot}$ (see below). Note that the second solution of its SED fitting is not reliable because of a very large $\chi_2^2 \sim 900$ with $z_{p2} = 1.75$. It is detected in both F606W and F850LP bands, with its morphology classified visually as a bulge LBG (see Section 4). Its Sérsic indices are 0.7 and 2.2, respectively, with similar sizes of $\sim 5$ kpc in the two bands (see Section 5). This implies that the distribution of its star formation activity is extended while that of stars is compact. Because LBGs are selected according to their rest-frame UV luminosities (star formation activities), early-type galaxies with SFRs higher than the selection criterion can be selected as LBGs, although these are rare (e.g. there is only one in our sample).

To investigate the statistical properties of different SpTs in the following studies, we always divide LBGs into the starburst (SB) group for starburst-type LBGs, the irregular (Irr) group for irregular-type LBGs and the S+E (EST) group for spiral- and elliptical-type LBGs. Note that SpTs are later to earlier from the SB, Irr to EST groups. The numbers of LBGs are 264 (227 blue and 37 red), 107 (92 blue and 15 red) and 12 (5 blue and 7 red) for the SB, Irr and EST groups respectively. The fraction of blue/red LBGs decreases/increases from the SB, Irr to EST groups, consistent with the SpTs obtained. Because the number of LBGs in the EST group is too small to have statistical significance, we mainly focus on the SB and Irr groups for statistical discussions, and we only take the EST group for illustration in the following studies.

## 3.2 SEDs and UV luminosity function

Because of the restriction on the length of this paper, we do not show the results of the individual SED fits for the LBGs. Instead, in Table 6, we compile figures of the results of the SED fitting for LBGs with images detected, available in both the F606W and F850LP bands, together with their IDs, morphological parameters and images (see Section 5 for details).

The ages of the LBGs are spread from several Myr to 1.5 Gyr with a median value of $\sim 50$ Myr, which is comparable to Haberzettl et al. (2012) for LBGs at $z \sim 2$ and to Papovich, Dickinson & Ferguson (2001) for LBGs at $z \sim 3$. Similar to Shapley et al. (2001), we find that $\sim 10$ per cent of LBGs, classified as SB types, have ages younger than 10 Myr, which is different from Haberzettl et al. (2012) for LBGs at $z \sim 2$. To be more specific, the median ages for LBGs in the SB and Irr groups are $\sim 30$ and $\sim 800$ Myr, respectively. This is consistent with the fact that the ages of galaxies are generally younger for later types. Similar to B07, the median ages of red and blue LBGs are $\sim 400$ and $\sim 250$ Myr, respectively. Moreover, the intrinsic reddening of individual LBGs can be simply obtained through the adopted dust extinction law of Calzetti et al. (2000). We find that the dust extinctions in their rest-frame FUV for blue and

<figure><img src="image_12.png" /><figcaption>**Figure 10.** SpT distribution of LBGs obtained through their SED fittings with the corresponding templates (i.e. SpT numbers) listed in Table 2. The solid and dashed histograms denote all LBGs and LBGs for morphological studies (see Section 5), respectively.</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Figure 11.** Averaged rest-frame SEDs of LBGs in the SB group with the upper and lower panels denoting results without (blue LBGs) and with (red LBGs) 24-µm detections by MIPS, respectively.</figcaption></figure>

red LBGs show narrow distributions, with median values $A_{\text{FUV}}$ of 1.7 and 2.5 mag, respectively, which agrees well with B07.

For easy comparison with observations, we plot the averaged SEDs in the rest frame for red and blue LBGs (i.e. with and without 24-µm detections) in the SB and Irr groups at $z \sim 1$ in Figs 11 and 12, respectively. These figures also show the mean observed data with error bars. Section 2.2, the IRAC data of the two reddest bands (i.e. 5.8 and 8 µm) are plotted as dashed lines for comparison with the extrapolations of SEDs, because the templates adopted do not consider the dust-emission features.

It can be seen from the figures that LBGs in the SB group are, on average, brighter in their rest-frame UV than LBGs in the Irr group. This is consistent with the fact that starburst galaxies are, on average, more active in star formation than irregular galaxies.

<figure><img src="image_14.png" /><figcaption>**Figure 12.** Same as Fig. 11, but for LBGs in the Irr group.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>**Figure 13.** Rest-frame FUV LF of LBGs at $z \sim 1$ shown as points with solid and dotted error bars denoting the complete and incomplete parts, respectively. The best-fitting Schechter function is plotted as the solid line. The result of B07 is shown as the dash-dotted histogram with error bars and the best-fitting Schechter function of NUV-selected galaxies at $z \sim 1$ by Arnouts et al. (2005) is shown as the dashed line (see text for details).</figcaption></figure>

Within each group, LBGs with 24-µm detections (i.e. red LBGs) are, on average, brighter in their rest-frame NIR (i.e. on average, more massive in their stellar masses) than LBGs without 24-µm detections (i.e. blue LBGs; see also Section 3.3).

The luminosities of individual LBGs in the rest-frame FUV can be estimated through their SEDs. As pointed out by B07, the completeness of the LBG sample is 83 per cent down to NUV = 24.5. To be more accurate, we adopt NUV = 24.2 for the completion of the LBG sample (see also Burgarella et al. 2008), which corresponds to the absolute magnitude $M_{1800\AA} \sim -19$ at $z \sim 0.7$. Based on the widely adopted $1/V_{\text{max}}$ method suggested by Schmidt (1968) – and considering the uncertainties of individual luminosity bins as a result of the uncertainties of photometric redshifts – we obtain the rest-frame FUV (1800 Å) luminosity function (LF) of LBGs at $z \sim 1$. This is shown as points in Fig. 13, simply taking the error bars as Poisson statistics. Note that the points with solid and dashed error bars denote the complete and incomplete parts of the LBG sample, respectively. Adopting the Levenberg-Marquardt method (see Zaninetti 2008), we fit the rest-frame FUV LF of the LBGs by a Schechter function, shown as the solid line in Fig. 13, without considering its incomplete part. We obtain $\alpha = -1.61 \pm 0.40$, $M^* = -20.40 \pm 0.22$ and $\phi^* = (0.89 \pm 0.30) \times 10^{-3} \text{ Mpc}^{-3} \text{ dex}^{-1}$. For ease of comparison, the rest-frame FUV LF for LBG candidates in B07 and the NUV-selected galaxies at $z \sim 1$ by Arnouts et al. (2005) are also plotted as the dash-dotted histogram with error bars and the dashed line, respectively. We find that our FUV LF agrees well with that of B07 for $M_{1800\AA} < -18.5$. It decreases faster for $M_{1800\AA} > -18.5$ because the number of LBGs in our sample is 10 per cent less and the redshift range is wider in the present paper. We also find that the rest-frame FUV LF of LBGs at the bright end matches well the LF of NUV-selected galaxies at $z \sim 1$ (Arnouts et al. 2005). It is also lower by a factor of less than 2, even at the complete magnitude of $M_{1800\AA} = -19$. This implies that most NUV-selected galaxies at
<figure><img src="image_16.png" /><figcaption>**Figure 16.** Comparison between stellar masses $M_*$ of LBGs obtained in the present study and $M_{*,const}$ by taking a constant $M_*/L_K = 0.5 M_{\odot}/L_{\odot,K}$. The solid diagonal line indicates where they would agree.</figcaption></figure>

Zheng et al. 2007), because $M_*/L_K$ varies very slowly for young stellar populations and increases within a factor of 2 for stellar populations older than ~3 Gyr (Drory et al. 2004). Fig. 16 shows a comparison between the stellar masses $M_*$ of LBGs obtained in the present paper and $M_{*,const}$, by simply taking a constant $M_*/L_K = 0.5 M_{\odot}/L_{\odot,K}$ for all LBGs. It can be seen from the figure that the stellar masses obtained by these two methods are in good agreement with each other within a factor of 2, because the ages of the LBGs in the present study are younger than 1.5 Gyr. This also further confirms that the rest-frame K-band luminosity is a good indicator to estimate the stellar mass of a galaxy, especially when its age is less than 3 Gyr (Drory et al. 2004).

The distribution of stellar masses $M_*$ for LBGs at $z \sim 1$ is shown in Fig. 17 as the solid histogram. As in Fig. 15, the corresponding distribution of LBGs for the morphological studies in Section 5 is also plotted as the dashed histogram. We can see that the stellar masses $M_*$ of LBGs in our sample are distributed from $2.3 \times 10^8$ to $7.7 \times 10^{11} M_{\odot}$, with a median value of $\sim 10^{10} M_{\odot}$, which is slightly smaller than those for LBGs at $z \sim 2$ (Haberzettl et al. 2012) and at $z \sim 3$ (Papovich et al. 2001; Shapley et al. 2001), and about a factor of 10 larger than that of LBGs at $z \sim 5$ (Verma et al. 2007). Moreover, the mean stellar masses for red LBGs in the SB and Irr groups are $6.6 \times 10^{10}$ and $7.8 \times 10^{10} M_{\odot}$, larger than those of $7.7 \times 10^9$ and $1.7 \times 10^{10} M_{\odot}$ for blue LBGs in the SB and Irr groups, respectively. This is consistent with the discussion in Section 3.2 concerning the fact that red LBGs are, on average, brighter in their rest-frame NIR (i.e. they have more massive stellar masses) than blue LBGs.

The correlation between SFRs and $M_*$ for the LBGs in our sample is shown in Fig. 18, with dots, crosses and squares denoting LBGs in the SB, Irr and EST groups, respectively. The LBGs of individual groups show clear trends that the SFRs increase with the increase of $M_*$. Studying the IR SEDs of galaxies at $0 < z < 2.5$ in detail, Elbaz et al. (2011) have established a prescription of the main sequence for star-forming galaxies with the uncertainty of $\sim 0.3$ dex (i.e. a factor of 2), and have defined starbursts at different redshifts. A similar result has also been obtained by Sargent et al. (2012). For

<figure><img src="image_17.png" /><figcaption>**Figure 17.** Same as Fig. 15, but for the stellar mass $M_*$ distribution of LBGs.</figcaption></figure>

<figure><img src="image_18.png" /><figcaption>**Figure 18.** The correlation between SFRs and $M_*$ for LBGs, with dots, crosses and squares denoting LBGs in the SB, Irr and EST groups, respectively. For illustration, the lower limits of the defined starbursts (equation (14) in Elbaz et al. 2011) at $z = 1.4, 1$ and $0.7$ are plotted as dash-dotted, solid and dotted lines, respectively (see text for details).</figcaption></figure>

comparison, in Fig. 18, we plot, for illustration, the lower limits of the defined starbursts (equation (14) in Elbaz et al. 2011) at $z = 1.4, 1$ and $0.7$ as dash-dotted, solid and dotted lines, respectively. For a given line in the figure (i.e. a given redshift), its upper-left part denotes the starburst region and its lower-right region within a factor of 4 in the SFR (y-axis) of the line denotes the main sequence of star-forming galaxies.

It can be seen from the figure that LBGs in the EST and Irr groups are distributed along the main sequence of star-forming galaxies, as suggested by Elbaz et al. (2011), considering the uncertainties and
the redshift range. Most LBGs in the SB group are located in the starburst region, away from the main sequence. This is consistent with the SpT classifications of these LBGs during SED fittings. Distances to the main sequence (i.e. the inverse way of star-forming time-scales) increase, on average, for LBGs from the EST, Irr to SB groups. Greater distances to the main sequence imply more dramatic star formation. Because two-thirds of LBGs are classified as belonging to the SB group, this means that LBGs are dominated by active star-forming galaxies from the point of view of either a SpT or a star-forming time-scale.

It is interesting that the median $M_*$ of LBGs increases very rapidly from $z \sim 5$ to $z \sim 3$ and then varies slowly to $z \sim 1$, while median SFRs are similar for LBGs at different redshifts (see earlier the second paragraph of this subsection). If we simply extrapolate the prescription suggested by Elbaz et al. (2011) and Sargent et al. (2012) to $z \gtrsim 2.5$, the main sequence of star-forming galaxies changes by a factor of $\sim 3.5$ while the median SFR/$M_*$ changes by a factor of $\sim 10$ from $z \sim 5$ to $z \sim 3$. This implies that the star formation mode for LBGs at $z > 3$ might be mainly starburst, with the significance increasing with $z$. Also, it might evolve to be more significant to the quenching mode after $z \sim 3$. This possible picture is consistent with the fact that the epoch of major mergers ends at $z \sim 2$ (Conselice et al. 2003, 2011). We suggest that the fraction of starburst galaxies increases with redshift for LBGs at $z > 3$.

## 3.4 Downsizing effect

The correlation between specific SFRs (sSFRs), SFR/$M_*$, and the stellar masses of LBGs is plotted in the upper panel of Fig. 19 with the same notations as the points in Fig. 18. Dashed and solid lines,

<figure><img src="image_19.png" /><figcaption>**Figure 19.** Upper panel: similar to Fig. 18, but for the correlation between sSFR and $M_*$ for LBGs, with dashed and solid lines denoting the observational limit of NUV $\lesssim 26.2$ mag and the simple least-squares fit to the data. Lower panel: results of the least-squares fit to data at different redshifts (see text for details).</figcaption></figure>

respectively, denote the observational limit of NUV $\lesssim 26.2$ mag and the simple least-squares fitting to the data with the slope of $-0.68$. A clear trend for LBGs can be seen from the figure (i.e. the downsizing effect), such that the sSFR decreases with the increase of stellar mass $M_*$, except for the observational limit in NUV. It can also be seen from the figure that the significance of this downsizing effect decreases for LBGs from the SB, Irr to EST groups. This is consistent with the fact that the distance to the main sequence decreases for LBGs from the SB, Irr to EST groups (see Section 3.3). The downsizing effect indicates that LBGs with less massive stellar masses build up their stellar blocks relatively faster. This effect has been already found by Zheng et al. (2007) and Buat et al. (2008) for star-forming galaxies at $z \lesssim 0.9$; see also Elbaz et al. (2007), Rodighiero et al. (2010) and Gruppioni et al. (2010) for galaxies at $z \sim 2$.

Usually, the downsizing effect can be parametrized as sSFR $\propto M_*^{-\gamma}$, with $\gamma \sim 0$ for the extreme case of quenching (quiescent) star formation. Together with the observed correlation between sizes $R$ and stellar masses $M_*$ for galaxies, the downsizing effect, in fact, has many physical implications for galaxy formation and evolution, in particular closely related to the structural parameters of galaxies, such as compactness and the surface densities of galaxies. A more qualitative discussion follows.

The observed correlation of $R-M_*$ for galaxies can be described as $R \propto M_*^{\beta}$. For active star formation galaxies, such as merging galaxies, $\beta \sim 0.2$ at the redshift range of $0 \lesssim z \lesssim 3.5$ (Shen et al. 2003; Mosleh et al. 2011; Ichikawa, Kajisawa & Akhlaghi 2012). For quenching star formation by gas accretion, such as local quiescent galaxies, the mean stellar surface densities in the central regions of galaxies display small dynamic ranges, for example, with $\beta \sim 0.5$ (Shen et al. 2003; see also Chang et al. 2010).

If we simply assume that the stellar sizes and star formation distributions of a galaxy are similar (see also Section 5), we obtain $R \propto \text{SFR}^{\beta/(1-\gamma)}$. For LBGs dominated by star-forming galaxies, $R \sim \text{SFR}^{0.6}$ with $\gamma \sim 0.68$ and $\beta \sim 0.2$. This implies that LBGs with larger SFRs have, on average, more compact star-forming regions, which is consistent with the existence of a very weak correlation with scatters between Sérsic indices/sizes in the F606W images and SFRs for LBGs in Section 6.2 and in Elbaz et al. (2011). Furthermore, for the mean SFR surface density (i.e. the mean rest-frame UV brightness) in the central regions $\Sigma_{*,c}$ of a galaxy, we have $\Sigma_{*,c} \propto \text{SFR}^{1-2\beta/(1-\gamma)}$. For our LBGs, for $\alpha \sim 0.68$ and $\beta \sim 0.2$, $\Sigma_* \propto \text{SFR}^{-0.2}$, which implies that the mean rest-frame UV surface densities in the central regions of LBGs display a small dynamic range (i.e. they are insensitive to their total SFRs). This is consistent with previous studies on the surface brightness for LBGs at $z \gtrsim 3$ and star-forming galaxies at $z \lesssim 1.1$ (Giavalisco et al. 1996; Barden et al. 2005; Akiyama et al. 2008). So, if we simply take $\Sigma_{*,c}$ to be constant for LBGs, their $R-M_*$ relation at $z \sim 1$ can be a direct result of the downsizing fitting with the slope $\beta \sim 0.68/2 = 0.34$. This is consistent with the slopes of $\sim 0.36$ for the correlations obtained in Section 6 between sizes in F606W and F850LP and stellar masses for LBGs, respectively (see Fig. 30).

It is interesting to compare the downsizing effect of our LBG sample with other previous studies on star-forming galaxies at different redshifts from $z \sim 0.3$ to $z \sim 3$, as shown in the lower panel of Fig. 19. For simplicity, in the figure we present the results of least-squares fitting to the data. From top to bottom, the lines denote the results of $z \sim 3$, 1.4, 1, 0.9, 0.7, 0.5 and 0.3, with the slopes $\gamma$ of 0.68, 0.67, 0.68, 0.53, 0.45, 0.32 and 0.25, respectively. Data are from Zheng et al. (2007) for $z \sim 0.3$, 0.5, 0.7 and 0.9, from Dunne et al. (2009) for $z \sim 1.4$ and from Magdis et al. (2010a) for $z \sim 3$.
As can be seen from the figure, the significance of the downsizing effect for star-forming galaxies increases (i.e. slope $\gamma$ increases) with redshift until $z \sim 1$. It is also very interesting that slope $\gamma$ remains unchanged for $z \gtrsim 1$. According to the above analysis, this implies that distributions of star formation for galaxies are, on average, less and less compact after $z \sim 1$, while they show similar compactness before $z \sim 1$. Because star formation is dominated by galaxy merging at high $z$, this also implies that galaxy mergers become rare after $z \sim 1$. Although slope $\gamma$ decreases from 0.68 at $z \gtrsim 1$ to 0.25 at $z \sim 0.3$, the power index of the mean star formation activities in the central region of a galaxy $|1 - 2\beta/(1 - \gamma)| < 0.5$, which implies that the mean central rest-frame UV surface brightness of a galaxy is not sensitive to its total SFR.

## 3.5 Colours and stellar masses

Based on the DEEP2 (Davis et al. 2003) and COMBO-17 surveys, Faber et al. (2007) have studied in detail the LFs of galaxies to $z \sim 1$. Their sample contains 39 000 galaxies in total, with 15 000 beyond $z = 0.8$. They have found that galaxies display clear bimodality in the plane of rest-frame colour $U - B$ versus stellar mass (i.e. the red sequence and the blue cloud). The number and total stellar masses of blue galaxies are substantially constant since $z \sim 1$, while the number densities of red galaxies (near $L^*$) significantly increase. Some suggestions are given for the ways in which galaxies evolve from the blue cloud to the red sequence. Because the results of Faber et al. (2007) provide strong constraints on galaxy formation and evolution, there have been many further studies since then, together with discussions on the green valley between the blue cloud and the red sequence (Cattaneo et al. 2010; Cortese & Hughes 2009; Schawinski et al. 2010; Lemaux et al. 2012; Skelton, Bell & Somerville 2010).

In Fig. 20, we plot the distributions and the corresponding density contours for LBGs in the SB (upper panel) and Irr (middle panel) groups, in the rest-frame colour $U - B$ versus stellar mass $M_*$ diagram. Contours from the outside to the inside of the figure correspond to the mean number densities of LBGs and the overdense contrasts of two and three times the mean densities, respectively. The lower panel in the figure shows the mean values and the corresponding standard deviations for LBGs in the SB and Irr groups. Results for LBGs in the EST group are plotted in the lower panel as dashes, just for simple illustration. For ease of comparison with Faber et al. (2007), the solid lines in the upper and middle panels correspond exactly to the position and direction of the major axis of the blue cloud in fig. 10 of Faber et al. (2007). The rest-frame colour $U - B$ is in the Vega system and the stellar mass in the x-axis decreases from left to right.

We can see that the LBGs in our sample are located in the blue cloud, consistent with the fact that they are star-forming galaxies selected by rest-frame UV luminosities. The LBGs in the SB group have a wider range in stellar masses, as also seen in Fig. 18. It is interesting that most LBGs are located in the lower part if we divide the blue cloud into two parts along the major axis. This implies that LBGs, on average, display lower dust attenuation than star-forming (blue) galaxies selected by the other methods. This is in agreement with the analysis of dust properties for star-forming galaxies by Buat et al. (2005, 2012) and Reddy et al. (2012) (see also references therein). Although the number of LBGs in the EST group is too small to have statistical significance, there is a clear trend that LBGs of earlier types are, on average, redder and have more massive stellar masses. As claimed by Conselice et al. (2003, 2011), the epoch of major mergers ends at $z \sim 2$. LBGs at $z \sim 1$,

<figure><img src="image_20.png" /><figcaption>**Figure 20.** The distributions and the corresponding density contours for LBGs in the SB (upper panel) and Irr (middle panel) groups, respectively, in the diagram of rest-frame colour $U - B$ versus stellar mass $M_*$. Contours from outside to inside correspond to the mean number densities of LBGs and the overdense contrasts of two and three times the mean densities, respectively.</figcaption></figure>

especially starburst types with low stellar masses, seem to maintain their star formation activities for a relatively long time, so they can evolve to have masses such as the local $L^*$ present-day galaxies. We suggest that LBGs at $z \sim 1$ evolve along the blue cloud and then finally arrive in the red sequence as red galaxies. Other mechanisms have also been investigated, such as the re-merging processes in particular at $z \sim 1$ by Firmani, Avila-Reese & Rodríguez-Puebla (2010).

## 3.6 Summary of the photometric properties

We list the detailed photometric properties of the 383 LBGs in our sample at the redshift range of $0.7 \lesssim z_p \lesssim 1.4$ in Table 5. Columns 1–3 denote the corresponding IDs of individual LBGs in UV from B07, and from the COMBO 17 and MUSYC catalogues, respectively. Re-estimated photometric redshifts $z_p$ of individual LBGs, together with their corresponding error estimates in $3\sigma$, are listed in Column 4. Note that $z_p$ are replaced by spectroscopic redshifts $z_{spe}$ if available, except for two with bad spectroscopic observations (see Section 2.1). SFRs and stellar masses are given in Columns 5 and 6, respectively. The 24-µm detections of MIPS compiled by B07 are given in Column 7, with the number 99 indicating that there are no detections. Column 8 shows the goodness of SED fittings (reduced $\chi^2$), with the resulting SpTs in Column 9. The full table can be found at http://202.121.53.133/ZhuChen/. Because the figures of SED fittings for individual LBGs are not easy to compile in the table, we show the SEDs of those LBGs for morphological studies in Table 6, together with their HST images (see below).
<table><caption>**Table 5.** Detailed photometric properties of LBGs in the redshift range 0.7 ≲ z<sub>p</sub> ≲ 1.4. Columns 1–3 denote the corresponding IDs of individual LBGs in UV from B07, and from the COMBO 17 and MUSYC catalogues, respectively. Column 4 lists the re-estimated photometric redshifts z<sub>p</sub> of individual LBGs, together with their corresponding error estimates in 3σ. Note that z<sub>p</sub> are replaced by spectroscopic redshifts z<sub>spe</sub> if available, except for two with bad spectroscopic observations. SFRs and stellar masses are given in Columns 5 and 6. Column 7 lists the 24-µm detections of MIPS compiled by B07, with 99 indicating that there were no detections. Column 8 shows the goodness of SED fittings (reduced χ<sup>2</sup>), with the resulting SpTs (SpT numbers) in Column 9. The complete table is available as Supporting Information in the online version of this article.</caption><thead><tr><th>ID<sub>UV</sub></th><th>ID<sub>COMBO17</sub></th><th>ID<sub>MUSYC</sub></th><th>z<sub>p</sub> (z<sub>spe</sub>)</th><th>SFR (M<sub>☉</sub> yr<sup>-1</sup>)</th><th>Log (M<sub>*</sub>/M<sub>☉</sub>)</th><th>24 µm</th><th>χ<sup>2</sup></th><th>SpT</th></tr></thead><tbody><tr><td>81517</td><td>63245</td><td>79110</td><td>0.974<sup>+0.009</sup><sub>-0.006</sub></td><td>37.1</td><td>11.1</td><td>19.74</td><td>7.80</td><td>13</td></tr><tr><td>47798</td><td>20406</td><td>25420</td><td>1.330<sup>+0.010</sup><sub>-0.008</sub></td><td>124.3</td><td>11.4</td><td>99</td><td>3.09</td><td>13</td></tr><tr><td>47839</td><td>20506</td><td>25134</td><td>0.980<sup>+0.037</sup><sub>-0.035</sub></td><td>18.8</td><td>10</td><td>99</td><td>0.51</td><td>13</td></tr><tr><td>47853</td><td>20341</td><td>25125</td><td>1.145<sup>+0.020</sup><sub>-0.036</sub></td><td>32.2</td><td>9.9</td><td>99</td><td>0.76</td><td>13</td></tr><tr><td>47897</td><td>20480</td><td>—</td><td>0.930<sup>+0.011</sup><sub>-0.030</sub></td><td>14.2</td><td>10.6</td><td>99</td><td>8.34</td><td>13</td></tr><tr><td>49702</td><td>22572</td><td>27552</td><td>0.861<sup>+0.073</sup><sub>-0.049</sub></td><td>19.8</td><td>10.7</td><td>20.33</td><td>2.66</td><td>13</td></tr><tr><td>50258</td><td>23164</td><td>28404</td><td>0.974<sup>+0.060</sup><sub>-0.023</sub></td><td>17.5</td><td>10.0</td><td>99</td><td>2.24</td><td>13</td></tr><tr><td>50616</td><td>23587</td><td>29012</td><td>(1.046)</td><td>10.0</td><td>10.8</td><td>99</td><td>1.18</td><td>11</td></tr><tr><td>52269</td><td>25529</td><td>31702</td><td>0.867<sup>+0.011</sup><sub>-0.012</sub></td><td>34.6</td><td>10.9</td><td>20.40</td><td>2.41</td><td>13</td></tr><tr><td>...</td><td>...</td><td>...</td><td>...</td><td>...</td><td>...</td><td>...</td><td>...</td><td>...</td></tr></tbody></table>

# 4 HST IMAGES FOR THE LBG SAMPLE

## 4.1 Images

The GEMS survey (Rix et al. 2004), together with the central region from the GOODS project (Dickinson et al. 2004), is centred on the CDF-S area. This is the 90 per cent field of the present study for LBGs that is covered by the GEMS and GOODS surveys. So, we can find HST optical images for LBGs with very high quality from both the GEMS and GOODS surveys using the coordinates of their optical counterparts to study quantitatively their morphological properties. Here, we include a brief introduction to the GEMS and GOODS surveys.

The GEMS survey is a large-area (i.e. 800 arcmin²), two-colour – F606W (V band) and F850LP (z band) – imaging survey with the ACS on the HST (Rix et al. 2004). Centred on the CDF-S, it covers an area of ~28 × 28, arcmin² to a depth of $m_{AB}$(F606W) = 28.3 and $m_{AB}$(F850LP) = 27.1 in 5σ for compact sources. In its central (~1/4) field, there are 15 tiles for which GEMS incorporates ACS imaging from the GOODS project. The GOODS survey covers approximately 300 arcmin² in two fields of the HDF-N and CDF-S. It provides ACS images in four bands: F435W(B), F606W(V), F775W(i) and F850LP(z). To ensure data homogeneity, the GEMS group has reduced the first epoch of GOODS-S data at the centre of the overall GEMS area, taking exactly the same processes as the reduction for the GEMS data, although the GOODS images are slightly shallower than those of GEMS. Details of the GEMS<fnref n="4" /> and GOODS<fnref n="5" /> surveys can be found from their websites.

According to the coordinates of optical counterparts of individual LBGs through the cross-identifications in Section 2, we find 336 LBGs in the GEMS and GOODS-S fields. The fraction of LBGs within the fields of the GEMS and GOODS-S surveys is similar to the fraction of the sky coverage, which implies that the morphological studies of these 336 LBGs can represent all the LBGs of our sample. Images of F606W and F850LP from the GOODS-S

and GEMS surveys can be downloaded from the GEMS website. In the overlapping region between GOODS-S and GEMS, which is a small fraction, images with better qualities are chosen. Note that for our LBG sample at redshift 0.7 ≲ z ≲ 1.4, images of F606W are intended to represent star-forming regions, while images of F850LP mainly reflect the distributions of colder stars.

## 4.2 Visual classifications of morphologies

Here, we first establish an image gallery of LBGs in a variety of shapes, with classifications by eye. We cut areas of 5 × 5 arcsec² individually as counter images of LBGs centred on their COMBO-17 coordinates. Because the LBGs in our sample are at z ~ 1, images of 5 × 5 arcsec² (corresponding to 40 × 40 kpc² at z ~ 1) are large enough to study their morphologies. The widely used software SExTRACTOR<fnref n="6" /> (Bertin & Arnouts 1996) is used to detect LBGs in images, with the detection threshold chosen as 3σ for F606W and F850LP, respectively (see also Section 5). As pointed out by Caldwell et al. (2008), GEMS can reliably detect most COMBO 17 galaxies with magnitudes R < 24, but its detection ability decreases significantly for objects with R > 24. Because there are 59 LBGs in the GEMS and GOOD-S fields with R-band magnitudes fainter than 24, the images of some LBGs located within the GEMS and GOODS surveys are missed.

In total, there are 309 and 279 LBGs detected by SExTRACTOR successfully in their F606W and F850LP counter images, respectively, with 277 LBGs being commonly detected in both bands. The fact that more LBGs are detected in F606W is consistent with the fact that LBGs are bright in F606W, because they are star-forming galaxies at z ~ 1 and because the depth of F606W is slightly deeper. The spatial distributions of detected/undetected LBGs in the GOOD-S and GEMS fields are roughly the same. This implies that the slightly different observational depths between GOODS and GEMS do not influence the following morphological studies for our LBG sample.

For ease of comparison, in this paper, the LBGs detected in the images of both bands are used for the morphological studies. For

<footnote n="4">⁴ http://archive.stsci.edu/prepds/gems/</footnote>

<footnote n="5">⁵ http://www.stsci.edu/science/goods/</footnote>

<footnote n="6">⁶ http://www.astromatic.net/software/extractor</footnote>
<table><caption>**Table 6.** Details of the morphological properties of 277 LBGs with images detected in both the F606W and F850LP bands. Column 1 in the first row lists the UV IDs of LBGs in B07, which is the same as the first column in Table 5. Columns 2 and 3 in the first row list the Sérsic indices in F606W and F850LP, respectively, with their corresponding half-light radii (sizes) obtained by GALFIT in Columns 4 and 5. Columns 6 and 7 in the first row list the indicator flags of the GALFIT results and the visual classifications of LBG images. Note that flag (0/1) denotes a reliable/unreliable result of the GALFIT fitting, and types 1–5 denote tadpole, chain, clump, bulge and spiral LBGs, respectively, with 6 denoting bad pixel images. The second row shows LBG images detected by SEXTRACTOR, modelled by GALFIT, and residual images with the upper and lower panels for F606W and F850LP bands, respectively. The figures of the corresponding SED fittings are on the right-hand side of the second row, with arrows denoting upper limits of the flux detections. The complete table is available as Supporting Information in the online version of this article.</caption><thead><tr><th>ID<sub>UV</sub></th><th>n<sub>F606W</sub></th><th>n<sub>F850LP</sub></th><th>r<sub>F606W(kpc)</sub></th><th>r<sub>F850LP(kpc)</sub></th><th>flag</th><th>type</th></tr></thead><tbody><tr><td>47839</td><td>99</td><td>99</td><td>99</td><td>99</td><td>1</td><td>3</td></tr></tbody></table><table><thead><tr><th>ID<sub>UV</sub></th><th>n<sub>F606W</sub></th><th>n<sub>F850LP</sub></th><th>r<sub>F606W(kpc)</sub></th><th>r<sub>F850LP(kpc)</sub></th><th>flag</th><th>type</th></tr></thead><tbody><tr><td>47897</td><td>5.69</td><td>3.92</td><td>7.10</td><td>4.56</td><td>0</td><td>4</td></tr></tbody></table><table><thead><tr><th>ID<sub>UV</sub></th><th>n<sub>F606W</sub></th><th>n<sub>F850LP</sub></th><th>r<sub>F606W(kpc)</sub></th><th>r<sub>F850LP(kpc)</sub></th><th>flag</th><th>type</th></tr></thead><tbody><tr><td>49702</td><td>99</td><td>99</td><td>99</td><td>99</td><td>1</td><td>3</td></tr></tbody></table>
<table><caption>Table 6 – continued</caption><thead><tr><th>ID<sub>UV</sub></th><th>n<sub>F606W</sub></th><th>n<sub>F850LP</sub></th><th>r<sub>F606W(kpc)</sub></th><th>r<sub>F850LP(kpc)</sub></th><th>flag</th><th>type</th></tr></thead><tbody><tr><td>50258</td><td>0.55</td><td>1.40</td><td>4.70</td><td>2.60</td><td>0</td><td>3</td></tr></tbody></table><table><thead><tr><th>ID<sub>UV</sub></th><th>n<sub>F606W</sub></th><th>n<sub>F850LP</sub></th><th>r<sub>F606W(kpc)</sub></th><th>r<sub>F850LP(kpc)</sub></th><th>flag</th><th>type</th></tr></thead><tbody><tr><td>50616</td><td>99</td><td>99</td><td>99</td><td>99</td><td>1</td><td>3</td></tr></tbody></table><table><thead><tr><th>ID<sub>UV</sub></th><th>n<sub>F606W</sub></th><th>n<sub>F850LP</sub></th><th>r<sub>F606W(kpc)</sub></th><th>r<sub>F850LP(kpc)</sub></th><th>flag</th><th>type</th></tr></thead><tbody><tr><td>52269</td><td>99</td><td>99</td><td>99</td><td>99</td><td>1</td><td>3</td></tr></tbody></table>
<figure><img src="image_21.png" /><figcaption>**Figure 21.** Images of three chain LBGs with UV IDs of 60187, 75408 and 43975 in B07, from left to right. The upper and lower panels denote the images in F606W and F850LP, respectively.</figcaption></figure>

59 LBGs undetected or only detected in one band, they display low SFRs/stellar masses, as expected. Most are classified as starburst-type LBGs, consistent with Fig. 18. For visual classifications of LBG morphologies, we follow the prescriptions of Elmegreen et al. (2009). Note that two LBGs (with UV IDs 74154 and 81517 in B07) are found to have many bad pixels in their images in both bands, although they are detected by *SEXTRACTOR*. These two LBGs are excluded from the following morphological studies.

It is found, by eye, that 10 LBGs show anomalous morphologies that display very chaotic shapes and chain-like structures, and contain multiple bright knots. These LBGs are classified visually as chain LBGs in the present paper. As examples, images of three chain LBGs in F606W and F850LP are displayed in Fig. 21. Several bright knots of star-forming regions in the images for chain LBGs suggest that they are ongoing merging processes in a very young and active dynamical stage (Abraham et al. 1999). Their visual morphologies are consistent with their SpTs of starbursts or irregulars, with relatively high SFRs and massive stellar masses.

Seven LBGs can clearly be seen by eye in their counter images of both bands as large spiral galaxies individually, with two prominent components of discs and bright central bulges. These galaxies are classified as spiral LBGs, with examples shown in Fig. 22. It is found that all of these have high SFRs and massive stellar masses, and are classified in the EST group as Sbc/Scd/E-type galaxies during their SED fittings in Section 2.

It is also easily found that 23 LBGs clearly show very close companions in their images of either the F606W or F850LP band.

<figure><img src="image_22.png" /><figcaption>**Figure 22.** Same as Fig. 21, but for three spiral LBGs with UV IDs of 47798, 36485 and 59934 in B07, from left to right, respectively.</figcaption></figure>

<figure><img src="image_23.png" /><figcaption>**Figure 23.** Same as Fig. 21, but for three tadpole LBGs with UV IDs of 79716, 81474 and 73352 in B07, from left to right, respectively.</figcaption></figure>

We call these galaxies ‘tadpole’ LBGs, with three examples shown in Fig. 23. They are classified as starburst/Arp 220/irregular types, consistent with their morphological classifications by eye.

There are 132 LBGs that display very clumpy star-forming regions spreading across wide areas in their images. We call these ‘clump’ LBGs, with three examples shown in Fig. 24. In fact, these are marginally detected by *SEXTRACTOR*. All of them are classified as starburst-type LBGs with relatively low SFRs and low stellar masses, very similar to those LBGs without detections or with detections available in only one band.

Except for the above-classified LBGs, the remaining 103 LBGs show very regular shapes in both counter images, and are called bulge LBGs. Examples of these are shown in Fig. 25. In their SED fittings, most of these are classified as starburst galaxies.

Among the 277 commonly detected LBGs in both bands, 224 (81 per cent) and 53 (19 per cent) LBGs are classified as blue and red LBGs, respectively. Although the fractions are similar to the whole LBG sample, blue and red LBGs display very different fractions in the visual classifications. For instance, four and three red LBGs are spiral and chain LBGs, respectively. Five of the 23 tadpole LBGs and 28 of the 132 clump LBGs are red LBGs. To conclude, red LBGs are always more structured visually than blue LBGs.

We compile an image gallery of the 277 LBGs detected in both F606W and F850LP in Table 6, with their visual classifications. Quantitative studies of the morphological properties can be found in Section 5.

<figure><img src="image_24.png" /><figcaption>**Figure 24.** Same as Fig. 21, but for three clump LBGs with UV IDs of 78053, 36914 and 58532 in B07, from left to right, respectively.</figcaption></figure>
<figure><img src="image_25.png" /><figcaption>**Figure 25.** Same as Fig. 21, but for three bulge LBGs with UV IDs of 69074, 57264 and 54502 in B07, from left to right, respectively.</figcaption></figure>

# 5 MORPHOLOGICAL PROPERTIES OF LBGs

## 5.1 Morphological analysis

The powerful software GALFIT, with the one-Sérsic-component model, is adopted to analyse the morphological properties of LBGs. GALFIT is a galaxy/point-source fitting algorithm developed by Peng et al. (2002, 2010),<fnref n="7" /> which fits two-dimensional parametrized axis- ally symmetric functions directly to images. We refer to Peng et al. (2002, 2010) for details.

In the present paper, we focus on 277 LBGs with images detected in both F606W and F850LP bands (see Section 4). The Sérsic index and the half-light radius, the most important structural parameters of a galaxy, are obtained by analysing the images with GALFIT. Instead of the details for the processes and other parameters chosen, simple prescriptions are given, as follows. SExTRACTOR (Bertin & Arnouts 1996) is applied first to the individual counter images of LBGs to obtain their initial parameters and to mask files required by GALFIT, with the detection threshold being chosen as 3σ for both F606W and F850LP bands. The point spread function (PSF) adopted here is generated by the TINYTIM software (Krist 1993), which is designed to reproduce the HST modelled PSF.

Although for the 277 LBGs with images detected successfully by SExTRACTOR in both F606W and F850LP bands, we can obtain their Sérsic indices and half-light radii through GALFIT with the one- Sérsic-component model, the results for half of the LBGs are not physically reliable. For instance, the seven spiral LBGs classified in Section 4.2 cannot be reliably fitted, because they display at least two components in their images. Obviously, the 10 chain LBGs can- not be fitted using GALFIT, because they show multiple components with different shapes. There are 87 of the 132 clump LBGs whose residual images still have several bright knots after using GALFIT. This implies that they display complicated structures. Another five of the 132 clump LBGs are very faint, with low surface brightness, and are detected marginally by SExTRACTOR. Thus, it is very difficult for GALFIT to obtain their true physical parameters subtracted from the sky background. These LBGs are also excluded from our mor- phological studies, although the results for these from GALFIT seem to be acceptable. Moreover, two LBGs (with UV IDs 74154 and 81517 in B07) must be ruled out because their images show many bad pixels in both bands. In total, 142 LBGs, consisting of 127 blue LBGs and 15 red LBGs, have reliable results from GALFIT, with their images detected in both F606W and F850LP bands.

It is important to understand whether these 142 LBGs can re- present the morphological properties of all LBGs at $z \sim 1$ without significant bias. The distributions of redshifts, SpTs, SFRs and stel- lar masses $M_*$ for these 142 LBGs with reliable GALFIT results are also plotted in Figs 9, 10, 15 and 17 as dashed histograms, in order to compare with the results for all LBGs. It can be seen that the distributions of these 142 LBGs are very similar to those for all LBGs. So, it is reasonable for us to take these as the morphological sample, to represent the whole LBG sample, in order to study the morphological properties of LBGs.

The details of the morphological properties of 277 LBGs with images detected in both F606W and F850LP bands are summarized individually in Table 6. The table consists of two rows for each LBG. Column 1 in the first row denotes the UV IDs of LBGs in B07, which is the same as the first column in Table 5. Columns 2 and 3 in the first row list the Sérsic indices in F606W and F850LP, with their corresponding half-light radii (sizes) obtained by GALFIT in Columns 4 and 5, respectively. Columns 6 and 7 in the first row list the indicator flags of the GALFIT results and the visual classifications of LBG images. Note that flag (0/1) denotes a reliable/unreliable result of the GALFIT fitting, and types 1–5 denote tadpole, chain, clump, bulge and spiral LBGs, respectively, with 6 denoting a bad pixel image. The second row shows the LBG images detected by SExTRACTOR, modelled by GALFIT, and the residual images with the upper and lower panels for F606W and F850LP bands, respectively. The figures of the corresponding SED fittings are shown in the right- hand side of the second row. The full table can be downloaded at http://202.121.53.133/ZhuChen/.

## 5.2 Sérsic indices and sizes

The Sérsic profile is one of the most popular ways of studying the morphologies of galaxies. The Sérsic index $n$ is often used to describe the concentration of the light profile for a galaxy. It displays a steeper inner profile and a more extended outer wing when $n$ is larger. The effective radius $r_{\text{eff}}$ describes the size of a galaxy, which contains its half of the light within $r_{\text{eff}}$. Usually, galaxies with $n \lesssim 2$ and $n > 2$ are classified as disc types and elliptical types respectively (see Peng et al. 2002, 2010).

In Fig. 26, the distributions of Sérsic indices for 142 LBGs (i.e. the morphology sample), obtained by GALFIT in the images of F606W and F850LP bands, are shown as hatched and empty histograms, respectively. It is found that about 80 per cent of the LBGs have Sérsic indices smaller than 2, with a median value of around 1 for both bands. We conclude that most LBGs are disc-like late- type galaxies. This is consistent with Burgarella et al. (2006, 2008) who, after partly analysing their counterparts from GOODS data, have suggested that ~75 per cent of LBGs at $z \sim 1$ are disc-like. Haberzettl et al. (2012) have found a similar result (i.e. the majority of LBGs in the sample at $z \sim 2$ show disc-like structures).

Giavalisco et al. (1996) have found that star-forming galaxies (LBGs at $z \sim 3$) in their sample are compact, with sizes and scale- lengths similar to present-day bulges or intermediate-luminosity spheroids, while they are often surrounded by lower surface bright- ness nebulosities. However, the multicolour morphologies of some of their LBGs clearly show similar irregularities. Here, deeper im- ages with higher resolution and signal-to-noise ratio, provided by the GOODS and GEMS surveys, make it possible for us to obtain more reliable conclusions, especially for the outskirts of LBGs, which could not be detected clearly then and which play important roles in the morphology analysis.

<footnote n="7">7 http://usersobs.carnegiescience.edu/peng/work/galfit/galfit.html</footnote>
<figure><img src="image_26.png" /><figcaption>**Figure 26.** The distributions of Sérsic indices *n* for 142 LBGs with reliable GALFIT results in F606W and F850LP, shown as the hatched and empty histograms, respectively.</figcaption></figure>

<figure><img src="image_27.png" /><figcaption>**Figure 27.** Comparison between Sérsic indices for LBGs obtained in F606W and F850LP bands, with dots, crosses and squares denoting LBGs in the SB, Irr and EST groups, respectively. The solid line denotes where they would agree.</figcaption></figure>

For a disc galaxy, the central concentration of its stars in the space distribution always increases with age, and that of star formation activity evolves in an inverse way. For interacting disc galaxies, the induced star formation activities usually occur more in the outskirts than their stars (Rawat, Wadadekar & De Mello 2009). Note that Sérsic indices in F606W and F850LP reflect the central concentrations of star formation activities and stars in space distributions, respectively, for LBGs at $z \sim 1$. It is interesting to compare the resulting Sérsic indices of 142 LBGs in F606W images with those in F850LP images, as shown in Fig. 27. As in Fig. 18, the LBGs in the SB, Irr and EST groups are labelled with different symbols for illustration.

<figure><img src="image_28.png" /><figcaption>**Figure 28.** The distributions of sizes for 142 LBGs with reliable GALFIT results in F606W and F850LP, shown as empty and hatched histograms, respectively.</figcaption></figure>

It can be seen from the figure that LBGs in individual groups have similar distributions. Most of the disc-like LBGs with $n_{850LP} \lesssim 2$ have Sérsic indices in F850LP that are larger than those in F606W. This implies that their stellar distributions are more centrally concentrated than their star formation activities, similar to the local old disc galaxies or interacting disc galaxies. Furthermore, LBGs with $n_{850LP} \lesssim n_{F606W} \lesssim 2$ might be very young disc galaxies, according to the above analysis.

Another important physical quantity obtained by GALFIT is the effective radius (size) of an LBG, within which half of its luminosity is included. As for Sérsic indices, in Fig. 28, we plot the distributions of sizes in F606W and F850LP for the LBG morphology sample as the hatched and empty histograms, respectively. It can be seen that the median values for the sizes of the LBGs are 2.34 and 2.68 kpc in F606W and F850LP, respectively. These values are larger than the median size of LBGs in UV at $z \sim 3$ (Shu et al. 2001) and similar to that of local disc galaxies (Shen et al. 2003) if the cosmic evolution is simply taken into account as $(1+z)^{-1}$. Assuming that the sizes of LBGs result from the angular momentum of their host haloes, the Gaussian-like distributions of the logarithmic sizes for two bands can be easily explained by the Gaussian distribution for the spin parameters in the logarithm of haloes (Cole & Lacey 1996).

As in Fig. 27, a comparison between the sizes of LBGs obtained in F606W and F850LP is shown in Fig. 29. It can be seen that the distributions of LBGs in individual groups are similar. For LBGs with $r_{F606W} \gtrsim 3$ kpc, $r_{F606W}$ agrees well with $r_{F850LP}$. For LBGs with $r_{F606W} \lesssim 2$ kpc, $r_{F606W}$ is often smaller than $r_{F850LP}$. This implies that smaller LBGs must be displayed as more compact in F606W (i.e. more compact star formation regions), so that they can have SFRs high enough to be selected as LBGs.

# 6 CORRELATIONS BETWEEN PHOTOMETRIC AND MORPHOLOGICAL PROPERTIES

As mentioned in Section 5, 142 LBGs with reliable morphological parameters in the GOODS and GEMS fields can represent all LBGs
<figure><img src="image_29.png" /><figcaption>**Figure 29.** Comparison between the sizes of LBGs obtained in F606W and F850LP, with dots, crosses and squares denoting LBGs in the SB, Irr and EST groups, respectively. The solid line denotes where they would agree.</figcaption></figure>

at z ~ 1 for the morphological studies. Among these 142 LBGs, 79, 59 and four are in the SB, Irr and EST groups, respectively, classified according to their SpTs during their SED fittings (see Section 3.1). Here, we investigate the correlations between their photometric and morphological properties. To limit the length of the paper, only figures with strong correlations are shown.

**6.1 Spectral types versus Sérsic indices and sizes**

Because the number of LBGs in the EST group is too small to
have statistical significance, we only focus on LBGs in the SB and
Irr groups. The mean Sérsic indices of LBGs in the SB and Irr
groups are 1.30/1.43 and 1.07/1.17 for the F606W/F850LP bands,
respectively. Moreover, the mean sizes for LBGs in the SB and
Irr groups are 1.82/2.15 and 3.09/3.35 kpc for the F606W/F850LP
bands, respectively. This implies that LBGs in the SB group, on
average, show more compact distributions of both star formation
activities and stars than LBGs in the Irr group. This is consistent with
the implications obtained from the downsizing effect in Section 3.4
and with the recent study of Elbaz et al. (2011). In fact, the same
conclusion can be drawn from the visual classifications in Section 4.

6.2 SFRs versus Sérsic indices and sizes

Because the number of LBGs in the morphological sample is only 142, we discuss the correlations between SFRs and morphological parameters (i.e. Sérsic indices and sizes) as a whole, instead of grouping LBGs into different SpTs. It is found that both Sérsic indices $n_{\text{F606W}}$ and sizes $r_{\text{F606W}}$ of LBGs in the F606W band display weak correlations with scatters of increasing with SFRs. Because images of the F606W band represent distributions of star formation activities for LBGs at $z \sim 1$, this implies that more compact LBGs in F606W, on average, have higher SFRs. This is also consistent with Elbaz et al. (2011) and the implication from the downsizing effect that LBGs with higher SFRs are, on average, more compact (see Section 3.4).

However, neither Sérsic indices $n_{\text{F850LP}}$ nor sizes $r_{\text{F850LP}}$ of LBGs in the F850LP band show any significant correlations with SFRs with large scatters. This is consistent with the images of the 850LP band mainly represent stellar distributions for LBGs at $z \sim 1$ rather than their star formation activities, as discussed in Section 4.1.

6.3 Stellar masses versus Sérsic indices and sizes

Because of large scatters, no significant correlations between Sérsic
indices in the F606W/F850LP bands and stellar masses $M_*$ for
LBGs can be found. For the F606W band, this can be simply ex-
plained as its irrelevance to stellar distributions for LBGs at $z \sim 1$.
Because images of the F850LP band reflect stellar distributions of
LBGs, the insignificant correlation between $n_{\text{F850LP}}$ and $M_*$ suggests
that their stellar distributions might have similar profiles. LBGs are
more massive in their stellar masses mainly because they have larger
sizes (see below). This implies that mean stellar surface densities in
the central regions of LBGs similarly have a small dynamic range
as their SFR surface densities, which is consistent with implications
from the downsizing effect in Section 3.4.

Finally, we find strong correlations between sizes $r_{\text{F606W}}/r_{\text{F850LP}}$ in the F606W/F850LP bands and stellar masses $M_*$ (i.e. the size-stellar mass relations) for LBGs, as shown in Fig. 30. In the figure, we plot the distributions of LBGs and the corresponding density contours, together with the mean values and the standard deviations as error bars for different mass bins. As in Fig. 20, contours from outside to inside correspond to the mean number densities of LBGs and the overdense contrasts of two and three times the mean densities, respectively.

It can be found that the sizes in both F606W and F850LP bands
increase with stellar masses for LBGs significantly. Because there is
no significant correlation between Sérsic indices and stellar masses,
LBGs with more massive stellar masses certainly have larger sizes

<figure><img src="image_30.png" /><figcaption>**Figure 30.** The distributions of LBGs and the corresponding density contours, together with the mean values and the standard deviations as error bars for different mass bins in the size-stellar mass plans, with the results of the F606W and F850LP bands in the upper and lower panels. Contours from outside to inside in the figure correspond to the mean number densities of LBGs and the overdense contrasts of two and three times the mean densities, respectively (see text for details).</figcaption></figure>
in the F850LP band. The correlation between sizes in the F606 band and their stellar masses for LBGs can be easily obtained from their size-stellar mass relation in the F850LP band and the downsizing effect in Section 3.4. Ignoring the contribution of the most massive bin due to a few LBGs in Fig. 30, we can easily obtain the slopes of 0.37 ± 0.04 and 0.36 ± 0.04 with the same corresponding coefficient of 0.99 for the size-stellar mass relations in the F606W and F850LP bands, respectively. This is in agreement with Mosleh et al. (2011) and Ichikawa et al. (2012). Together with the least-squares fitting for the downsizing effect, this again implies that the mean rest-frame UV surface densities (i.e. the mean SFR surface densities) of LBGs in their central regions display a small dynamic range.

## 7 CONCLUSIONS

B07 have defined a preliminary sample of 420 LBG candidates, with the completeness ~80 per cent down to NUV ~24.8, at $z \sim 1$ using FUV - NUV >2 from the deep *GALEX* observation in the CDF-S field, according to their photometric redshifts from COMBO-17. Four confirmed AGNs have been included. Although the global accuracies of photometric redshifts in both the COMBO 17 and MUSYC catalogues are high, the differences of the photometric redshifts for LBG candidates between these two catalogues are significant. Together with the MUSYC *BVR*-selected catalogue, we re-estimate their photometric redshifts using broader multiwavelength data from UV, optical to NIR using the code *HYPERZ*. Comparisons between re-estimated photometric redshifts and spectroscopic redshifts, if available, show that our photometric redshift determinations are good. After carefully cross-identifying LBG candidates one-to-one with their optical counterparts, and considering their re-estimated photometric redshifts and SEDs, we refine a new updated sample of 383 LBGs at $0.7 \lesssim z \lesssim 1.4$, excluding two AGNs in this redshift range. This means that the AGN contamination is about 0.5 per cent, which is smaller than that of ~3 per cent for LBGs at $z \sim 3$ (Steidel et al. 2003; Lehmer et al. 2008).

According to SED fittings, 260 and 111 LBGs are classified as starburst galaxies and irregulars, respectively, with 11 classified as Sbc/Scd types and only one classified as an elliptical. For easy comparison by future studies, the averaged SEDs of LBGs are presented for SB/Irr types and blue/red LBGs, respectively. The ages of the LBGs range from several Myr to 1.5 Gyr, with a median value of ~50 Myr, comparable to Haberzettl et al. (2012) for LBGs at $z \sim 2$ and to Papovich et al. (2001) for LBGs at $z \sim 3$. The median dust attenuations in the rest-frame FUV for blue and red LBGs are 1.7 and 2.5 mag, respectively, in good agreement with the previous study by B07. Based on the $1/V_{max}$ method and considering the uncertainties of individual luminosity bins because of the uncertainties of photometric redshifts, we obtain the rest-frame FUV LF of LBGs with completion down to $M_{1800\AA} = -19$. By adopting the Levenberg-Marquardt method with the consideration of the two-dimensional error bars, a Schechter parametrization is applied to the LF and we obtain the best-fitting parameters of $\alpha = -1.61 \pm 0.40$, $M^* = -20.40 \pm 0.22$ and $\phi^* = (0.89 \pm 0.30) \times 10^{-3}$ Mpc$^{-3}$ dex$^{-1}$. According to Arnouts et al. (2005), this implies that more than half of the NUV-selected galaxies at $z \sim 1$ are selected as LBGs. We have presented simple comparisons with previous studies on FUV LFs at different redshifts, and brief discussions.

The dust-corrected SFRs of LBGs are from 4 to 220 $M_{\odot}$ yr$^{-1}$, with a median value of ~25 $M_{\odot}$ yr$^{-1}$, which is similar to that for LBGs at $z \sim 3$ (Shapley et al. 2001; Steidel et al. 2003) and slightly smaller than those for LBGs at $z \sim 2$ (Haberzettl et al. 2012) and $z \sim 5$ (Verma et al. 2007). The stellar masses of individual LBGs

are obtained according to their rest-frame K-band luminosities and the calculated mass-to-light ratios in the K band through their SED fittings. The results are similar to those when simply taking a constant mass-to-light ratio in the K band of $M_*/L_K = 0.5 M_{\odot}/L_{\odot, K}$. The stellar masses $M_*$ of LBGs range from $2.3 \times 10^8$ to $4 \times 10^{11} M_{\odot}$, with a median value of ~$10^{10} M_{\odot}$ which is comparable to those of LBGs at $z \sim 2$ (Haberzettl et al. 2012) and $z \sim 3$ (Papovich et al. 2001; Shapley et al. 2001) and about 10 times larger than that of LBGs at $z \sim 5$ (Verma et al. 2007). As expected, the mean SFR of LBGs in the SB group is larger than that of LBGs in the Irr group. The mean stellar mass of red LBGs is larger than that of blue LBGs.

It is found that LBGs in the Irr and EST groups are distributed in the SFR-$M_*$ diagram along the main sequence of star-forming galaxies suggested by Elbaz et al. (2011), with the considerations of uncertainties and the redshift range. Most LBGs in the SB group are located in the starburst region, away from the main sequence. Distances to the main sequence (i.e. the inverse way of star-forming time-scales) increase from earlier to later SpTs for LBGs, on average. Because the median $M_*$ of LBGs increases very rapidly from $z \sim 5$ to $z \sim 3$ and then varies slowly to $z \sim 1$, and because the median SFRs are similar for LBGs at different redshifts, this suggests that the star formation mode for LBGs at $z > 3$ is mainly starburst, with the significance increasing with $z$, and evolving to be more significant to the quenching mode after $z \sim 3$. This is consistent with the fact that the epoch of major mergers ends at $z \sim 2$ (Conselice et al. 2003, 2011). We predict that the fraction of starburst galaxies will increase with redshift for LBGs at $z > 3$.

Taking the observational limit of NUV $\lesssim 26.2$ mag into account, the downsizing effect is clearly found for LBGs at $z \sim 1$, and the significance of the effect decreases for LBGs from the SB, Irr to EST groups. We have presented detailed discussions on the implications of the downsizing effect for galaxy formation and evolution, especially for structural parameters, such as the compactness and surface densities of galaxies. Taking the observed size-stellar mass relation into account, we suggest that LBGs with larger SFRs are, on average, more compact. We suggest that the mean rest-frame UV surface densities in the central regions of LBGs are insensitive to their SFRs, in agreement with Giavalisco et al. (1996), Barden et al. (2005) and Akiyama et al. (2008). The size-stellar mass relation can be a direct result of the downsizing effect. We have presented comparisons with previous studies on the downsizing effect at different redshifts. We have found that the downsizing effect is more significant with the increase of $z$ up to $z \sim 1$.

In the rest-frame colour $(U - B)-M_*$ diagram, LBGs at $z \sim 1$ are distributed in the blue cloud. It is interesting that most LBGs are located in the lower part of the blue cloud, which suggests that LBGs display lower dust attenuation, on average, than star-forming galaxies selected by other methods. This is consistent with Buat et al. (2005, 2012) and Reddy & Steidel (2009), and references therein. Considering LBGs of starburst types with low stellar masses, we suggest that LBGs might evolve along the blue cloud and finally arrive in the red sequence.

Because the GEMS and GOODS-S surveys by the *HST* cover ~90 per cent of the field of our LBG sample, high-quality images in F606W (V band) and F850LP (z band) have been downloaded from the Multimission Archive at the Space Telescope Science Institute (STScI) for morphological studies of LBGs. The fraction of LBGs within the GEMS and GOODS-S fields is similar to the fraction of the sky coverage. The counter images of LBGs are analysed using *SEXTRACTOR*. There are 309 and 279 LBGs successfully detected in the images of F606W and F850LP, respectively, with 277 commonly detected in both bands. We establish an image gallery of
these 277 LBGs by visually classifying their images individually as chain, spiral, tadpole, bulge and clump types, respectively. We have concluded that red LBGs are more structured than blue LBGs.

The powerful software GALFIT of the one-component model is applied to the LBG images available, detected in both F606W and F850LP bands. A morphological sample of 142 LBGs with reliable GALFIT results has been established to represent all LBGs. We have found that more than 80 per cent of LBGs in the morphological sample have Sérsic indices smaller than 2 in both bands. We have concluded that LBGs at z ~ 1 are dominated by disc-like galaxies, consistent with Burgarella et al. (2006, 2008). A similar result has been obtained by Haberzettl et al. (2012) for LBGs at z ~ 2. The distributions of half-light radii (sizes) in F606W and F850LP are similar, with median values of 2.34 and 2.68 kpc, respectively. If the cosmic evolution factor is simply taken as (1 + z)^-1, the median sizes of LBGs are comparable to local disc galaxies.

We have investigated the correlations between photometric and morphological properties of LBGs. Note that LBGs display strong correlations between their half-light radii and $M_*$ (i.e. the size-stellar mass relation) in both bands. Similar slopes of 0.036 ± 0.04 in the two bands, consistent with Mosleh et al. (2011) and Ichikawa et al. (2012), have been obtained, with their linear fitting coefficient of 0.99. For Sérsic indices, a weak correlation between $n_{F606W}$ and SFRs suggests that more compact LBGs have higher SFRs. An insignificant correlation between $n_{F850LP}$ and $M_*$ implies that the stellar distributions of LBGs might have similar profiles. It should be pointed out that correlations between photometric and morphological properties for LBGs can be explained by the implications of the downsizing effect.

## ACKNOWLEDGEMENTS

We are grateful to an anonymous referee for a thorough review of the manuscript and for valuable comments. ZC is very grateful for the financial support provided by DB and to the Laboratoire d'Astrophysique de Marseille for a visit. ZC is also grateful to DB and VB for their hospitality during the stay in Marseille. This work is partly supported by the Chinese National Nature Science Foundation (Nos 10878003, 10833005 and 10778725), the Shanghai Science Foundation and the Leading Academic Discipline Project of Shanghai Normal University (DZL805). Some/all of the data presented in this paper were obtained from the Multimission Archive at the Space Telescope Science Institute (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Support for MAST for non-HST data is provided by the NASA Office of Space Science via grant NNX09AF08G and by other grants and contracts.

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## SUPPORTING INFORMATION

Additional Supporting Information may be found in the online version of this article:
**Table 5.** Detailed photometric properties of LBGs in the redshift range 0.7 ≲ z_p ≲ 1.4.

**Table 6.** Details of the morphological properties of 277 LBGs with images detected in both the F606W and F850LP bands (http://mnras.oxfordjournals.org/lookup/suppl/doi:10.1093/ mnras/stt230/-/DC1).

Please note: Oxford University Press are not responsible for the
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directed to the corresponding author for the article.

This paper has been typeset from a TeX/LATEX file prepared by the author.