Proceedings Title IAU Symposium
Proceedings IAU Symposium No. 245, 2004
A.C. Editor, B.D. Editor & C.E. Editor, eds.

c 2004 International Astronomical Union
DOI: 00.0000/X000000000000000X

Adaptive Optics Imaging of Lyman BreakGalaxies as Progenitors of Spheroids in theLocal Universe †M. Akiyama1 Y.Minowa2 N.Kobayashi3 K.Ohta4 and I.Iwata5
1

Subaru Telescope, National Astronomical Observatory of Japan, 650 North A’ohoku Place,Hilo, HI, 96720, USAemail: akiyama@naoj.org
2Optical and Infrared Astronomy Division, NAOJ, Osawa, Mitaka, 181-8588, Japan
3
Institute of Astronomy, University of Tokyo, Osawa, Mitaka, 181-0015, Japan
4
Department of Astronomy, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
5Okayama Astrophysical Observatory, NAOJ, Okayama 719-0232, Japan

Abstract. In order to reveal the stellar mass distribution of z ∼ 3 galaxies, we are conducting
deep imaging observations of U-dropout Lyman Break Galaxies (LBGs) with Adaptive Optics(AO) systems in K-band, which corresponds to rest-frame V -band of z ∼ 3 galaxies. The resultsof the Subaru intensive-program observations with AO36/NGS/IRCS indicate that 1) the Kband peaks of some of the LBGs brighter than K = 22.0 mag show signiﬁcant oﬀset from thosein the optical images, 2) the z ∼ 3 MV∗ LBGs and serendipitously observed Distant Red Galaxies(DRGs) have ﬂat proﬁles similar to disk galaxies in the local universe (i.e., Sérsic with n < 2),and 3) the surface stellar mass densities of the MV∗ LBGs are 3-6 times larger than those of
disk galaxies at z = 0 − 1. Considering the lack of n > 2 systems among the luminous z ∼ 3
LBGs and DRGs, and their strong spatial clustering, we infer that the dence n < 2 disk-likestructures evolve into the n > 2 spheroids of nearby galaxies through relaxations due to major
merger events.
Keywords. galaxies:formation, galaxies: high-redshift, cosmology:observations

1. Introduction
The statistical evolution of the morphology of galaxies is one of the most fundamental observational clues to understand the establishing process of the Hubble sequence ofgalaxies seen in the local universe. Especially rest-frame optical morphology is importantbecause it reﬂects stellar mass distribution of galaxies. High-resolution imaging observations with Hubble Space Telescope (HST), covering rest-frame optical wavelength up toz ∼ 1, have shown that the Hubble sequence is already established at z ∼ 1. Morphological studies on galaxies at higher redshifts started following the discovery of a large
number of high redshift galaxies using Lyman Break selection (Steidel et al. 2003). HSTNICMOS observations of the Hubble Deep Field North (HDFN) show that the morphology of the LBGs is essentially independent of the wavelength up to the rest-frame 4000Å
(Giavalisco 2002).
However, the NICMOS observations are not suﬃcient to conclude the distribution of
the stellar mass in z ∼ 3 LBGs. The NICMOS observations covers up to the H-band,† Based on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan.

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Figure 1. Left) K-band images (upper) and optical images with K-band contour of the 4brightest z ∼ 3 LBGs in the sample. A 3. 5times3. 5 (27kpc×27kpc at z = 3). FoV is shown.
North is to the top and east is to the left. We do not match the PSF size of the K-band contour
to that of the optical images, thus only the positions of the peaks should be compared. Right)
Distance between K-band and optical peaks as a function of K-band magnitudes for the z ∼ 3
LBGs (ﬁlled squares) and ﬁeld object (crosses).

which is just at 4000Å in the rest-frame of z ∼ 3 galaxies. In addition, the NICMOS observations are mostly limited to the z ∼ 3 LBGs in the small HDFN area and only includeobject as bright as MB ∼ −22mag, which is still 0.5mag fainter than the characteristic
absolute magnitude of the z ∼ 3 LBGs (MV∗ = −24.0 mag).
In order to examine the rest-frame V -band morphology of the z ∼ 3 LBGs covering awide luminosity range (MV∗ −0.5 mag − MV∗ +3.0 mag), we conduct AO-assisted imaging
observations of 36 of them in the K-band, which corresponds to the rest-frame V -bandin z ∼ 3 galaxies (Akiyama et al. 2007). Thirty one of the LBGs are detected in the deep
imaging observations with typical eﬀective integration time of 5 hours.

2. Results of AO imaging observations of the z ∼ 3 LBGs
The AO-assisted K-band images of the 4 brightest z ∼ 3 LBGs are shown in the upper
left panel of Figure 1 as examples. The AO observations clearly resolve most of the 31z ∼ 3 LBGs at the resolution of FWHM∼ 0. 2.
In order to examine their total J −K colors, which corresponds to the rest-frame U −Vcolors, we obtain J-band data of thirty of the LBGs under bad seeing condition. Thebright LBGs show red rest-frame U − V colors (average of 0.2 mag), while most of thefainter LBGs show blue rest-frame U − V color (average of −0.4 mag). The color distribution of the LBGs show that the LBG sample covers not only blue less massive galaxies
but also red massive galaxies at among 2 < z < 3 galaxies selected with photometricredshifts.
In the lower left panels of Figure 1, the K-band contours of the LBGs are comparedwith the optical images with the seeing-limited resolution. The peaks in the K-band
images of the LBGs show signiﬁcant oﬀset from those in the optical images. In the rightpanel of Figure 1, the shifts of the LBGs are shown as a function of K-band magnitudewith ﬁlled squares for the whole sample. In order to evaluate the uncertainties of the shiftmeasurements, we also plot the diﬀerences of K-band and optical positions of compact
ﬁeld objects with crosses. The peaks in the K-band images of 7 of the LBGs with K < 22
mag show signiﬁcant or marginal shifts from those in the optical images. The presence

AO Observations of LBGs at z ∼ 3121Figure 2. Left) K-band proﬁles of an LBG along the semi-major axis. The red-solid,blue-dashed, and green-dotted lines show the best ﬁt Sérsic (with free n), r 1/4 (Sérsic withn = 4), and exponential (Sérsic with n = 1) proﬁles, respectively. Long dashed line show theestimated proﬁles of the PSFs at the object positions. Right) Re vs. n for the z ∼ 3 LBGs
(ﬁlled squares) and DRGs (open squares) with K < 21.5 mag. Simulated z = 3 galaxies thatare brighter than K = 21.5 mag in the 2 mag PLE model are plotted with blue small crosses.

of the shifts among red luminous LBGs implies that the UV-bright star-forming regions
are not necessarily centered at their main body observed in the K-band.
In order to examine whether their light proﬁles are similar to those of local galaxies
or not, we apply one component Sérsic proﬁle ﬁtting to the AO-assisted K-band imagesof the LBGs with K < 21.5mag. An example of the Sérsic proﬁle ﬁtting is shown in theleft panel of Figure 2 (DSF2237b-MD81 with K = 20.2 mag at z = 2.82). As can be seenin the panel, the exponential proﬁle (Sérsic proﬁle with n = 1) ﬁt the proﬁle better than
the r1/4 proﬁle (with n = 4), and the best-ﬁt n is 0.9 with free n ﬁtting. The results ofthe Sérsic proﬁle ﬁtting of the LBGs and DRGs with K < 21.5 mag are shown in theright panel of Figure 2 with ﬁlled and open squares, respectively. The images of all but
one of the LBGs and DRGs with K < 21.5 mag are ﬁtted well with Sérsic proﬁle with nindex less than 2, similar to disk galaxies in the local universe.
In order to directly compare the best-ﬁt parameters with those of galaxies at intermediate redshifts, we make simulated images of z ∼ 3 galaxies by ”cloning” HST/ACS
images of galaxies at z = 0.4 − 0.7. Their distribution is shown with blue crosses in thepanel. The LBGs and DRGs locate similar part of the Re vs. n plane to the disk galaxies
at intermediate redshifts.
Assuming that the z ∼ 3 LBGs and DRGs have a disk shape, we compare their sizeluminosity and size-stellar mass relations with those of disk galaxies at low/intermediateredshifts in the left and right panels of Figure 3, respectively. The z ∼ 3 LBGs arebrighter than z = 0 and z = 1 disk galaxies at the same Re . The surface brightnessof the LBGs, which are estimated from MV and Re , are 2.2-2.9 mag and 1.2-1.9 magbrighter than those of the disk galaxies at z = 0 and z = 1, respectively. The size-stellarmass relation indicates that the z ∼ 3 LBGs brighter than MV∗ have the average surface
stellar mass density 3-6 times larger than those of the z = 0 and z = 1 disk galaxies. On
the contrary, for less-luminous z ∼ 3 LBGs, their size-stellar mass relation is consistent
with those of z = 0 and z = 1 disk galaxies.

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Figure 3. Left) MV vs. Re for the LBG sample (ﬁlled squares). Large and small symbols
indicate Re obtained with the Sérsic proﬁle ﬁtting with free n and ﬁxed n = 1, respectively.
Open squares show the DRG sample. Contours show the distributions of z = 0 (dashed bluecontour) and z = 1 (solid red contour) disk-galaxies from Barden et al. (2005). The think blue
and red dashed lines shown < µV > of disk galaxies at z = 0 (20.84 mag arcsec−2 ) and z = 1(19.84 mag arcsec−2 ), respectively. Right) M∗ vs. Re of the LBG (ﬁlled squares) and DRG (opensquares) samples. Contours show the distributions of z = 0 and z = 1 disk-galaxies from Bardenet al. (2005). The solid line represents the relation log ΣM (M kpc−2 = 8.50 derived from thedisk galaxies at z = 0 − 1 (Barden et al. 2005).

3. Speculation: LBGs before z ∼ 3 and after
Considering the diﬀerence between the mass of the dark matter halos the z ∼ 3 LBGs
reside (2 − 6 × 1011 M ; Adelberger et al. 2005) and the Jeans mass at the time ofreionization (∼ 1010 M ), we naively expect that the z ∼ 3 LBGs form from builingblocks through several major merges. The high fraction of disk-like galaxies in the highredshift universe would be explained with a hypothesis that they have been going throughonly gas-rich ”wet” merges at that time (e.g., Springel & Hernquist 2005).
Because in the local universe, such high surface stellar mass density disk as the massivez ∼ 3 LBGs is rare, we expect that the disks of the massive z ∼ 3 LBGs are destroyed
between z ∼ 3 to z ∼ 1. The strong spatial clustering of the LBGs implies that they
reside in massive dark halos and that they evolve into local elliptical galaxies (Adelbergeret al. 2005). Therefore, the high surface stellar mass density disks would evolve into local
elliptical galaxies through ”dry” merging events.
We start an AO observing program with laser guide stars in order to extend the sampleof bright z ∼ 3 galaxies, and to establish the morphological evolution between z = 3 toz = 0. The high-resolution K-band imaging observations of the galaxies at z  3 should
be one of unique ﬁelds which are explored only with LGS AO on ground-based 8-10mclass telescopes until the launch of the James Webb Space Telescope.
References
Adelberger, K.L., Steidel, C., Pettini, M., Shapley, A.E., Reddy, N.A., & Erb, D.K. 2005, ApJ619, 697
Akiyama, M., Minowa, Y., Kobayashi, N., Ohta, K., Ando, M., Iwata, I. 2007, ApJS submitted
Barden, M., et al. 2005, ApJ 635, 959
Giavalisco, M. 2002, ARA&A 40, 579
Springel, V., & Hernquist, L. 2005, ApJ 622, L9Steidel, C., et al. 2003, ApJ 592, 728

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