# The Low-Mass End of theMBH/Mhost Relation in Quasars

Authors: Roberto Decarli; Renato Falomo; Jari K. Kotilainen; Tomi Hyvönen; Michela Uslenghi; Aldo Treves
Journal: Advances in Astronomy
 (2012)

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## Abstract

TheMBH-Mhost relation in quasars has been probed only in a limited parameter space, namely, at
MBH~109 M⊙ and Mhost~1012  M⊙. Here we present a study of 26 quasars laying in the low-mass
end of the relation, down to MBH~107 M⊙. We selected quasars from the SDSS and HST-FOS
archives, requiring modest MBH (as derived through the virial paradigm). We imaged our sources in H band from the Nordic Optical Telescope. The quasar host galaxies have been resolved in 25 out
of 26 observed targets. Host galaxy luminosities and stellar masses are computed, under reasonable
assumptions on their star formation histories. Combining these results with those from our previous
studies, we manage to extend the sampled parameter space of the MBH-Mhost relation in quasars.
The relation holds over 2 dex in both the parameters. For the first time, we are able to measure the slope of the MBH-Mhost relation in quasars. We find that it is consistent with the linear case (similarly to what observed in
quiescent galaxies). We do not find any evidence of a population of massive black holes lying below
the relation.


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## Body

## 1. Introduction

Massive black holes (BHs) are ubiquitously found in the centre of massive galaxies [1–3]. Their masses (MBH) show strong correlations with large-scale properties of the host galaxies, namely, the stellar velocity dispersion (σ*, [4–7]), the luminosity, and mass of the spheroidal component (Lhost, Mhost; see [8–10]). These relations have been interpreted as the outcome of a joint evolution between BHs and their host galaxies [11–18]. In this scenario, the growth of BHs through accretion regulates the gas cooling in the outskirt of host galaxies through energy or momentum injection (feedback), thus quenching the formation of stars. Galaxy mergers may also play a role in this scenario, as gravitationally induced dynamical instabilities may trigger both star formation bursts and gas inflows fuelling the BH activity ([12, 19–21], see also [22]).TheMBH-host galaxy relations have been pinned down on an albeit small set of local, mostly quiescent galaxies. The sampled parameter space ranges over 3 dex in masses, from a few million to a few billion solar masses in terms of MBH. Extending these studies beyond the local Universe is challenging. On one side, the influence radius of BHs, Rinf, that is, the radius where the gravitational potential is dominated by the singularity, is resolved only in very nearby objects (distances < few tens Mpc) with high MBH values. For any other sources, indirect tracers of MBH are required. The most commonly adopted indirect estimator of MBH is based on the width of broad emission lines and the size of the broad line region (BLR). This can be done only in type-1 AGN, where broad lines are observed [23–26]. This approach allows to estimate MBH from single-epoch spectra in ~100 000 quasars up to z~5 from SDSS spectra [27], and in most of the z~6 quasars known to day [28–31]. (Caveats to this technique arise as BLR clouds may be supported by radiation pressure [32] or move nonvirially; projection effects depending on the geometry and orientation of the BLR may hinder our ability to actually measure the orbital velocity of clouds [33, 34]; different emission lines may be produced in regions where the gas dynamics are different [35–39].)On the other side, the properties of host galaxies are hard to measure in distant sources. Bright active nuclei (necessary to measureMBH) can easily outshine the light of their host galaxies. Observations in excellent seeing conditions (e.g., [40, 41])or based on adaptive optics (e.g., [42, 43]) are required. Diffraction-limited observations with HST have also significantly contributed in this field [44–54], although some concerns about the reproducibility of the PSF have been arisen [51]. Up to now, ~300 quasar host galaxies have been resolved up to z≈3, and most of them at z<0.5 (see [40], and references therein).In order to understand the processes and timescales which led to the onset of the BH-host galaxy relations, two key observational tests are required. The first one consists intracing the evolution of theBH-to-host galaxy mass ratio (Γ≡MBH/Mhost) as a function of Cosmic Time. If, for example, Γ(z>0)<Γ(z≈0), then we can argue that the BHs in these systems still have to accrete in already formed bulges. Vice versa, Γ(z>0)>Γ(z≈0) could suggest a rapid growth of the BHs, followed by a slower build-up of the spheroids. Most of the studies on the evolution of the BH-host galaxy relations suggest that at high redshift, for a given mass of the host spheroid, the harbored BH is more massive than at low-z [50, 53, 55–58], with Γ(z)∝(1+z)3/2. It is interesting to note that the host galaxy of J1148 + 5251, the highest-z SDSS quasar, at z=6.42, shows an MBH-host galaxy dynamical mass ratio of ≈0.13 [59, 60], in an order-of-magnitude agreement with the extrapolation from the z<3 studies.A second observational test to probe the onset of the BH-host galaxy relations is totrace the low-mass end of the BH-host galaxy relations. Different initial host galaxy mass, BH seed mass, and build-up processes produce different slopes of the relations, especially at low mass. Light seeds (10-100M⊙) are expected as the remnants of metal-pure stars in the early Universe, while heavier seeds (up to ~105M⊙) can result from the direct collapse of primordial gas clouds. The former ones would produce a larger scatter in the MBH-host relations, a higher occupation fration in relatively small galaxies, and a lower cutoff in the minimum MBH with respect to the latter (see, e.g., [61], and references therein). Some authors even claim that the MBH-σ* relation itself is just the upper limit of a broader distribution, with a number of (hard-to-detect) modest-mass BHs embedded in relatively massive galaxies (e.g., [62]).The low-mass end of theMBH-host galaxy relations has been probed down to few 105MBH in quiescent galaxies or in low-luminosity AGN at low-z [54, 63–67], suggesting that the MBH-host relations hold in quiescent or mildly active galaxies. However, no effort has been attempted so far to extend this test to higher luminosity AGN. Quasars are ideal probes of the BH-host galaxy relations at z>0. However, while Γ in quasars has been measured up to very high redshift, the ranges of MBH and Mhost investigated up to date are limited, and comparable with the observed scatter of the BH-host galaxy relations. Filling the low-mass end of the MBH-Mhost relation therefore represents a main challenge and a fundamental step in our comprehension of the BH-host galaxy evolution. In this paper, we present ground-based NIR observations of quasars at z<0.5 selected so that virial MBH<109MBH. Our imaging campaign successfully resolved 25 quasar host galaxies. This enables us to directly probe the slope of the MBH-Mhost relation in quasars.The structure of this work is the following: in Section2 we describe the sample. In Section 3 we present the analysis of the spectra and we derive MBH in all our sources. The new observations, the data reduction and analysis, and the results from the imaging campaign are presented in Section 4. In Section 5 we discuss our results. Conclusions are summarized in Section 6. Throughout the paper we will assume a standard cosmology with H0=70kms−1 Mpc−1, Ωm=0.3, and ΩΛ=0.7.


## 2. The Sample

We selected quasars atz<0.5 with available Hβ observations in the Sloan Digital Sky Survey (SDSS [68]) spectroscopic database or CIV or MgII observations in the HST-Faint Object Spectrograph (FOS) archive. We require that black hole virial masses, computed as described in Section 3, range between 107 and 109 M⊙. Note that, out of the 62 z<0.5 quasars examined in our previous study [26], only 8 (13%) had MBH<109M⊙. We then selected quasars having at least 3 relatively bright (mH=10–15 mag) stars within 2′ (corresponding to the half size of the NOTCam field of view), and a number of fainter stars in order to have an accurate description of the Point Spread Function (PSF). Observability constraints and modest weather losses further limited our analysis to 26 targets (see Table 1), mostly detected in radio wavelengths according to the [69] catalogue. This new sample is then matched with the 62 z<0.5 targets presented in our previous study of the MBH-Mhost relation [26, 58]. Figure 1 compares the distribution of MBH in the general SDSS sample at z<0.5, computed from the continuum luminosities and Hβ width estimates reported by [27], with the ones presented in [26] and in the present study.Table 1
The sample. (1) Target name. (2-3) Right ascension and declination (J2000). (4) Is the target detected in radio wavelengths, according to the [69] catalogue? (5) Catalogue redshift. (6) Apparent visual magnitude of the quasar.
NameRADECRadio?zV [mag](1)(2)(3)(4)(5)(6)PB572300 05 47.5+02 03 02N0.23416.60PG0026+1200 29 13.7+13 16 04Y0.14515.41PG0052+25100 54 52.1+25 25 39Y0.15515.43B20110+2901 13 24.2+29 58 16Y0.36317.00PKS0214+1002 17 07.6+11 04 10Y0.40816.46J02321+000802 32 11.8+00 08 03Y0.43219.10J02331-090902 33 10.6−09 09 40Y0.38818.45J03010+000403 01 00.2+00 04 29Y0.48619.33J03323+010603 32 18.0+01 06 48N0.48218.91J03579-055003 57 59.0−05 50 15Y0.43918.93B20752+25A07 55 37.0+25 42 39Y0.44618.00J08044+190408 04 42.1+19 04 26Y0.34619.40J08285+274808 28 53.5+27 48 33N0.33020.00J08305+080208 30 57.4+08 02 34Y0.31919.20PG0844+34908 47 42.4+34 45 03Y0.06414.50J09010+353809 01 00.9+35 38 09N0.30219.20PG0947+39609 50 48.3+39 26 51Y0.20616.39PG0953+41509 56 52.4+41 15 23Y0.23415.32TON118710 13 03.1+35 51 23N0.07914.75TEX1156+21311 59 26.2+21 06 56Y0.34916.90Q1214+180412 16 49.0+17 48 04N0.37417.30PG1404+22614 06 21.9+22 23 47Y0.09815.82PG1415+45114 17 00.8+44 56 06Y0.11415.24PG1626+55416 27 56.1+55 22 31Y0.13215.684C73.1819 27 48.5+73 58 02Y0.30216.50PKS2251+1122 54 10.4+11 36 39Y0.32515.82Comparison among theMBH distributions in the SDSS quasars at z<0.5 [27] (a), in our 2010 study (b) and in the present work (c). The latter is clearly more representative of the general quasar population at low redshift.
(a)(b)(c)


## 3. The Spectroscopic Dataset

The spectroscopic dataset consists of pipeline-processed, publicly available spectra from the SDSS or FOS archive. SDSS spectra haveλ/Δλ~2000 and a spectral range between 3800 and 9000 Å. Uncertainties on wavelength calibration amount to 0.05 Å, while flux calibration formal errors account to 5%. FOS spectra are taken from the compilation of recalibrated quasar and AGN FOS spectra by [70]. Observations were performed with a number of different gratings at both high spectral resolution (1–6 Å diode−1, λ/Δλ≈1300) and low spectral resolution (6–25 Å diode−1, λ/Δλ≈250) covering various spectral windows from 1140 Å to 3275 Å. Photometric uncertainties are usually 5–10%, while typical wavelength calibration uncertainties are around 0.5 channels rms (see [70]).Spectral analysis follows the same approach described in [26] and [31]. Briefly, continua are modeled as a superposition of a power-law component, the host galaxy emission (only in the optical spectra; the Elliptical model by [71] was adopted as a template), and the FeII multiplets (modeled on the template by [72]). Relevant broad emission lines (Hβ, MgII, and CIV) are modeled with the superposition of two Gaussian curves with the same peak wavelength. The spectral resolution of the SDSS data is adequate to allow an easy identification of the narrow Hβ emission with respect to the broad line. Examples of the line fitting are shown in Figure 2.Examples ofHβ line fitting. The observed spectra, shifted to the rest frame, are plotted in solid, black lines. The various components of the models are shown: the power-law (black, dotted line), the host galaxy and FeII templates (green, solid line), the line model (red, solid line), and its components (red, dotted lines). Fit residuals are shown in blue, dashed lines.
(a)(b)We measure the continuum luminosity at 1350, 3000, and 5100 Å from the fitted power-law. The broad line luminosity and FWHM are measured from the line model (see [36], for a discussion on the fitting technique and the parametrization of the relevant quantities). Virial black hole masses are computed with the same recipes used in [26], adopting geometrical factors of 1.6 for Hβ and MgII and 2.4 for CIV. Table 2 lists the main measurements and inferred quantities from the spectroscopic analysis. (We note that, after a careful reanalysis of the spectra, two objects (B20110+29 and B20752+25A) show values of MBH slightly exceeding the initial selection criteria. Nevertheless, we include these sources in the present analysis.)Table 2
Results from the spectroscopic analysis. (1) Quasar name. (2) Redshift. (3) Line used in theMBH estimate. (4) Continuum monochromatic luminosity at 1350 (for CIV), 3000 (for MgII) or 5100 Å (for Hβ), in ergs−1. (5) Line FWHM in km s−1. (6) Virial estimate of the black hole mass, in solar units.
NamezLinelogλLλ  [erg s−1]FWHM [km s−1]logMBH [M⊙](1)(2)(3)(4)(5)(6)PB57230.234CIV44.7137158.15PG0026+120.145CIV45.2220627.92PG0052+2510.155CIV45.3359148.90B20110+290.363Hβ44.8161499.33PKS0214+100.408CIV45.7141228.79J02321+00080.432Hβ44.4117277.56J02331-09090.388Hβ44.6318637.77J03010+00040.486Hβ44.4566348.76J03323+01060.482Hβ44.7742828.59J03579-05500.439Hβ44.6240058.43B20752+25A0.446Hβ45.0697389.50J08044+19040.346Hβ44.0973228.60J08285+27480.330Hβ44.1453858.37J08305+08020.319Hβ44.1461498.48PG0844+3490.064MgII44.5732098.20J09010+35380.302Hβ44.2984958.86PG0947+3960.206CIV45.1740908.49PG0953+4150.234CIV45.4534908.50TON11870.079Hβ44.2021417.60TEX1156+2130.349Hβ44.9356638.94Q1214+18040.374Hβ45.0437288.65PG1404+2260.098Hβ44.1410366.93PG1415+4510.114Hβ44.1932447.96PG1626+5540.132CIV45.1840578.494C73.180.302CIV45.9241558.91PKS2251+110.325CIV45.5450288.87


## 4. The Imaging Dataset

All the objects in our study have been observed inH band in a campaign at the 2.5 m Nordic Optical Telescope (Roque de Los Muchachos, Spain) using NOTCam. Observations were carried out during three observing runs in May 2007 and April and November 2008. The average seeing in H band was 0.7”. The 1024×1024 pixel NOTCam detector has a pixel scale of 0.235” pxl−1, yielding a field of view size of ~4×4 arcmin2. Usual observing techniques for broad-band NIR imaging of point-like sources were adopted. Observations were split in 1 min long individual frames. Random jittering patterns within a 20” box were adopted in order to perform optimal sky subtraction. The total time on each source was 45 min.Data reduction was performed using IRAF. (IRAF is distributed by the National Optical Astronomy Observatories, which are operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.) Bad pixels were corrected for in each image using a mask made from the ratio of two sky flats with different illumination levels. Sky subtraction was performed for each science image using a median averaged frame of all the other temporally close frames in a grid of eight exposures. Flat fielding was made using normalized median averaged twilight sky frames with different illumination levels. Finally, images were aligned to subpixel accuracy using field stars as reference points and combined after removing spurious pixel values to obtain the final reduced coadded image. Zero point calibration is achieved by cross-matching the photometry of field stars with the 2MASS database. This approach yields typical photometric uncertainties <0.1 mag.We analyzed our data using the Astronomical Image Decomposition and Analysis package (AIDA; [73]), an IDL-based software designed to perform two-dimensional model fitting of quasar images. Details on the procedure are presented in [40, 74] and briefly summarized here.A careful modeling of the Point Spread Function (PSF) is crucial to disentangle the extended host galaxy light from the nuclear source. To model the PSF shape, we used suitable field stars. Each star was modeled with four two-dimensional Gaussians, representing the core of the PSF, and an exponential feature, representing the extended wing of the PSF. Regions contaminated by nearby sources, saturated pixels, and other defects affecting the images were masked out.In order to discriminate between resolved and unresolved targets, we first fit the images of our sources with the pure PSF model. In most of the cases, an extended halo was clearly observed in the residuals. We then reperformed the fits using a Sersic law, (describing the host galaxy) plus a point-source (the nucleus), convolved to the PSF model. In all but one case(J02331-0909), this second fit is significantly better than the fit with the pure PSF, as confirmed by the χ2 ratio between the two fits (see Table 3).Table 3
Results from the imaging analysis. (1) Quasar name. (2) Redshift. (3) Apparent observedH-band magnitude of the nucleus. (4) Apparent observed H-band magnitude of the host galaxy. (5) Apparent central surface brightness, as extrapolated from the host galaxy model. (6) Chi-square ratio between the best fit using a pure PSF and the best fit using a galaxy+quasar model. (7) Effective radius of the host galaxy. (8) Ellipticity of the host galaxy model. (9) Sersic index of the host galaxy model. (10) Rest-frame R-observed H color correction. (11) Resulting absolute rest-frame R-band magnitude of the host galaxy. (12) Adopted mass-to-light ratio. (13) Stellar mass of the host galaxy. (14) BH-to-host galaxy mass ratio.
Namezmnuc [mag]mhost  [mag]μ0 [mag ”-2]χpsf/χgqReEll[”]nsR-H [mag]MR [mag]logM/L [M⊙/L⊙]logMhost[M⊙]logΓ(1)( 2)(3)(4)(5)(6)(7)(8)(9)(10)(11(12)(13)(14)PB57230.23415.9915.815.411.410.320.575.002.59−21.930.7111.38−3.61PG0026+120.14512.9014.9416.101.771.440.151.202.58−21.640.7511.30−3.76PG0052+2510.15513.4614.3716.346.552.190.191.222.58−22.380.7411.59−3.07B20110+290.36316.0416.2112.072.513.890.095.002.60−22.610.6711.61−2.48PKS0214+100.40814.6016.1013.981.881.160.002.772.59−23.030.6511.77−3.36J02321+00080.43299.9016.809.931.430.370.003.892.58−22.500.6411.54−4.18J02331-09090.38815.98——1.01————————J03010+00040.48699.9017.3513.421.170.260.241.892.57−22.250.6311.43−2.87J03323+01060.48218.6617.1511.551.910.540.613.172.57−22.430.6311.50−3.11J03579-05500.43918.7817.228.061.140.450.355.002.58−22.120.6411.39−3.16B20752+25A0.44614.8016.1412.511.390.710.072.952.58−23.240.6411.83−2.53J08044+19040.34617.0717.4017.731.550.980.371.002.60−21.300.6711.09−2.69J08285+27480.33016.9817.2016.631.580.810.061.532.60−21.380.6811.13−2.96J08305+08020.31917.5018.2517.661.190.570.330.902.60−20.250.6810.68−2.40PG0844+3490.06413.3414.5917.551.922.820.270.902.56−20.130.7810.73−2.73J09010+35380.30217.0816.3514.102.790.910.202.432.60−22.010.6911.39−2.73PG0947+3960.20614.4215.1816.174.451.920.201.592.59−22.240.7211.52−3.41PG0953+4150.23412.9315.8019.361.333.390.120.902.59−21.930.7111.38−3.26TON11870.07913.9714.9516.833.701.910.201.102.56−20.250.7710.77−3.37TEX1156+2130.34915.3815.7814.862.501.070.121.982.60−22.950.6711.75−3.01Q1214+18040.37416.3616.4812.051.110.250.401.972.59−22.440.6611.54−3.09PG1404+2260.09814.3514.7017.087.622.990.521.042.57−20.970.7711.05−4.32PG1415+4510.11413.6514.0215.2210.561.500.041.302.57−22.010.7611.46−3.70PG1626+5540.13213.8014.7816.238.552.040.041.532.57−21.600.7511.29−3.184C73.180.30213.5516.4217.581.441.160.180.902.60−21.940.6911.36−2.83PKS2251+110.32513.5315.7018.622.942.430.050.902.60−22.860.6811.72−3.23An example of the outputs of our analysis is shown in Figure3.An example of the analysis of the quasar host galaxies, shown in the case of the quasarJ08285+2748. Top panels: observed quasar image (left), contour plot (center), and light profile (right; the PSF and galaxy models are plotted in dotted and dashed lines, resp.). Central panels: similarly, the images (left), contour plots (center), and light profile (right) of the PSF and the galaxy model. Bottom panels: image (left), contour plot (center), and light profile (top right) of residuals after model subtraction. The radial distribution of χ2 values is also plotted (bottom right).
(a)(b)(c)(d)(e)(f)(g)(h)(i)(j)(k)(l)Host galaxy apparentH-band magnitudes are then converted into rest-frame R-band. We use the Elliptical galaxy template by [71] to estimate k-corrections. The host galaxy R-band absolute magnitude is then converted into a stellar mass by adopting the mass-to-light ratio (M/L) of a single stellar population originated at zburst=5 and passively evolving down to z=0 (see [58], for details). Table 3 summarizes the relevant results from the modeling of the quasar host galaxies described here.


## 5. Discussion

In Figure4 we show the MBH-Mhost relation for quasar host galaxies at z<0.5. The dataset (62 objects from [26, 58], plus 25 objects with resolved host galaxies from the present study) span over 2 dex in both MBH and Mhost. The same MBH-Mhost relation observed for inactive galaxies appears to hold through all the sampled range, from ~3×1010 to ~3×1012M⊙ in terms of Mhost. We find that 〈logΓ〉=-2.843 (in excellent agreement with the MBH=0.0015Mhost value reported by [9], for inactive galaxies) with a 0.44 dex scatter. Only 3 sources (J02321+0008, PG1404+226 from this study; 1001+291 from the old sample) lie more than 2-σ  below the relation. Since the sampled parameter space is about 5 times larger than the dispersion of the relation, we can exclude that the observed MBH-Mhost relation is the upper envelope of a population of quasars with relatively small black holes hosted by very massive galaxies. The best bilinear regression fit of the relation is(1)log⁡MBH109M⊙=(1.26±0.29)×log⁡Mhost7⋅1011M⊙+(0.04±0.03),
consistent with the relations with a constant MBH/Mhost ratio, as observed in quiescent galaxies in the local Universe [9, 10].Figure 4
TheMBH-Mhost relation in the objects in our sample (filled symbols), as compared with the z<0.5 sample from [58] (empty symbols) and the local relation (MBH/Mhost=0.0015; see [9]). Squares, triangles, and circles refer to MBH estimates derived from Hβ, MgII, and CIV respectively. Our new data substantially extend the sampled range of the MBH-Mhost relation. The relation holds down to quasar host masses of ~1011M⊙ (BH masses of ~108M⊙). There is no evidence of a population of objects lying below the relation, as claimed by [62]. The overall scatter of the MBH-Mhost relation is 0.44 dex, and it extends over 2 dex both in terms of host galaxy and BH mass. Dotted, horizontal lines show how our objects would move if the correction for the bulge-to-total luminosity ratio is taken into account (see the text for details). The solid line is the best fit to our data.When considering subsets of our data, the high-mass end shows a slightly smaller scatter (see Table4).Table 4
Average values of theMBH/Mhost ratio in various subsets of our sample. (1) Subsample. (2) Number of objects. (3) Average value of log Γ. (4) Root Mean Square of log Γ.
SubsampleN〈logΓ〉RMSdexdex(1)(2)(3)(4)WithoutB/TcorrectionALL87−2.8420.444MBH≥109M⊙33−2.6270.351MBH<109M⊙54−2.9740.445Mhost≥4×1011M⊙49−2.8890.401Mhost<4×1011M⊙38−2.7820.492WithB/TcorrectionALL87−2.6970.524MBH≥109M⊙33−2.6230.356MBH<109M⊙54−2.8860.603Mbulge≥4×1011M⊙46−2.8860.419Mbulge<4×1011M⊙41−2.4850.554For twelve objects the host galaxies are found to be best described by a Sersic law with small index (ns<1.5), suggesting the presence of significant disc components. In particular, PG1404+226 (incidentally, the object showing the smallest MBH and the largest deviation with respect to the MBH-Mhost relation in our sample) shows clear spiral arms in the residuals of the host galaxy model. From local galaxy studies, MBH is found to be more sensitive to the properties of the spheroidal stellar component rather than of the whole galaxy. On the other hand, a bulge + disc decomposition is practically impossible with ground-based images of quasar host galaxies at relatively high redshift. Here we attempt a rule-of-thumb correction starting from the Sersic index value. We assume that the bulge-to-total luminosity ratio in the rest-frame R band, B/T, scales with the Sersic index as follows:(2)B/T={(ns-0.5)/3.5ifns<4,1ifns≥4.
This simple analytical form roughly traces the bulk of the B/T values for ns<4, as found by [75], who performed accurate image deconvolution for ~1 million galaxies from the SDSS. Furthermore, it is consistent with the operative hypothesis that all the galaxies well described by a de Vaucouleurs profile (ns=4) are bulge dominated (B/T≈1), as assumed in our previous study. The effect of this correction is to move disc-dominated host galaxies towards the left side of Figure 4. In particular, all but one source at Mbulge<1010M⊙ would lie above the local relation. The best fit relation is indeed flatter (0.88±0.18 instead of 1.26±0.29), but still consistent with the linear case. The scatter is also increased (0.53 dex, computed over the whole sample; 0.61 and 0.55 dex in the small MBH and small Mbulge subsets, resp.). We stress that the correction reported in (2) is uncertain, because of the wide range of B/T values reported for any given ns. However, we remark that any correction for the B/T would make the case against a population of under-massive black holes in very massive galaxies even stronger.A similar argument can be used to evaluate how our results are affected by different assumptions on the star formation history. In our study, we adopted the mass-to-light ratio (M/L) of a single stellar population originated at zburst=5 and passively evolving down to z=0. However, objects with significant disc contaminations are expected to have a younger stellar population than old, passive spheroids. This would imply smaller M/L, that is, less massive host galaxies for a given observed host luminosity. This would make the case against a quasar population lying below the observed MBH-Mhost relation even more robust.


## 6. Conclusions

We measured black hole masses and host galaxy luminosities in a sample of 25 low-redshift (z<0.5) quasars selected to have modest (<109M⊙) black hole masses.For each object we inferred stellar masses. This allowed us to significantly expand the sampled range ofMhost and MBH for quasars. We found the following.(i)
TheMBH-Mhost relation holds over all the 2 dex both in terms of MBH and Mhost. The relation has a scatter of 0.44 dex; that is, the sampled parameter space is ~5 times larger.(ii)
The slope of theMBH-Mhost relation in quasars is consistent with unity (in a log-log plane), consistently with what observed in quiescent galaxies.(iii)
The scatter of the relation increases by~0.9 dex at the low-mass end.(iv)
After applying a simplistic correction for the disc contribution in objects with low Sersic indexes, the slope of theMBH-Mhost relation is smaller but still consistent with the linear case.(v)
No evidence of a population of quasars with relatively modestMBH and very high Mhost values is found.Further studies at even lowerMBH masses (≲107M⊙) and at higher redshift could provide further constraints on the early black hole growth and the nature of the seeds, and to pin down the evolution of the MBH-host galaxy relations along the Cosmic Time. This requires extremely high-quality imaging of quasar host galaxies that would become possible with the next generation of ELT and laser assisted AO imagers.



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*Source: 782528-2012-02-13.xml*