The low z quasar hosts and environments


The low redshift quasar host galaxies and enviroments

Both ground-based and HST studies have shown that virtually all luminous low redshift (z<0.5) quasars reside in massive, spheroid-dominated host galaxies, whereas at lower luminosities quasars can also be found in early- type spiral hosts at (e.g. Bahcall et al. 1997; Dunlop et al. 2003; Pagani et al. 2003; Floyd et al. 2004; Jahnke et al. 2004). This is in good agreement with the BH – bulge relationship in inactive galaxies (e.g. Gultekin et al. 2009), since very massive BHs power luminous quasars. Only a small fraction of the host galaxies (!15% ) are found in merger systems but it is difficult to determine clear merger signatures from morphology alone. At low redshifts a major contribution to the properties of quasar host galaxies has been provided by images from the Hubble Space Telescope (HST). The improved spatial resolution has allowed the characterization of the structure and the detailed morphology of the host galaxies (Bahcall et al. 1997; Kukula et al. 2001; Ridgway et al. 2001; Dunlop et al. 2003; Peng et al. 2006; Zakamska et al. 2006). It turned out that QSO are hosted in luminous galaxies that are often dominated by the spheroidal component. Most of the old studies of quasar host considered few tens of objects therefore in order to derive a picture of the host properties at various redshift one should combine many different samples often obtained with different telescopes and filters. Observations carried out by HST are certainly more homogeneous (although different filters were used) and allow to investigate a somewhat large sample based on high quality data. Nevertheless the size of these samples remain relatively small For instance in the range 0.25 < z < 0.5 about 50 QSO were imaged by HST (see references above).

In order to explore a significantly larger dataset of QSO one should refer to large surveys that include both imaging and spectroscopic data. In this respect one of the most productive recent surveys is the Sloan Digital Sky Survey that allowed to find 105783 quasars (Schneider et al. 2010) from (DR-7). Standard SDSS images are, however, too shallow and the faint nebulosity around the nucleus of quasars is not detected. This problem has been overcome in the case of the special sky region mapped by SDSS for the SDSS Legacy Survey.

 We investigate the properties of the galaxies hosting quasars in about 400 low redshift (z < 0.5) SDSS QSO that are in the "Stripe 82" sky area. For this region deep (r~22.4 mag) u, b,v,r and i images are available and allow us to study both the host galaxy and the Mpc scale environments. This sample greatly outnumber previous studies of low-z QSOs. We present preliminary results of the properties of quasars activity and in particular we focus on the relationships among host galaxy luminosity, black hole mass, radio emission and the surrounding galaxy environments. We select from the SDSS - QSO Catalogue all the QSOs in the range of redshift 0.1<z<0.5 and in the Stripe82 region. This gives a total of 416 QSO. In this sample we are dominated by radio quiet quasars (about 5% are radio loud). We implemented an automated procedure using AIDA to decompose the QSO images into nucleus and host galaxy luminosity. After masking of all contaminating sources in the field a 2D fitting is performed using PSF + galaxy model.

 fig confr a4


We have retrieved all images of the selected QSO from SDSS Stripe 82 dataset (Annis et al. 2011) in the i band. This corresponds to observe in the R filter at rest frame at the average redshift of the dataset. In order to derive the properties of the galaxies hosting the QSO we performed a 2D fit of the image of the QSO assuming it is the superposition of two components. The nucleus in the center and the surrounding nebulosity. The first is described by the local Point Spread Function of the image while for the second component we assumed a galaxy model described by a Sersic law convolved with the proper PSF. The analysis of these images was performed using the Astronomical Image Decomposition Analysis (AIDA, Uslenghi & Falomo (2008)) that was used in our previous studies of QSO host galaxies (Falomo et al. 2008; Decarli et al. 2012; Kotilainen et al. 2007, 2009).

The most critical aspect of the image decomposition is he determination of a suitable PSF. In the case of SDSS images the field of view is large enough that there are always many stars in the field to properly derive the PSF. To derive the most suitable PSF of each field we have selected a number of stars (between 5 and 15) in the field that are distributed around the target. Selection of these PSF stars was based on various parameters as their magnitude, FWHM, ellipticity and presence of close companions. We then define a radius to compute the PSF model and a ring around each star where to compute the sky background. All extra sources that were found inside these areas were masked out with an automated procedure. The PSF model was then obtained from the simultaneous fit of all selected stars using a multi function 2D model composed by 3 gaussians and one exponential function.







FIGURE 1 : Example of QSO in the sample: Left panels show the SDSS DR7 data; Right panels the corresponding data from Stripe 82 ( image resulting combining 35 individual images of 54 sec). Top panels yield the grey scale images in the i band; central panels give contour plots of the region and in the bottom panels we show the luminosity radial profiles together with the AIDA fit. 

 Host galaxy properties

mhz fig2a 

The distribution of host galaxies in the redshift luminosity plane (see Fig 7) confirms previous claims that they are encompassed between M* and M*-3 with more frequent distribution in the range M*-1 and M*-2 (Kukula et al. 2001; Dunlop et al. 2003; Falomo et al. 2004; Peng et al. 2006; Kotilainen et al. 2007; Decarli et al. 2010b). There is a small, but significant, increasing of the host luminosity with the redshift (from M(R) ! -22.5 at z !0.2 to M(R) ! -23.1 at z ! 0.5 ) that is consistent with passive evolution of the underlying stellar population. A similar trend was also reported over a wider redshift range by Kotilainen et al. (2009). 

A long debated question concerning the properties of the galaxies hosting quasar is its morphology (see e.g. B¨ohm et al. (2013) and references therein). Do quasars inhabit both disc and bulge dominated galaxies ? This question was debated for long time since the poor spatial resolution of the observations combined with the bright nuclei hindered the clear nature of the QSO hosts. The original idea that considered radio loud QSO being hosted by ellipticals while radio quiet quasars hosted in spiral galaxies is clearly not consistent with the observations that show a more complex scenario. 



FIGURE 2: The absolute magnitude of QSO (RQQ circles; RLQ squares) host galaxies versus redshift. Resolved quasars (filled points), marginally resolved (open points) and luminosity lower limits (red crosses with arrows). For comparison we include a compilation of about 100 QSO host galaxies from HST observations (Decarli et al. 2010b) (filled green triangles: inverted triangles for radio loud objects )





People: Renato Falomo  , Daniela Bettoni

Collaboration:  J. Kotilainen (Tuorla,FIN), K. Karhunen


Falomo et al 2012,  IAU Symp 295 - XXVIII  IAU-GA , Beijing  (PDF); 

Low redshift quasars in the Stripe82: The host galaxies 
Falomo, R. , Bettoni, D., Karhunen K, Kotilainen J.K.,Uslenghi, M 2014, MNRAS 440, 476 (PDF

Low redshift quasars in the Stripe82: The local environments
Karhunen K, Kotilainen J.K.,, Falomo, R. , Bettoni, 2014, MNRAS 441, 1802 





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