Quasar Environments Research Articles

The Sightline to Q2343–BX415: Clues to Galaxy Formation in a Quasar Environment

We have discovered a strong DLA coincident in redshift with the faint QSO Q2343-BX415 (R = 20.2, z_(em) = 2.57393). Follow-up observations at intermediate spectral resolution reveal that the metal lines associated with this proximate DLA (PDLA) consist of two sets of absorption components. One set is moving toward the quasar with velocities of ~150-600 km s^(-1); this gas is highly ionized and does not fully cover the continuum source, suggesting that it is physically close to the active nucleus. The other, which accounts for most of the neutral gas, is blueshifted relative to the QSO, with the strongest component at ~-160 km s^(-1). We consider the possibility that the PDLA arises in the outflowing interstellar medium of the host galaxy of Q2343-BX415, an interpretation supported by strong C IV and N V absorption at nearby velocities, and by the intense radiation field longward of the Lyman limit implied by the high C II*/H I ratio. If Q2343-BX415 is the main source of these UV photons, then the PDLA is located at either ~8 or ~37 kpc from the active nucleus. Alternatively, the absorber may be a foreground star-forming galaxy unrelated to the quasar and coincidentally at the same redshift, but our deep imaging and follow-up spectroscopy of the field of Q2343-BX415 has not yet produced a likely candidate. We measure the abundances of 14 elements in the PDLA, finding an overall metallicity of ~1/5 solar and a normal pattern of relative element abundances for this metallicity. Thus, in this PDLA there is no evidence for the supersolar metallicities that have been claimed for some proximate, high-ionization, systems.

Dust in the Wind: Crystalline Silicates, Corundum, and Periclase in PG 2112+059

We have determined the mineralogical composition of dust in the broad absorption line (BAL) quasar PG 2112+059 using mid-infrared spectroscopy obtained with the Spitzer Space Telescope. From spectral fitting of the solid state features, we find evidence for Mg-rich amorphous silicates with olivine stoichiometry, as well as the first detection of corundum (Al 2O3) and periclase (MgO) in quasars. This mixed composition provides the first direct evidence for a clumpy density structure of the grain-forming region. The silicates in total encompass 56.5% ± 1.4% of the identified dust mass, while corundum takes up 38 ± 3 wt.%. Depending on the choice of continuum, a range of mass fractions is observed for periclase ranging from 2.7% ± 1.7% in the most conservative case to 9% ± 2% in a less-constrained continuum. In addition, we identify a feature at 11.2 μm as the crystalline silicate forsterite, with only a minor contribution from polycyclic aromatic hydrocarbons. The 5% ± 3% crystalline silicate fraction requires high temperatures such as those found in the immediate quasar environment in order to counteract rapid destruction from cosmic rays.

On the Connection between Metal Absorbers and Quasar Nebulae

We establish a simple model for the distribution of cool (~10^4 K) gas around L* galaxies using the properties of strong Mg II absorption line systems as observational constraints. Our analysis suggests that the halos of L* galaxies are filled with cool gas clouds having sizes of order 1 kpc and densities of ~10^−2 cm^−3. We then investigate the physical conditions of a similar ensemble of clouds situated around a central quasar. We show that the flux from the quasar gives rise to (1) extended narrow line emission on ~100 kpc scales and (2) an anisotropy in the properties of the absorbing gas arising from the geometry of the quasar radiation field. Provided that quasars reside in gaseous halos more massive than those of L* galaxies, our predictions agree with the results from detections of both narrow emission line nebulae and ~100 kpc Mg II-absorbing halos around quasars, suggesting a common origin for these phenomena. We discuss the implications of our results for understanding quasar absorption line systems, quasar environments at high redshifts, and the quasar unification scheme.

An Investigation into the Effects of Luminosity on the Mid‐Infrared Spectral Energy Distributions of Radio‐quiet Quasars

We present an analysis of the effects of luminosity on the shape of the mid-infrared spectral energy distributions (SEDs) of 234 radio-quiet quasars originally presented by Richards et al. In quasars without evident dust extinction, the spectrally integrated optical and infrared luminosities are linearly correlated over nearly three decades in luminosity. We find a significant (≳99.99% confidence) correlation between the 1.8-8.0 μm spectral index and infrared luminosity that indicates an enhancement of the mid-infrared continuum with increasing luminosity. Coupled with strong evidence for spectral curvature in more luminous quasars, we conclude that this trend is likely a manifestation of the near-infrared (3-5 μm) bump noted in earlier quasar SED surveys. The strength of this feature is indicative of the contribution of emission from the hottest (≳1000 K) dust to the mid-infrared spectrum; higher luminosity quasars tend to show more hot dust emission. Finally, the comparable distribution of bolometric corrections from the monochromatic 3 μm luminosity as well as its lack of sensitivity to dust extinction as compared to the standard bolometric correction from νL_(5100 A) suggest that the former may be a more robust indicator of bolometric quasar luminosity. The close link between the power in the mid-infrared and optical and the effect of luminosity on the shape of the mid-infrared continuum indicate that considering mid-infrared emission independent of the properties of the quasar itself is inadequate for understanding the parsec-scale quasar environment.

Clustering Analyses of 300,000 Photometrically Classified Quasars. I. Luminosity and Redshift Evolution in Quasar Bias

Using 300,000 photometrically classified quasars, by far the largest quasar sample ever used for such analyses,we study the redshift and luminosity evolution of quasar clustering on scales of 50 h-1 kpc to 20 h-1 Mpc from redshifts of z 0:75-2:28. We parameterize our clustering amplitudes using realistic dark matter models and find that a ΛCDM power spectrum provides a superb fit to our data with a redshift-averaged quasar bias of bz =1:40Q = 2:41±0:08 (P 99.6%using our data set alone, increasing to >99.9999% if stellar contamination is not explicitly parameterized. We measure the quasar classification efficiency across our full sample as a = 95:6 ± 4:41:9 %, a star-quasar separation comparable to the star-galaxy separation in many photometric studies of galaxy clustering.We derive the mean mass of the dark matter halos hosting quasars as MDMH = (5:2 ± 0:6) x 1012 h-1 M⊙. At z ∼ 1:9 we find a 1.5 σ deviation from luminosity independent quasar clustering; this suggests that increasing our sample size by a factor of ∼1.8 could begin to constrain any luminosity dependence in quasar bias at z ∼ 2. Our results agree with recent studies of quasar environments at z < 0:4, which detected little luminosity dependence to quasar clustering on proper scales ≳50 h-1 kpc. At z < 1:6,our analysis suggests that bQ is constant with luminosity to within ΔbQ ∼ 0:6, and that, for g < 21, angular quasar autocorrelation measurements are unlikely to have sufficient statistical power at z ≲ 1:6 to detect any luminosity dependence in quasars’ clustering.

Quasars Probing Quasars. II. The Anisotropic Clustering of Optically Thick Absorbers around Quasars

With close pairs of quasars at different redshifts, a background quasar sightline can be used to study a foreground quasar's environment in absorption. We used a sample of 17 Lyman limit systems with column density N_HI > 10^19 cm^-2 selected from 149 projected quasar pair sightlines, to investigate the clustering pattern of optically thick absorbers around luminous quasars at z ~ 2.5. Specifically, we measured the quasar-absorber correlation function in the transverse direction, and found a comoving correlation length of r_0=9.2_{+1.5}_{-1.7} Mpc/h (comoving) assuming a power law correlation function, with gamma=1.6. Applying this transverse clustering strength to the line-of-sight, would predict that ~ 15-50% of all quasars should show a N_HI > 10^19 cm^-2 absorber within a velocity window of v < 3000 km/s. This overpredicts the number of absorbers along the line-of-sight by a large factor, providing compelling evidence that the clustering pattern of optically thick absorbers around quasars is highly anisotropic. The most plausible explanationfor the anisotropy is that the transverse direction is less likely to be illuminated by ionizing photons than the line-of-sight, and that absorbers along the line-of-sight are being photoevaporated. A simple model for the photoevaporation of absorbers subject to the ionizing flux of a quasar is presented, and it is shown that absorbers with volume densities n_H < 0.1 cm^-3 will be photoevaporated if they lie within ~ 1 Mpc (proper) of a luminous quasar. Using this simple model, we illustrate how comparisons of the transverse and line-of-sight clustering around quasars can ultimately be used to constrain the distribution of gas in optically thick absorption line systems.

The DEEP2 Galaxy Redshift Survey: Clustering of Quasars and Galaxies atz= 1

We present the clustering of Deep Extragalactic Evolutionary Probe 2 (DEEP2) galaxies at 0.7 < z < 1.4 around quasars identified using both the Sloan Digital Sky Survey (SDSS) and DEEP2 surveys. We measure the two-point cross-correlation of a sample of 36 optically selected, spectroscopically identified quasars from the SDSS and 16 more found in the DEEP2 survey with the full DEEP2 galaxy sample over scales 0.1 h-1 Mpc < rp < 10 h-1 Mpc. The clustering amplitude is found to be similar to the autocorrelation function of DEEP2 galaxies, with a relative bias of b = 0.89 ± 0.24 between quasars and DEEP2 galaxies at z ~ 1. No significant dependence is found on scale, quasar luminosity, or redshift over the ranges we probe here. The clustering amplitude errors are comparable to those from significantly larger quasar samples, such as the 2dF (Two Degree Field) QSO Redshift Survey. This results from the statistical power of cross-correlation techniques, which exploit the fact that galaxies are much more numerous than quasars. We also measure the local environments of quasars using the third-nearest-neighbor surface density of surrounding DEEP2 galaxies. Quasars are found in regions of similar mean overdensity to blue DEEP2 galaxies; they differ in environment from the red DEEP2 galaxy population at 2 σ significance. Our results imply that quasars do not reside in particularly massive dark matter halos at these redshifts, with a mean dark matter halo mass of M200 ~ 3 × 1012 M☉ in a concordance ΛCDM cosmology.

Quasars Probing Quasars. I. Optically Thick Absorbers near Luminous Quasars

With close pairs of quasars at different redshifts, a background quasar sightline can be used to study a foreground quasar's environment in absorption. We search 149 moderate resolution background quasar spectra, from Gemini, Keck, the MMT, and the SDSS to survey Lyman Limit Systems (LLSs) and Damped Ly-alpha systems (DLAs) in the vicinity of 1.8 < z < 4.0 luminous foreground quasars. A sample of 27 new quasar-absorber pairs is uncovered with column densities, 17.2 < log (N_HI/cm^2) < 20.9, and transverse (proper) distances of 22 kpc/h < R < 1.7 Mpc/h, from the foreground quasars. If they emit isotropically, the implied ionizing photon fluxes are a factor of ~ 5-8000 times larger than the ambient extragalactic UV background over this range of distances. The observed probability of intercepting an absorber is very high for small separations: six out of eight projected sightlines with transverse separations R < 150 kpc/h have an absorber coincident with the foreground quasar, of which four have log N_HI > 10^19. The covering factor of log N_HI > 10^19 absorbers is thus ~ 50 % (4/8) on these small scales, whereas < 2% would have been expected at random. There are many cosmological applications of these new sightlines: they provide laboratories for studying fluorescent Ly-alpha recombination radiation from LLSs, constrain the environments, emission geometry, and radiative histories of quasars, and shed light on the physical nature of LLSs and DLAs.

Photoelectric Emission from Dust Grains Exposed to Extreme Ultraviolet and X‐Ray Radiation

Photoelectric emission from dust plays an important role in grain charging and gas heating. To date, detailed models of these processes have focused primarily on grains exposed to soft radiation fields. We provide new estimates of the photoelectric yield for neutral and charged carbonaceous and silicate grains, for photon energies exceeding 20 eV. We include the ejection of electrons from both the band structure of the material and the inner shells of the constituent atoms, as well as Auger and secondary electron emission. We apply the model to estimate gas-heating rates in planetary nebulae and grain charges in quasar environments. For these applications, secondary emission can be neglected; the combined effect of inner shell and Auger emission is small, although not always negligible. Finally, we investigate the survivability of dust in quasar host galaxies. For unobscured lines of sight, dust can be destroyed out to distances ~1 kpc.

The DEEP2 Galaxy Redshift Survey: Clustering of Galaxies as a Function of Luminosity atz= 1

We present the clustering of DEEP2 galaxies at 0.7<z<1.4 around quasars identified using both the SDSS and DEEP2 surveys. We measure the two-point cross-correlation of a sample of 36 optically-selected, spectroscopically-identified quasars from the SDSS and 16 more found in the DEEP2 survey with the full DEEP2 galaxy sample over scales 0.1 < r_p < 10 Mpc/h. The clustering amplitude is found to be similar to the auto-correlation function of DEEP2 galaxies, with a relative bias of b=0.89 +/-0.24 between quasars and DEEP2 galaxies at z~1. No significant dependence is found on scale, quasar luminosity, or redshift over the ranges we probe here. The clustering amplitude errors are comparable to those from significantly larger quasar samples, such as the 2dF QSO Redshift Survey. This results from the statistical power of cross-correlation techniques, which exploit the fact that galaxies are much more numerous than quasars. We also measure the local environments of quasars using the 3rd-nearest-neighbor surface density of surrounding DEEP2 galaxies. Quasars are found in regions of similar mean overdensity as blue DEEP2 galaxies; they differ in environment from the red DEEP2 galaxy population at 2 sigma significance. Our results imply that quasars do not reside in particularly massive dark matter halos at these redshifts, with a mean dark matter halo mass of M_200 ~ 3 10^12 solar masses in a concordance LCDM cosmology.

Properties of galaxies in Sloan Digital Sky Survey quasar environments at z &lt; 0.2

We analyse the environment of low redshift, z < 0.2, Sloan Digital Sky Survey (SDSS) quasars using the spectral and photometric information of galaxies from the Sloan Digital Sky Survey Third Data Release (SDSS-DR3). We compare quasar neighbourhoods with field and highdensity environments through an analysis on samples of typical galaxies and groups. We compute the surrounding surface number density of galaxies finding that quasar environments systematically avoid high-density regions. Their mean environments correspond to galaxy density enhancements similar to those of typical galaxies. We have also explored several galaxy properties in these environments, such as spectral types, specific star formation rates (SFRs), concentration indexes, colours and active nuclei activity. We find a higher relative fraction of blue galaxies in quasar environments compared to groups and typical galaxy neighbourhoods. Consistent with this picture, the distribution of the concentration index of these galaxies also indicate a larger fraction of late-type objects. By analysing the available information of galaxy spectra we have also studied the distribution of the SFRs of these neighbour galaxies finding that quasar environments are populated by objects with an enhanced star formation activity. An analysis of the relative flux ratios of [O III]λ5700/Hβ and [N II]λ6583/Hα of emission-line galaxies shows no excess of nuclei activity in quasar neighbourhood with respect to the environment of a typical galaxy. We conclude that low-redshift quasar neighbourhoods (r p < 1 h −1 Mpc, � V < 500 km s −1 ) are populated by bluer and more intense star forming galaxies of disc-type morphology than galaxies in groups and in the field. Although star formation activity is thought to be significantly triggered by interactions, we find that quasar fuelling may not require the presence of a close companion galaxy (r p < 100 h −1 kpc, � V < 350 km s −1 ). As a test of the unified active galactic nucleus (AGN) model, we have performed a similar analysis to the neighbours of a sample of active galaxies. The results indicate that these neighbourhoods are comparable to those of quasars giving further support to this unified scenario.

The FSVS Cluster Catalogue: galaxy clusters and groups in the Faint Sky Variability Survey

We describe a large sample of 598 galaxy clusters and rich groups discovered in the data of the Faint Sky Variability Survey (FSVS). The clusters have been identified using a fully automated, semiparametric technique based on a maximum likelihood approach applied to Voronoi tessellation, and enhanced by colour discrimination. The sample covers a wide range of richness, has a density of ∼28 clusters deg −2 , and spans a range of estimated redshifts of 0.05 0.6, the catalogue contributes substantially to the current total of optically selected, intermediate-redshift clusters, and complements the existing, usually X-ray selected, samples. The FSVS fields are accessible for observation throughout the whole year, making them particularly suited for large follow-up programmes. The construction of this FSVS Cluster Catalogue completes a fundamental component of our continuing programmes to investigate the environments of quasars and the chemical evolution of galaxies. We publish here the list of all clusters with their basic parameters, and discuss some illustrative examples in more detail. The full FSVS Cluster Catalogue, together with images and lists of member galaxies etc., will be issued as part of the ‘NOAO (National Optical Astronomy Observatories) data products’, and accessible at http://www.noao.edu/dpp/. We describe the format of these data and access to them.

The Small‐Scale Environment of Quasars

Where do quasars reside? Are quasars located in environments similar to those of typical L* galaxies, and, if not, how do they differ? An answer to this question will help shed light on the triggering process of quasar activity. We use the Sloan Digital Sky Survey to study the environment of quasars and compare it directly with the environment of galaxies. We find that quasars (Mi ≤ -22, z ≤ 0.4) are located in higher local overdensity regions than are typical L* galaxies. The enhanced environment around quasars is a local phenomenon; the overdensity relative to that around L* galaxies is strongest within 100 kpc of the quasars. In this region, the overdensity is a factor of 1.4 larger than around L* galaxies. The overdensity declines monotonically with scale to nearly unity at ~1 h Mpc, where quasars inhabit environments comparable to those of L* galaxies. The small-scale density enhancement depends on quasar luminosity, but only at the brightest end: the most luminous quasars reside in higher local overdensity regions than do fainter quasars. The mean overdensity around the brightest quasars (Mi ≤ -23.3) is nearly 3 times larger than around L* galaxies, while the density around dimmer quasars (Mi = -22.0 to -23.3) is ~1.4 times that of L* galaxies. By ~0.5 Mpc, the dependence on quasar luminosity is no longer significant. The overdensity on all scales is independent of redshift to z ≤ 0.4. The results suggest a picture in which quasars typically reside in L* galaxies but have a local excess of neighbors within ~0.1-0.5 Mpc; this local density excess likely contributes to the triggering of quasar activity through mergers and other interactions.

Nanodiamond Dust and the Far‐Ultraviolet Quasar Break

We explore the possibility that the steepening observed shortward of 1000 A in the energy distribution of quasars may result from absorption by dust, being either intrinsic to the quasar environment or intergalactic. We find that a dust extinction curve consisting of nanodiamonds, composed of terrestrial cubic diamonds or with surface impurities as found in carbonaceous chondrite meteorites, such as Allende, is successful in reproducing the sharp break observed. The intergalactic dust model is partially successful in explaining the shape of the composite energy distribution but must be discarded in the end, as the amount of crystalline dust required is unreasonable and would imply an improbable fine-tuning among the dust formation processes. The alternative intrinsic dust model requires a mixture of both cubic diamonds and Allende nanodiamonds and provides a better fit of the UV break. The gas column densities implied are of the order 1020 cm-2, assuming solar metallicity for carbon and full depletion of carbon into dust. The absorption only occurs in the ultraviolet and is totally negligible in the visible. The minimum dust mass required is of the order ~0.003r M☉, where rpc is the distance in parsecs between the dust screen and the continuum source. The intrinsic dust model reproduces the flux rise observed around 660 A in key quasar spectra quite well. We present indirect evidence of a shallow continuum break near 670 A (18.5 eV), which would be intrinsic to the quasar continuum.

No Evidence of "Gray" Dust from Composite Quasar Spectra

Two recent studies based on composite reddened quasar spectra have indicated the presence of `gray' dust in quasar environments. This gray dust has a relatively flat extinction law in the UV, consistent with the theoretical expectation of a lack of small dust grains close to a quasar. In contrast, individual reddened quasars in the Sloan Digital Sky Survey tend to have steep extinction laws in the UV, similar to that in the SMC. We analyze the method used in determining extinction laws from composite quasar spectra in order to resolve this discrepancy. We show that quasars reddened by SMC-type dust that are present in quasar samples have a negative correlation between E_{B-V} and redshift, due to selection effects. The fact that the highest redshift quasars (which contribute to the UV part of a composite spectrum) are less extincted leads to shallower extinction in the UV. We construct a composite quasar spectrum from a simulated sample of quasars reddened by SMC-type dust and show that the extinction curve derived from the composite does not recover the intrinsic extinction law. We conclude there is no evidence of gray dust in quasar environments.

NICMOS Coronagraphy

ABSTRACTThe NICMOS coronagraphic capability is a unique resource. It has been used to search for and/or confirm the presence of planetary or brown dwarf companions around nearby stars, to probe the structure of circumstellar disks, and to study the host environment of quasars with damped Lyα systems. These and other programs are expected to continue with NICMOS, using the recently installed NICMOS cooling system. In this paper we present an operational overview of NICMOS coronagraphy from its inception on 1998 February 6 through 1998 December 18. During this time interval, which corresponds to H ubble Space Telescope (HST) cycles 7 and 7N, coronagraphic observations of 329 targets for 22 programs were obtained. The flight software was enhanced to locate the position of the hole as well as the target during onboard acquisitions. Coronagraphic images are affected by changes in the point‐spread function (PSF) due to thermally induced HST focus changes, NICMOS cold‐mask movement, and light scatter around the coronagraphic hole. Proper PSF subtraction requires a well‐planned observing strategy that is appropriate for the target, and a matched PSF target. For example, for point sources, observations with the same filter taken back‐to‐back in the same orbit with a roll of the spacecraft between observations yields good results. For circumstellar disks, contemporaneous observations of a matched PSF target are required.

The cluster environments of radio-loud quasars at 0.6 &lt; z &lt; 1.1

We have carried out multicolour imaging of a complete sample of radio-loud quasars at 0.6 < z < 1.1 and find groups or clusters of galaxies in the fields of at least eight and possibly 13 of the 21 sources. There is no evidence for an evolution in the richness of the environments of radio-loud quasars from other low-redshift studies to z≳ 0.9. The quasars associated with groups and clusters in our sample do not necessarily reside in the centre of the galaxy distribution, which rarely displays a spherical geometry. Clustering is preferentially associated with small or asymmetric steep-spectrum radio sources. The quasars with the largest projected angular size are, in nearly all cases, found in non-clustered environments. Radio-based selection (including source size) of high-redshift groups and clusters can be a very efficient method of detecting rich environments at these redshifts. We find that in optical searches for galaxy overdensities above z∼ 0.6, multiple filters must be used. If the single-filter counting statistics used by groups at lower redshift are applied to our data, uncertainties are too large to make accurate quantifications of cluster richness. This means that genuine clustering of galaxies about quasars will be missed and, in ∼10 per cent of cases, putative clusters turn out to be false detections. The statistics are further diluted by the fact that galaxy overdensities are generally not centred on the quasar.

Gemini-South+FLAMINGOS Demonstration Science: Near-Infrared Spectroscopy of the z = 5.77 Quasar SDSS J083643.85+005453.3

We report an infrared 1-1.8 μm (J+H-band), low-resolution (R = 450) spectrogram of the highest redshift radio-loud quasar currently known, SDSS J083643.85+005453.3, obtained during the spectroscopic commissioning run of the FLAMINGOS multiobject, near-IR spectrograph at the 8 m Gemini-South Observatory. These data show broad emission from both C IV λ1549 and C III] λ1909, with strengths comparable to lower redshift quasar composite spectra. The implication is that there is substantial enrichment of the quasar environment, even at times less than a billion years after the big bang. The redshift derived from these features is z = 5.774 ± 0.003, more accurate and slightly lower than the z = 5.82 reported in the discovery paper based on the partially absorbed Lyα emission line. The infrared continuum is significantly redder than lower redshift quasar composites. Fitting the spectrum from 1.0 to 1.7 μm with a power law fν ∝ ν-α, the derived power-law index is α = 1.55 compared to the average continuum spectral index α = 0.44 derived from the first Sloan Digital Sky Survey composite quasar. Assuming an SMC-like extinction curve, we infer a color excess of E(B-V) = 0.09 ± 0.01. Only ≈6% of quasars in the optically selected Sloan Digital Sky Survey show comparable levels of dust reddening.

Relations between Galaxy Formation and the Environments of Quasars

Abstract We investigate the environments of quasars using a semi-analytic model which includes both galaxy and quasar formations based on a hierarchical clustering scenario. We assume that a supermassive black hole is formed and fueled by the accretion of cold gas during a major merger of galaxies. Our model can reproduce not only the general form of the galaxy luminosity functions in the local Universe, but also the observed relation between a supermassive black hole mass and a spheroid luminosity, the present black hole mass function and the quasar luminosity functions at different redshifts. Using this model, we predict the mean number of quasars per halo, the bias parameter of quasars and the probability distribution of the number of galaxies around quasars. The results of our model show that the spatial distribution of galaxies is different from that of quasars. Furthermore, we found that at $0.2 \lesssim z \lesssim 0.5$ most quasars are likely to reside in galaxy groups. On the other hand, at $1 \lesssim z \lesssim 2$ most quasars seem to reside in environments ranging from small groups of galaxies to clusters of galaxies. Comparing these predictions with observations in the future will enable us to constrain our model of quasar formation.

The environment of active objects in the nearby Universe

We study the galaxy environment of active galaxies, radio-loud and radio-quiet quasars in the redshift range 0.1 ≤ z ≤ 0.25. We use galaxies from the APM survey in order to explore the local galaxy overdensity and the b J - R colour distribution of neighbouring galaxies of these target samples. For comparison, we perform similar analysis on samples of Abell clusters with X-ray emission, and samples of Abell clusters with richness R = 1 and 0. The projected cross-correlations show that the samples of quasars and active galaxies reside in regions of galaxy density enhancements lower than those typical of R = 0 clusters. We also find that in the nearby Universe the local galaxy overdensities of radio-loud and radio-quiet quasars are comparable. The analysis of the distribution of b J - R galaxy colour indices suggests that the environment of quasars is not strongly dominated by a population of red galaxies, characteristic of rich Abell clusters, an effect that is more clearly appreciated for our sample of radio-loud quasars.