Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe Environment of AGNs in the Sloan Digital Sky Survey
92
0
0.0
(
0
)
Authors
Christopher J. Miller
Ask ChatGPT about the research
No Arabic abstract
We present the observed fraction of galaxies with an Active Galactic Nucleus (AGN) as a function of environment in the Early Data Release of the Sloan Digital Sky Survey (SDSS). Using 4921 galaxies between 0.05 <= z <= 0.095, and brighter than M_r* = -20.0 (or M* +1.45), we find at least ~ 20% of these galaxies possess an unambiguous detection of an AGN, but this fraction could be as high as ~40% after we model the ambiguous emission line galaxies in our sample. We have studied the environmental dependence of galaxies using the the distance to the 10^th nearest neighbor. As expected, we observe that the fraction of star--forming galaxies decreases with density, while the fraction of passive galaxies increases with density. In contrast, the fraction of galaxies with an AGN remains constant from the cores of galaxy clusters to the rarefied field population. We conclude that the presence of an AGN is independent of the disk component of a galaxy. Our analyses are robust against measurement error, definition of an AGN, aperture bias, stellar absorption, survey geometry and signal--to--noise. Our observations are consistent with the hypothesis that a supermassive black hole resides in the bulge of all massive galaxies and ~40% of these black holes are seen as AGNs in our sample. A high fraction of local galaxies with an AGN suggests that either the mean lifetime of these AGNs is longer than previously thought (>10^8 years), or that the AGN burst more often than expected; ~40 times over the redshift range of our sample.
rate research
Read More
We use the Fourth Data Release of the Sloan Digital Sky Survey to investigate the relation between galaxy rest frame u-r colour, morphology, as described by the concentration and Sersic indices, and environmental density, for a sample of 79,553 galaxies at z < ~0.1. We split the samples according to density and luminosity and recover the expected bimodal distribution in the colour-morphology plane, shown especially clearly by this subsampling. We quantify the bimodality by a sum of two Gaussians on the colour and morphology axes and show that, for the red/early-type population both colour and morphology do not change significantly as a function of density. For the blue/late-type population, with increasing density the colour becomes redder but the morphology again does not change significantly. Both populations become monotonically redder and of earlier type with increasing luminosity. There is no significant qualitative difference between the behaviour of the two morphological measures. We supplement the morphological sample with 13,655 galaxies assigned Hubble types by an artificial neural network. We find, however, that the resulting distribution is less well described by two Gaussians. Therefore, there are either more than two significant morphological populations, physical processes not seen in colour space, or the Hubble type, particularly the different subtypes of spirals Sa-Sd, has an irreducible fuzziness when related to environmental density. For each of the three measures of morphology, on removing the density relation due to it, we recover a strong residual relation in colour. However, on similarly removing the colour-density relation there is no evidence for a residual relation due to morphology. [Abridged]
The influence of the Cosmic Web on galaxy formation and evolution is of great observational and theoretical interest. We investigate whether the Cosmic Web leaves an imprint in the spatial clustering of galaxies in the Sloan Digital Sky Survey (SDSS), using the group catalog of Yang et al. and tidal field estimates at $sim2h^{-1}$Mpc scales from the Mass-Tides-Velocity data set of Wang et al. We use the $textit{tidal anisotropy}$ $alpha$ (Paranjape et al.) to characterise the tidal environment of groups, and measure the redshift-space 2-point correlation function (2pcf) of group positions and the luminosity- and colour-dependent clustering of group galaxies using samples segregated by $alpha$. We find that all the 2pcf measurements depend strongly on $alpha$, with factors of $sim20$ between the large-scale 2pcf of objects in the most and least isotropic environments. To test whether these strong trends imply `beyond halo mass effects for galaxy evolution, we compare our results with corresponding 2pcf measurements in mock catalogs constructed using a halo occupation distribution that only uses halo mass as an input. We find that this prescription qualitatively reproduces $textit{all}$ observed trends, and also quantitatively matches many of the observed results. Although there are some statistically significant differences between our `halo mass only mocks and the data -- in the most and least isotropic environments -- which deserve further investigation, our results suggest that if the tidal environment induces additional effects on galaxy properties other than those inherited from their host halos, then these must be weak.
We have conducted a systematic search of AGNs with IMBHs from the SDSS DR4. As results we found 245 candidates of broad-line AGN with M_{BH}<10^6 Msun estimated from the luminosity and width of the broad Halpha component. Compared to the pioneer Greene & Ho (2004) sample of 19 IMBH AGNs, our sample has improved in covering a larger range of the Eddington ratio, as well as black hole mass and redshift, taking the advantage of our AGN-galaxy spectral decomposition algorithm. Among these, thirty-six have L_{bol}/L_{Edd} < 0.1, hinting that a significant fraction of IMBHs might exist with weak or no nuclear activity.
The astrometric calibration of the Sloan Digital Sky Survey is described. For point sources brighter than r ~ 20 the astrometric accuracy is 45 milliarcseconds (mas) rms per coordinate when reduced against the USNO CCD Astrograph Catalog, and 75 mas rms when reduced against Tycho-2, with an additional 20 - 30 mas systematic error in both cases. The rms errors are dominated by anomalous refraction and random errors in the primary reference catalogs. The relative astrometric accuracy between the r filter and each of the other filters (u g i z) is 25 - 35 mas rms. At the survey limit (r ~ 22), the astrometric accuracy is limited by photon statistics to approximately 100 mas rms for typical seeing. Anomalous refraction is shown to contain components correlated over two or more degrees on the sky.
We quantify the variability of faint unresolved optical sources using a catalog based on multiple SDSS imaging observations. The catalog covers SDSS Stripe 82, and contains 58 million photometric observations in the SDSS ugriz system for 1.4 million unresolved sources. In each photometric bandpass we compute various low-order lightcurve statistics and use them to select and study variable sources. We find that 2% of unresolved optical sources brighter than g=20.5 appear variable at the 0.05 mag level (rms) simultaneously in the g and r bands. The majority (2/3) of these variable sources are low-redshift (<2) quasars, although they represent only 2% of all sources in the adopted flux-limited sample. We find that at least 90% of quasars are variable at the 0.03 mag level (rms) and confirm that variability is as good a method for finding low-redshift quasars as is the UV excess color selection (at high Galactic latitudes). We analyze the distribution of lightcurve skewness for quasars and find that is centered on zero. We find that about 1/4 of the variable stars are RR Lyrae stars, and that only 0.5% of stars from the main stellar locus are variable at the 0.05 mag level. The distribution of lightcurve skewness in the g-r vs. u-g color-color diagram on the main stellar locus is found to be bimodal (with one mode consistent with Algol-like behavior). Using over six hundred RR Lyrae stars, we demonstrate rich halo substructure out to distances of 100 kpc. We extrapolate these results to expected performance by the Large Synoptic Survey Telescope and estimate that it will obtain well-sampled 2% accurate, multi-color lightcurves for ~2 million low-redshift quasars, and will discover at least 50 million variable stars.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
