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Register a new userThe spatial clustering of X-ray selected AGN in the XMM-COSMOS field
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We study the spatial clustering of 538 X-ray selected AGN in the 2 deg^2 XMM-COSMOS field that are spectroscopically identified to I_{AB}<23 and span the redshift range z=0.2-3.0. The median redshift and luminosity of the sample are z = 0.98 and L_{0.5-10}=6.3 x 10^{43} erg/s, respectively. A strong clustering signal is detected at ~18sigma level, which is the most significant measurement obtained to date for clustering of X-ray selected AGN. By fitting the projected correlation function w(r_p) with a power law on scales of r_p=0.3-40 Mpc/h, we derive a best fit comoving correlation length of r_0 = 8.6 +- 0.5 Mpc/h and slope of gamma=1.88 +- 0.07 (Poissonian errors; bootstrap errors are about a factor of 2 larger). An excess signal is observed in the range r_p~5-15 Mpc/h, which is due to a large scale structure at z ~ 0.36 containing about 40 AGN. When removing the z ~ 0.36 structure, or computing w(r_p) in a narrower range around the peak of the redshift distribution (e.g. z=0.4-1.6), the correlation length decreases to r_0 ~ 5-6 Mpc/h, which is consistent with that observed for bright optical QSOs at the same redshift. We investigate the clustering properties of obscured and unobscured AGN separately. Within the statistical uncertainties, we do not find evidence that AGN with broad optical lines (BLAGN) cluster differently from AGN without broad optical lines (non-BLAGN). The correlation length measured for XMM-COSMOS AGN at z~1 is similar to that of massive galaxies (stellar mass M_*> 3 x 10^{10} M_sun) at the same redshift. This suggests that AGN at z~1 are preferentially hosted by massive galaxies, as observed both in the local and in the distant (z~2) Universe. (shortened)
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We study the spatial clustering through the projected two-point correlation function of $632$ $(1130)$ XMM-COSMOS Active Galactic Nuclei (AGNs) with known spectroscopic (spectroscopic or photometric) redshifts in the range $z = [0.1 - 2.5]$ in order to measure the AGN bias and estimate the typical mass of the hosting dark matter (DM) halo as a function of AGN host galaxy properties. We create AGN subsamples in terms of stellar mass $M_*$ and specific black hole accretion rate $L_X/M_*$, to probe how AGN environment depends on these quantities. For the full spectroscopic AGN sample, we measure a typical DM halo mass of $log (M_mathrm{halo} / h^{-1}mathrm{M}_odot)= 12.79_{-0.43}^{+0.26}$, similar to galaxy group environments and in line with previous studies for moderate-luminosity X-ray selected AGN. We find no significant dependence on $L_X/M_*$, with $log (M_mathrm{halo} / h^{-1}mathrm{M}_odot) = 13.06_{-0.38}^{+0.23}$ ($12.97_{-1.26}^{+0.39}$) for the low (high) $L_X/M_*$ subsample. We also find no difference in the hosting halos in terms of $M_*$ with $log (M_mathrm{halo} / h^{-1}mathrm{M}_odot) = 12.93_{-0.62}^{+0.31}$ ($12.90_{-0.62}^{+0.30}$) for the low (high) $M_*$ subsample. By comparing the $M_*-M_mathrm{halo}$ relation derived for XMM-COSMOS AGN subsamples with what is expected for normal non-active galaxies by abundance matching and clustering results, we find that the typical DM halo mass of our high $M_*$ AGN subsample is similar to that of non-active galaxies. However, AGNs in our low $M_*$ subsample are found in more massive halos than non-active galaxies. By excluding AGNs in galaxy groups from the clustering analysis, we find evidence that the result for low $M_*$ may be due a larger fraction of AGNs as satellites in massive halos.
We took advantage of the observations carried out by XMM in the COSMOS field during 3.5 years, to study the long term variability of a large sample of AGN (638 sources), in a wide range of redshift (0.1<z<3.5) and X-ray luminosity ($10^{41}<$L(2-10)$<10^{45.5}$). Both a simple statistical method to asses the significance of variability, and the Normalized Excess Variance ($sigma^{2}_{rms}$) parameter, where used to obtain a quantitative measurement of the variability. Variability is found to be prevalent in most AGN, whenever we have good statistic to measure it, and no significant differences between type-1 and type-2 AGN were found. A flat (slope -0.23+/-0.03) anti-correlation between $sigma^{2}_{rms}$ and X-ray luminosity is found, when significantly variable sources are considered all together. When divided in three redshift bins, the anti-correlation becomes stronger and evolving with z, with higher redshift AGN being more variable. We prove however that this effect is due to the pre-selection of variable sources: considering all the sources with available $sigma^{2}_{rms}$ measurement, the evolution in redshift disappears. For the first time we were also able to study the long term X-ray variability as a function of $M_{rm BH}$ and Eddington ratio, for a large sample of AGN spanning a wide range of redshift. An anti-correlation between $sigma^{2}_{rms}$ and $M_{rm BH}$ is found, with the same slope of the anti-correlation between $sigma^{2}_{rms}$ and X-ray luminosity, suggesting that the latter can be a byproduct of the former one. No clear correlation is found between $sigma^{2}_{rms}$ and the Eddington ratio in our sample. Finally, no correlation is found between the X-ray $sigma^{2}_{rms}$ and the optical variability.
We investigate the spatial clustering of X-ray selected sources in the two deepest X-ray fields to date, namely the 2Msec Chandra Deep Field North (CDFN) and the 1Msec Chandra Deep Field South (CDFS). The projected correlation function w(r_p), measured on scales ~0.2-10 h^-1 Mpc for a sample of 240 sources with spectroscopic redshift in the CDFN and 124 sources in the CDFS at a median redshift of z~0.8, is used to constrain the amplitude and slope of the real space correlation function xi(r)=(r/r0)^-gamma. The clustering signal is detected at high confidence (>~ 7 sigma) in both fields. The amplitude of the correlation is found to be significantly different in the two fields, the correlation length r0 being 8.6 +- 1.2 h^-1 Mpc in the CDFS and 4.2 +- 0.4 h^-1 Mpc in the CDFN, while the correlation slope gamma is found to be flat in both fields: gamma=1.33 +- 0.11 in the CDFS and gamma=1.42 +- 0.07 in the CDFN (a flat Universe with Omega_m=0.3 and Omega_L=0.7 is assumed; 1 sigma Poisson error estimates are considered). The correlation function has been also measured separately for sources classified as AGN or galaxies. In both fields AGN have a median redshift of z~0.9 and a median 0.5-10 keV luminosity of L_x~10^43 erg s^-1, i.e. they are generally in the Seyfert luminosity regime. As in the case of the total samples, we found a significant difference in the AGN clustering amplitude between the two fields, the best fit correlation parameters being r0=10.3 +- 1.7 h^-1 Mpc, gamma=1.33 +- 0.14 in the CDFS, and r0=5.5 +- 0.6 h^-1 Mpc, gamma=1.50 +- 0.12 in the CDFN. Within each field no statistically significant difference is found between soft and hard X-ray selected sources or between type 1 and type 2 AGN. (abridged)
We present our statistical study of near infrared (NIR) variability of X-ray selected Active Galactic Nuclei (AGN) in the COSMOS field, using UltraVISTA data. This is the largest sample of AGN light curves in YJHKs bands, making possible to have a global description of the nature of AGN for a large range of redshifts, and for different levels of obscuration. To characterize the variability properties of the sources we computed the Structure Function. Our results show that there is an anti-correlation between the Structure Function $A$ parameter (variability amplitude) and the wavelength of emission, and a weak anti-correlation between $A$ and the bolometric luminosity. We find that Broad Line (BL) AGN have a considerably larger fraction of variable sources than Narrow Line (NL) AGN, and that they have different distributions of the $A$ parameter. We find evidence that suggests that most of the low luminosity variable NL sources correspond to BL AGN, where the host galaxy could be damping the variability signal. For high luminosity variable NL, we propose that they can be examples of True type II AGN or BL AGN with limited spectral coverage which results in missing the Broad Line emission. We also find that the fraction of variable sources classified as unobscured in the X-ray is smaller than the fraction of variable sources unobscured in the optical range. We present evidence that this is related to the differences in the origin of the obscuration in the optical and X-ray regimes.
This paper presents a survey of X-ray selected active galactic nuclei (AGN) with optical spectroscopic follow-up in a $sim 18, rm{deg^2}$ area of the equatorial XMM-XXL north field. A sample of 8445 point-like X-ray sources detected by XMM-Newton above a limiting flux of $F_{rm 0.5-10, keV} > 10^{-15} rm,erg, cm^{-2}, s^{-1}$ was matched to optical (SDSS) and infrared (WISE) counterparts. We followed up 3042 sources brighter than $r=22.5$ mag with the SDSS BOSS spectrograph. The spectra yielded a reliable redshift measurement for 2578 AGN in the redshift range $z=0.02-5.0$, with $0.5-2rm, keV$ luminosities ranging from $10^{39}-10^{46}rm,erg,s^{-1}$. This is currently the largest published spectroscopic sample of X-ray selected AGN in a contiguous area. The BOSS spectra of AGN candidates show a bimodal distribution of optical line widths allowing a separation between broad- and narrow-emission line AGN. The former dominate our sample (70 per cent) due to the relatively bright X-ray flux limit and the optical BOSS magnitude limit. We classify the narrow emission line objects (22 per cent of full sample) using standard BPT diagnostics: the majority have line ratios indicating the dominant source of ionization is the AGN. A small number (8 per cent of full sample) exhibit the typical narrow line ratios of star-forming galaxies, or only have absorption lines in their spectra. We term the latter two classes elusive AGN. We also compare X-ray, optical and infrared color AGN selections in this field. X-ray observations reveal, the largest number of AGN. The overlap between the selections, which is a strong function of the imaging depth in a given band, is also remarkably small. We show using spectral stacking that a large fraction of the X-ray AGN would not be selectable via optical or IR colours due to host galaxy contamination.
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