Article

# The role of AGN in the colour transformation of galaxies at redshifts z~1

[more]
(Impact Factor: 5.23). 01/2008; 385. DOI: 10.1111/j.1365-2966.2008.12962.x
Source: arXiv

ABSTRACT We explore the role of AGN in establishing and/or maintaining the bimodal colour distribution of galaxies by quenching their star-formation and hence, causing their transition from the blue to the red cloud. Important tests for this scenario include (i) the X-ray properties of galaxies in the transition zone between the two clouds and (ii) the incidence of AGN in post-starbursts, i.e. systems observed shortly after (<1Gyr) the termination of their star-formation. We perform these tests by combining deep Chandra observations with multiwavelength data from the AEGIS survey. Stacking the X-ray photons at the positions of galaxies (0.4<z<0.9) not individually detected at X-ray wavelengths suggests a population of obscured AGN among sources in the transition zone and in the red cloud. Their mean X-ray and mid-IR properties are consistent with moderately obscured low-luminosity AGN, Compton thick sources or a mix of both. Morphologies show that major mergers are unlikely to drive the evolution of this population but minor interactions may play a role. The incidence of obscured AGN in the red cloud (both direct detections and stacking results) suggests that BH accretion outlives the termination of the star-formation. This is also supported by our finding that post-starburst galaxies at z~0.8 and AGN are associated, in agreement with recent results at low-z. A large fraction of post-starbursts and red cloud galaxies show evidence for at least moderate levels of AGN obscuration. This implies that if AGN outflows cause the colour transformation of galaxies, then some nuclear gas and dust clouds either remain unaffected or relax to the central galaxy regions after the quenching their star-formation. Comment: Accepted for publication in MNRAS

### Full-text

Available from: Joel Primack, Nov 25, 2014
0 Followers
·
90 Views
• Source
##### Article: On halo formation times and assembly bias
[Hide abstract]
ABSTRACT: In this paper we use the Millennium Simulation'' to re-examine the mass assembly history of dark matter halos and the age dependence of halo clustering. We use eight different definitions of halo formation times to characterize the different aspects of the assembly history of a dark matter halo. We find that these formation times have different dependence on halo mass. While some formation times characterize well the hierarchical nature of halo formation, in the sense that more massive halos have later formation, the trend is reversed for other definitions of the formation time. In particular, the formation times that are likely to be related to the formation of galaxies in dark halos show strong trends of down-sizing'', in that lower-mass halos form later. We also investigate how the correlation amplitude of dark matter halos depends on the different formation times. We find that this dependence is quite strong for some definitions of formation time but weak or absent for other definitions. In particular, the correlation amplitude of halos of a given mass is almost independent of their last major merger time. For the definitions that are expected to be more related to the formation of galaxies in dark halos, a significant assembly bias is found only for halos less massive than M_*. We discuss our results in connection to the hierarchical assembly of dark matter halos, the archaeological down-sizing'' observed in the galaxy population, and the observed color-dependence of the clustering strength of galaxy groups and clusters. Comment: 9 pages, 4 figures, submitted to MNRAS
Monthly Notices of the Royal Astronomical Society 03/2008; 389(3). DOI:10.1111/j.1365-2966.2008.13667.x · 5.23 Impact Factor
• Source
##### Article: The X-ray luminosity function of AGN at z~3
[Hide abstract]
ABSTRACT: We combine Lyman-break colour selection with ultradeep (> 200 ks) Chandra X-ray imaging over a survey area of ~0.35 deg^2 to select high redshift AGN. Applying careful corrections for both the optical and X-ray selection functions, the data allow us to make the most accurate determination to date of the faint end of the X-ray luminosity function (XLF) at z~3. Our methodology recovers a number density of X-ray sources at this redshift which is at least as high as previous surveys, demonstrating that it is an effective way of selecting high z AGN. Comparing to results at z=1, we find no evidence that the faint slope of the XLF flattens at high z, but we do find significant (factor ~3.6) negative evolution of the space density of low luminosity AGN. Combining with bright end data from very wide surveys we also see marginal evidence for continued positive evolution of the characteristic break luminosity L*. Our data therefore support models of luminosity-dependent density evolution between z=1 and z=3. A sharp upturn in the the XLF is seen at the very lowest luminosities (Lx < 10^42.5 erg s^-1), most likely due to the contribution of pure X-ray starburst galaxies at very faint fluxes. Comment: 16 pages, 9 figures, accepted for publication in MNRAS
Monthly Notices of the Royal Astronomical Society 04/2008; 387. DOI:10.1111/j.1365-2966.2008.13286.x · 5.23 Impact Factor
• Source
##### Article: Searching for mid-IR obscured AGN in the Chandra deep field North
[Hide abstract]
ABSTRACT: The efficiency of mid-infrared selection methods for finding obscured AGN is investigated using data in the Chandra Deep Field North. It is shown that samples of AGN candidates compiled on the basis of mid-infrared colours only suffer substantial contamination from normal galaxies. X-ray stacking analysis reveals a soft mean X-ray spectrum for these sources, consistent with $\Gamma\approx$ 2.1. This suggests that star-forming galaxies and not obscured AGN dominate the stacked signal. In contrast AGN selection methods that combine mid-infrared with optical criteria are more successful in finding heavily obscured AGN candidates. A method similar to the one proposed by Fiore et al. (2008) is adopted to select extremely red objects ($R$ - [3.6] > 3.7 mag) with high 24 $\mu$m to optical flux ratio $(f_{\rm 24~\mu m}/f_{R}>1000)$. About 80% of these sources are not detected at X-ray wavelengths. Stacking the X-ray photons at the positions of these sources shows a flat mean X-ray spectrum ($\Gamma$$\approx$ 0.8), which suggests Compton-thick sources, low-luminosity and moderately obscured ($N_{\rm H}$ ~ 8 $\times$ $10^{22}$ cm$^{-2}$) AGN, or a combination of the two. The mid-infrared colours and luminosities of these sources are consistent with ULIRGs at $z$ $\approx$ 2, while HST/ACS images, available for the optically brighter of these sources, show disturbed optical morphologies in many of them. The evidence above suggests that this population includes systems in the process of formation.
Astronomy and Astrophysics 06/2008; 484(3). DOI:10.1051/0004-6361:20078790 · 4.48 Impact Factor