X-ray emission from faint galaxies and quasars

In this thesis a study of faint X-ray sources is undertaken to understand the nature and origin of the Cosmic X-ray Background (XRB). A study of the X-ray variability characteristics of the QSO population is also presented. The optical identification of faint ROSAT sources is described. A large fraction of these sources are identified as QSOs. A number of the remaining sources appear to be associated with faint galaxies with a range of morphological types. The discovery of a rare, high redshift obscured QSO is also described and discussed. A method is developed to study the X-ray variability of faint QSOs. Low amplitude variability (~ 25% rms) on timescales of hours to days appears to be characteristic. Contrary to the trends apparent in local AGN, the QSO population show no evidence for a decrease in variability amplitude with luminosity. There is also no trend in variability with redshift, which may favour a short lived model for QSO evolution. A bright, highly variable Seyfert galaxy is analysed in detail. To constrain the galaxy contribution to the XRB a cross-correlation analysis is presented. Cross-correlating faint galaxy catalogues with unidentified X-ray sources suggests that faint b(_i) < 23 galaxies account for ~ 21 ± 6% of all X-ray sources to a flux limit S(0.5 - 2.0 keV) ~ 4 x l0(^-15)ergs(^-1)cm(^-2). Galaxies are then cross-correlated with the remaining unresolved XRB. A highly significant signal is obtained on 3 deep ROSAT fields. Using a formalism to model the galaxy population and its clustering, an estimate of the local X-ray volume emissivity is obtained. The results suggest that b(_j) < 23 galaxies contribute ~ 20% of the unresolved XRB. Comparing the cross-correlation of faint and bright galaxies with theoretical models reveals strong evolution in the X-ray luminosity of galaxies with the form L(_x) oc (1 + z)(^3±1). Extrapolation to high redshift suggests that faint galaxies contribute 50 - 100% of the unresolved XRB. Finally, a study of the X-ray spectra of faint ROSAT sources is presented. The mean source spectra harden significantly at lower flux. Separating the QSOs from the unidentified sources and galaxies, we find no trend in the 0.5 - 2 keV spectra of QSOs. The remaining population produces the spectral hardening. Taking a subset of the most probable X-ray luminous galaxies, these show significantly harder spectra than QSOs with a mean photon index of Γ ~ 1.5 ± 0.1. X-ray luminous galaxies could therefore account for the missing component of the XRB

The prevalence of AGN feedback in massive galaxies at z~1

We use the optical--infrared imaging in the UKIDSS Ultra Deep Survey field, in combination with the new deep radio map of Arumugam et al., to calculate the distribution of radio luminosities among galaxies as a function of stellar mass in two redshift bins across the interval 0.4<z<1.2. This is done with the use of a new Bayesian method to classify stars and galaxies in surveys with multi-band photometry, and to derive photometric redshifts and stellar masses for those galaxies. We compare the distribution to that observed locally and find agreement if we consider only objects believed to be weak-lined radio-loud galaxies. Since the local distribution is believed to be the result of an energy balance between radiative cooling of the gaseous halo and mechanical AGN heating, we infer that this balance was also present as long ago as z~1. This supports the existence of a direct link between the presence of a low-luminosity ('hot-mode') radio-loud active galactic nucleus and the absence of ongoing star formation.Comment: 10 pages, MNRAS, in pres

Obscured quasars at high redshift in the UKIDSS Ultra Deep Survey

Obscured quasars hidden in deep X-ray surveys can be recovered by looking at mid-infrared wavelengths, where dust re-radiates the absorbed radiation. Here we present a sample of obscured quasars in the redshift range 1 < z < 4 based on data from the UKIDSS Ultra-Deep Survey (UDS), the deepest near-IR survey over ~ 1 sq. deg. to date. Candidates that are primarily selected by their 24 μm emission are probed by decomposing their spectral energy distribution (SED) to disentangle the emission from the AGN and its host galaxy. We show preliminary results on their host galaxy properties as well as their clustering, showing that obscured quasars are found in galaxies located in the green valley, residing in dark matter haloes not different from normal galaxies at those redshift

Evidence for a large fraction of Compton-thick quasars at high redshift

Using mid-infrared and radio selection criteria, we pre-select a sample of candidate high-redshift type-2 quasars in the Subaru XMM-Newton Deep Field (SXDF). To filter out starburst contaminants, we use a bayesian method to fit the spectral energy distributions (SEDs) between 24-microns and B-band, obtain photometric redshifts, and identify the best candidates for high-z type-2 quasars. This leaves us with 12 z_phot >= 1.7 type-2 quasar candidates in an area ~0.8 deg^2, of which only two have secure X-ray detections. The two detected sources have estimated column densities N_H~2 & 3x10^27 m^-2, i.e. heavily obscured but Compton-thin quasars. Given the large bolometric luminosities and redshifts of the undetected objects, the lack of X-ray detections suggests extreme absorbing columns N_H >= 10^28 m^-2 are typical. We have found evidence for a population of ``Compton-thick'' high-redshift type-2 quasars, at least comparable to, and probably larger than the type-1 quasar population, although spectroscopic confirmation of their AGN nature is important.Comment: 6 pages, 2 colour figures. Accepted by MNRAS. Full resolution version and supplementary figures can be found at: http://www.mpia.de/homes/martinez/publications.htm

The Nature, Evolution, Clustering and X-ray Properties of Extremely Red Galaxies in the CDFS/GOODS field

We identify a deep sample of 198 extremely red objects (EROs) in (50.4 sq. arcmin of) the Chandra Deep Field South, selected as I-K>3.92 galaxies to a limit K=22. The ERO number counts remain well below the predictions for pure luminosity evolution, and fall below even a non-evolving model, suggesting the comoving number density of passive/very red galaxies decreases with redshift. The angular correlation function of these EROs indicates stronger clustering than that of other galaxies at the same magnitudes, and is best-fitted by models in which the EROs have a comoving correlation radius 12.5/h Mpc, or 21.4/h Mpc in a stable clustering model. We find a 40-arcsec diameter grouping of 10 EROs, centered on the Chandra source (and ERO) XID:58, with colours suggesting a cluster of mostly passive EROs at approx. z=1.5. The 942 ksec Chandra survey detected 73 X-ray sources in the area of our ERO sample, of which 17 coincide with EROs. Of these sources, 13 have X-ray properties indicative of obscured AGN, while the faintest 4 may be starburst galaxies. In addition, we find evidence that Chandra sources and EROs are positively cross-correlated at non-zero (2-20 arcsec) separations, implying that they tend to trace the same large-scale structures.Comment: 17 pages, latex, 14 figures, accepted for publication in MNRAS. Major revisions from original version, with a new, reselected sample of ERO

Minor versus major mergers: the stellar mass growth of massive galaxies from z=3 using number density selection techniques

We present a study on the stellar mass growth of the progenitors of local massive galaxies with a variety of number density selections with n≤1×10−4 Mpc−3 (corresponding to M*=1011.24 M⊙ at z=0.3) in the redshift range 0.3<z<3.0. We select the progenitors of massive galaxies using a constant number density selection, and one which is adjusted to account for major mergers. We find that the progenitors of massive galaxies grow by a factor of 4 in total stellar mass over this redshift range. On average the stellar mass added via the processes of star formation, major and minor mergers account for 24±8, 17±15 and 34±14per cent, respectively, of the total galaxy stellar mass at z=0.3. Therefore 51±20per cent of the total stellar mass in massive galaxies at z=0.3 is created externally to their z=3 progenitors. We explore the implication of these results on the cold gas accretion rate and size evolution of the progenitors of most massive galaxies over the same redshift range. We find an average gas accretion rate of∼66±32 M⊙ yr−1 over the redshift range of 1.5<z<3.0. We find that the size evolution of a galaxy sample selected this way is on average lower than the findings of other investigation

A consistent measure of the merger histories of massive galaxies using close-pair statistics I:Major mergers at z &lt;3.5

We use a large sample of $\sim 350,000$ galaxies constructed by combining the UKIDSS UDS, VIDEO/CFHT-LS, UltraVISTA/COSMOS and GAMA survey regions to probe the major merging histories of massive galaxies ($>10^{10}\ \mathrm{M}_\odot$) at $0.005 < z < 3.5$. We use a method adapted from that presented in Lopez-Sanjuan et al. (2014) using the full photometric redshift probability distributions, to measure pair $\textit{fractions}$ of flux-limited, stellar mass selected galaxy samples using close-pair statistics. The pair fraction is found to weakly evolve as $\propto (1+z)^{0.8}$ with no dependence on stellar mass. We subsequently derive major merger $\textit{rates}$ for galaxies at $> 10^{10}\ \mathrm{M}_\odot$ and at a constant number density of $n > 10^{-4}$ Mpc$^{-3}$, and find rates a factor of 2-3 smaller than previous works, although this depends strongly on the assumed merger timescale and likelihood of a close-pair merging. Galaxies undergo approximately 0.5 major mergers at $z < 3.5$, accruing an additional 1-4 $\times 10^{10}\ \mathrm{M}_\odot$ in the process. Major merger accretion rate densities of $\sim 2 \times 10^{-4}$ $\mathrm{M}_\odot$ yr$^{-1}$ Mpc$^{-3}$ are found for number density selected samples, indicating that direct progenitors of local massive ($>10^{11}\mathrm{M}_\odot$) galaxies have experienced a steady supply of stellar mass via major mergers throughout their evolution. While pair fractions are found to agree with those predicted by the Henriques et al. (2014) semi-analytic model, the Illustris hydrodynamical simulation fails to quantitatively reproduce derived merger rates. Furthermore, we find major mergers become a comparable source of stellar mass growth compared to star-formation at $z < 1$, but is 10-100 times smaller than the SFR density at higher redshifts.Comment: 26 pages, 18 figures, accepted to MNRA

Exploring the progenitors of brightest cluster galaxies at z ∼ 2

We present a new method for tracing the evolution of brightest cluster galaxies (BCGs) from z ∼ 2 to z ∼ 0. We conclude on the basis of semi-analytical models that the best method to select BCG progenitors at z ∼ 2 is a hybrid environmental density and stellar mass ranking approach. Ultimately, we are able to retrieve 45 per cent of BCG progenitors. We apply this method on the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey, Ultra Deep Survey data to construct a progenitor sample at high redshift. We furthermore populate the comparisons in local Universe by using Sloan Digital Sky Survey data with statistically likely contamination to ensure a fair comparison between high and low redshifts. Using these samples we demonstrate that the BCG sizes have grown by a factor of ∼3.2 since z ∼ 2, and BCG progenitors are mainly late-type galaxies, exhibiting less concentrated profiles than their early type local counterparts. We find that BCG progenitors have more disturbed morphologies. In contrast, local BCGs have much smoother profiles. Moreover, we find that the stellar masses of BCGs have grown by a factor of ∼2.5 since z ∼ 2, and the star formation rate of BCG progenitors has a median value of 13.5 Mʘ yr‾¹, much higher than their quiescent local descendants. We demonstrate that over z = 1–2 star formation and merging contribute equally to BCG mass growth. However, merging plays a dominant role in BCG assembly at z ≲ 1. We also find that BCG progenitors at high z are not significantly different from other galaxies of similar mass at the same epoch. This suggests that the processes which differentiate BCGs from normal massive elliptical galaxies must occur at z ≲ 2

Correcting for the overabundance of low-mass quiescent galaxies in semi-analytic models

We compare the L-Galaxies semi-analytic model to deep observational data from the UKIDSS Ultra Deep Survey (UDS) across the redshift range 0.5 < z < 3. We find that the overabundance of low-mass, passive galaxies at high redshifts in the model can be attributed solely to the properties of 'orphan' galaxies, i.e. satellite galaxies where the simulation has lost track of the host dark matter subhalo. We implement a simple model that boosts the star-formation rates in orphan galaxies by matching them to non-orphaned satellite galaxies at a similar evolutionary stage. This straightforward change largely addresses the discrepancy in the low-mass passive fraction across all redshifts. We find that the orphan problem is somewhat alleviated by higher resolution simulations, but the preservation of a larger gas reservoir in orphans is still required to produce a better fit to the observed space density of low-mass passive galaxies. Our findings are also robust to the precise definition of the passive galaxy population. In general, considering the vastly different prescriptions used for orphans in semi-analytic models, we recommend that they are analysed separately from the resolved satellite galaxy population, particularly with JWST observations reigniting interest in the low-mass regime in which they dominate