Majority Of Galaxies Research Articles

The UCD Population of the Coma Cluster

AbstractUCDs are super massive star clusters found largely in dense regions but have also been found around individual galaxies and in smaller groups. Their origin is still under debate but consensus is that they formed either during major galaxy mergers as mergers of super massive star clusters, are simply the high mass end of the globular cluster luminosity function and formed in the same way as globular clusters, or that they formed from the threshing of galaxies and are remnant nuclear star clusters, which themselves may have formed from the mergers of globular star clusters within galaxies. We are attempting to disentangle these competing formation scenarios with a large survey of UCDs in the Coma cluster. Using ACS two-passband imaging from the HST/ACS Coma Cluster Treasury Survey, we are using colors and sizes to identify the UCD cluster members. With a large sample within the core region of the Coma cluster, we will use the population size, properties, and spatial distribution, and comparison with the Coma globular cluster and nuclear star cluster populations to discriminate between the threshing and globular cluster scenarios. In particular, previously we have found a possible correlation of UCD colors with host galaxy and a possible excess of UCDs around a non-central giant galaxy with an unusually large globular cluster population, both suggestive of a globular cluster origin. With a larger sample size, we are investigating whether the color correlation with host persists and whether the UCD population is consistent with, or in excess of, the bright end of the GCLF. We present initial results from the survey.

HENIZE 2–10: THE ONGOING FORMATION OF A NUCLEAR STAR CLUSTER AROUND A MASSIVE BLACK HOLE

The central region of the galaxy Henize 2-10 has a central black hole (BH) with a mass of about $2\times 10^6$ M$_\odot$. While this black hole does not appear to coincide with any central stellar over density, it is surrounded by 11 young massive clusters with masses above $10^5$ M$_\odot$. The availability of high quality data on the structure of the galaxy and the age and mass of the clusters provides excellent initial conditions for studying the dynamical evolution of Henize 2-10's nucleus. Here we present a set of $N$-body simulations of the central clusters and black hole to understand whether and how they will merge to form a nuclear star cluster. Nuclear star clusters (NSCs) are present in a majority of galaxies with stellar mass similar to Henize 2-10. Despite the results depend on the choice of initial conditions, we find that a NSC with mass $M_{NSC}\simeq 4-6\times 10^6$ M$_\odot$ and effective radius $r_{NSC}\simeq 2.6-4.1$ pc will form within $0.2$ Gyr. This work is the first showing, in a realistic realization of the host galaxy and its star cluster system, that the formation of a bright nucleus is a process that can happen after the formation of a central massive BH leading to a composite NSC+BH central system. The merging process of the clusters does not affect significantly the kinematics of the BH, whose motion, after the globular cluster merger, is limited to a $\sim 1$ pc oscillation at less than $2$ kms$^{-1}$ speed.

Galaxy pairs in the Sloan Digital Sky Survey – XII. The fuelling mechanism of low-excitation radio-loud AGN

Abstract We investigate whether the fuelling of low-excitation radio galaxies (LERGs) is linked to major galaxy interactions. Our study utilizes a sample of 10 800 spectroscopic galaxy pairs and 97 post-mergers selected from the Sloan Digital Sky Survey with matches to multiwavelength data sets. The LERG fraction amongst interacting galaxies is a factor of 3.5 higher than that of a control sample matched in local galaxy density, redshift and stellar mass. However, the LERG excess in pairs does not depend on projected separation and remains elevated out to at least 500 $h_{70}^{-1}$ kpc, suggesting that major mergers are not their main fuelling channel. In order to identify the primary fuelling mechanism of LERGs, we compile samples of control galaxies that are matched in various host galaxy and environmental properties. The LERG excess is reduced, but not completely removed, when halo mass or D4000 are included in the matching parameters. However, when bothMhalo and D4000 are matched, there is no LERG excess and the 1.4 GHz luminosities (which trace jet mechanical power) are consistent between the pairs and control. In contrast, the excess of optical and mid-IR selected active galactic nuclei (AGN) in galaxy pairs is unchanged when the additional matching parameters are implemented. Our results suggest that whilst major interactions may trigger optically and mid-IR selected AGN, the gas which fuels the LERGs has two secular origins: one associated with the large-scale environment, such as accretion from the surrounding medium or minor mergers, plus an internal stellar mechanism, such as winds from evolved stars.

THE STRUCTURE AND STELLAR CONTENT OF THE OUTER DISKS OF GALAXIES: A NEW VIEW FROM THE Pan-STARRS1 MEDIUM DEEP SURVEY

We present the results of an analysis of Pan-STARRS1 Medium Deep Survey multi-band (grizy) images of a sample of 698 low-redshift disk galaxies that span broad ranges in stellar mass, star-formation rate, and bulge/disk ratio. We use population synthesis spectral energy distribution fitting techniques to explore the radial distribution of the light, color, surface mass density, mass/light ratio, and age of the stellar populations. We characterize the structure and stellar content of the galaxy disks out to radii of about twice Petrosian r90, beyond which the halo light becomes significant. We measure normalized radial profiles for sub-samples of galaxies in three bins each of stellar mass and concentration. We also fit radial profiles to each galaxy. The majority of galaxies have down-bending radial surface brightness profiles in the bluer bands with a break radius at roughly r90. However, they typically show single unbroken exponentials in the reddest bands and in the stellar surface mass density. We find that the mass/light ratio and stellar age radial profiles have a characteristic "U" shape. There is a good correlation between the amplitude of the down-bend in the surface brightness profile and the rate of the increase in the M/L ratio in the outer disk. As we move from late- to early-type galaxies, the amplitude of the down-bend and the radial gradient in M/L both decrease. Our results imply a combination of stellar radial migration and suppression of recent star formation can account for the stellar populations of the outer disk.

Should we believe the results of ultraviolet–millimetre galaxy spectral energy distribution modelling?

Galaxy spectral energy distribution (SED) modelling is a powerful tool, but constraining how well it is able to infer the true values for galaxy properties (e.g. the star formation rate, SFR) is difficult because independent determinations are often not available. However, galaxy simulations can provide a means of testing SED modelling techniques. Here, we present a numerical experiment in which we apply the SED modelling code MAGPHYS to ultraviolet (UV)--millimetre (mm) synthetic photometry generated from hydrodynamical simulations of an isolated disc galaxy and a major galaxy merger by performing three-dimensional dust radiative transfer. We compare the properties inferred from the SED modelling with the true values and find that MAGPHYS recovers most physical parameters of the simulated galaxies well. In particular, it recovers consistent parameters irrespective of the viewing angle, with smoothly varying results for neighbouring time steps of the simulation, even though each viewing angle and time step is modelled independently. The notable exception to this rule occurs when we use an SMC-type intrinsic dust extinction curve in the radiative transfer calculations. In this case, the two-component dust model used by MAGPHYS is unable to effectively correct for the attenuation of the simulated galaxies, which leads to potentially significant errors (although we obtain only marginally acceptable fits in this case). Overall, our results give confidence in the ability of SED modelling to infer physical properties of galaxies, albeit with some caveats.

A FAR-INFRARED SPECTROSCOPIC SURVEY OF INTERMEDIATE REDSHIFT (ULTRA) LUMINOUS INFRARED GALAXIES

We present Herschel far-IR photometry and spectroscopy as well as ground based CO observations of an intermediate redshift (0.21 < z < 0.88) sample of Herschel-selected (ultra)-luminous infrared galaxies (L_IR > 10^11.5L_sun). With these measurements we trace the dust continuum, far-IR atomic line emission, in particular [CII]\,157.7microns, as well as the molecular gas of z~0.3 (U)LIRGs and perform a detailed investigation of the interstellar medium of the population. We find that the majority of Herschel-selected intermediate redshift (U)LIRGs have L_CII/L_FIR ratios that are a factor of about 10 higher than that of local ULIRGs and comparable to that of local normal and high-$z$ star forming galaxies. Using our sample to bridge local and high-z [CII] observations, we find that the majority of galaxies at all redshifts and all luminosities follow a L_CII-L_FIR relation with a slope of unity, from which local ULIRGs and high-z AGN dominated sources are clear outliers. We also confirm that the strong anti-correlation between the L_CII/L_FIR ratio and the far-IR color L_60/L_100 observed in the local Universe holds over a broad range of redshifts and luminosities, in the sense that warmer sources exhibit lower L_CII/L_FIR at any epoch. Intermediate redshift ULIRGs are also characterised by large molecular gas reservoirs and by lower star formation efficiencies compared to that of local ULIRGs. The high L_CII/L_FIR ratios, the moderate star formation efficiencies (L_LIR/L_CO or L_IR/M_gas) and the relatively low dust temperatures of our sample (which are also common characteristics of high-z star forming galaxies with ULIRG-like luminosities) indicate that the evolution of the physical properties of (U)LIRGs between the present day and z > 1 is already significant by z ~ 0.3.

The middle infrared properties of OH megamaser host galaxies

We compiled all 119 OH maser galaxies (110 out of them are megamasers, i.e., L-OH > 10 L-circle dot) published so far and cross-identified these OH masers with the Wide-Field Infrared Survey Explorer (WISE) catalog, to investigate the middle infrared (MIR) properties of OH maser galaxies. The WISE magnitude data at the 3.4, 4.6, 12 and 22 mu m (W1 to W4) are collected for the OH maser sample and one control sample, which are non-detection sources. The color-color diagrams show that both OH megamaser (OHM) and non-OHM (ultra) luminous infrared galaxies ((U) LIRGs) are far away from the single blackbody model line and many of them can follow the path described by the power-law model. The active galaxy nuclei (AGN) fraction is about similar to 40% for both OHM and non-OHM (U) LIRGs, according to the AGN criteria W1-W2 >= 0.8. Among the Arecibo survey sample, OHM sources tend to have a lower luminosity at short MIR wavelengths (e. g., 3.4 mu m and 4.6 mu m) than that of non-OHM sources, which should come from the low OHM fraction among the survey sample with large 3.4 mu m and 4.6 mu m luminosity. The OHM fraction tends to increase with cooler MIR colors (larger F-22 (mu m)/F-3.4 (mu m)). These may be good for sample selection when searching OH megamasers, such as excluding extreme luminous sources at short MIR wavelengths, choosing sources with cooler MIR colors. In the case of the power-law model, we derived the spectral indices for our samples. For the Arecibo survey sample, OHM (U) LIRGs tend to have larger spectral index alpha(22-12) than non-OHM sources, which agrees with previous results. One significant correlation exists between the WISE infrared luminosity at 22 mu m and the color [W1]-[W4] for the Arecibo OHM hosts. These clues should provide suitable constraints on the sample selection for OH megamaser surveys by future advanced telescopes (e.g., FAST). In addition, the correlation of maser luminosity and the MIR luminosity of maser hosts tends to be non-significant, which may indirectly support the pumping of OHM emission that is dominated by the far infrared radiation, instead of the MIR radiation.

The properties of early-type galaxies in the Ursa Major cluster

Using SDSS-DR7 and NASA/IPAC Extragalactic Database spectroscopic data, we identify 166 galaxies as members of the Ursa Major cluster with Mr < −13.5 mag. We morphological classify all galaxies by means of carefully inspecting g-, r-, i-band colour and monochromatic images. We show that the Ursa Major cluster is dominated by late-type galaxies, but also contains a significant number of early-type galaxies, particularly in the dwarf regime. We present further evidence for the existence of several subgroups in the cluster, consistent with previous findings. The early-type fraction is found to correlate with the mass of the subgroup. We also investigate environmental effects by comparing the properties of the Ursa Major early-type dwarf galaxies to those of the Virgo cluster. In contrast to the Virgo, the red sequence of the Ursa Major cluster is only sparsely populated in the optical and ultraviolet colour–magnitude relations. It also shows a statistically significant gap between −18 < Mr < −17 mag, i.e. the Ursa Major cluster lacks early-type dwarf galaxies at the bright end of their luminosity function. We discover that the majority of early-type dwarf galaxies in the Ursa Major cluster have blue cores with hints of recent or ongoing star formation. We suggest that gravitational tidal interactions can trigger central blue star-forming regions in early-type dwarfs. After that, star formation would only fade completely when the galaxies experience ram-pressure stripping or harassment, both of which are nearly absent in the Ursa Major cluster.

The ATLAS3D project – XXV. Two-dimensional kinematic analysis of simulated galaxies and the cosmological origin of fast and slow rotators

We present a detailed two-dimensional stellar dynamical analysis of a sample of 44 cosmological hydrodynamical simulations of individual central galaxies and their satellites. Kinematic maps of the stellar line-of-sight velocity, velocity dispersion, and higher-order Gauss-Hermite moments $h_3$ and $h_4$ are constructed for each central galaxy and for the most massive satellites. The amount of rotation is quantified using the $\lambda_{\mathrm{R}}$-parameter. The velocity, velocity dispersion, $h_3$, and $h_4$ fields of the simulated galaxies show a diversity similar to observed kinematic maps of early-type galaxies in the ATLAS$^{\rm{3D}}$ survey. This includes fast (regular), slow, and misaligned rotation, hot spheroids with embedded cold disk components as well as galaxies with counter-rotating cores or central depressions in the velocity dispersion. We link the present day kinematic properties to the individual cosmological formation histories of the galaxies. In general, major galaxy mergers have a significant influence on the rotation properties resulting in both a spin-down as well as a spin-up of the merger remnant. Lower mass galaxies with significant in-situ formation of stars, or with additional gas-rich major mergers - resulting in a spin-up - in their formation history, form elongated fast rotators with a clear anti-correlation of $h_3$ and $v/\sigma$. An additional formation path for fast rotators includes gas poor major mergers leading to a spin-up of the remnants. This formation path does not result in anti-correlated $h_3$ and $v/\sigma$. The galaxies most consistent with the rare class of non-rotating round early-type galaxies grow by gas-poor minor mergers alone. In general, more massive galaxies have less in-situ star formation since $z \sim 2$, rotate slower and have older stellar populations. (shortened)

DEEPCHANDRAOBSERVATIONS OF HCG 16. I. ACTIVE NUCLEI, STAR FORMATION, AND GALACTIC WINDS

We present new, deep Chandra X-ray and Giant Metrewave Radio Telescope 610~MHz observations of the spiral-galaxy-rich compact group HCG 16, which we use to examine nuclear activity, star formation and the high luminosity X-ray binary populations in the major galaxies. We confirm the presence of obscured active nuclei in NGC 833 and NGC 835, and identify a previously unrecognized nuclear source in NGC 838. All three nuclei are variable on timescales of months to years, and for NGC 833 and NGC 835 this is most likely caused by changes in accretion rate. The deep Chandra observations allow us to detect for the first time an Fe-K$\alpha$ emission line in the spectrum of the Seyfert 2 nucleus of NGC 835. We find that NGC 838 and NGC 839 are both starburst-dominated systems, with only weak nuclear activity, in agreement with previous optical studies. We estimate the star formation rates in the two galaxies from their X-ray and radio emission, and compare these results with estimates from the infra-red and ultra-violet bands to confirm that star formation in both galaxies is probably declining after galaxy-wide starbursts were triggered ~400-500 Myr ago. We examine the physical properties of their galactic superwinds, and find that both have temperatures of ~0.8 keV. We also examine the X-ray and radio properties of NGC 848, the fifth largest galaxy in the group, and show that it is dominated by emission from its starburst.

Dense Molecular Gas and H2O Maser Emission in Galaxies

Extragalactic H2O masers have been found in dense gas circumstance in off-nuclear star formation regions or within parsecs of Active Galactic Nuclei (AGNs). HCN molecular (one of the best dense gas tracers) emission has been detected in more than 60 galaxies. For HCN-detected galaxy sample, the relation of maser and gas emission was investigated here to identify physical observable properties that differentiate maser and non-maser galaxies. Our analysis results show that there is no significant difference on the infrared and gas emission between maser galaxies and galaxies without maser detection. For maser host HCN-galaxies, maser luminosity is found to be correlated to CO luminosity (a proxy of the total molecular gas) and HCN luminosity, i.e., kilomasers ( $L_{\mathrm{H}_{2}\mathrm{O}}$ < 10 L ⊙) with low maser luminosity having low gas emission luminosity, with respect to megamasers ( $L_{\mathrm{H}_{2}\mathrm{O}}$ > 10 L ⊙). For normalized maser and HCN luminosity (for removing distance effect), the correlation is still apparent. However, for normalized maser and CO luminosity, the correlation disappeared completely. Thus one proposition that the amount of dense molecular gas should be a good tracer of H2O maser emission can be made.

The Megamaser Cosmology Project: precise black hole mass measurement and the implication for the MBH–σ∗ relation

AbstractWe made dynamical black hole mass measurements from nineteen Seyfert 2 galaxies which host sub-parsec H2O maser disks using the H2O megamaser technique. The nearly perfect Keplerian rotation curves in many of these maser systems guarantee the high accuracy and precision of the black hole mass measurements. With the stellar velocity dispersion (σ∗) of the galaxy bulges measured with the Dupont 2.5 m telescope at Las Campanas Observatory in the South and the Apache Point Observatory (APO) 3.5m telescope in the North, we found that H2O maser galaxies, most of which host pseudo bulges rather than classical bulges, do not all follow the MBH–σ∗ relation shown in the literature. This result is well consistent with the latest findings by Kormendy &amp; Ho (2013) that only early type galaxies and galaxies with classical bulges follow a tight MBH–σ∗ relation. Such a tight correlation may not exist in pseudo bulge galaxies.

Co-orbiting satellite galaxy structures are still in conflict with the distribution of primordial dwarf galaxies

Both major galaxies in the Local Group host planar distributions of co-orbiting satellite galaxies, the Vast Polar Structure (VPOS) of the Milky Way and the Great Plane of Andromeda (GPoA). The $\Lambda$CDM cosmological model did not predict these features. However, according to three recent studies the properties of the GPoA and the flattening of the VPOS are common features among sub-halo based $\Lambda$CDM satellite systems, and the GPoA can be naturally explained by satellites being acquired along cold gas streams. We point out some methodological issues in these studies: either the selection of model satellites is different from that of the observed ones, or an incomplete set of observational constraints has been considered, or the observed satellite distribution is inconsistent with basic assumptions. Once these issues have been addressed, the conclusions are different: features like the VPOS and GPoA are very rare (each with probability $\lesssim 10^{-3}$, and combined probability $< 10^{-5}$) if satellites are selected from a $\Lambda$CDM simulation combined with semi-analytic modelling, and accretion along cold streams is no natural explanation of the GPoA. The origin of planar dwarf galaxy structures remains unexplained in the standard paradigm of galaxy formation.

CO-ORBITING PLANES OF SUB-HALOS ARE SIMILARLY UNLIKELY AROUND PAIRED AND ISOLATED HOSTS

Sub-halos in dark-matter-based cosmological simulations tend to be distributed approximately isotropically around their host. The existence of highly flattened, co-orbiting planes of satellite galaxies has therefore been identified as a possible problem for these cosmological models, but so far studies have not considered the hosts' environments. That satellite planes are now known around both major galaxies in the Local Group raises the question whether they are more likely around paired hosts. In a first attempt to investigate this possibility we focus on the flattening and orbital coherence of the 11 brightest satellite galaxies of the vast polar structure (VPOS) around the Milky Way (MW). We search for VPOS analogs in the ELVIS suite of cosmological simulations, which consist of 24 paired and 24 isolated host halos. We do not find significant differences between the properties of sub-halo distributions around paired and isolated hosts. The observed flattening and the observed orbital alignment are each reproduced by only 0.2 to 2 per cent of paired and isolated systems incorporating the obscuration of satellites by randomly oriented galactic discs. Only one of all 4800 analyzed realizations (0.02 per cent) reproduces both parameters simultaneously, but the average orbital pole of this sub-halo system does not align as well with the normal to the plane fit as observed. That the MW is part of a galaxy pair thus does not help in explaining the existence of the VPOS if the satellite galaxies are identified with sub-halos found in dissipationless simulations.

The pre-processing of subhaloes in SDSS groups and clusters

We investigate pre-processing using the observed quenched fraction of group and cluster galaxies in the Yang et al. (2007) SDSS-DR7 group catalogue in the redshift range of 0.01 < z < 0.045. We categorize group galaxies as virialized, infall or backsplash and we apply a combination of the Dressler-Shectman statistic and group member velocities to identify subhaloes. On average the fraction of galaxies that reside in subhaloes is a function of host halo mass, where more massive systems have a higher fraction of subhalo galaxies both in the overall galaxy and infall populations. Additionally, we find that between 2 < r_200 < 3 the quiescent fraction is higher in the subhalo population with respect to both the field and non-subhalo populations. At these large radii (2 < r_200 < 3), the majority of galaxies (~ 80 %) belong to the infall population and therefore, we attribute the enhanced quenching to infalling subhalo galaxies, indicating that pre-processing has occurred in the subhalo population. We conclude that pre-processing plays a significant role in the observed quiescent fraction, but only for the most massive (M_halo > 10^14.5 M_sun) systems in our sample.

Perseus I and the NGC 3109 association in the context of the Local Group dwarf galaxy structures

The recently discovered dwarf galaxy Perseus I appears to be associated with the dominant plane of non-satellite galaxies in the Local Group (LG). We predict its velocity dispersion and those of the other isolated dSphs Cetus and Tucana to be 6.5, 8.2, and 5.5 km/s, respectively. The NGC 3109 association, including the recently discovered dwarf galaxy Leo P, aligns with the dwarf galaxy structures in the LG such that all known nearby non-satellite galaxies in the northern Galactic hemisphere lie in a common thin plane (rms height 53 kpc; diameter 1.2 Mpc). This plane has an orientation similar to the preferred orbital plane of the Milky Way (MW) satellites in the vast polar structure. Five of seven of these northern galaxies were identified as possible backsplash objects, even though only about one is expected from cosmological simulations. This may pose a problem, or instead the search for local backsplash galaxies might be identifying ancient tidal dwarf galaxies expelled in a past major galaxy encounter. The NGC 3109 association supports the notion that material preferentially falls towards the MW from the Galactic south and recedes towards the north, as if the MW were moving through a stream of dwarf galaxies.

The evolution of the dust and gas content in galaxies

We use deep Herschel PACS and SPIRE observations in GOODSS, GOODSN and COSMOS to estimate the average dust mass (Mdust) of galaxies on a redshift-stellar mass (Mstar)-SFR grid. We study the scaling relations between Mdust, Mstar and SFR at z<=2.5. No clear evolution of Mdust is observed at fixed SFR and Mstar. We find a tight correlation between SFR and Mdust, likely a consequence of the Schmidt-Kennicutt (S-K) law. The Mstar-Mdust correlation observed by previous works flattens or sometimes disappears when fixing the SFR. Most of it likely derives from the combination of the Mdust-SFR and Mstar-SFR correlations. We then investigate the gas content as inferred by converting Mdust by assuming that the dust/gas ratio scales linearly with the gas metallicity. All galaxies in the sample follow, within uncertainties, the same SFR-Mgas relation (integrated S-K law), which broadly agrees with CO-based results for the bulk of the population, despite the completely different approaches. The majority of galaxies at z~2 form stars with an efficiency (SFE=SFR/Mgas) ~5 times higher than at z~0. It is not clear what fraction of such variation is an intrinsic redshift evolution and what fraction arises from selection effects. The gas fraction (fgas) decreases with Mstar and increases with SFR, and does not evolve with z at fixed Mstar and SFR. We explain these trends by introducing a universal relation between fgas, Mstar and SFR, non-evolving out to z~2.5. Galaxies move across this relation as their gas content evolves in time. We use the 3D fundamental fgas-Mstar-SFR relation and the redshift evolution of the Main Sequence to estimate the evolution of fgas in the average population of galaxies as a function of z and Mstar, and we find evidence a downsizing scenario.

22 GHz water maser search in 37 nearby galaxies

We report four new 22 GHz water masers found in a Green Bank Telescope search toward 37 nearby objects. Our goal was to find new maser galaxies, AGN disk-masers, and objects where both hydroxyl and water maser species coexist. We observed 37 sources within 250 Mpc that were selected by high X-ray luminosity (L_X > 10^40 W) and high absorbing column density (N_H ~ 10^22 cm^-2). Sources also included dual or triple AGN and interacting systems. We further included objects detected in hydroxyl (OH). The selection consisted of 16 new sources, 13 previous non-detections to follow up with a factor ten higher sensitivity, 10 OH masers and one deep OH absorber, of which 37 were observed. Water megamasers were detected towards the Sy 2 galaxy 2MFGC 13581 (a disk-maser), towards the 6 GHz OH absorber NGC 4261 (3C 270 with a twin-jet and dusty torus; broad water maser emission), and towards the two 1.6 GHz OH maser sources IRAS 17526+3253 and IRAS 20550+1656 (possible star formation water masers). We set upper limits on 33 non-detections. The search increased the number of known "dual-species" objects containing both OH and H2O masers to eight. The detection rate was 25 % in OH galaxies and 11 % overall. Combined with other searches, a total of 95 objects have now been searched for both OH and H2O masers. We found the overall dual-species detection rate (8 in 95) to be of the order of the joint probability of both species independently occurring in the same object (1 % lower bound). However, this needs to be verified by a more detailed analysis of selection criteria. Lastly, we see a lack of H2O kilomasers in OH megamaser objects already noted by Tarchi et al. (2011). This may be due to sensitivity bias rather than astrophysical reasons.

New Compton-thick AGN in the circumnuclear H2O maser hosts UGC 3789 and NGC 6264

Large column densities, derived from X-ray studies, are typically measured towards AGN hosting water masers, especially when the H2O emission is associated with the nuclear accretion disk. In addition, possible correlations between the intrinsic X-ray luminosity and the characteristics of the H2O maser emission have been put forward that, however, require confirmation. We have performed high-sensitivity XMM-Newton observations of a sample of five H2O maser sources confidently detected in our ongoing survey with the Swift satellite of all known water masers in AGN, in order to obtain detailed X-ray information of these promising targets and to set up a systematic detailed study of the X-ray/H2O-maser relation in AGN. For three galaxies, NGC 613, VIIZw73, and IRAS 16288+3929, the amount of intrinsic absorption has been estimated, indicating column densities of 4-6x10^23 cm^-2. For UGC 3789 and NGC 6264 (the two confirmed disk-maser galaxies in our sample), column densities in excess of 1x10^24 cm^-2 are inferred from the large EW of the Fe Kalpha line. By adding our results to those obtained in past similar studies, we find that the percentage of water masers sources that host highly-obscured (NH>10^23 cm^-2) and Compton-thick AGN is 96% (45/47) and 57% (27/47), respectively. In addition, 86%, 18/21, of disk maser galaxies host Compton-thick AGN. The correlation between the galaxies' bolometric luminosity and accretion disk radius, suggested in previous works, is also confirmed.

The Multiwavelength AGN Population and the X-ray Background

AbstractIn order to fully understand galaxy formation we need to know when in the cosmic history are supermassive black holes (SMBHs) growing more intensively, in what type of galaxies this growth is happening and what fraction of these sources are invisible at most wavelengths due to obscuration. Active Galactic Nuclei (AGN) population synthesis models that can explain the spectral shape and intensity of the cosmic X-ray background (CXRB) indicate that most of the SMBH growth occurs in moderate-luminosity (LX~ 1044 erg/s) sources (Seyfert-type AGN), at z~ 0.5−1 and in heavily obscured but Compton-thin, NH~ 1023cm−2, systems. However, this is not the complete history, as a large fraction of black hole growth does not emit significantly in X-rays either due to obscuration, intrinsic low luminosities or large distances. The integrated intensity at high energies indicates that a significant fraction of the total black hole growth, 22%, occurs in heavily-obscured systems that are not individually detected in even the deepest X-ray observations. We further investigate the AGN triggering mechanism as a function of bolometric luminosity, finding evidence for a strong connection between significant black hole growth events and major galaxy mergers from z~ 0 to z~ 3, while less spectacular but longer accretion episodes are most likely due to other (stochastic) processes. AGN activity triggered by major galaxies is responsible for ~60% of the total black hole growth. Finally, we constrain the total accreted mass density in supermassive black holes at z &gt; 6, inferred via the upper limit derived from the integrated X-ray emission from a sample of photometrically selected galaxy candidates. We estimate an accreted mass density &lt;1000 M⊙Mpc−3 at z~ 6, significantly lower than the previous predictions from some existing models of early black hole growth and earlier prior observations.