We explore the question of whether low and moderate luminosity active galactic nuclei (AGNs) are preferentially found in galaxies that are undergoing a transition from active star formation (SF) to ...quiescence. This notion has been suggested by studies of the UV-optical colors of AGN hosts, which find them to be common among galaxies in the so-called Green Valley, a region of galaxy color space believed to be composed mostly of galaxies undergoing SF quenching. Combining the deepest current X-ray and Herschel/PACS far-infrared (FIR) observations of the two Chandra Deep Fields with redshifts, stellar masses, and rest-frame photometty derived from the extensive and uniform multi-wavelength data in these fields, we compare the rest-frame U - V color distributions and star formation rate distributions of AGNs and carefully constructed samples of inactive control galaxies. The UV-to-optical colors of AGNs are consistent with equally massive inactive galaxies at redshifts out to z ~ 2, but we show that such colors are poor tracers of SF. While the FIR disutibutions of both star-forming AGNs and star-forming inactive galaxies are statistically similar, we show that AGNs are preferentially found in star-forming host galaxies, or, in other words, AGNs are less likely to be found in weakly star-forming or quenched galaxies. We postulate that, among X-ray-selected AGNs of low and moderate accretion luminosities, the supply of cold gas primarily determines the accretion rate distribution of the nuclear black holes.
Deep Herschel PACS/SPIRE imaging and 12CO(2−1) line luminosities from the IRAM Plateau de Bure Interferometer are combined for a sample of 17 galaxies at z > 1 from the GOODS-N field. The sample ...includes galaxies both on and above the main sequence (MS) traced by star-forming galaxies in the SFR-M∗ plane. The far-infrared data are used to derive dust masses, Mdust, following the Draine & Li (2007, ApJ, 657, 810) models. Combined with an empirical prescription for the dependence of the gas-to-dust ratio on metallicity (δGDR(μ0)), the CO luminosities and Mdust values are used to derive for each galaxy the CO-to-H2 conversion factor, αCO. Like in the local Universe, the value of αCO is a factor of ~5 smaller in starbursts compared to normal star-forming galaxies (SFGs). We additionally uncover a relation between αCO and dust temperature (Tdust; αCO decreasing with increasing Tdust) as obtained from modified blackbody fits to the far-infrared data. While the absolute normalization of the αCO(Tdust) relation is uncertain, the global trend is robust against possible systematic biases in the determination of Mdust, δGDR(μ0) or metallicity. Although we cannot formally distinguish between a step and a smooth evolution of αCO with the dust temperature, we can unambiguously conclude that in galaxies of near-solar metallicity, a critical value of Tdust = 30 K can be used to determine whether the appropriate αCO is closer to the “starburst” value (1.0 M⊙ (K km s-1 pc2)-1, when Tdust > 30 K) or closer to the Galactic value (4.35 M⊙ (K km s-1 pc2)-1, when Tdust < 30 K). This indicator has the great advantage of being less subjective than visual morphological classifications of mergers/SFGs, which can be difficult at high z because of the clumpy nature of SFGs. Using Tdust to select the appropriate αCO is also more indicative of ISM conditions than a fixed LIR criterion. In the absence of far-infrared data, the offset of a galaxy from the star formation main sequence (i.e., Δlog (SSFR)MS = log SSFR(galaxy)/SSFRMS(M∗,z)) can be used to identify galaxies requiring the use of an αCO conversion factor lower than the Galactic value (i.e., when Δlog (SSFR)MS ≳ 0.3 dex).
We take advantage of the sensitivity and resolution of the Herschel Space Observatory at 100 and 160 mum to directly image the thermal dust emission and investigate the infrared luminosities (LsubIR) ...and dust obscuration of typical star-forming (L*) galaxies at high redshift. The result is similar to that inferred from previous investigations of the UV, Halpha, 24 mum, radio, and X-ray properties of the same galaxies studied here. Stacking in bins of UV slope (beta) implies that L* galaxies with redder spectral slopes are also dustier and that the correlation between beta and dustiness is similar to that found for local starburst galaxies. Hence, the rest-frame ~30 and 50 mum fluxes validate on average the use of the local UV attenuation curve to recover the dust attenuation of typical star-forming galaxies at high redshift.
We study relationships between star-formation rate (SFR) and the accretion luminosity and nuclear obscuration of X-ray selected active galactic nuclei (AGNs) using a combination of deep far-infrared ...(FIR) and X-ray data in three key extragalactic survey fields (GOODS-South, GOODS-North and COSMOS), as part of the PACS Evolutionary Probe (PEP) program. The use of three fields with differing areas and depths enables us to explore trends between the global FIR luminosity of the AGN hosts and the luminosity of the active nucleus across 4.5 orders of magnitude in AGN luminosity (LAGN) and spanning redshifts from the Local Universe to z = 2.5. Using imaging from the Herschel/PACS instrument in 2−3 bands, we combine FIR detections and stacks of undetected objects to arrive at mean fluxes for subsamples in bins of redshift and X-ray luminosity. We constrain the importance of AGN-heated dust emission in the FIR and confirm that the majority of the FIR emission of AGNs is produced by cold dust heated by star-formation in their host galaxies. We uncover characteristic trends between the mean FIR luminosity (L60) and accretion luminosity of AGNs, which depend both on LAGN and redshift. At low AGN luminosities, accretion and SFR are uncorrelated at all redshifts, consistent with a scenario where most low-luminosity AGNs are primarily fueled by secular processes in their host galaxies. At high AGN luminosities, a significant correlation is observed between L60 and LAGN, but only among AGNs at low and moderate redshifts (z < 1). We interpret this observation as a sign of the increasing importance of major-mergers in driving both the growth of super-massive black holes (SMBHs) and global star-formation in their hosts at high AGN luminosities. We also find evidence that the enhancement of SFR in luminous AGNs weakens or disappears at high redshifts (z > 1) suggesting that the role of mergers is less important at these epochs. At all redshifts, we find essentially no relationship between L60 and nuclear obscuration across five orders of magnitude in obscuring Hydrogen column density (NH), suggesting that various mechanisms are likely to be responsible for obscuring X-rays in active galaxies. We discuss a broad scenario which can account for these trends: one in which two different modes of AGN fueling operate in the low- and high-luminosity regimes of SMBH accretion. We postulate that the dominant mode of accretion among high-luminosity AGNs evolves with redshift. Our study, as well as a body of evidence from the literature and emerging knowledge about the properties of high redshift galaxies, supports this scenario.
We present a catalog of 129 X-ray galaxy groups, covering a redshift range 0.04 <z< 1.23, selected in the ~3 deg2 part of the CFHTLS W1 field overlapping XMM observations performed under the XMM-LSS ...project. We carry out a statistical study of the redshift evolution out to redshift one of the magnitude gap between the first and the second brightest cluster galaxies of a well defined mass-selected group sample. We find that the slope of the relation between the fraction of groups and the magnitude gap steepens with redshift, indicating a larger fraction of fossil groups at lower redshifts. We find that 22.2 ± 6% of our groups at z ≤ 0.6 are fossil groups. We compare our results with the predictions of three semi-analytic models based on the Millennium simulation. The intercept of the relation between the magnitude of the brightest galaxy and the value of magnitude gap becomes brighter with increasing redshift. This trend is steeper than the model predictions which we attribute to the younger stellar age of the observed brightest cluster galaxies. This trend argues in favor of stronger evolution of the feedback from active galactic nuclei at z< 1 compared to the models. The slope of the relation between the magnitude of the brightest cluster galaxy and the value of the gap does not evolve with redshift and is well reproduced by the models, indicating that the tidal galaxy stripping, put forward as an explanation of the occurrence of the magnitude gap, is both a dominant mechanism and sufficiently well modeled.
ABSTRACT
The paper presents the analysis of optically selected GAMA groups and clusters in the SRG/eROSITA X-ray map of eROSITA Final Equatorial Depth Survey, in the halo mass range 1013−5 × 1014 M⊙ ...and at z < 0.2. All X-ray detections have a clear GAMA counterpart, but most of the GAMA groups in the halo mass range 1013−1014 M⊙ remain undetected. We compare the X-ray surface brightness profiles of the eROSITA detected groups with the mean stacked profile of the undetected low-mass haloes at fixed halo mass. Overall, we find that the undetected groups exhibit less concentrated X-ray surface brightness, dark matter, and galaxy distributions with respect to the X-ray-detected haloes. The mean gas mass fraction profiles are consistent in the two samples within 1.5σ, indicating that the gas follows the dark matter profile. The low-mass concentration and the magnitude gap indicate that these systems are young. They reside with a higher probability in filaments while X-ray-detected groups favour the nodes of the Cosmic Web. Because of the lower central emission, the undetected systems tend to be X-ray underluminous at fixed halo mass and to lie below the LX−Mhalo relation. Interestingly, the X-ray-detected systems inhabiting the nodes scatter the less around the relation, while those in filaments tend to lie below it. We do not observe any strong relationship between the system X-ray appearance and the active galactic nucleus (AGN) activity. We cannot exclude the role of the past AGN feedback in affecting the gas distribution over the halo lifetime. However, the data suggests that the observed differences might be related to the halo assembly bias.
In order to investigate the far-infrared (FIR) properties of radio-active active galactic nuclei (AGN), we have considered three different fields where both radio and FIR observations are the deepest ...to date: GOODS-South, GOODS-North and the Lockman Hole. Out of a total of 92 radio-selected AGN, ∼64 per cent are found to have a counterpart in Herschel maps. The percentage is maximum in the GOODS-North (72 per cent) and minimum (∼50 per cent) in the Lockman Hole, where FIR observations are shallower. Our study shows that in all cases FIR emission is associated with star-forming activity within the host galaxy. Such an activity can even be extremely intense, with star-forming rates as high as ∼103–104 M⊙ yr−1. AGN activity does not inhibit star formation in the host galaxy, just as on-site star formation does not seem to affect AGN properties, at least those detected at radio wavelengths and for z ≳ 1. Given the very high rate of FIR detections, we stress that this refers to the majority of the sample: most radio-active AGN are associated with intense episodes of star formation. However, the two processes proceed independently within the same galaxy, at all redshifts but in the local universe, where powerful enough radio activity reaches the necessary strength to switch off the on-site star formation. Our data also show that for z ≳ 1 the hosts of radio-selected star-forming galaxies and AGN are indistinguishable from each other in terms of both mass and IR luminosity distributions. The two populations only differentiate in the very local universe, whereby the few AGN which are still FIR-active are found in galaxies with much higher masses and luminosities.
Quasi-stellar objects (QSOs) occur in galaxies in which supermassive black holes (SMBHs) are growing substantially through rapid accretion of gas. Many popular models of the co-evolutionary growth of ...galaxies and black holes predict that QSOs are also sites of substantial recent star formation (SF), mediated by important processes, such as major mergers, which rapidly transform the nature of galaxies. A detailed study of the star-forming properties of QSOs is a critical test of these models. We present a far-infrared Herschel/PACS study of the mean star formation rate (SFR) of a sample of spectroscopically observed QSOs to z ~ 2 from the COSMOS extragalactic survey. This is the largest sample to date of moderately luminous QSOs (with nuclear luminosities that lie around the knee of the luminosity function) studied using uniform, deep far-infrared photometry. We study trends of the mean SFR with redshift, black hole mass, nuclear bolometric luminosity, and specific accretion rate (Eddington ratio). To minimize systematics, we have undertaken a uniform determination of SMBH properties, as well as an analysis of important selection effects of spectroscopic QSO samples that influence the interpretation of SFR trends. We find that the mean SFRs of these QSOs are consistent with those of normal massive star-forming galaxies with a fixed scaling between SMBH and galaxy mass at all redshifts. No strong enhancement in SFR is found even among the most rapidly accreting systems, at odds with several co-evolutionary models. Finally, we consider the qualitative effects on mean SFR trends from different assumptions about the SF properties of QSO hosts and from redshift evolution of the SMBH-galaxy relationship. While currently limited by uncertainties, valuable constraints on AGN-galaxy co-evolution can emerge from our approach.
We present an analysis of 109 moderate-luminosity AGNs in the Extended Chandra Deep Field-South survey, which is drawn from 5549 galaxies from the COMBO-17 and GEMS surveys having image. These ...obscured or optically weak AGNs facilitate the study of their host galaxies since the AGNs provide an insubstantial amount of contamination to the galaxy light. We find that the color distribution of AGN host galaxies is highly dependent on (1) the strong color-evolution of luminous galaxies, and (2) the influence of image10 Mpc scale structures. When excluding galaxies within the redshift range image, a regime dominated by sources in large-scale structures at image and image, we observe a bimodality in the host galaxy colors. Galaxies hosting AGNs at image preferentially have bluer (rest-frame image) colors than their image counterparts (many of which fall along the red sequence). The fraction of galaxies hosting AGNs peaks in the 'green valley'; this is primarily due to enhanced AGN activity in the redshift interval image. The AGN fraction in this redshift and color interval is 12.8% (compared to its 'field' value of 7.8%) and reaches a maximum of 14.8% at image. We further find that blue, bulge-dominated (Sersic index image) galaxies have the highest fraction of AGN (21%) in our sample. We explore the scenario that the evolution of AGN hosts is driven by galaxy mergers and illustrate that an accurate assessment requires a larger area survey since only three hosts may be undergoing a merger with timescales image1 Gyr following a starburst phase.
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