The X-Ray Zurich Environmental Study (X-ZENS). I. Chandra and XMM-Newton observations of active galactic nuclei in galaxies in nearby groups

We describe X-ray observations with Chandra and XMM-Newton of 18 galaxy groups (M_group ~ 1-6x10^13 Msolar, z~0.05) from the Zurich Environmental Study (ZENS). We aim to establish the frequency and properties, unaffected by host galaxy dilution and obscuration, of AGNs in central and satellite galaxy members, also as a function of halo-centric distance. X-ray point-source detections are reported for 22 of 177 observed galaxies, down to a limit of f_(0.5-8 keV) ~ 5x10^-15 erg cm^-2 s^-1, corresponding to a limiting luminosity of L_(0.5-8 keV)~3x10^40 erg s^-1. With the majority of the X-ray sources attributed to AGNs of low-to-moderate levels (L/L_Edd>~10^-4), we discuss the detection rate in the context of the occupation of AGNs to halos of this mass scale and redshift, and compare the structural/morphological properties between AGN-active and non-active galaxies of different rank and location within the group halos. We see a slight tendency for AGN hosts to have either relatively brighter/denser disks (or relatively fainter/diffuse bulges) than non-active galaxies of similar mass. At galaxy mass scales <10^11 Msolar, central galaxies appear to be a factor ~4 more likely to host AGNs than satellite galaxies of similar mass. This effect, coupled with the tendency for AGNs to reside in massive galaxies, explains the (weak) trend for AGNs to be preferentially found in the inner regions of groups, with no detectable trend with halo-centric distance in the frequency of AGNs within the satellite population. Finally, our data support other analyses in finding that the rate of decline with redshift of AGN activity in groups matches that of the global AGN population, indicating that either AGNs occur preferentially in groups, or that the evolution rate is independent of halo mass. These trends are of potential importance, and require X-ray coverage of a larger sample to be solidly confirmed.Comment: 18 pages, 13 figures, submitted to The Astrophysical Journal, this is a revised version that addresses the referee's comment

The Zurich Environmental Study (ZENS) of galaxies in groups along the cosmic web. V. properties and frequency of merging satellites and centrals in different environments

We use the Zurich ENvironmental Study (ZENS) database to investigate the environmental dependence of the merger fraction $\Gamma$ and merging galaxy properties in a sample of ~1300 group galaxies with $M>10^{9.2}M_\odot$ and 0.05<z<0.0585. In all galaxy mass bins investigated in our study, we find that $\Gamma$ decreases by a factor of ~2-3 in groups with halo masses $M_{HALO}>10^{13.5} M_\odot$ relative to less massive systems, indicating a suppression of merger activity in large potential wells. In the fiducial case of relaxed groups only, we measure a variation $\Delta\Gamma/\Delta \log (M_{HALO}) \sim - 0.07$ dex$^{-1}$, which is almost independent of galaxy mass and merger stage. At galaxy masses $>10^{10.2} M_\odot$, most mergers are dry accretions of quenched satellites onto quenched centrals, leading to a strong increase of $\Gamma$ with decreasing group-centric distance at these mass scales.Both satellite and central galaxies in these high mass mergers do not differ in color and structural properties from a control sample of nonmerging galaxies of equal mass and rank. At galaxy masses $<10^{10.2} M_\odot$, where we mostly probe satellite-satellite pairs and mergers between star-forming systems, close pairs (projected distance $<10-20$ kpc) show instead $\sim2\times$ enhanced (specific) star formation rates and $\sim1.5\times$ larger sizes than similar mass, nonmerging satellites. The increase in both size and SFR leads to similar surface star-formation densities in the merging and control-sample satellite populations.Comment: Published in ApJ, 797, 12

A Survey of Atomic Carbon [C I] in High-redshift Main-Sequence Galaxies

We present the first results of an ALMA survey of the lower fine structure line of atomic carbon [C I](^3P_1\,-\,^{3}P_0) in far infrared-selected galaxies on the main sequence at $z\sim1.2$ in the COSMOS field. We compare our sample with a comprehensive compilation of data available in the literature for local and high-redshift starbursting systems and quasars. We show that the [C I]($^3P_1$$\rightarrow$$^3P_0$) luminosity correlates on global scales with the infrared luminosity $L_{\rm IR}$ similarly to low-$J$ CO transitions. We report a systematic variation of L'_{\rm [C\,I]^3P_1\,-\, ^3P_0}/$L_{\rm IR}$ as a function of the galaxy type, with the ratio being larger for main-sequence galaxies than for starbursts and sub-millimeter galaxies at fixed $L_{\rm IR}$. The L'_{\rm [C\,I]^3P_1\,-\, ^3P_0}/$L'_{\rm CO(2-1)}$ and $M_{\rm{[C I]}}$/$M_{\rm dust}$ mass ratios are similar for main-sequence galaxies and for local and high-redshift starbursts within a 0.2 dex intrinsic scatter, suggesting that [C I] is a good tracer of molecular gas mass as CO and dust. We derive a fraction of $f_{\rm{[C\,I]}} = M_{\rm{[C\,I]}} / M_{\rm{C}}\sim3-13$% of the total carbon mass in the atomic neutral phase. Moreover, we estimate the neutral atomic carbon abundance, the fundamental ingredient to calibrate [C I] as a gas tracer, by comparing L'_{\rm [C\,I]^3P_1\,-\, ^3P_0} and available gas masses from CO lines and dust emission. We find lower [C I] abundances in main-sequence galaxies than in starbursting systems and sub-millimeter galaxies, as a consequence of the canonical $\alpha_{\rm CO}$ and gas-to-dust conversion factors. This argues against the application to different galaxy populations of a universal standard [C I] abundance derived from highly biased samples.Comment: 14 pages + Appendix. Accepted for publication in ApJ. All the data tables in Appendix will be also released in electronic forma

Early- and late-stage mergers among main sequence and starburst galaxies at 0.2 ≤ z ≤ 2

We investigate the fraction of close pairs and morphologically identified mergers on and above the star-forming main sequence (MS) at 0.2 ≤ z ≤2.0. The novelty of our work lies in the use of a non-parametric morphological classification performed on resolved stellar mass maps, reducing the contamination by non-interacting, high-redshift clumpy galaxies. We find that the merger fraction rapidly rises to ≥70 per cent above the MS, implying that - already at z ≳ 1 - starburst (SB) events (∆MS ≥ 0.6) are almost always associated with a major merger (1:1 to 1:6 mass ratio). The majority of interacting galaxies in the SB region are morphologically disturbed, late-stage mergers. Pair fractions show little dependence on MS offset and pairs are more prevalent than late-stage mergers only in the lower half of the MS. In our sample, major mergers on the MS occur with a roughly equal frequency of ̃5-10 per cent at all masses ≳ 1010 M☉. The MS major merger fraction roughly doubles between z = 0.2 and 2, with morphological mergers driving the overall increase at z ≳ 1. The differential redshift evolution of interacting pairs and morphologically classified mergers on the MS can be reconciled by evolving observability time-scales for both pairs and morphological disturbances. The observed variation of the late-stage merger fraction with ∆MS follows the perturbative 2-Star Formation Mode model, where any MS galaxy can experience a continuum of different star formation rate enhancements. This points to an SB-merger connection not only for extreme events, but also more moderate bursts which merely scatter galaxies upward within the MS, rather than fully elevating them above it

Bailing Out the Milky Way: Variation in the Properties of Massive Dwarfs Among Galaxy-Sized Systems

Recent kinematical constraints on the internal densities of the Milky Way's dwarf satellites have revealed a discrepancy with the subhalo populations of simulated Galaxy-scale halos in the standard CDM model of hierarchical structure formation. This has been dubbed the "too big to fail" problem, with reference to the improbability of large and invisible companions existing in the Galactic environment. In this paper, we argue that both the Milky Way observations and simulated subhalos are consistent with the predictions of the standard model for structure formation. Specifically, we show that there is significant variation in the properties of subhalos among distinct host halos of fixed mass and suggest that this can reasonably account for the deficit of dense satellites in the Milky Way. We exploit well-tested analytic techniques to predict the properties in a large sample of distinct host halos with a variety of masses spanning the range expected of the Galactic halo. The analytic model produces subhalo populations consistent with both Via Lactea II and Aquarius, and our results suggest that natural variation in subhalo properties suffices to explain the discrepancy between Milky Way satellite kinematics and these numerical simulations. At least ~10% of Milky Way-sized halos host subhalo populations for which there is no "too big to fail" problem, even when the host halo mass is as large as M_host = 10^12.2 h^-1 M_sun. Follow-up studies consisting of high-resolution simulations of a large number of Milky Way-sized hosts are necessary to confirm our predictions. In the absence of such efforts, the "too big to fail" problem does not appear to be a significant challenge to the standard model of hierarchical formation. [abridged]Comment: 12 pages, 3 figures; accepted by JCAP. Replaced with published versio

A contribution of star-forming clumps and accreting satellites to the mass assembly of z ∼ 2 galaxies

We investigate the contribution of clumps and satellites to the galaxy mass assembly. We analysed spatially resolved HubbleSpace Telescope observations (imaging and slitless spectroscopy) of 53 star-forming galaxies at z ∼ 1–3. We created continuum and emission line maps and pinpointed residual ‘blobs’ detected after subtracting the galaxy disc. Those were separated into compact (unresolved) and extended (resolved) components. Extended components have sizes ∼2 kpc and comparable stellar mass and age as the galaxy discs, whereas the compact components are 1.5 dex less massive and 0.4 dex younger than the discs. Furthermore, the extended blobs are typically found at larger distances from the galaxy barycentre than the compact ones. Prompted by these observations and by the comparison with simulations, we suggest that compact blobs are in situ formed clumps, whereas the extended ones are accreting satellites. Clumps and satellites enclose, respectively, ∼20 per cent and ≲80 per cent of the galaxy stellar mass, ∼30 per cent and ∼20 per cent of its star formation rate. Considering the compact blobs, we statistically estimated that massive clumps (M⋆ ≳ 109 M⊙) have lifetimes of ∼650 Myr, and the less massive ones (108 < M⋆ < 109 M⊙) of ∼145 Myr. This supports simulations predicting long-lived clumps (lifetime ≳ 100 Myr). Finally, ≲30 per cent (13 per cent) of our sample galaxies are undergoing single (multiple) merger(s), they have a projected separation ≲10 kpc, and the typical mass ratio of our satellites is 1:5 (but ranges between 1:10 and 1:1), in agreement with literature results for close pair galaxies

CO excitation of normal star forming galaxies out to z=1.5 as regulated by the properties of their interstellar medium

We investigate the CO excitation of normal star forming galaxies at z=1.5 using IRAM PdBI observations of the CO[2-1], CO[3-2] and CO[5-4] transitions for 4 galaxies, and VLA observations of CO[1-0] for 3 of them, measuring reliable line fluxes with S/N>4-7 for individual transitions. While the average CO Spectral Line Energy Distribution (SLED) has a sub-thermal excitation similar to the Milky Way (MW) up to CO[3-2], we show that the average CO[5-4] emission is 4 times stronger than assuming MW excitation. This demonstrates the presence of an additional component of more excited, denser and possibly warmer molecular gas. The ratio of CO[5-4] to lower-J CO emission is however lower than in local (U)LIRGs and high-redshift starbursting SMGs, and appears to correlate closely with the average intensity of the radiation field and with the star formation surface density, but not with SF efficiency (SFE). This suggests that the overall CO excitation is at least indirectly affected by the metallicity of the ISM. The luminosity of the CO[5-4] transition is found to correlate linearly with the bolometric infrared luminosity over 4 orders of magnitudes, with BzK galaxies following the same linear trend as local spirals and (U)LIRGs and high redshift star bursting sub-millimeter galaxies. The CO[5-4] luminosity is thus related to the dense gas, and might be a more convenient way to probe it than standard high--density tracers. We see excitation variations among our sample galaxies, linked to their evolutionary state and clumpiness in optical rest frame images. In one galaxy we see spatially resolved excitation variations, the more highly excited part corresponds to the location of massive SF clumps. This provides support to models that suggest that giant clumps are the main source of the high excitation CO emission in high redshift disk-like galaxies

The main sequence at z ∼ 1.3 contains a sizable fraction of galaxies with compact star formation sizes: a new population of early post-starbursts?

Atacama Large Millimeter/submillimeter Array (ALMA) measurements for 93 Herschel-selected galaxies at 1.1 ≤ z ≤ 1.7 in COSMOS reveal a sizable (>29%) population with compact star formation (SF) sizes, lying on average >×3.6 below the optical stellar mass (M ⋆)─size relation of disks. This sample widely spans the star-forming main sequence (MS), having 108 ≤ M ⋆ ≤ 1011.5 M ☉ and 20 ≤ star formation rate (SFR) ≤ 680 M ☉ yr−1. The 32 size measurements and 61 upper limits are measured on ALMA images that combine observations of CO(5─4), CO(4─3), CO(2─1), and λ obs ∼ 1.1─1.3 mm continuum, all tracing the star-forming molecular gas. These compact galaxies have instead normally extended K band sizes, suggesting strong specific SFR gradients. Compact galaxies comprise the 50 ± 18% of MS galaxies at M ⋆ > 1011 M ☉. This is not expected in standard bimodal scenarios, where MS galaxies are mostly steadily growing extended disks. We suggest that compact MS objects are early post-starburst galaxies in which the merger-driven boost of SF has subsided. They retain their compact SF size until either further gas accretion restores premerger galaxy-wide SF, or until becoming quenched. The fraction of merger-affected SF inside the MS seems thus larger than anticipated and might reach ∼50% at the highest M ⋆. The presence of large galaxies above the MS demonstrates an overall poor correlation between galaxy SF size and specific SFR

The HDUV Survey: A Revised Assessment of the Relationship between UV Slope and Dust Attenuation for High-redshift Galaxies

We use a newly assembled sample of 3545 star-forming galaxies with secure spectroscopic, grism, and photometric redshifts at z = 1.5–2.5 to constrain the relationship between UV slope (β) and dust attenuation (L IR/L UV ≡ IRX). Our sample significantly extends the range of L UV and β probed in previous UV-selected samples, including those as faint as M 1600 = −17.4 ($\simeq 0.05{L}_{\mathrm{UV}}^{* }$) and −2.6 lesssim β lesssim 0.0. IRX is measured using stacks of deep Herschel data, and the results are compared with predictions of the IRX−β relation for different assumptions of the stellar population model and obscuration curve. We find that z = 1.5–2.5 galaxies have an IRX−β relation that is consistent with the predictions for an SMC curve if we invoke subsolar-metallicity models currently favored for high-redshift galaxies, while the commonly assumed starburst curve overpredicts the IRX at a given β by a factor of gsim3. IRX is roughly constant with L UV for L UV gsim 3 × 109 L ⊙. Thus, the commonly observed trend of fainter galaxies having bluer β may simply reflect bluer intrinsic slopes for such galaxies, rather than lower obscurations. The IRX−β relation for young/low-mass galaxies at z gsim 2 implies a dust curve that is steeper than the SMC. The lower attenuations and higher ionizing photon output for low-metallicity stellar populations point to Lyman continuum production efficiencies, ξ ion, that may be elevated by a factor of ≈2 relative to the canonical value for L* galaxies, aiding in their ability to keep the universe ionized at z ~ 2