ALBERT

Description: We present the first of a series of studies of the evolution of galaxies in compact groups over the past 3 Gyr. This article focuses on the evolution of nuclear activity and how it has been affected by the dense environment of the groups. Our analysis is based on the largest multiwavelength compact-group sample to date, containing complete ultraviolet–infrared (UV–IR) photometry for 1770 isolated groups (7417 galaxies). We classified the nuclear activity of the galaxies based on optical emission-line and mid-infrared (mid-IR) diagnostic methods, as well as using spectral energy distribution fitting. We observe a 15 per cent increase in the number of active galactic nucleus (AGN)-hosting late-type galaxies found in dynamically old groups over the past 3 Gyr, accompanied by a corresponding decrease in their circumnuclear star formation. Comparing our compact group results with those for local isolated field and interacting pair galaxies, we find no differences in the AGNs at the same redshift range. Based on both optical and mid-IR colour classifications, we report the absence of Seyfert 1 nuclei and attribute this to the low accretion rates caused by depletion of gas. We propose that the observed increase in low-ionization nuclear emission-line regions (LINERs) and Seyfert 2 nuclei (at low z ) in early-type galaxies of dynamically young groups is due to the morphological transformation of lenticular galaxies into elliptical ones. Finally, we show that, at any given stellar mass, galaxies found in dynamically old groups are more likely to host an AGN. Our findings suggest that depletion of gas due to past star formation and tidal stripping is the major mechanism driving the evolution of nuclear activity in compact groups of galaxies.

Description: We report on the long- and short-term X-ray spectral analysis of the polar-scattered Seyfert 1.2 galaxy ESO 323–G77, observed in three epochs between 2006 and 2013 with Chandra and XMM–Newton . Four high-resolution Chandra observations give us a unique opportunity to study the properties of the absorbers in detail, as well as their short time-scale (days) variability. From the rich set of absorption features seen in the Chandra data, we identify two warm absorbers with column densities and ionizations that are consistent with being constant on both short and long time-scales, suggesting that those are the signatures of a rather homogeneous and extended outflow. A third absorber, ionized to a lesser degree, is also present and it replaces the strictly neutral absorber that is ubiquitously inferred from the X-ray analysis of obscured Compton-thin sources. This colder absorber appears to vary in column density on long time-scales, suggesting a non-homogeneous absorber. Moreover, its ionization responds to the nuclear luminosity variations on time-scales as short as a few days, indicating that the absorber is in photoionization equilibrium with the nuclear source on these time-scales. All components are consistent with being co-spatial and located between the inner and outer edges of the so-called dusty, clumpy torus. Assuming co-spatiality, the three phases also share the same pressure, suggesting that the warm / hot phases confine the colder, most likely clumpy, medium. We discuss further the properties of the outflow in comparison with the lower resolution XMM–Newton data.

Description: We report on a systematic investigation of the cold and mildly ionized gaseous baryonic metal components of our Galaxy, through the analysis of high-resolution Chandra and XMM–Newton spectra of two samples of Galactic and extragalactic sources. The comparison between lines of sight towards sources located in the disc of our Galaxy and extragalactic sources allows us for the first time to clearly distinguish between gaseous metal components in the disc and halo of our Galaxy. We find that a warm ionized metal medium (WIMM) permeates a large volume above and below the Galaxy's disc, perhaps up to the circum-galactic space. This halo WIMM imprints virtually the totality of the O i and O ii absorption seen in the spectra of our extragalactic targets, has a temperature of T $_{{\rm WIMM}}^{{\rm Halo}}=2900 \pm 900$  K, a density $\langle n_{\rm H}\rangle _{{\rm WIMM}}^{{\rm Halo}} = 0.023 \pm 0.009$  cm –3 and a metallicity $Z_{{\rm WIMM}}^{{\rm Halo}} = (0.4 \pm 0.1)$ Z . Consistently with previous works, we also confirm that the disc of the Galaxy contains at least two distinct gaseous metal components, one cold and neutral (the CNMM: cold neutral metal medium) and one warm and mildly ionized, with the same temperature of the halo WIMM, but higher density ( $\langle n_{\rm H}\rangle _{{\rm WIMM}}^{{\rm Disc}} = 0.09 \pm 0.03$  cm –3 ) and metallicity ( $Z_{{\rm WIMM}}^{{\rm Disc}} = 0.8 \pm 0.1$ Z ). By adopting a simple disc+sphere geometry for the Galaxy, we estimate masses of the CNMM and the total (disc + halo) WIMM of M CNMM 8 x 10 8 M and M WIMM ~= 8.2 x 10 9 M .

Description: We analyse the mid-infrared (MIR) spectra of 39 Seyfert galaxies observed with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope . Our sample consists of 19 Seyfert type 1 (Sy1) galaxies, three intermediate Seyfert galaxies and 17 type 2 Seyfert (Sy2) galaxies in the nearby Universe. This sample was extracted from a larger sample, the circumgalactic environment of which was studied in a previous work by Dultzin-Hacyan et al., who found that Sy2 objects are in interaction more frequently than normal galaxies, while Sy1s are not. In this article, we study the silicate dust distribution that dominates the mid-infrared (MIR) spectra. This dust produces spectral features at 10 and 18 μm that are sensitive to the clumpiness of the dust. We measure the strength of the emission or absorption of the silicate features to find whether there is a correlation between the clumpiness of the circumnuclear dust and the active galactic nucleus (AGN) type and incidence of companions. All isolated Sy1 galaxies have clumpy dust distributions, whereas Sy1s with a close companion can have either clumpy or smooth distributions. In the case of Sy2 galaxies, those with one or more companions have mostly smooth dust distributions, whereas isolated Sy2s may have clumpy or smoothly distributed dust. Our result is at odds with the simplest formulation of the unified scheme for Seyferts and supports an evolutionary sequence where the influence of an interaction triggers a type 2 AGN, which evolves into a type 1.

Description: In this Letter, we demonstrate that the two claims of z ~= 0.03 O  vii K α absorption lines from Warm Hot Intergalactic Medium (WHIM) along the lines of sight to the blazars H 2356-309 (Buote et al.; Fang et al.) and Mkn 501 (Ren, Fang & Buote) are likely misidentifications of the z  = 0 O  ii K β line produced by a diffuse Low-Ionization Metal Medium in the Galaxy's interstellar and circum-galactic mediums. We perform detailed modelling of all the available high signal-to-noise Chandra Low Energy Transmission Grating (LETG) and XMM–Newton Reflection Grating Spectrometer (RGS) spectra of H 2356-309 and Mkn 501 and demonstrate that the z ~= 0.03 WHIM absorption along these two sightlines is statistically not required. Our results, however, do not rule out a small contribution from the z ~= 0.03 O  vii K α absorber along the line of sight to H 2356-309. In our model the temperature of the putative z  = 0.031 WHIM filament is T  = 3  x  10 5  K and the O  vii column density is ${\rm N}_{\rm O\,\small {VII}} \lesssim 4\times 10^{15}$  cm –2 , twenty times smaller than the O  vii column density previously reported, and now more consistent with the expectations from cosmological hydrodynamical simulations.

Description: We analysed 385 Sloan Digital Sky Survey Data Release 7 (SDSS-DR7) galactic spectra that belong to the catalogue of isolated pairs of galaxies by Karachentsev. The spectra correspond to physical pairs of galaxies defined by a difference in velocity ≤1200 km s –1 and a pair separation ≤100 kpc. We study the incidence of nuclear activity, both star formation and non-thermal – active galactic nuclei (AGNs). After a careful extraction of the nuclear spectra, we use diagnostic diagrams and find that the incidence of AGNs is 48 per cent in emission line paired galaxies and 40 per cent for the total sample (as compared to ~43 per cent and 41 per cent, respectively, in a sample of isolated galaxies). These results remain after dissecting the effects of morphological type and galactic stellar mass (with only a small, non significant, enhancement of the AGN fraction in galaxy pairs). These results suggest that weak interactions are not necessary and/or sufficient to trigger low-luminosity AGN. Since the fraction of AGN is predominant in early-type spiral galaxies, we conclude that the role of a bulge, and a large gas reservoir are both essential for the triggering of nuclear activity. The most striking result is that Type 1 nuclei are absent from the AGN sample. This result is in conflict with the Unified Model, and suggests that high accretion rates are essential to form the broad line region in active galaxies.

Description: We present an in depth study on the evolution of galaxy properties in compact groups over the past 3 Gyr. We are using the largest multiwavelength sample to-date, comprised 1770 groups (containing 7417 galaxies), in the redshift range of 0.01 〈 z 〈 0.23. To derive the physical properties of the galaxies, we rely on ultraviolet (UV)-to-infrared spectral energy distribution modelling, using cigale . Our results suggest that during the 3 Gyr period covered by our sample, the star formation activity of galaxies in our groups has been substantially reduced (3 to 10 times). Moreover, their star formation histories as well as their UV-optical and mid-infrared colours are significantly different from those of field and cluster galaxies, indicating that compact group galaxies spend more time transitioning through the green valley. The morphological transformation from late-type spirals to early-type galaxies occurs in the mid-infrared transition zone rather than in the UV-optical green valley. We find evidence of shocks in the emission line ratios and gas velocity dispersions of the late-type galaxies located below the star forming main sequence. Our results suggest that in addition to gas stripping, turbulence and shocks might play an important role in suppressing the star formation in compact group galaxies.

Description: We present a spectroscopic study of the incidence of active galactic nucleus (AGN) nuclear activity in two samples of isolated galaxies. Our results show that the incidence of non-thermal nuclear activity is about 43 and 31 per cent for galaxies with emission lines and 40 and 27 per cent for the total sample, respectively. For the first time we have a large number of bona fide isolated galaxies (513 objects), with statistically significant number of all morphological types. A large fraction (~70 per cent) of elliptical galaxies or early-type spirals have an AGN and ~70 per cent of them are low-ionization nuclear emission-line regions. We find a larger fraction of AGN in early morphological types, as also found in the general population of galaxies. Only 3 per cent of the AGN show the presence of broad lines (not a single one can be classified as type 1 AGN). This is an important result which is at odds with the unified model even if we consider warped or clumpy tori. Finally, we interpret the large fraction of AGN in isolated galaxies as the result of secular accretion.

Description: We report results from multi-epoch (2006–2013) X-ray observations of the polar-scattered Seyfert 1 galaxy ESO 323–G77. The source exhibits remarkable spectral variability from months to years timescales. The observed spectral variability is entirely due to variations of the column density of a neutral absorber towards the intrinsic nuclear continuum. The column density is generally Compton-thin ranging from a few times 10 22  cm –2 to a few times 10 23  cm –2 . However, one observation reveals a Compton-thick state with column density of the order of 1.5 10 24  cm –2 . The observed variability offers a rare opportunity to study the properties of the X-ray absorber(s) in an active galaxy. We identify variable X-ray absorption from two different components, namely (i) a clumpy torus whose individual clumps have a density of ≤1.7 10 8  cm –3 and an average column density of ~4 10 22  cm –2 , and (ii) the broad-line region (BLR), comprising individual clouds with density of 0.1-8 x 10 9  cm –3 and column density of 10 23 -10 24  cm –2 . The derived properties of the clumpy torus can also be used to estimate the torus half-opening angle, which is of the order of 47°. We also confirm the previously reported detection of two highly ionized warm absorbers with outflow velocities of 1000–4000 km s –1 . The observed outflow velocities are consistent with the Keplerian/escape velocity at the BLR. Hence, the warm absorbers may be tentatively identified with the warm/hot intercloud medium which ensures that the BLR clouds are in pressure equilibrium with their surroundings. The BLR line-emitting clouds may well be the cold, dense clumps of this outflow, whose warm/hot phase is likely more homogeneous, as suggested by the lack of strong variability of the warm absorber(s) properties during our monitoring.