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Register a new userNoSOCS in SDSS. VI. The Environmental Dependence of AGN in Clusters and Field in the Local Universe
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We investigated the variation in the fraction of optical active galactic nuclei (AGN) hosts with stellar mass, as well as their local and global environments. Our sample is composed of cluster members and field galaxies at $z le 0.1$ and we consider only strong AGN. We find a strong variation in the AGN fraction ($F_{AGN}$) with stellar mass. The field population comprises a higher AGN fraction compared to the global cluster population, especially for objects with log $M_* > 10.6$. Hence, we restricted our analysis to more massive objects. We detected a smooth variation in the $F_{AGN}$ with local stellar mass density for cluster objects, reaching a plateau in the field environment. As a function of clustercentric distance we verify that $F_{AGN}$ is roughly constant for R $> $ R$_{200}$, but show a steep decline inwards. We have also verified the dependence of the AGN population on cluster velocity dispersion, finding a constant behavior for low mass systems ($sigma_P lesssim 650-700$ km s$^{-1}$). However, there is a strong decline in $F_{AGN}$ for higher mass clusters ($>$ 700 km s$^{-1}$). When comparing the $F_{AGN}$ in clusters with or without substructure we only find different results for objects at large radii (R $> $ R$_{200}$), in the sense that clusters with substructure present some excess in the AGN fraction. Finally, we have found that the phase-space distribution of AGN cluster members is significantly different than other populations. Due to the environmental dependence of $F_{AGN}$ and their phase-space distribution we interpret AGN to be the result of galaxy interactions, favored in environments where the relative velocities are low, typical of the field, low mass groups or cluster outskirts.
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We present the analysis of simultaneous NuSTAR and XMM-Newton data of 8 Compton-thick (CT-) active galactic nuclei (AGN) candidates selected in the Swift-Burst Alert Telescope (BAT) 100 month survey. This work is part of an ongoing effort to find and characterize all CT-AGN in the local ($zleq$0.05) Universe. We used two physically motivated models, MYTorus and borus02, to characterize the sources in the sample, finding 5 of them to be confirmed CT-AGN. These results represent an increase of $sim19$% over the previous NuSTAR-confirmed, BAT-selected CT-AGN at $zleq0.05$, bringing the total number to 32. This corresponds to an observed fraction of $sim 8$% of all AGN within this volume-limited sample, although it increases to $20pm5$% when limiting the sample to $zleq0.01$. Out of a sample of 48 CT-AGN candidates, selected using BAT and soft (0.3$-$10 keV) X-ray data, only 24 are confirmed as CT-AGN with the addition of the NuSTAR data. This highlights the importance of NuSTAR when classifying local obscured AGN. We also note that most of the sources in our full sample of 48 Seyfert 2 galaxies with NuSTAR data have significantly different line-of-sight and average torus column densities, favouring a patchy torus scenario.
The neutral hydrogen~(HI) gas is an important barometer of recent star formation and metal enrichment activities in galaxies. I develop a novel statistical method for predicting the HI-to-stellar mass ratio $f_{gas}$ of galaxies from their stellar mass and optical colour, and apply it to a volume-limited galaxy sample jointly observed by the Sloan Digital Sky Survey and the Arecibo Legacy Fast ALFA survey. I eliminate the impact of the Malmquist bias against HI-deficient systems on the $f_{gas}$ predictor by properly accounting for the HI detection probability of each galaxy in the analysis. The best-fitting $f_{gas}$ predictor, with an estimated scatter of $0.272$ dex, provides excellent description to the observed HI mass function. After defining an HI excess parameter as the deviation of the observed $f_{gas}$ from the expected value, I confirm that there exists a strong secondary dependence of the mass-metallicity relation on HI excess. By further examining the 2D metallicity distribution on the specific star formation rate vs. HI excess plane, I show that the metallicity dependence on HI is likely more fundamental than that on specific star formation rate. In addition, I find that the environmental dependence of HI in the local Universe can be effectively described by the cross-correlation coefficient between HI excess and the red galaxy overdensity $rho_{cc}{=}-0.18$. This weak anti-correlation also successfully explains the observed dependence of HI clustering on $f_{gas}$. My method provides a useful framework for learning HI gas evolution from the synergy between future HI and optical galaxy surveys.
Within the standard model of hierarchical galaxy formation in a {Lambda}CDM Universe, the environment of galaxies is expected to play a key role in driving galaxy formation and evolution. In this paper we investigate whether and how the gas metallicity and the star formation surface density ({Sigma}_SFR) depend on galaxy environment. To this end we analyse a sample of 1162 local, star-forming galaxies from the galaxy survey Mapping Nearby Galaxies at APO (MaNGA). Generally, both parameters do not show any significant dependence on environment. However, in agreement with previous studies, we find that low-mass satellite galaxies are an exception to this rule. The gas metallicity in these objects increases while their {Sigma}SFR decreases slightly with environmental density. The present analysis of MaNGA data allows us to extend this to spatially resolved properties. Our study reveals that the gas metallicity gradients of low-mass satellites flatten and their {Sigma}SFR gradients steepen with increasing environmental density. By extensively exploring a chemical evolution model, we identify two scenarios that are able to explain this pattern: metal-enriched gas accretion or pristine gas inflow with varying accretion timescales. The latter scenario better matches the observed {Sigma}SFR gradients, and is therefore our preferred solution. In this model, a shorter gas accretion timescale at larger radii is required. This suggests that outside-in quenching governs the star formation processes of low-mass satellite galaxies in dense environments.
We are able to extend the investigation of the color-morphology-density-radius relations, for bright and faint galaxies, to $R gtrsim 3 times R_{200}$ and to very low density regions, probing the transition region between cluster and field galaxies, and finding a smooth variation between these two populations. We investigate the environmental variation of galaxy properties (and their relations), such as color, spectral type and concentration. Our sample comprises 6,415 galaxies that were previously selected as cluster members from 152 systems with $z le 0.100$. Our main findings are: (i) The fraction of discs is generally higher than the ones for blue and star-forming galaxies, indicating a faster transformation of color and star-formation compared to structural parameters. (ii) Regarding the distance to the cluster center we find a small variation in the galaxy populations outside the virial radius. Once within that radius the fractions of each population change fast, decreasing even faster within $R sim 0.3 times R_{200}$. (iii) We also find a small increase in the fraction of blue faint galaxies within $R sim 0.4 times R_{200}$, before decreasing again to the most central bin. (iv) Our results do not indicate a significant dependence on cluster mass, except for the disc fraction in the core of clusters. (v) The relations between galaxy properties also point to no dependence on cluster mass, except for the scatter of the color stellar mass relation. Our results corroborate a scenario on which pre-processing in groups leads to a strong evolution in galaxy properties, before they are accreted by large clusters (abridged).
We explore the dependence of UV upturn colours in early type cluster galaxies on the properties of their parent clusters (such as velocity dispersion and X-ray luminosity) and on the positions and kinematics of galaxies within them. We use a sample of 24 nearby clusters with highly complete spectroscopy and optical/infrared data to select a suitable sample of red sequence galaxies, whose FUV and NUV magnitudes we measure from archival GALEX data. Our results show that the UV upturn colour has no dependence on cluster properties and has the same range in all clusters. There is also no dependence on the projected position within clusters or on line-of-sight velocity. Therefore, our conclusion is that the UV upturn phenomenon is an intrinsic feature of cluster early type galaxies, irrespective of their cluster environment.
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