ABSTRACT
We study environmental quenching using the spatial distribution of current star formation and stellar population ages with the full SAMI Galaxy Survey. By using a star formation ...concentration index C-index, defined as log10(r50, H α/r50, cont), we separate our sample into regular galaxies (C-index ≥−0.2) and galaxies with centrally concentrated star formation (SF-concentrated; C-index <−0.2). Concentrated star formation is a potential indicator of galaxies currently undergoing ‘outside-in’ quenching. Our environments cover ungrouped galaxies, low-mass groups (M200 ≤ 1012.5M⊙), high-mass groups (M200 in the range 1012.5–14 M⊙) and clusters (M200 > 1014M⊙). We find the fraction of SF-concentrated galaxies increases as halo mass increases by 9 ± 2 per cent, 8 ± 3 per cent, 19 ± 4 per cent, and 29 ± 4 per cent for ungrouped galaxies, low-mass groups, high-mass groups, and clusters, respectively. We interpret these results as evidence for ‘outside-in’ quenching in groups and clusters. To investigate the quenching time-scale in SF-concentrated galaxies, we calculate light-weighted age (AgeL) and mass-weighted age (AgeM) using full spectral fitting, as well as the Dn4000 and HδA indices. We assume that the average galaxy age radial profile before entering a group or cluster is similar to ungrouped regular galaxies. At large radius (1–2 Re), SF-concentrated galaxies in high-mass groups have older ages than ungrouped regular galaxies with an age difference of 1.83 ± 0.38 Gyr for AgeL and 1.34 ± 0.56 Gyr for AgeM. This suggests that while ‘outside-in’ quenching can be effective in groups, the process will not quickly quench the entire galaxy. In contrast, the ages at 1–2 Re of cluster SF-concentrated galaxies and ungrouped regular galaxies are consistent (difference of 0.19 ± 0.21 Gyr for AgeL, 0.40 ± 0.61 Gyr for AgeM), suggesting the quenching process must be rapid.
Abstract
The existence of a kinematic morphology–density relation remains uncertain, and instead stellar mass appears the more dominant driver of galaxy kinematics. We investigate the dependence of ...the stellar spin parameter proxy
λ
R
e
on environment using a marked cross-correlation method with data from the Sydney Australian Astronomical Observatory Multi-object Integral-field Spectrograph (SAMI) Galaxy Survey. Our sample contains 710 galaxies with spatially resolved stellar velocity and velocity dispersion measurements. By utilizing the highly complete spectroscopic data from the Galaxy and Mass Assembly Survey, we calculate marked cross-correlation functions for SAMI galaxies using a pair count estimator and marks based on stellar mass and
λ
R
e
. We detect an anticorrelation of stellar kinematics with environment at the 3.2
σ
level, such that galaxies with low
λ
R
e
values are preferably located in denser galaxy environments. However, a significant correlation between stellar mass and environment is also found (correlation at 2.4
σ
), as found in previous works. We compare these results to mock observations from the cosmological Evolution and Assembly of Galaxies and their Environments (EAGLE) simulations, where we find a similar significant
λ
R
e
anticorrelation with environment, and a mass and environment correlation. We demonstrate that the environmental correlation of
λ
R
e
is not caused by the mass–environment relation. The significant relationship between
λ
R
e
and environment remains when we exclude slow rotators. The signals in SAMI and EAGLE are strongest on small scales (10–100 kpc) as expected from galaxy interactions and mergers. Our work demonstrates that the technique of marked correlation functions is an effective tool for detecting the relationship between
λ
R
e
and environment.
Abstract
Current methods of identifying the ionizing source of nebular emission in galaxies are well defined for the era of single-fiber spectroscopy, but still struggle to differentiate the complex ...and overlapping ionization sources in some galaxies. With the advent of integral field spectroscopy, the limits of these previous classification schemes are more apparent. We propose a new method for distinguishing the ionizing source in resolved galaxy spectra by use of a multidimensional diagnostic diagram that compares emission-line ratios with velocity dispersion on a spaxel-by-spaxel basis within a galaxy. This new method is tested using the Sydney-Australian-Astronomical-Observatory Multi-object Integral-Field Spectrograph Galaxy Survey (SAMI) Data Release 3 (DR3), which contains 3068 galaxies at
z
< 0.12. Our results are released as ionization maps available alongside the SAMI DR3 public data. Our method accounts for a more diverse range of ionization sources than the standard suite of emission-line diagnostics; we find 1433 galaxies with a significant contribution from non-star-forming ionization using our improved method as compared to 316 galaxies identified using only emission-line ratio diagnostics. Within these galaxies, we further identify 886 galaxies hosting unique signatures inconsistent with standard ionization by H
ii
regions, active galactic nuclei, or shocks. These galaxies span a wide range of masses and morphological types and comprise a sizable portion of the galaxies used in our sample. With our revised method, we show that emission-line diagnostics alone do not adequately differentiate the multiple ways to ionize gas within a galaxy.
Abstract
We present the discovery of a new H
i
structure in the NGC 7194 group from the observations using the Karl G. Jansky Very Large Array. NGC 7194 group is a nearby (
z
∼ 0.027) small galaxy ...group with five quiescent members. The observations reveal a 200 kpc long H
i
plume that spans the entire group with a total mass of
M
H I
= 3.4 × 10
10
M
⊙
. The line-of-sight velocity of the H
i
gas gradually increases from south (7200 km s
−1
) to north (8200 km s
−1
), and the local velocity dispersion is up to 70 km s
−1
. The structure is not spatially coincident with any member galaxies but it shows close associations with a number of blue star-forming knots. Intragroup H
i
gas is not rare, but this particular structure is still one of the unusual cases in the sense that it does not show any clear connection with sizable galaxies in the group. We discuss the potential origins of this large-scale H
i
gas in the NGC 7194 group and its relation with the intergalactic star-forming knots. We propose that this H
i
feature could have originated from tidal interactions among group members or the infall of a late-type galaxy into the group. Alternatively, it might be leftover gas from flyby intruders.
ABSTRACT
In this work, we investigate how the central stellar metallicity (Z/H) of 1363 galaxies from the SAMI galaxy survey is related to their stellar mass and a proxy for the gravitational ...potential, $\Phi = \log _{10}\left(\frac{M_*}{M_{\odot }} \right) - \log _{10}\left(\frac{r_e}{\mathrm{kpc}} \right)$. In agreement with previous studies, we find that passive and star-forming galaxies occupy different areas of the Z/H–M* plane, with passive galaxies having higher Z/H than star-forming galaxies at fixed mass (a difference of 0.23 dex at log10(M*/M⊙) = 10.3). We show for the first time that all galaxies lie on the same relation between Z/H and Φ, and show that the offset in Z/H between passive and star-forming galaxies at fixed Φ is smaller than or equal to the offset in Z/H at fixed mass (an average ΔZ/H of 0.11 dex at fixed Φ compared to 0.21 dex at fixed mass). We then build a simple model of galaxy evolution to explain and understand our results. By assuming that Z/H traces Φ over cosmic time and that the probability that a galaxy quenches depends on both its mass and size, we are able to reproduce these offsets in stellar metallicity with a model containing instantaneous quenching. We therefore conclude that an offset in metallicity at fixed mass cannot by itself be used as evidence of slow quenching processes, in contrast to previous studies. Instead, our model implies that metal-rich galaxies have always been the smallest objects for their mass in a population. Our findings reiterate the need to consider galaxy size when studying stellar populations.
ABSTRACT
We explore local and global dynamical differences between the kinematics of ionized gas and stars in a sample of galaxies from Data Release 3 of the SAMI Galaxy Survey. We find better ...agreement between local (i.e. comparing on a spaxel-to-spaxel basis) velocities and dispersion of gas and stars in younger systems as with previous work on the asymmetric drift in galaxies, suggesting that the dynamics of stars and ionized gas are initially coupled. The intrinsic scatter around the velocity and dispersion relations increases with increasing stellar age and mass, suggesting that subsequent mechanisms, such as internal processes, divergent star formation, and assembly histories, also play a role in setting and altering the dynamics of galaxies. The global (flux-weighted) dynamical support of older galaxies is hotter than in younger systems. We find that the ionized gas in galaxies is almost always dynamically colder than the stars with a steeper velocity gradient. In absolute terms, the local difference in velocity dispersion is more pronounced than the local difference in velocity, possibly reflecting inherent differences in the impact of turbulence, inflow and/or feedback on gas compared to stars. We suggest how these findings may be taken into account when comparing high and low redshift galaxy samples to infer dynamical evolution.
This paper presents a sample of 'cold front' clusters selected from the Chandra archive. The clusters are selected based purely on the existence of surface brightness edges in their Chandra images ...which are modeled as density jumps. A combination of the derived density and temperature jumps across the fronts is used to select nine robust examples of cold front clusters: 1ES0657 - 558, Abell 1201, Abell 1758N, MS1455.0+2232, Abell 2069, Abell 2142, Abell 2163, RXJ1720.1+2638, and Abell 3667. This sample is the subject of an ongoing study aimed at relating cold fronts to cluster merger activity, and understanding how the merging environment affects the cluster constituents. Here, temperature maps are presented along with the Chandra X-ray images. A dichotomy is found in the sample in that there exists a subsample of cold front clusters which are clearly mergers based on their X-ray morphologies, and a second subsample of clusters which harbor cold fronts, but have surprisingly relaxed X-ray morphologies, and minimal evidence for merger activity at other wavelengths. For this second subsample, the existence of a cold front provides the sole evidence for merger activity at X-ray wavelengths. We discuss how cold fronts can provide additional information which may be used to constrain merger histories, and also the possibility of using cold fronts to distinguish major and minor mergers.
We present a detailed study of emission-line systems in the Galaxy And Mass Assembly (GAMA) G23 region, making use of Wide-field Infrared Survey Explorer (WISE) photometry that includes carefully ...measured resolved sources. After applying several cuts to the initial catalog of ∼41,000 galaxies, we extract a sample of 9809 galaxies. We then compare the spectral diagnostic Baldwin, Philips & Terlevich (BPT) classification of 1154 emission-line galaxies (38% resolved in W1) to their location in the WISE color-color diagram, leading to the creation of a new zone for mid-infrared "warm" galaxies located 2 above the star-forming sequence, below the standard WISE active galactic nucleus (AGN) region. We find that the BPT and WISE diagrams agree on the classification for 85% and 8% of the galaxies as non-AGN (star-forming = SF) and AGN, respectively, and disagree on ∼7% of the entire classified sample. Thirty-nine percent of the AGNs (all types) are broad-line systems for which the N ii and H fluxes can barely be disentangled, giving in most cases spurious N ii/H flux ratios. However, several optical AGNs appear to be completely consistent with SF in WISE. We argue that these could be low-power AGNs, or systems whose hosts dominate the IR emission. Alternatively, given the sometimes high O iii luminosity in these galaxies, the emission lines may be generated by shocks coming from super-winds associated with SF rather than AGN activity. Based on our findings, we have created a new diagnostic: W1 - W2 versus N ii/H , which has the virtue of separating SF from AGNs and high-excitation sources. It classifies 3 to ∼5 times more galaxies than the classic BPT.
A combination of BRI photometry and archival Chandra X-ray data have been used to analyse the effects a minor merger has on the galaxy population of A1664. We utilize adaptive smoothing techniques in ...the 2D spatial distribution of cluster galaxies to reveal substructure ∼800 kpc south of the cluster core. We identify this substructure as most likely the remnant core of a merging group which has passed pericentre and responsible for triggering a cold front in the cluster core. We define two samples to represent two different environments within A1664 in accordance with the location of the substructure. We apply a morphological analysis using concentration-asymmetry-clumpiness, M
20 and Gini to these samples to deduce if there has been any significant effect on the cluster galaxies due to this interaction. We find that there are more asymmetric galaxies found in the inner sample (at the 3.7σ level) which are likely due to galaxy-galaxy interactions as the merging group passed through core passage. No other differences were found between the inner and outer cluster in our morphological analysis, which we attribute to the limited resolution of our imagery. The colour profiles of the galaxies are found to be consistent with the morphology-density relation suggesting that there is no unique environmental effect in A1664 that has enhanced galaxy transformations. This study favours the star formation of cluster galaxies being quenched well before they are able to interact with the merging group and demonstrates that a minor cluster merger has little effect on the observable parameters of cluster galaxies such as morphology and colour.
ABSTRACT
We explore a sample of 1492 galaxies with measurements of the mean stellar population properties and the spin parameter proxy, $\lambda _{R_{\rm {e}}}$, drawn from the SAMI Galaxy Survey. We ...fit a global $\alpha /\rm {Fe}$–σ relation, finding that ${\alpha /\rm {Fe}}=(0.395\pm 0.010)\rm {log}_{10}(\sigma)-(0.627\pm 0.002)$. We observe an anti-correlation between the residuals $\Delta \alpha /\rm {Fe}$ and the inclination-corrected $\lambda _{\, R_{\rm {e}}}^{\rm {\, eo}}$, which can be expressed as ${\Delta \alpha /\rm {Fe}}=(-0.057\pm 0.008){\lambda _{\, R_{\rm {e}}}^{\rm {\, eo}}}+(0.020\pm 0.003)$. The anti-correlation appears to be driven by star-forming galaxies, with a gradient of ${\Delta \alpha /\rm {Fe}}\sim (-0.121\pm 0.015){\lambda _{\, R_{\rm {e}}}^{\rm {\, eo}}}$, although a weak relationship persists for the subsample of galaxies for which star formation has been quenched. We take this to be confirmation that disc-dominated galaxies have an extended duration of star formation. At a reference velocity dispersion of 200 km s−1, we estimate an increase in half-mass formation time from ∼0.5 Gyr to ∼1.2 Gyr from low- to high-$\lambda _{\, R_{\rm {e}}}^{\rm {\, eo}}$ galaxies. Slow rotators do not appear to fit these trends. Their residual α-enhancement is indistinguishable from other galaxies with ${\lambda _{\, R_{\rm {e}}}^{\rm {\, eo}}}\lessapprox 0.4$, despite being both larger and more massive. This result shows that galaxies with ${\lambda _{\, R_{\rm {e}}}^{\rm {\, eo}}}\lessapprox 0.4$ experience a similar range of star formation histories, despite their different physical structure and angular momentum.
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