arXiv subjects

S. L. McGee

Publications and source records attributed to S. L. McGee.

21 records · Page 2Linked to original sources

arXiv2009

The Diverse Nature of Optical Emission Lines in Brightest Cluster Galaxies: IFU Observations of the Central Kiloparsecs

We present integral field spectroscopy of the nebular line emission in a sample of 9 brightest cluster galaxies (BCGs). The sample was chosen to probe both cooling flow and non-cooling flow clusters, as well as a range of cluster X-ray luminosities. The line emission morphology and velocity gradients suggest a great diversity in the properties of the line emitting gas. While some BGCs show evidence for filamentary or patchy emission (Abell 1060, Abell 1668 and MKW3s), others have extended emission (Abell 1204, Abell 2199), while still others have centrally concentrated emission (Abell 2052). We examine diagnostic line ratios to determine the dominant ionization mechanisms in each galaxy. Most of the galaxies show regions with AGN-like spectra, however for two BCGs, Abell 1060 and Abell 1204, the emission line diagnostics suggest regions which can be described by the emission from young stellar populations. The diversity of emission line properties in our sample of BCGs suggests that the emission mechanism is not universal, with different ionization processes dominating different systems. Given this diversity, there is no evidence for a clear distinction of the emission line properties between cooling flow and non-cooling flow BCGs. It is not always cooling flow BCGs which show emission (or young stellar populations), and non-cooling flow BCGs which do not.

astro-ph.CO↗

arXiv2008

Morphological Composition of z~0.4 groups: The site of S0 formation

The low redshift Universe (z<~0.5) is not a dull place. Processes leading to the suppression of star formation and morphological transformation are prevalent: this is particularly evident in the dramatic upturn in the fraction of S0-type galaxies in clusters. However, until now, the process and environment of formation has remained unidentified. We present a HST-based morphological analysis of galaxies in the redshift-space selected group and field environments at z~0.4. Groups contain a much higher fraction of S0s at fixed luminosity than the lower density field, with >99.999% confidence. Indeed the S0 fraction in groups is at least as high as in z~0.4 clusters and X-ray selected groups, which have more luminous Intra Group Medium (IGM). An 97% confident excess of S0s at >=0.3Mpc from the group centre at fixed luminosity, tells us that formation is not restricted to, and possibly even avoids, the group cores. Interactions with a bright X-ray emitting IGM cannot be important for the formation of the majority of S0s in the Universe. In contrast to S0s, the fraction of elliptical galaxies in groups at fixed luminosity is similar to the field, whilst the brightest ellipticals are strongly enhanced towards the group centres (>99.999% confidence within 0.3Mpc). We conclude that the group and sub-group environments must be dominant for the formation of S0 galaxies, and that minor mergers, galaxy harassment and tidal interactions are the most likely responsible mechanisms. This has implications not only for the inferred pre-processing of cluster galaxies, but also for the global morphological and star formation budget of galaxies: as hierarchical clustering progresses, more galaxies will be subject to these transformations as they enter the group environment.

astro-ph↗

arXiv2008

Unveiling the Important Role of Groups in the Evolution of Massive Galaxies: Insights from an Infrared Passive Sequence at Intermediate Redshift

The most massive galaxies in the Universe are also the oldest. To overturn this apparent contradiction with hierarchical growth models, we focus on the group scale haloes which host most of these galaxies. A stellar mass selected M_* >~ 2x10^10M_sol sample at z~0.4 is constructed within the CNOC2 redshift survey. A sensitive Mid InfraRed (MIR) IRAC colour is used to isolate passive galaxies. It produces a bimodal distribution, in which passive galaxies (highlighted by morphological early-types) define a tight MIR colour sequence (Infrared Passive Sequence, IPS). This is due to stellar atmospheric emission from old stellar populations. Significantly offset from the IPS are galaxies where reemission by dust boosts emission at 8microns (InfraRed-Excess or IRE galaxies). They include all known morphological late-types. Comparison with EW[OII] shows that MIR colour is highly sensitive to low levels of activity, and allows us to separate dusty-active from passive galaxies. The fraction of IRE galaxies, f(IRE) drops with M_*, such that f(IRE)=0.5 at a ``crossover mass'' of ~1.3x10^11M_sol. Within our optically-defined group sample there is a strong and consistent deficit in f(IRE) at all masses, and most clearly at M_* >~10^11M_sol. Using a mock galaxy catalogue derived from the Millenium Simulation we show that the observed trend of f(IRE) with M_* can be explained if suppression of star formation occurs primarily in the group environment, and particularly for M_*>~10^11M_sol galaxies. In this way, downsizing can be driven solely by structure growth in the Universe.

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