The Impact of the Group Environment on the OVI Circumgalactic Medium

TL;DR

This study compares OVI absorption in the circumgalactic medium of galaxy groups versus isolated galaxies, revealing narrower velocity spreads in groups and suggesting different ionization conditions.

Contribution

It extends previous CGM studies to galaxy groups, providing new insights into OVI absorption properties and their dependence on environment.

Findings

Group galaxy OVI equivalent width is smaller than in isolated galaxies.

Velocity spread of OVI is narrower in group environments.

Covering fractions are similar between groups and isolated galaxies.

Abstract

We present a study comparing OVI $\lambda\lambda$1031, 1037 doublet absorption found towards group galaxy environments with that of isolated galaxies. The OVI absorption in the circumgalactic medium (CGM) of isolated galaxies has been studied previously by the "Multiphase Galaxy Halos" survey, where the kinematics and absorption properties of the CGM have been investigated. We extend these studies to group environments. We define a galaxy group to have two or more galaxies having a line-of-sight velocity difference of no more than 1000 km/s and located within 350 kpc (projected) of a background quasar sightline. We identified a total of six galaxy groups associated with OVI absorption $W_{\rm r}>0.06$ {\AA} that have a median redshift of $\langle z_{\rm gal} \rangle = 0.1669$ and a median impact parameter of $\langle D \rangle = 134.1$ kpc. An additional 12 non-absorbing groups were identified with a median redshift of $\langle z_{\rm gal} \rangle = 0.2690$ and a median impact parameter of $\langle D \rangle = 274.0$ kpc. We find the average equivalent width to be smaller for group galaxies than for isolated galaxies $(3\sigma)$. However, the covering fractions are consistent with both samples. We used the pixel-velocity two-point correlation function method and find that the velocity spread of OVI in the CGM of group galaxies is significantly narrower than that of isolated galaxies $(10\sigma)$. We suggest that the warm/hot CGM does not exist as a superposition of halos, instead, the virial temperature of the halo is hot enough for OVI to be further ionised. The remaining OVI likely exists at the interface between hot, diffuse gas and cooler regions of the CGM.

Tables

Table 1. Table 1 Quasar Observations

J-Name	RA (J2000)	DEC (J2000)	$z_{\mathrm{qso}}$	Instrument	REFaaReference for the position and redshift of the quasars are: $\left(1\right)$ Beasley et al. (2002), $\left(2\right)$ Beuermann et al. (1999), $\left(3\right)$ Zickgraf et al. (2003), $\left(4\right)$ Fey et al. (2004), $\left(5\right)$ Li & Jin (1996), $\left(6\right)$ Wu et al. (2012), $\left(7\right)$ Healey et al. (2007), $\left(8\right)$ Chen et al. (2001)	COS/FUSE PID
J004706$+$031955	$00:47:05.93$	$+03:19:54.90$	$0.6233$	COS	$\left(3\right)$	$12275$
J012529$-$000556	$01:25:28.84$	$-00:05:55.93$	$1.0748$	COS	$\left(1\right)$	$13398$
J022815$-$405714	$02:28:15.17$	$-40:57:14.29$	$0.4934$	COS	$\left(2\right)$	$11541$
J035129$-$142909	$03:51:28.54$	$-14:29:08.71$	$0.6163$	COS	$\left(5\right)$	$13398$
J040748$-$121137	$04:07:48.43$	$-12:11:36.66$	$0.5726$	COS, FUSEbbThese quasar spectra were obtained from Wakker (2016, private communication).	$\left(4\right)$	$11541$, B$087$
J045609$-$215909	$04:56:08.92$	$-21:59:09.40$	$0.5335$	COS	$\left(7\right)$	$13398$
J092838$+$602521	$09:28:37.98$	$+60:25:21.02$	$0.2959$	COS	$\left(3\right)$	$11598$
J111909$+$211918	$11:19:08.68$	$+21:19:18.01$	$0.1765$	FUSEbbThese quasar spectra were obtained from Wakker (2016, private communication).	$\left(3\right)$	P$101$
J113328$+$032719	$11:33:27.78$	$+03:27:19.17$	$0.5245$	COS	$\left(6\right)$	$11598$
J113910$-$135044	$11:39:10.70$	$-13:50:43.64$	$0.5565$	COS	$\left(1\right)$	$12275$
J130112$+$590206	$13:01:12.93$	$+59:02:06.75$	$0.4778$	COS	$\left(8\right)$	$11541$
J131956$+$272808	$13:19:56.23$	$+27:28:08.22$	$1.0147$	COS	$\left(8\right)$	$11667$
J135704$+$191907	$13:57:04.43$	$+19:19:07.37$	$0.7200$	COS	$\left(8\right)$	$13398$
J170441$+$604430	$17:04:41.37$	$+60:44:30.50$	$0.3719$	COS	$\left(8\right)$	$12276$
J182157$+$642037	$18:21:57.31$	$+64:20:36.37$	$0.2970$	COS	$\left(8\right)$	$12038$

Full text

††slugcomment: Accepted to ApJ, June 2, 2017

1. Introduction

The environment in which galaxies exist plays a significant part in the way they will evolve. While unperturbed isolated galaxies have a disk-like structure, interactions between galaxies can result in other interesting phenomena such as tidal streams, shells, and increased star formation rates (e.g., Barnes & Hernquist, 1992; Veilleux et al., 2005; Poggianti et al., 2016). In the most extreme cases interactions remove the galaxy gas reservoir, leading to the quenching of star formation, and the original disk-like structures of the merging galaxies deform into elliptical galaxies (e.g., Gunn & Gott, 1972; Cowie & Songaila, 1977; Larson et al., 1980; Nulsen, 1982; Moore et al., 1996; Cen & Ostriker, 1999; Oppenheimer & Davé, 2008; Lilly et al., 2013). These phenomena are observed in images taken of interacting galaxies with, for example, bursts of star formation activity (e.g., Keel et al., 1985; Barnes, 2004) or streams of HI gas connecting galaxies can be seen (e.g., Bridge et al., 2010). However, we know very little about the impact of a merger or interaction on the diffuse gas around galaxies.

Of particular interest is the circumgalactic medium (CGM), which is a vast halo of diffuse gas surrounding galaxies out to radii $\approx 200$ kpc (e.g., Kacprzak et al., 2008; Chen et al., 2010; Steidel et al., 2010; Kacprzak et al., 2011; Tumlinson et al., 2011; Rudie et al., 2012; Burchett et al., 2013; Nielsen et al., 2013b, a; Werk et al., 2013). The mass of this halo is comparable to the gas mass of the galaxy itself (i.e., the ISM; Thom et al., 2011; Tumlinson et al., 2011; Werk et al., 2013) and hence plays an important role in the evolution of galaxies. Models of the CGM indicate that gas can flow in and out of this reservoir, which in turn controls the star formation rate of the galaxy and the metallicity of stars formed from this gas (Oppenheimer & Davé, 2008; Lilly et al., 2013; Kacprzak et al., 2016). While a more concrete model of how this gas drives isolated galaxy evolution is being built, we are still only just beginning to study the effects an interaction or merger can have on the CGM.

Since the visible components of galaxies are clearly affected by galaxy interactions, it is reasonable to expect that such effects would also take place in the CGM. Indeed, due to the large radius of the CGM, two galaxies could have overlapping gaseous halos even if they do not yet show any signs that an interaction is taking place (Tully et al., 2009; Bordoloi et al., 2011; Stocke et al., 2014). Thus, it is possible to use the CGM to understand the very first processes that take place in an interaction. Similarly, as interactions often result in the ISM being stripped from the galaxy leading to gas depletion (e.g., Gunn & Gott, 1972; Larson et al., 1980; Fujita & Nagashima, 1999), the CGM could also be susceptible to gas removal through tidal interactions (Chen & Mulchaey, 2009). This, in turn, would affect the ability of galaxies to form stars later in life as accretion of gas from the CGM is what sustains star formation.

Previous studies of the CGM in group environments have used the Mgii $\lambda\lambda 2796,2803$ absorption doublet. Bordoloi et al. (2011) stacked background galaxy spectra and found that Mgii absorption at a given equivalent width was located at higher impact parameters in group environments compared to isolated environments. This could be explained by constructing a simple model where the CGM of the group galaxies were superimposed. Thus the extended CGM is due to more gas along the line of sight, and hence, the Mgii component of the CGM is minimally affected by the interactions in the group. Recent work by Nielsen et al. (2017, in prep) has also compared Mgii absorption between group and isolated environments. They found that the absorption associated with group galaxies had larger equivalent widths than for isolated environments. The authors also found an increased fraction of Mgii components with higher velocities in group environments, which indicated that there could be some interaction between the CGM halos of the group galaxies.

Another ideal absorption doublet to trace the CGM is Civ, which traces gas with temperatures $T\approx 10^{4-5}$ K. Burchett et al. (2016) compared Civ absorption in isolated and group/cluster environments and found that for a massive and dense group environment with seven group member galaxies ($M_{h}>10^{12.7}M_{\odot}$ and ${M}_{r}\leq-19$), no Civ was detected despite associated Hi absorbers. They find that the cause of Civ depletion in group environments is not clear due to the limited sample size. However, if larger studies of Civ absorption follow similar trends, the authors suggest that the CGM of the group galaxies may be experiencing ram pressure or tidal stripping, and that Civ traces gas with higher temperatures than HI absorbers.

The warmer, more diffuse phase of the CGM gas is traced by Ovi due to its higher ionisation state and temperature $T\approx 10^{5.5}$ K. Ovi can be both collisionally ionised and photo-ionised, which makes interpretation more difficult, because it traces multiphase structures (Mo & Miralda-Escude, 1996; Maller & Bullock, 2004; Dekel & Birnboim, 2006). Previous studies by Stocke et al. (2014, 2017) compared Ovi absorption in group environments to that found in isolated environments. They found that Ovi absorption profiles had to be modeled using fewer, broader components in group environments, indicating a warmer environment. However, the total absorption profile was narrower from which the authors reasoned that Ovi could not be distributed over the ”circum-group” medium, especially since Ovi is unstable due to rapid cooling. This suggests that a diffuse halo of warm CGM gas would be difficult to maintain, and that Ovi exists at the interface of the diffuse, hot ($T>10^{6}$ K) and cooler, photoionized regions embedded in the ”circum-group” medium. The existence of interfaces between hot and cold gas within the CGM has also been found by Churchill et al. (2012, 2013) and Stern et al. (2016) for isolated environments.