In order for galaxies to continuously form stars throughout the age of the Universe, they must acquire a sufficient amount of gas from their surroundings. In fact, roughly 50% of a galaxy's gas mass is in the circumgalactic medium (Zheng et al., 2015; Wolfe et al., 2005). Using background quasars as probes of gas surrounding foreground galaxies, we have discovered that galaxies have an abundance of multi-phased circumgalactic gas.
2.1. Circumgalactic Gas Radial Distribution
The quantity and extend of the circumgalactic medium has been traced using a range of absorption features such as Ly α (Tripp et al., 1998; Chen et al., 2001b; Wakker & Savage, 2009; Chen & Mulchaey, 2009; Steidel et al., 2010; Stocke et al., 2013; Richter et al., 2016), Mg ii (Steidel et al., 1994; Steidel, 1995; Guillemin & Bergeron, 1997; Zibetti et al., 2007; Kacprzak et al., 2008; Chen et al., 2010; Nielsen et al., 2013a), C iv (Chen et al., 2001a; Adelberger et al., 2005; Steidel et al., 2010; Bordoloi et al., 2014a; Liang & Chen, 2014; Burchett et al., 2015; Richter et al., 2016), and O vi (Savage et al., 2003; Sembach et al., 2004; Stocke et al., 2006; Danforth & Shull, 2008; Tripp et al., 2008; Wakker & Savage, 2009; Prochaska et al., 2011; Tumlinson et al., 2011; Johnson et al., 2013; Stocke et al., 2013; Johnson et al., 2015; Kacprzak et al., 2015). These studies all have shown that regardless of redshift (at least between z = 0 − 3), galaxies typically have hydrogen gas detected out to ∼ 500 kpc with “metal-enriched” gas within 100–200 kpc. Furthermore, the data show an anti-correlation with equivalent width and impact parameter, with the covering fraction being unity close to the galaxy and declining with increasing distance. This is demonstrated in Figure 1, which shows the ∼ 200 kpc extent of Mg ii absorbing gas around typical galaxies from the MAGIICAT catalog (Nielsen et al., 2013b) along with well fit anti-correlation between equivalent width and impact parameter (logWr(2796) = 0.015 × D + 0.27). Note also that the gas covering fraction, for an equivalent width limit of 0.3 Å, is roughly unity near the galaxy and decreases to about 20% beyond 100 kpc.
Figure 1. Top: the rest-frame Mg ii equivalent width as a function of impact parameter for the MAGIICAT catalog for “isolated” galaxies (Nielsen et al., 2013a; Nielsen et al., 2013b). The closed symbols are detections while open symbols are 3σ are upper limits. The fit, and 1σ confidence levels are shown (logWr(2796) = [0.015 ± 0.002] × D + [0.27 ± 0.11]). Note the large extent of gas surrounding galaxies, which begs the question of what is the origin of this gas and is it some combination of gas outflows and accretion. Bottom: the radial decline of the gas covering fraction as a function of impact parameter for an equivalent detection limit of 0.3 Å. Image courtesy of Nikole Nielsen.
Interestingly, Richter (2012) demonstrated, using the distribution of high-velocity clouds around the Milky Way and M31, that high-velocity clouds could give rise to the majority of the absorption systems seen around other galaxies. The accretion rate of high-velocity gas at z = 0 is almost equivalent to the star formation density of the local Universe and thus, at least at low redshifts, high-velocity clouds could provide a significant fraction of the gas mass accreted onto galaxies (see chapter by Philipp Richter for further discussion of gas accretion onto the Milky Way).
Simulations predict that gas accretion should occur via a “hot" or “cold" mode, which is dependent on a galaxy being above or below a critical halo mass ranging between log(Mh) = 11–12 (Birnboim & Dekel, 2003; Kereš et al., 2005; Dekel & Birnboim, 2006; Ocvirk et al., 2008; Brooks et al., 2009; Dekel et al., 2009; Kereš et al., 2009; Stewart et al., 2011; van de Voort et al., 2011). A repercussion of these models is that the covering fraction of cool accreting gas should drop significantly to almost zero for massive galaxies (Stewart et al., 2011). However, observational evidence shows that the covering fraction of cold gas is constant over a larger range of halo masses 10.7 ≤ log(Mh) ≤ 13.9 within a given impact parameter (or impact parameter normalized by the virial radius) and that gaseous galaxy halos are self-similar (Churchill et al., 2013a; Churchill et al., 2013b). This is suggestive that either outflows and/or other substructures contribute to absorption in high-mass halos such that low- and high-mass gas halos are observationally indistinguishable or the data indicate that predictions of a mass dependent shutdown of cold-mode accretion may require revision. This area needs to be examined further in order to address the discrepancy between observations and simulations.
Although the average radial gas profiles around star-forming galaxies are well quantified, it does not provide much insight into the nature of the circumgalactic gas. Thus, it is critical to determine the geometric distribution of gas relative to its host galaxy to help improved our understanding of its origins.
2.2. Circumgalactic Gas Spatial Distribution
In the mid-90s, the exploration of circumgalactic medium geometry started with Steidel (1995) who acquired a large sample of 51 galaxy-quasar pairs. The data were suggestive that, independent of galaxy spectroscopic/morphological type, Mg ii gas resided within a spherical halo with unity gas covering fraction. The data fit well to a Holmberg-like luminosity scaling between a characteristic halo radius and galaxy K-band luminosity. However, even Steidel noted that spherical halos were likely a tremendous over-simplification of the true situation, however, the data did not disprove it. Using simple geometric models, Charlton & Churchill (1996) determined that both spherical halos and extended monolithic thick-disk models could be made consistent with the current data. They suggested that the kinematic structure of the absorption profiles could be used to further constrain the gas geometry, which we further discuss in Section 3.
Cosmological simulations commonly show that gas accretion should occur along filaments that are co-planar to the galaxy disk, whereas gas outflows are expected to be expelled along the galaxy projected minor axis (e.g., Shen et al., 2012; Stewart et al., 2013). Reminiscent of Charlton & Churchill (1996); Kacprzak et al. (2011b) reported that the Mg ii equivalent width measured from high resolution quasar spectra was dependent on galaxy inclination, suggesting that the circumgalactic medium has a co-planer geometry that is coupled to the galaxy inclination. It was noted however that the absorbing gas could arise from tidal streams, satellites, filaments, etc., which could also have somewhat co-planer distributions. By stacking over 5000 background galaxies to probe over 4000 foreground galaxies, Bordoloi et al. (2011) found a strong azimuthal dependence of the Mg ii absorption within 50 kpc of inclined disk-dominated galaxies (also see Lan et al., 2014). They found elevated equivalent width along the galaxy minor axis and lower equivalent width along the major axis. Their data are indicative of bipolar outflows with possible flows along the major axis. Later, Bordoloi et al. (2014b) presented models of the circumgalactic medium azimuthal angle distribution by using joint constraints from: the integrated Mg ii absorption from stacked background galaxy spectra (Bordoloi et al., 2011) and Mg ii absorption from individual galaxies as seen from background quasar spectra (Kacprzak et al., 2011b). They determined that either composite models consisting of a bipolar outflow component plus a spherical or disk component, or a single highly softened bipolar distribution, could well represent data within 40 kpc.
Bouché et al. (2012), using 10 galaxies, first showed that the azimuthal angle distribution of absorbing gas traced by Mg ii appeared to be bimodal with half of the Mg ii sight-lines showing a co-planar geometry. Kacprzak et al. (2012a) further confirmed the bimodality in the azimuthal angle distribution of gas around galaxies, where cool dense circumgalactic gas prefers to exist along the projected galaxy major and minor axes where the gas covering fraction are enhanced by 20% – 30% as shown in Figure 2. Also shown in Figure 2 is that blue star-forming galaxies drive the bimodality while red passive galaxies may contain gas along their projected major axis. The lower equivalent width detected along the projected major axis is suggestive that accretion would likely contain metal poor gas with moderate velocity width profiles.
Figure 2. Binned azimuthal angle mean probability distribution function (PDF) for Mg ii absorbing galaxies (solid line). The binned PDFs are normalized such that the total area is equal to unity, yielding an observed frequency in each azimuthal bin. Absorption is detected with increased frequency toward the major (Φ = 0 degrees) and minor (Φ = 90 degrees) axes. Also shown is the galaxy color dependence of the distribution split by a B − R ≤ 1.1 representing late-type galaxies (dashed blue line) and B − R > 1.1 representing early-type galaxies (dotted red line). The data are consistent with star-forming galaxies accreting gas and producing large-scale outflows while quiescent galaxies have much less gas activity.
The aforementioned results provide a geometric picture that is consistent with galaxy evolution scenarios where star-forming galaxies accrete co-planer gas within a narrow streams with opening angles of about 40 degrees, providing fuel for new stars that produce metal-enriched galactic scale outflows with wide opening angles of 100 degrees, while red galaxies exist passively due to reduced gas reservoirs. These conclusions are based on Mg ii observations, however both infalling gas and outflowing are expected to contain multi-phased gas.
Mathes et al. (2014) first attempted to address the azimuthal angle dependence for highly ionized gas traced by O vi and found it to have a spatially uniform distribution out to 300 kpc. Using a larger sample of O vi absorption selected galaxies, Kacprzak et al. (2015) reported a bimodality in the azimuthal angle distribution of gas around galaxies within 200 kpc. Similar to Mg ii, they found that O vi is commonly detected within opening angles of 20–40 degrees of the galaxy projected major axis and within opening angles of at least 60 degrees along the projected minor axis. Again similar to Mg ii, weaker equivalent width systems tend to reside on along the project major axis. This would be expected for either lower column density, lower kinematic dispersion or low metallicity (or a combination thereof) gas accreting towards the galaxy major axis. Different from the Mg ii results, non-detections of O vi exist almost exclusively between 20–60 degrees, suggesting that O vi is not mixed throughout the circumgalactic medium and remains confined within the accretion filaments and the gas outflows.
Further supporting this bimodality accretion/outflow picture is the recent work using H i 21-cm absorption to probe the circumgalactic medium within impact parameters of < 35 kpc around z < 0.4 galaxies. (Dutta et al., 2016) found that the majority of their absorbers (nine) exists along the projected major axis and a few (three) exists along the projected minor axis. The data are supportive of high column density co-planer H i thick-disk around these galaxies. In addition, the three minor axis absorption systems all reside within 15 kpc, therefore they conclude that these low impact parameter minor axis systems could originate from warps in these thick and extended H i disks.
Although gas geometry is highly suggestive of (and consistent with) our exception of gas accretion onto star-forming galaxies, it alone is not sufficient enough to determine if gas is actually fact accreting onto galaxies. Gas and relative gas-galaxy kinematics can provide additional data that can be used to address whether we are detecting gas accretion or not.