A key advantage of 21 cm and CO imaging surveys is their ability to resolve the spatial distribution of the gas around individual galaxies. The morphologies of the detected H I and CO gas span a broad range, from regular disk- or ring-like structures to irregular distributions of clumps and/or streams (e.g. Oosterloo et al., 2007, Serra et al., 2012) with roughly 1/4 displaying centralized disk or ring-like structures (Serra et al., 2012). Such a wide range of morphologies suggests different origins of the gas in different galaxies, including leftover materials from previous mergers and newly accreted gas from the CGM/IGM. On the other hand, QSO absorption spectroscopy, while offering extremely sensitive probes of tenuous gas beyond the limits of 21 cm/CO surveys, reveals gas properties along a single sightline per halo and does not provide constraints for spatial variations in individual halos. Nevertheless, considering an ensemble of close QSO and galaxy pairs over a wide range of projected separations provides a spherical average of halo absorption properties for the entire galaxy sample.
Figure 3 shows the spatial distribution of the observed Mg II absorption strength [Wr(2796)] and annular average of Mg II gas covering fraction (⟨κ⟩MgII) as a function of projected distance (d) for a sample of 13330 LRGs at z ≈ 0.5 from Huang et al. (2016). These LRGs are selected to have a background QSO within d ≈ 500 kpc, the virial radius of a typical LRG halo, with sufficiently high signal-to-noise (S/N) absorption spectra available in the SDSS archive for detecting Mg II absorbers of Wr(2796) ≳ 0.3 Å. The LRGs are grouped into passive and [O II]-emitting subsamples based on the observed strength of [O II] emission lines (Figure 1). A number of unique features are apparent in the observed spatial distribution of chemically-enriched cool gas in LRG halos:
Figure 3. Spatial distribution of the observed Mg II absorption strength (Wr(2796); left panel) and covering fraction (⟨κ⟩MgII; right panel) as a function of projected distance d from LRGs at z ≈ 0.5. The figures are adapted from Huang et al. (2016). The LRGs are selected to have a background QSO at d < 500 kpc (the virial radius of a typical LRG halo) with sufficiently high signal-to-noise (S / N) absorption spectra available for constraining the presence/absence of Mg II absorbers with absorption strength exceeding W0 = 0.3 Å. A total of 13330 LRGs in the SDSS data archive satisfy this criterion, 1575 of these exhibit [O II]-emission features and the rest are labeled as passive galaxies. For non-detections, a 2-σ upper limit of Wr(2796) is presented for the LRG. For a few sightlines, the QSO spectra are of sufficiently high S/N for detecting Mg II absorption lines as weak as Wr(2796) ≈ 0.1 Å (red and green data points with Wr(2796) < 0.3 Å in the left panel). These are by definition included in the covering fraction calculations in the right panel, although LRGs with Wr(2796) < 0.3 Å are considered non-absorbing galaxies at the W0 = 0.3-Å threshold. The upper-right panel displays the numbers of LRGs, passive (in red) and [O II]-emitting (in green), contributing to the ⟨κ⟩MgII calculation in each 40-kpc bin (horizontal bars in the right panel). Uncertainties in ⟨κ⟩MgII are shown in vertical error bars, which represent the 68% confidence intervals. Possible contributions due to random background absorbers and gas-rich satellite galaxies are also shown in the orange dashed line and the purple dash-dotted curve, respectively.
The observed flat Wr(2796) versus d trend for Mg II absorbers and the large fraction of strong Mg II absorbers with Wr(2796) > 1 Å in LRG halos (see also Bowen & Chelouche, 2011) are both in stark contrast to what is known for star-forming, L* and sub-L* galaxies. Specifically, CGM observations of star-forming galaxies at both low and high redshifts have shown steadily declining absorption strength, both in hydrogen and in heavy ions, with increasing projected distance (e.g. Lanzetta & Bowen, 1990, Bowen et al., 1995, Chen et al., 1998, Chen et al., 2001a, Steidel et al., 2010, Liang & Chen, 2014). Strong Mg II absorbers with Wr(2796) > 1 Å are only found at d ≲ 60 kpc from isolated L* galaxies (e.g. Chen & Tinker, 2008, Chen et al., 2010a). In addition, the extent of halo gas at a fixed absorption equivalent width threshold, Rgas, is found to scale with galaxy luminosity (e.g. Chen et al., 1998, Chen et al., 2001b, Kacprzak et al., 2008, Chen & Tinker, 2008, Chen et al., 2010a). Therefore, the relation between Wr(2796) versus d / Rgas exhibits a significantly smaller scatter than the relation between Wr(2796) and d. This luminosity–Rgas scaling relation is understood as such that more luminous (and presumably more massive) halos possess more extended halo gas. For the LRGs, however, the scatter in the observed Wr(2796) versus luminosity-normalized d relation is the same as in the observed Wr(2796) versus d distribution. Figure 3 shows that, aside from a declining ⟨κMg II ⟩ at d > 120 kpc, the cool clouds that give rise to the observed Mg II absorption features do not appear to depend on either the luminosity (or equivalently mass) or projected distance of the LRGs 1.
The lack of dependence of the observed Mg II absorbing clouds on the LRG properties appears to rule out the possibility of these absorbers being connected to outgoing materials from the LRGs. However, the observed flat mean covering fraction of Mg II absorbers at d ≲ 120 kpc for passive LRGs is also inconsistent with the expectation for accreted materials from the IGM, which are expected to show an increasing covering fraction with decreasing projected distance (e.g. Faucher-Giguère & Kereš, 2011, Fumagalli et al., 2011, Shen et al., 2013a, Ford et al., 2014). Such a discrepancy indicates that IGM accretion alone cannot explain the spatial distribution of chemically-enriched cool gas in these massive quiescent halos, at least not in the inner 120 kpc.
Contributions from the interstellar and circumgalactic gas of satellite galaxies to the observed Mg II absorbers in LRG halos have also been considered (e.g. Gauthier et al., 2010, Huang et al., 2016). In particular, observations have shown that LRGs reside in overdense environments with neighboring satellites (e.g. Tal et al., 2012). Roughly 20% of satellite galaxies are blue and presumably gas-rich (e.g. Hansen et al., 2009, Prescott et al., 2011). Under the assumption that these blue satellites can retain their gaseous halos, the expected contributions to the observed covering fraction of Mg II gas can be estimated based on the known luminosity scaling relation (e.g. Chen & Tinker, 2008). The expectation is shown in the purple curve in the right panel of Figure 3. It is clear that blue satellites alone cannot account for the observed 15% covering fraction of Mg II absorbing gas at d ∼ 100 kpc in LRG halos.
A promising explanation for the observed Mg II absorbers in LRG halos are cool clumps condensing out of thermally unstable hot halos (e.g. Mo & Miralda-Escudé, 1996, Maller & Bullock, 2004, Voit et al., 2015). In particular, hot halos are a common feature among the LRG population, because efforts in search of a Sunyaev-Zel’dovich signal (Sunyaev & Zeldovich, 1970) around high-mass LRGs (Mh ≳ 8 × 1013 M⊙) have yielded detections in all mass bins studied (Hand et al., 2011). In addition, Mg II absorbers are frequently resolved into multiple components with the number of components being proportional to both the rest-frame absorption equivalent width Wr(2796) (e.g. Petitjean & Bergeron, 1990, Churchill et al., 2003) and the minimum line-of-sight velocity spread (e.g. Churchill et al., 2000). The Mg II absorption equivalent width is therefore driven by the line-of-sight cloud motion rather than by the underlying total gas column density. Recall also that the Mg II absorbers found in the vicinities of passive and [O II]-emitting LRGs display a constant Wr(2796) across the entire projected distance range shown in Figure 3 with a mean and dispersion of ⟨ log Wr(2796) / Å ⟩ = −0.050 ± 0.25.
Interpreting the scatter as due to Poisson noise in the number of individual clumps intercepted along a line of sight leads to an estimate of the mean number of clumps per galactic halo per sightline, nclump (e.g. Lanzetta & Bowen, 1990, Chen et al., 2010a). In this simple toy model, each absorber is characterized as Wr = nclump × ω0, where ω0 is the mean absorption equivalent width per component. Following Poisson counting statistics, the intrinsic scatter δ(log Wr) ≡ δ(Wr) / (Wr ln 10) is related to the mean number of clumps ⟨ nclump ⟩ according to
which leads to ⟨ nclump ⟩ ∼ 3.8 for ⟨ Wr(2796) ⟩ ≈ 1 Å in the LRG halos with a corresponding mean absorption equivalent width per component of ω0 = 0.24 Å. The inferred ⟨ nclump ⟩ per sightline for LRG halos is more than four times smaller than what was inferred for lower-mass, star-forming galaxies at d ≈ 40 kpc (Chen et al., 2010a). With a halo size twice as big as L* galaxies, the significantly lower ⟨ nclump ⟩ therefore suggests that the volume filling factor of Mg II absorbing gas is ∼ 10 times lower in LRG halos than in L* halos.
1 Using stacked QSO spectra at the rest frames of all LRGs, it has been shown that the mean absorption equivalent width ⟨ W0 ⟩ of Mg II gas declines steadily with increasing projected distance in LRG halos (Zhu et al., 2014). The steady decline of ⟨ W0 ⟩ does not necessarily contradict the relatively flat Wr(2796) versus d distribution for individual Mg II absorbers in Figure 3, because ⟨ W0 ⟩ represents a weighted average of absorberstrength and gas covering fraction. A direct comparison between ⟨ W0 ⟩ and Figure 3 requires knowledge of the frequency distribution function of Mg II absorbers in LRG halos, f(Wr), and the underlying number density ratio between absorbing and non-absorbing LRGs. Back.