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Register a new userThe Structure of the Circumgalactic Medium of Galaxies: Cool Accretion Inflow Around NGC 1097
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We present Hubble Space Telescope far-UV spectra of 4 QSOs whose sightlines pass through the halo of NGC 1097 at impact parameters of 48 -165 kpc. NGC 1097 is a nearby spiral galaxy that has undergone at least two minor merger events, but no apparent major mergers, and is relatively isolated with respect to other nearby bright galaxies. This makes NGC 1097 a good case study for exploring baryons in a paradigmatic bright-galaxy halo. Lyman-alpha absorption is detected along all sightlines and Si III 1206 is found along the 3 smallest impact parameter sightlines; metal lines of C II, Si II and Si IV are only found with certainty towards the inner-most sightline. The kinematics of the absorption lines are best replicated by a model with a disk-like distribution of gas approximately planar to the observed 21 cm H I disk, that is rotating more slowly than the inner disk, and into which gas is infalling from the intergalactic medium. Some part of the absorption towards the inner-most sightline may arise either from a small-scale outflow, or from tidal debris associated with the minor merger that gives rise to the well known `dog-leg stellar stream that projects from NGC 1097. When compared to other studies, NGC 1097 appears to be a `typical absorber, although the large dispersion in absorption line column density and equivalent width in a single halo goes perhaps some way in explaining the wide range of these values seen in higher-redshift studies.
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The circumgalactic medium (CGM) close to ~L* star-forming galaxies hosts strong MgII 2796 absorption (with equivalent width W_2796>0.1 Ang) with a near-unity covering fraction. To characterize the spatial coherence of this absorption, we analyze the W_2796 distribution in the CGM of 27 star-forming galaxies detected in deep spectroscopy of bright background (b/g) galaxies first presented in Rubin et al. (2018). The sample foreground (f/g) systems have redshifts 0.35<z<0.8 and stellar masses 9.1<log M_*/M_sun<11.1, and the b/g galaxies provide spatially-extended probes with half-light radii 1.0 kpc<R_eff<7.9 kpc at projected distances R_perp<50 kpc. Our analysis also draws on literature W_2796 values measured in b/g QSO spectroscopy probing the halos of f/g galaxies with a similar range in M_* at z ~ 0.25. By making the assumptions that (1) samples of like galaxies exhibit similar circumgalactic W_2796 distributions; and that (2) the quantity log W_2796 has a Gaussian distribution with a dispersion that is constant with M_* and R_perp, we use this QSO-galaxy pair sample to construct a model for the log W_2796 distribution in the CGM. We then demonstrate the dependence of this distribution on the ratio of the surface area of the b/g probe to the projected absorber surface area (x_A=A_G/A_A), finding that distributions which assume x_A>=15 are statistically inconsistent with that observed toward our b/g galaxies at a 95% confidence level. This limit, in combination with the b/g galaxy sizes, requires that the length scale over which W_2796 does not vary (the coherence scale of MgII absorption) is l_A>1.9 kpc. This novel constraint on the morphology of cool, photoionized structures in the inner CGM suggests that either these structures each extend over kiloparsec scales, or that the numbers and velocity dispersion of these structures are spatially correlated over the same scales.
We analyse the properties of circumgalactic gas around simulated galaxies in the redshift range z >= 3, utilising a new sample of cosmological zoom simulations. These simulations are intended to be representative of the observed samples of Lyman-alpha emitters recently obtained with the MUSE instrument (halo masses ~10^10-10^11 solar masses). We show that supernova feedback has a significant impact on both the inflowing and outflowing circumgalactic medium by driving outflows, reducing diffuse inflow rates, and by increasing the neutral fraction of inflowing gas. By temporally stacking simulation outputs we find that significant net mass exchange occurs between inflowing and outflowing phases: none of the phases are mass-conserving. In particular, we find that the mass in neutral outflowing hydrogen declines exponentially with radius as gas flows outwards from the halo centre. This is likely caused by a combination of both fountain-like cycling processes and gradual photo/collisional ionization of outflowing gas. Our simulations do not predict the presence of fast-moving neutral outflows in the CGM. Neutral outflows instead move with modest radial velocities (~ 50 kms^-1), and the majority of the kinetic energy is associated with tangential rather than radial motion.
We survey the incidence and absorption strength of the metal-line transitions CII 1334 and CIV from the circumgalactic medium (CGM) surrounding z~2 quasars, which act as signposts for massive dark matter halos M_halo~10^12.5 Msun. On scales of the virial radius (Mvir~160kpc), we measure a high covering fraction fC=0.73+/-0.10 to strong CII absorption (rest equivalent width W1334>0.2A), implying a massive reservoir of cool (T~10^4K) metal enriched gas. We conservatively estimate a metal mass exceeding 10^8 Msun. We propose these metals trace enrichment of the incipient intragroup/intracluster medium that these halos eventually inhabit. This cool CGM around quasars is the pinnacle amongst galaxies observed at all epochs, as regards covering fraction and average equivalent width of HI Lya and low-ion metal absorption. We argue that the properties of this cool CGM primarily reflect the halo mass, and that other factors such as feedback, star-formation rate, and accretion from the intergalactic medium are secondary. We further estimate, that the CGM of massive, z~2 galaxies accounts for the majority of strong MgII absorption along random quasar sightlines. Lastly, we detect an excess of strong CIV absorption (W1548>0.3A) over random incidence to 1Mpc physical impact parameter and measure the quasar-CIV cross-correlation function: xi(r)=(r/r0)^-g with r0 = 7.5Mpc and g=1.7. Consistent with previous work on larger scales, we infer that this highly ionized CIV gas traces massive (10^12 Msun) halos.
In order to study the circumgalactic medium (CGM) of galaxies we develop an automated pipeline to estimate the optical continuum of quasars and detect intervening metal absorption line systems with a matched kernel convolution technique and adaptive S/N criteria. We process $sim$ one million quasars in the latest Data Release 16 (DR16) of the Sloan Digital Sky Survey (SDSS) and compile a large sample of $sim$ 160,000 MgII absorbers, together with $sim$ 70,000 FeII systems, in the redshift range $0.35<z_{abs}<2.3$. Combining these with the SDSS DR16 spectroscopy of $sim1.1$ million luminous red galaxies (LRGs) and $sim 200,000$ emission line galaxies (ELGs), we investigate the nature of cold gas absorption at $0.5<z<1$. These large samples allow us to characterize the scale dependence of MgII with greater accuracy than in previous work. We find that there is a strong enhancement of MgII absorption within $sim 50$ kpc of ELGs, and the covering fraction within $0.5r_{rm vir}$ of ELGs is 2-5 times higher than for LRGs. Beyond 50 kpc, there is a sharp decline in MgII for both kinds of galaxies, indicating a transition to the regime where the CGM is tightly linked with the dark matter halo. The MgII covering fraction correlates strongly with stellar mass for LRGs, but weakly for ELGs, where covering fractions increase with star formation rate. Our analysis implies that cool circumgalactic gas has a different physical origin for star forming versus quiescent galaxies.
The equivalent widths of MgII absorption in the circumgalactic medium (CGM) trace the global star formation rate up to $z<6$, are larger for star-forming galaxies than passively-evolving galaxies, and decrease with increasing distance from the galaxy. We delve further into the physics involved by investigating gas kinematics and cloud column density distributions as a function of galaxy color, redshift, and projected distance from the galaxy (normalized by galaxy virial radius, $D/R_{rm vir}$). For 39 isolated galaxies at $0.3<z_{rm gal}<1.0$, we have detected MgII absorption in high-resolution ($Delta vsimeq 6.6$ km/s) spectra of background quasars within a projected distance of $7<D<190$ kpc. We characterize the absorption velocity spread using pixel-velocity two-point correlation functions. Velocity dispersions and cloud column densities for blue galaxies do not differ with redshift nor with $D/R_{rm vir}$. This suggests that outflows continually replenish the CGM of blue galaxies with high velocity dispersion, large column density gas out to large distances. Conversely, absorption hosted by red galaxies evolves with redshift where the velocity dispersions (column densities) are smaller (larger) at $z_{rm gal}<0.656$. After taking into account larger possible velocities in more massive galaxies, we find that there is no difference in the velocity dispersions or column densities for absorption hosted by red galaxies with $D/R_{rm vir}$. Thus, a lack of outflows in red galaxies causes the CGM to become more quiescent over time, with lower velocity dispersions and larger column densities towards lower $z_{rm gal}$. The quenching of star formation appears to affect the CGM out to $D/R_{rm vir}=0.75$.
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