Relationship between the Metallicity of the Circumgalactic Medium and Galaxy Orientation

Abstract

We investigate the geometric distribution of gas metallicities in the circumgalactic medium (CGM) around 47, z<0.7 galaxies from the "Multiphase Galaxy Halos" Survey. Using a combination of quasar spectra from Hubble Space Telescope (HST)/COS and from Keck/HIRES or Very Large Telescope/UVES, we measure column densities of, or determine limits on, CGM absorption lines. We then use a Markov Chain Monte Carlo approach with Cloudy to estimate the metallicity of cool (T∼10 4 K) CGM gas. We also use HST images to determine hostgalaxy inclination and quasar-galaxy azimuthal angles. Our sample spans a H Icolumn density range of 13.8 cm −2 < N log H I <19.9 cm −2. We find (1) while the metallicity distribution appears bimodal, a Hartigan dip test cannot rule out a unimodal distribution (0.4σ). (2) CGM metallicities are independent of halo mass, spanning three orders of magnitude at a fixed halo mass. (3) The CGM metallicity does not depend on the galaxy azimuthal and inclination angles regardless of H Icolumn density, impact parameter, and galaxy color. (4) The ionization parameter does not depend on azimuthal angle. We suggest that the partial Lyman limit metallicity bimodality is not driven by a spatial azimuthal bimodality. Our results are consistent with simulations where the CGM is complex and outflowing, accreting, and recycled gas are well-homogenized at z<0.7. The presence of lowmetallicity gas at all orientations suggests that cold streams of accreting filaments are not necessarily aligned with the galaxy plane at low redshifts or intergalactic transfer may dominate. Finally, our results support simulations showing that strong metal absorption can mask the presence of low-metallicity gas in integrated line-of-sight CGM metallicities.

Predictions for the angular dependence of gas mass flow rate and metallicity in the circumgalactic medium

“…These works focused on the difference between the galaxy metallicity, based on emission from H II regions, and the neutral gas metallicity, from absorption, in order to control for the stellar mass trend. The direct comparison of absorption gas metallicity with azimuthal angle, however, also shows no strong correlation (Pointon et al 2019). We note that these latter samples contain systems with column densities in the range 13.8 < log (N HI /cm 2 ) < 19.9 and depend on non-negligible ionisation corrections.…”

“…As discussed earlier, we imagine that this is primarily because of the heterogeneous mix of the sample, and the broad range of impact parameters, stellar masses, and redshifts in the data set. Indeed, we would not expect to see the predicted trend, highlighting the current limits of our empirical constraints (Péroux et al 2016;Pointon et al 2019;Kacprzak et al 2019) and the utility of significantly larger data samples. We stress that an accurate measurement of the metallicity of the gas requires simultaneous knowledge of the hydrogen column density and an elemental abundance with minimal depletion on to dust grains.…”

“…In addition, gas-phase metals can be trapped in dust. As a result, if metallicity estimates are not corrected for dustdepletion the corresponding values may be biased (Kacprzak et al 2019;Pointon et al 2019).…”

Evidence for galaxy quenching in the green valley caused by a lack of a circumgalactic medium

“…We have identified a background galaxy at = 0.72450 and two foreground galaxies at = 0.09650 and = 0.13536 (G1547). The = 0.09650 galaxy has been discussed in Chen et al (2001) and in Pointon et al (2019) and has an impact parameter of 79.8 kpc. This is a blue ( − = 1.0) edge-on disk galaxy ( = 80.9 degrees) that has its major axis Φ = 54.7 degrees offset from the quasar sightline.…”

“…3. Galaxy surveys from the literature containing CGM measurements, limits and galaxy masses are plotted for comparison (Prochaska et al 2011;Werk et al 2013;Liang & Chen 2014;Bordoloi et al 2014b;Borthakur et al 2015;Burchett et al 2016;Johnson et al 2017;Pointon et al 2019). Given there exists a mass dependence on metal-line column densities and profiles (e.g., Chen et al 2010;Churchill et al 2013a,b;Tumlinson et al 2013;Oppenheimer et al 2016;Ng et al 2019), we only show galaxies between a stellar mass range of 9 ≤log(M * /M )≤11 (or luminosity range of 0.1 ≤log(L * /L )≤1 since masses are not available in Prochaska et al 2011).…”