The Relationship between Galaxy ISM and Circumgalactic Gas Metallicities

Abstract

We present ISM and CGM metallicities for 25 absorption systems associated with isolated star-forming galaxies ( z = 0.28) with 9.4≤log(M * /M ⊙ )≤10.9 and with absorption detected within 200 kpc. Galaxy ISM metallicities were measured using Hα/[N II] emission lines from Keck/ESI spectra. CGM single-phase lowionization metallicities were modeled using MCMC and Cloudy analysis of absorption from HST/COS and Keck/HIRES or VLT/UVES quasar spectra. We find that the star-forming galaxy ISM metallicities follow the observed stellar mass metallicity relation (1σ scatter 0.19 dex). CGM metallicity shows no dependence with stellar mass and exhibits a scatter of ∼2 dex. All CGM metallicities are lower than the galaxy ISM metallicities and are offset by log(dZ) = −1.17 ± 0.11. There is no obvious metallicity gradient as a function of impact parameter or virial radius (< 2.3σ significance). There is no relationship between the relative CGM-galaxy metallicity and azimuthal angle. We find the mean metallicity differences along the major and minor axes are −1.13±0.18 and −1.23±0.11, respectively. Regardless of whether we examine our sample by low/high inclination or low/high impact parameter, or low/high N(H I), we do not find any significant relationship with relative CGM-galaxy metallicity and azimuthal angle. We find that 10/15 low column density systems (logN(H I)< 17.2) reside along the galaxy major axis while high column density systems (logN(H I)≥ 17.2) reside along the minor axis. This suggest N(H I) could be a useful indicator of accretion/outflows. We conclude that CGM is not well mixed, given the range of galaxy-CGM metallicities, and that metallicity at low redshift might not be a good tracer of CGM processes. On the-other-hand, we should replace integrated line-of-sight, single phase, metallicities with multi-phase, cloud-cloud metallicities, which could be more indicative of the physical processes within the CGM.

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

“…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.…”

“…metallicity with azimuthal angle, although the small dataset indicated no sign of correlation. Kacprzak et al (2019) recently expanded the samples to lower column densities but find a similar lack of trend. 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.…”

“…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).…”

Project AMIGA: The Circumgalactic Medium of Andromeda*

“…We appreciate now that the CGM of typical star-forming or quiescent galaxies has a large share of galactic baryons and metals in relatively cool gas phases (10 4 -10 5.5 K; e.g., Stocke et al 2013;Bordoloi et al 2014;Liang & Chen 2014;Peeples et al 2014;Werk et al 2014;Johnson et al 2015;Burchett et al 2016;Prochaska et al 2017b;Chen et al 2019;Poisson et al 2020). We have come to understand that the CGM of galaxies at z1 is not just filled with metal-enriched gas ejected by successive galaxy outflows, but has also a large amount of metal-poor gas (<1%-2% solar) in which little net chemical enrichment has occurred over several billions of years (e.g., Ribaudo et al 2011;Thom et al 2011;Lehner et al 2013Lehner et al , 2018Lehner et al , 2019Wotta et al 2016Wotta et al , 2019Prochaska et al 2017b;Kacprzak et al 2019;Zahedy et al 2019;Poisson et al 2020). The photoionized gas around z1 galaxies is very chemically inhomogeneous, as shown by large metallicity ranges and the large metallicity variations among kinematically distinct components in a single halo (Lehner et al 2019;Wotta et al 2019; see also Crighton et al 2013;Muzahid et al 2015Muzahid et al , 2016Rosenwasser et al 2018).…”

Absorption-line Abundances in the SMC-like Galaxy UGC 5282: Evidence of ISM Dilution from Inflows on Kiloparsec Scales*

“…Some absorption line measurements of gas outside of the local ISM (along sightlines toward background QSOs) support the idea of infalling lowmetallicity gas (Ribaudo et al 2011) and even show evidence of the mixing of outflowing and inflowing material in their CGM (Frye et al 2019). At distances of many tens of kpc there appears to be significant offsets in Z H II and the metallicity of the CGM (Kacprzak et al 2019). Within highermass late-type galaxies, star formation processes are clearly modified by factors not experienced by dwarf galaxies, such as the density waves which define the former's spiral structure.…”