THE BIMODAL METALLICITY DISTRIBUTION OF THE COOL CIRCUMGALACTIC MEDIUM AT z ≲ 1

By N. Lehner, J. Howk, T. Tripp, Jason Tumlinson, J. Xavier Prochaska, John M. O’Meara, C. Thom, Jessica Werk, A. J. Fox, Joseph Ribaudo. 2013

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We assess the metal content of the cool (∼104 K) circumgalactic medium (CGM) about galaxies at z ≲ 1 using an H i-selected sample of 28 Lyman limit systems (LLS; defined here as absorbers with 16.2 ≲ log NH i ≲ 18.5) observed in absorption against background QSOs by the Cosmic Origins Spectrograph on board the Hubble Space Telescope. The NH i selection avoids metallicity biases inherent in many previous studies of the low-redshift CGM. We compare the column densities of weakly ionized metal species (e.g., O ii, Si ii, Mg ii) to NH i in the strongest H i component of each absorber. We find that the metallicity distribution of the LLS (and hence the cool CGM) is bimodal with metal-poor and metal-rich branches peaking at [X/H] ≃ −1.6 and −0.3 (or about 2.5% and 50% solar metallicities). The cool CGM probed by these LLS is predominantly ionized. The metal-rich branch of the population likely traces winds, recycled outflows, and tidally stripped gas; the metal-poor branch has properties consistent with cold accretion streams thought to be a major source of fresh gas for star forming galaxies. Both branches have a nearly equal number of absorbers. Our results thus demonstrate there is a significant mass of previously undiscovered cold metal-poor gas and confirm the presence of metal enriched gas in the CGM of z ≲ 1 galaxies.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/10adb90d57abea0859afa4c690aaa413ae1ddacc

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Star formation sustained by gas accretion (2014, methodology): “As we discuss above, cosmic web inflows are expected to be metal-poor so metal-rich moving gas is commonly interpreted as outflow rather than inflow (Lehner et al. 2013; Bregman et al. 2013).”
The Circumgalactic Medium (2017, background): “By building a sample of LLSs from high-quality COS sightlines, Lehner et al. (2013) and Wotta et al. (2016) found that the distribution of metals in LLS clearly exhibits two peaks near 4% solar and 50% solar metallicity (Figure 10a).”
THE COS-HALOS SURVEY: ORIGINS OF THE HIGHLY IONIZED CIRCUMGALACTIC MEDIUM OF STAR-FORMING GALAXIES (2016, background): “The resultant ionization parameters range from −2 < logU < −4, values which are common for lower-ionization state gas (e.g. Prochaska 1999; Lehner et al. 2013).”
THE COS-HALOS SURVEY: RATIONALE, DESIGN, AND A CENSUS OF CIRCUMGALACTIC NEUTRAL HYDROGEN (2013, methodology): “Finally, using data from a combination of COS programs with published absorbers, Lehner et al. (2013) have investigated the bimodal distribution of metallicities in Lyman-limit systems (LLSs) presumably tracing CGM gas.”
Neutral hydrogen in galaxy haloes at the peak of the cosmic star formation history (2014, background): “The most straightforward include predictions for metal absorption lines and gas kinematics, and at lower redshifts probed by the Cosmic Origins Spectrograph on the Hubble Space Telescope (e.g., Tumlinson et al. 2011; Lehner et al. 2013).”
THE COS-HALOS SURVEY: PHYSICAL CONDITIONS AND BARYONIC MASS IN THE LOW-REDSHIFT CIRCUMGALACTIC MEDIUM (2014, methodology): “Lehner et al. (2013) carried out a similar CLOUDY-based analysis for Lyman limit systems with log NHI > 16.0, and found a mean log U of -3.3±0.6.”
The COS-Halos Survey: Metallicities in the Low-redshift Circumgalactic Medium (2017, background): “…could arise primarily at R⊥ > 150 kpc, i.e. beyond the COS-Halos survey design (although high NHI values are more rarely observed at these separations Lehner et al. 2013); and (2) the median redshift of the W16 sample is z̃ ≈ 0.6, i.e. sampling an epoch 3.3 Gyr earlier than the COS-Halos sample.”