The Large, Oxygen-Rich Halos of Star-Forming Galaxies Are a Major Reservoir of Galactic Metals

Abstract

The circumgalactic medium (CGM) is fed by galaxy outflows and accretion of intergalactic gas, but its mass, heavy element enrichment, and relation to galaxy properties are poorly constrained by observations. In a survey of the outskirts of 42 galaxies with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope, we detected ubiquitous, large (150 kiloparsec) halos of ionized oxygen surrounding star-forming galaxies, but we find much less ionized oxygen around galaxies with little or no star formation. This ionized CGM contains a substantial mass of heavy elements and gas, perhaps far exceeding the reservoirs of gas in the galaxies themselves. It is a basic component of nearly all star-forming galaxies that is removed or transformed during the quenching of star formation and the transition to passive evolution.Galaxies grow by accreting gas from the intergalactic medium (IGM) and converting it to stars. Stellar winds and explosions release gas enriched with heavy elements (or metals, 1), some of which is ejected in galactic-scale outflows (2). The circumgalactic medium (CGM), 1 arXiv:1111.3980v1 [astro-ph.CO] 16 Nov 2011 loosely defined as gas surrounding galaxies within their own halos of dark matter (out to 100-300 kiloparsec), lies at the nexus of accretion and outflows, but the structure of the CGM and its relation to galaxy properties are still uncertain. Galactic outflows are observed at both low (2-4) and high (5-7) redshift, but it is unclear how far they propagate, what level of heavyelement enrichment they possess, and whether the gas escapes the halo or eventually returns to fuel later star formation. Models of galaxy evolution require significant outflows to explain observed galaxy masses and chemical abundances and to account for metals observed in the more diffuse IGM (8, 9). The CGM may also reflect the theoretically-predicted transition from filamentary streams of cold gas that feed low mass galaxies to hot, quasi-static envelopes that surround high mass galaxies (10, 11). Both outflow and accretion through the CGM may be intimately connected to the observed dichotomy between blue, star-forming, disk-dominated galaxies and red, passively evolving, elliptical galaxies with little or no star formation (12). However, the low density of the CGM makes it extremely difficult to probe directly, thus models of its structure and influences are typically constrained indirectly by its effects on the visible portions of galaxies, not usually by observations of the gas itself.We have undertaken a large program with the new Cosmic Origins Spectrograph (COS) aboard the Hubble Space Telescope to directly map the CGM using the technique of absorptionline spectroscopy, in which a diffuse gas is detected by its absorption of light from a background source. Our background sources are UV-bright quasi-stellar objects (QSOs), which are the luminous active nuclei of galaxies lying far behind the galaxies of interest. We focus on the ultraviolet 1032,1038Å doublet of O VI (O +5 ), the most accessible tracer of ...

The Properties of the Circumgalactic Medium in Red and Blue Galaxies: Results From the Cos-Gass+cos-Halos Surveys

“…There is also a significant fraction of the blue galaxies showing large velocity differences between the radial velocity of the absorber and central galaxy (Dv up to 500 km s −1 ). Taken together with the COS-Halos results on the presence (absence) of highly ionized gas (seen as O VI absorbers Tumlinson et al 2011) in blue (red) galaxies, there are therefore real differences between the CGM surrounding star-forming and quiescent galaxies. Unfortunately, as discussed in Section 3.1, the data from the COS-GASS do not cover O VI; thus, we are not able to do the same analysis as done by Tumlinson et al (2011) for the combined sample.…”

“…Similar ionization parameters were also estimated by Shull et al (2009) 2 that are seen in our sample. Therefore, it is likely that most of these highly ionized absorbers are different from those detected in various other CGM and QSO-absorption studies (Chen et al 2001a;Wakker & Savage 2009;Tumlinson et al 2011;Borthakur et al 2013;Bordoloi et al 2014, and references therein). However, as discussed in detail by Werk et al (2014), Fox et al (2013), Meiring et al (2013), Lehner et al (2013), Tripp et al (2011), and references therein, O VI may represent a different phase of gas that differs from the ones traced by lower ionization transitions.…”

In-N-Out: The Gas Cycle From Dwarfs to Spiral Galaxies

“…Future work will analyze the source of metals, their eventual location, and the relative rates of pristine versus enriched accretion. Such metal distribution mechanisms can be directly constrained by observations of metal absorption lines in quasar spectra passing near galaxies, such as that obtained from the COS-Halos program (Tumlinson et al 2011;Ford et al 2016).…”

Long way to go: how outflows from large galaxies propagate through the hot halo gas

“…In the case, if the starburst activity lasts long time ( 300 Myr) or there are repeated bursts at the centre, the outflow properties (viz. velocity, metallicity etc) at ∼ 100-200 kpc also can be connected to the ongoing star formation process, as observed by Tumlinson et al (2011). Suppose one had defined the outer mass loading factor as the ratio between averaged mass outflow rate and the SFR as…”

“…The mass loading factor estimated at the virial radius has important implications for understanding dynamical processes accompanying enrichment of the Universe. Recent observations of the extended circum-galactic haloes bearing metals up to distances 100 -200 kpc from the parent galaxy (Tumlinson et al 2011;Bordoloi et al 2014;Mathes et al 2014) call for a long-term numerical study covering passage of the outflows through the virial threshold. Hopkins et al (2012) also found that the outflowing gas is mostly dominated by warm gas (10 4 < T < 4 × 10 5 K), which is consistent with the results of Melioli et al (2013) who have simulated the outflow from M82.…”