Abstract
We analyze new far-ultraviolet spectra of 13 quasars from the
COS-Halos survey that cover the H
i
Lyman limit of 14 circumgalactic medium (CGM) systems. These data yield precise estimates or ...more constraining limits than previous COS-Halos measurements on the H
i
column densities
. We then apply a Monte-Carlo Markov chain approach on 32 systems from COS-Halos to estimate the metallicity of the cool (
K) CGM gas that gives rise to low-ionization state metal lines, under the assumption of photoionization equilibrium with the extragalactic UV background. The principle results are: (1) the CGM of field
L
* galaxies exhibits a declining H
i
surface density with impact parameter
(at
confidence), (2) the transmission of ionizing radiation through CGM gas alone is 70 ± 7%; (3) the metallicity distribution function of the cool CGM is unimodal with a median of
and a 95% interval
to
; the incidence of metal-poor (
) gas is low, implying any such gas discovered along quasar sightlines is typically unrelated to
L
* galaxies; (4) we find an unexpected increase in gas metallicity with declining
(at
confidence) and, therefore, also with increasing
; the high metallicity at large radii implies early enrichment; and (5) a non-parametric estimate of the cool CGM gas mass is
, which together with new mass estimates for the hot CGM may resolve the galactic missing baryons problem. Future analyses of halo gas should focus on the underlying astrophysics governing the CGM, rather than processes that simply expel the medium from the halo.
Without a source of new gas, our Galaxy would exhaust its supply of gas through the formation of stars. Ionized gas clouds observed at high velocity may be a reservoir of such gas, but their ...distances are key for placing them in the galactic halo and unraveling their role. We have used the Hubble Space Telescope to blindly search for ionized high-velocity clouds (iHVCs) in the foreground of galactic stars. We show that iHVCs with 90 < lVLSRl £ 170 kilometers per second (where VLSR is the velocity in the local standard of rest frame) are within one galactic radius of the Sun and have enough mass to maintain star formation, whereas iHVCs with lVLSRl > 170 kilometers per second are at larger distances. These may be the next wave of infalling material.
We demonstrate the presence of an extended and massive circumgalactic medium (CGM) around Messier 31 using archival HST Cosmic Origins Spectrograph ultraviolet spectroscopy of 18 QSOs projected ...within two virial radii of M31 (R sub(vir) = 300 kpc). We detect absorption from Si III at -300 <, ~ v sub(LSR) <, ~ -150 km s super(-1) toward all three sightlines at R <, ~ 0.2R sub(vir), 3 of 4 sightlines at 0.8 <, ~ R/R sub(vir) <, ~ 1.1, and possibly 1 of 11 at 1.1 < R/R sub(vir) <, ~ 1.8. We present several arguments that the gas at these velocities observed in these directions originates from the M31 CGM rather than the Local Group or Milky Way CGM or Magellanic Stream. We show that the dwarf galaxies located in the CGM of M31 have very similar velocities over similar projected distances from M31. We find a non-trivial relationship only at these velocities between the column densities (N) of all the ions and R, whereby N decreases with increasing R. At R < 0.8R sub(vir), the covering fraction is close to unity for Si III and C IV (functionof sub(c) ~ 60%-97% at the 90% confidence level), but drops to functionof sub(c) <, ~ 10%-20% at R > ~ R sub(vir). We show that the M31 CGM gas is bound, multiphase, predominantly ionized, and is more highly ionized gas at larger R. We estimate using Si II, Si III, and Si IV, a CGM metal mass of > ~2 x 10 super(6) M sub(middot in circle) and gas mass of > ~3 x 10 super(9)(Z sub(middot in circle)/Z) M sub(middot in circle) within 0.2R sub(vir), and possibly a factor of ~10 larger within R sub(vir), implying substantial metal and gas masses in the CGM of M31.
We analyze the physical conditions of the cool, photoionized (T similar to 10 super(4) K) circumgalactic medium (CGM) using the COS-Halos suite of gas column density measurements for 44 gaseous halos ...within 160 kpc of L ~ L super(*) galaxies at z similar to 0.2. These data are well described by simple photoionization models, with the gas highly ionized (n sub(HII)/n sub(H) gap 99%) by the extragalactic ultraviolet background. Scaling by estimates for the virial radius, R sub(vir), we show that the ionization state (tracked by the dimensionless ionization parameter, U) increases with distance from the host galaxy. The ionization parameters imply a decreasing volume density profile n sub(H) = (10 super(-4+ or -0.25))(R/R sub(vir)) super(-0+ or -0.3). Our derived gas volume densities are several orders of magnitude lower than predictions from standard two-phase models with a cool medium in pressure equilibrium with a hot, coronal medium expected in virialized halos at this mass scale. Applying the ionization corrections to the HI column densities, we estimate a lower limit to the cool gas mass M super(cool) sub(CGM) > 6.5 x 10 super(10) Mmiddot in circle for the volume within R < R sub(vir). Allowing for an additional warm-hot, O VI-traced phase, the CGM accounts for at least half of the baryons purported to be missing from dark matter halos at the 10 super(12) Mmiddot in circle scale.
ABSTRACT The total contribution of diffuse halo gas to the galaxy baryon budget strongly depends on its dominant ionization state. In this paper, we address the physical conditions in the highly ...ionized circumgalactic medium (CGM) traced by absorption lines observed in COS-Halos spectra. We analyze the observed ionic column densities, absorption-line widths and relative velocities, along with the ratios of / for 39 fitted Voigt profile components of O vi. We compare these quantities with the predictions given by a wide range of ionization models. Photoionization models that include only extragalactic UV background radiation are ruled out; conservatively, the upper limits to / and measurements of imply unphysically large path lengths 100 kpc. Furthermore, very broad absorption (b > 40 km s−1) is a defining characteristic of the CGM of star-forming L* galaxies. We highlight two possible origins for the bulk of the observed : (1) highly structured gas clouds photoionized primarily by local high-energy sources or (2) gas radiatively cooling on large scales behind a supersonic wind. Approximately 20% of circumgalactic O vi does not align with any low-ionization state gas within 50 km s−1 and is found only in halos with < 1012 . We suggest that this type of unmatched O vi absorption traces the hot corona itself at a characteristic temperature of K. We discuss the implications of these very distinct physical origins for the dynamical state, gas cooling rates, and total baryonic content of L* gaseous halos.
We present simulations from the new "Figuring Out Gas & Galaxies in Enzo" (FOGGIE) project. In contrast to most extant simulations of galaxy formation, which concentrate computational resources on ...galactic disks and spheroids with fluid and particle elements of fixed mass, the FOGGIE simulations focus on extreme spatial and mass resolution in the circumgalactic medium (CGM) surrounding galaxies. Using the Enzo code and a new refinement scheme, FOGGIE reaches spatial resolutions of 381 comoving h−1 pc and resolves extremely low masses ( 1-100 ) out to 100 comoving h−1 kpc from the central halo. At these resolutions, cloud and filament-like structures giving rise to simulated absorption are smaller, and better resolved, than the same structures simulated with standard density-dependent refinement. Most of the simulated absorption arises in identifiable and well-resolved structures with masses 104 , well below the mass resolution of typical zoom simulations. However, integrated quantities such as mass surface density and ionic covering fractions change at only the 30% level as resolution is varied. These relatively small changes in projected quantities-even when the sizes and distribution of absorbing clouds change dramatically-indicate that commonly used observables provide only weak constraints on the physical structure of the underlying gas. Comparing the simulated absorption features to the KODIAQ (Keck Observatory Database of Ionized Absorption toward Quasars) survey of z ∼ 2-3.5 Lyman limit systems, we show that high-resolution FOGGIE runs better resolve the internal kinematic structure of detected absorption and better match the observed distribution of absorber properties. These results indicate that circumgalactic medium resolution is key in properly testing simulations of galaxy evolution with circumgalactic observations.
Traditional cosmological hydrodynamics simulations fail to spatially resolve the circumgalactic medium (CGM), the reservoir of tenuous gas surrounding a galaxy and extending to its virial radius. We ...introduce the technique of Enhanced Halo Resolution (EHR), enabling more realistic physical modeling of the simulated CGM by consistently forcing gas refinement to smaller scales throughout the virial halo of a simulated galaxy. We investigate the effects of EHR in the tempest simulations, a suite of enzo-based cosmological zoom simulations following the evolution of an L* galaxy, resolving spatial scales of 500 comoving pc out to 100 comoving kpc in galactocentric radius. Among its many effects, EHR (1) changes the thermal balance of the CGM, increasing its cool gas content and decreasing its warm/hot gas content; (2) preserves cool gas structures for longer periods; and (3) enables these cool clouds to exist at progressively smaller size scales. Observationally, this results in a boost in "low ions" like H i and a drop in "high ions" like O vi throughout the CGM. These effects of EHR do not converge in the tempest simulations, but extrapolating these trends suggests that the CGM is actually a mist consisting of ubiquitous, small, long-lived, cool clouds suspended in a medium at the halo virial temperature. We find that EHR produces the above effects by (1) better sampling the distribution of CGM phases, enabling runaway cooling in the dense, cool tail of the phase distribution; and (2) preventing cool gas clouds from artificially mixing with the ambient hot halo and evaporating.
ABSTRACT We use the Hubble Space Telescope (HST) archive of ultraviolet (UV) quasar spectroscopy to conduct the first blind survey for damped Ly absorbers (DLAs) at low redshift ( ). Our statistical ...sample includes 463 quasars with spectral coverage spanning a total redshift path or an absorption path . Within this survey path, we identify 4 DLAs defined as absorbers with H i column density cm−2, which implies an incidence per absorption length at a median survey path redshift of z = 0.623. While our estimate of is lower than earlier estimates at from H i 21 cm emission studies, the results are consistent within the measurement uncertainties. Our data set is too small to properly sample the frequency distribution function , but the observed distribution agrees with previous estimates at . Adopting the shape of , we infer an H i mass density at of . This is significantly lower than previous estimates from targeted DLA surveys with the HST, but consistent with results from low-z H i 21 cm observations, and suggests that the neutral gas density of the universe has been decreasing over the past 10 Gyr.
ABSTRACT We present the first results from our KODIAQ Z survey aimed at determining the metallicity distribution and physical properties of the partial and full Lyman limit systems (pLLSs and LLSs; ...), which are probed of the interface regions between the intergalactic medium (IGM) and galaxies. We study 31 H i-selected pLLSs and LLSs at observed with Keck/HIRES in absorption against background QSOs. We compare the column densities of metal ions to H i and use photoionization models to assess the metallicity. The metallicity distribution of the pLLSs/LLSs at is consistent with a unimodal distribution peaking at . The metallicity distribution of these absorbers therefore evolves markedly with z since at it is bimodal with peaks at and −0.3. There is a substantial fraction (25%-41%) of pLLSs/LLSs with metallicities well below those of damped Ly absorbers (DLAs) at any studied z from to -4, implying reservoirs of metal-poor, cool, dense gas in the IGM/galaxy interface at all z. However, the gas probed by pLLSs and LLSs is rarely pristine, with a fraction of 3%-18% for pLLSs/LLSs with . We find C/ enhancement in several pLLSs and LLSs in the metallicity range , where C/ is 2-5 times larger than observed in Galactic metal-poor stars or high-redshift DLAs at similar metallicities. This is likely caused by preferential ejection of carbon from metal-poor galaxies into their surroundings.
In this third paper of a series studying the Magellanic gas in absorption, we analyze the gas ionization level using a sample of 69 Hubble Space Telescope/Cosmic Origins Spectrograph sightlines that ...pass through or within 30degrees of the 21 cm emitting regions. We find that 81% (56/69) of the sightlines show UV absorption at Magellanic velocities, indicating that the total cross-section of the Magellanic System is approximately 11,000 deg super(2), or around one-quarter of the entire sky. Using observations of the Si III/Si II ratio together with Cloudy photoionization modeling, we calculate the total gas mass (atomic plus ionized) of the Magellanic System to be approximately 2.0 x 10 super(9) M sub(middot in circle) (d/55 kpc) super(2), with the ionized gas contributing around three times as much mass as the atomic gas. This is larger than the current-day interstellar HI mass of both Magellanic Clouds combined, indicating that they have lost most of their initial gas mass.
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