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Register a new userThe COS Absorption Survey of Baryon Harbors (CASBaH): Warm-hot Circumgalactic Gas Reservoirs Traced by Ne VIII Absorption
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We survey the highly ionized circumgalactic media (CGM) of 29 blindly selected galaxies at 0.49 < z_(gal) < 1.44 based on high-S/N ultraviolet spectra of z > 1 QSOs and the galaxy database from the COS Absorption Survey of Baryon Harbors (CASBaH). We detect the Ne VIII doublet in nine of the galaxies, and for gas with N(Ne VIII) > 10^13.3 cm^-2 (> 10^13.5 cm^-2), we derive a Ne VIII covering fraction f_c = 75 +15/-25% (44 +22/-20%) within impact parameter (rho) < 200 kpc of M_* = 10^(9.5-11.5) Msol galaxies and f_c = 70 +16/-22% (f_c = 42 +20/-17%) within rho < 1.5 virial radii. We estimate the mass in Ne VIII-traced gas to be M_gas(Ne VIII) > 10^9.5 Msol (Z/Zsol)^-1, or 6-20% of the expected baryonic mass if the Ne VIII absorbers have solar metallicity. Ionizing Ne VII to Ne VIII requires 207 eV, and photons with this energy are scarce in the CGM. However, for the median halo mass and redshift of our sample, the virial temperature is close to the peak temperature for the Ne VIII ion, and the Ne VIII-bearing gas is plausibly collisionally ionized near this temperature. Moreover, we find that photoionized Ne VIII requires cool and low-density clouds that would be highly underpressured (by approximately two orders of magnitude) relative to the putative, ambient virialized medium, complicating scenarios where such clouds could survive. Thus, more complex (e.g., non-equilibrium) models may be required; this first statistical sample of Ne VIII absorber/galaxy systems will provide stringent constraints for future CGM studies.
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We describe the survey for galaxies in the fields surrounding 9 sightlines to far-UV bright, z~1 quasars that define the COS Absorption Survey of Baryon Harbors (CASBaH) program. The photometry and spectroscopy that comprise the dataset come from a mixture of public surveys (SDSS, DECaLS) and our dedicated efforts on private facilities (Keck, MMT, LBT). We report the redshifts and stellar masses for 5902 galaxies within ~10 comoving-Mpc (cMpc) of the sightlines with a median of z=0.28 and M_* ~ 10^(10.1) Msun. This dataset, publicly available as the CASBaH specDB, forms the basis of several recent and ongoing CASBaH analyses. Here, we perform a clustering analysis of the galaxy sample with itself (auto-correlation) and against the set of OVI absorption systems (cross-correlation) discovered in the CASBaH quasar spectra with column densities N(O^+5) >= 10^(13.5)/cm^2. For each, we describe the measured clustering signal with a power-law correlation function xi(r) = (r/r_0)^(-gamma) and find that (r_0,gamma) = (5.48 +/- 0.07 h_100^-1 Mpc, 1.33 +/- 0.04) for the auto-correlation and (6.00 +/- 1 h^-1 Mpc, 1.25 +/- 0.18) for galaxy-OVI cross-correlation. We further estimate a bias factor of b_gg = 1.3 +/- 0.1 from the galaxy-galaxy auto-correlation indicating the galaxies are hosted by halos with mass M_halo ~ 10^(12.1 +/- 0.05) Msun. Finally, we estimate an OVI-galaxy bias factor b_OVI = 1.0 +/- 0.1 from the cross-correlation which is consistent with OVI absorbers being hosted by dark matter halos with typical mass M_halo ~ 10^(11) Msun. Future works with upcoming datasets (e.g., CGM^2) will improve upon these results and will assess whether any of the detected OVI arises in the intergalactic medium.
We use the EAGLE (Evolution and Assembly of GaLaxies and their Environments) cosmological simulation to study the distribution of baryons, and far-ultraviolet (O VI), extreme-ultraviolet (Ne VIII) and X-ray (O VII, O VIII, Ne IX, and Fe XVII) line absorbers, around galaxies and haloes of mass $mathrm{M}_{200c}=10^{11}$-$10^{14.5},mathrm{M}_{odot}$ at redshift 0.1. EAGLE predicts that the circumgalactic medium (CGM) contains more metals than the interstellar medium across halo masses. The ions we study here trace the warm-hot, volume-filling phase of the CGM, but are biased towards temperatures corresponding to the collisional ionization peak for each ion, and towards high metallicities. Gas well within the virial radius is mostly collisionally ionized, but around and beyond this radius, and for O VI, photoionization becomes significant. When presenting observables we work with column densities, but quantify their relation with equivalent widths by analysing virtual spectra. Virial-temperature collisional ionization equilibrium ion fractions are good predictors of column density trends with halo mass, but underestimate the diversity of ions in haloes. Halo gas dominates the highest column density absorption for X-ray lines, but lower density gas contributes to strong UV absorption lines from O VI and Ne VIII. Of the O VII (O VIII) absorbers detectable in an Athena X-IFU blind survey, we find that 41 (56) per cent arise from haloes with $mathrm{M}_{200c}=10^{12.0}$-$10^{13.5},mathrm{M}_{odot}$. We predict that the X-IFU will detect O VII (O VIII) in 77 (46) per cent of the sightlines passing $mathrm{M}_{star}=10^{10.5}$-$10^{11.0},mathrm{M}_{odot}$ galaxies within 100 pkpc (59 (82) per cent for $mathrm{M}_{star}>10^{11.0},mathrm{M}_{odot}$). Hence, the X-IFU will probe covering fractions comparable to those detected with the Cosmic Origins Spectrograph for O VI.
We present basic data and modeling for a survey of the cool, photo-ionized Circum-Galactic Medium (CGM) of low-redshift galaxies using far-UV QSO absorption line probes. This survey consists of targeted and serendipitous CGM subsamples, originally described in Stocke et al. (2013, Paper 1). The targeted subsample probes low-luminosity, late-type galaxies at $z<0.02$ with small impact parameters ($langlerhorangle = 71$ kpc), and the serendipitous subsample probes higher luminosity galaxies at $zlesssim0.2$ with larger impact parameters ($langlerhorangle = 222$ kpc). HST and FUSE UV spectroscopy of the absorbers and basic data for the associated galaxies, derived from ground-based imaging and spectroscopy, are presented. We find broad agreement with the COS-Halos results, but our sample shows no evidence for changing ionization parameter or hydrogen density with distance from the CGM host galaxy, probably because the COS-Halos survey probes the CGM at smaller impact parameters. We find at least two passive galaxies with H I and metal-line absorption, confirming the intriguing COS-Halos result that galaxies sometimes have cool gas halos despite no on-going star formation. Using a new methodology for fitting H I absorption complexes, we confirm the CGM cool gas mass of Paper 1, but this value is significantly smaller than found by the COS-Halos survey. We trace much of this difference to the specific values of the low-$z$ meta-galactic ionization rate assumed. After accounting for this difference, a best-value for the CGM cool gas mass is found by combining the results of both surveys to obtain $log{(M/M_{odot})}=10.5pm0.3$, or ~30% of the total baryon reservoir of an $L geq L^*$, star-forming galaxy.
The baryon content around local galaxies is observed to be much less than is needed in Big Bang nucleosynthesis. Simulations indicate that a significant fraction of these missing baryons may be stored in a hot tenuous circum-galactic medium (CGM) around massive galaxies extending to or even beyond the virial radius of their dark matter halos. Previous observations in X-ray and Sunyaev-Zeldovich (SZ) signal claimed that $sim(1-50)%$ of the expected baryons are stored in a hot CGM within the virial radius. The large scatter is mainly caused by the very uncertain extrapolation of the hot gas density profile based on the detection in a small radial range (typically within 10%-20% of the virial radius). Here we report stacking X-ray observations of six local isolated massive spiral galaxies from the CGM-MASS sample. We find that the mean density profile can be characterized by a single power law out to a galactocentric radius of $approx 200rm~kpc$ (or $approx130rm~kpc$ above the 1~$sigma$ background uncertainty), about half the virial radius of the dark matter halo. We can now estimate that the hot CGM within the virial radius accounts for $(8pm4)%$ of the baryonic mass expected for the halos. Including the stars, the baryon fraction is $(27pm16)%$, or $(39pm20)%$ by assuming a flattened density profile at $rgtrsim130rm~kpc$. We conclude that the hot baryons within the virial radius of massive galaxy halos are insufficient to explain the missing baryons.
To characterize the absorption properties of this circumgalactic medium (CGM) and its relation to the LG we present the so-far largest survey of metal absorption in Galactic high-velocity clouds (HVCs) using archival ultraviolet (UV) spectra of extragalactic background sources. The UV data are obtained with the Cosmic Origins Spectrograph (COS) onboard the Hubble Space Telescope (HST) and are supplemented by 21 cm radio observations of neutral hydrogen. Along 270 sightlines we measure metal absorption in the lines of SiII, SiIII, CII, and CIV and associated HI 21 cm emission in HVCs in the velocity range |v_LSR|=100-500 km s^-1. With this unprecedented large HVC sample we were able to improve the statistics on HVC covering fractions, ionization conditions, small-scale structure, CGM mass, and inflow rate. For the first time, we determine robustly the angular two point correlation function of the high-velocity absorbers, systematically analyze antipodal sightlines on the celestial sphere, and compare the absorption characteristics with that of Damped Lyman alpha absorbers (DLAs) and constrained cosmological simulations of the LG. Our study demonstrates that the Milky Way CGM contains sufficient gaseous material to maintain the Galactic star-formation rate at its current level. We show that the CGM is composed of discrete gaseous structures that exhibit a large-scale kinematics together with small-scale variations in physical conditions. The Magellanic Stream clearly dominates both the cross section and mass flow of high-velocity gas in the Milky Ways CGM. The possible presence of high-velocity LG gas underlines the important role of the local cosmological environment in the large-scale gas-circulation processes in and around the Milky Way (abridged).
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