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Register a new userNIHAO VIII: Circum-galactic medium and outflows - The puzzles of HI and OVI gas distributions
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We study the hot and cold circum-galactic medium (CGM) of 86 galaxies of the cosmological, hydrodynamical simulation suite NIHAO. NIHAO allows a study of how the $z=0$ CGM varies across 5 orders of magnitude of stellar mass using OVI and HI as proxies for hot and cold gas. The cool HI covering fraction and column density profiles match observations well, particularly in the inner CGM. OVI shows increasing column densities with mass, a trend seemingly echoed in the observations. As in multiple previous simulations, the OVI column densities in simulations are lower than observed and optically thick HI does not extend as far out as in observations. We take a look at the collisional ionisation fraction of OVI as a function of halo mass. We make observable predictions of the bipolarity of outflows and their effect on the general shape of the CGM. Bipolar outflows can be seen out to around 40 kpc in intermediate and low mass halos ($M_{mathrm{halo}}<10^{11}M_{mathrm{sun}}$), but outside that radius, the CGM is too well mixed to detect an elongated shape. Larger halos have extended gas discs beyond the stellar disc that dominate the shape of the inner CGM. The simulated CGM is remarkably spherical even in low mass simulations. The chemical enrichment of both halo and disc gas follow expected increasing trends as a function of halo mass that are well fit with power laws. These relations can be used in non-hydrodynamic models, such as semi-analytic models.
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We investigate whether the dust content of the circum-galactic medium (CGM) depends on the location of the quasar sightline with respect to the galaxy major-axis using 13 galaxy-MgII absorber pairs (9 - 81 kpc distance) from the MusE GAs FLOw and Wind (MEGAFLOW) survey at 0.4 < z < 1.4. The dust content of the CGM is obtained from [Zn/Fe] using Ultraviolet and Visual Echelle Spectrograph (UVES) data. When a direct measurement of [Zn/Fe] is unavailable, we estimate the dust depletion from a method which consists in solving for the depletion from multiple singly ionized ions (e.g. MnII, CrII, ZnII) since each ion depletes on dust grains at different rates. We find a positive correlation between the azimuthal angle and [Zn/Fe] with a Pearsons r = 0.70 +/- 0.14. The sightlines along the major axis show [Zn/Fe] < 0.5, whereas the [Zn/Fe] is > 0.8 along the minor axis. These results suggest that the CGM along the minor axis is on average more metal enriched (by ~ 1 dex) than the gas located along the major axis of galaxies provided that dust depletion is a proxy for metallicity. This anisotropic distribution is consistent with recent results on outflow and accretion in hydro-dynamical simulations.
Galaxy halos appear to be missing a large fraction of their baryons, most probably hiding in the circumgalactic medium (CGM), a diffuse component within the dark matter halo that extends far from the inner regions of the galaxies. A powerful tool to study the CGM gas is offered by absorption lines in the spectra of background quasars. Here, we present optical (MUSE) and mm (ALMA) observations of the field of the quasar Q1130-1449 which includes a log [N(H I)/cm^-2]=21.71+/-0.07 absorber at z=0.313. Ground-based VLT/MUSE 3D spectroscopy shows 11 galaxies at the redshift of the absorber down to a limiting SFR>0.01 M_sun yr^-1 (covering emission lines of [OII], Hbeta, [OIII], [NII] and Halpha), 7 of which are new discoveries. In particular, we report a new emitter with a smaller impact parameter to the quasar line-of-sight (b=10.6 kpc) than the galaxies detected so far. Three of the objects are also detected in CO(1-0) in our ALMA observations indicating long depletion timescales for the molecular gas and kinematics consistent with the ionised gas. We infer from dedicated numerical cosmological RAMSES zoom-in simulations that the physical properties of these objects qualitatively resemble a small group environment, possibly part of a filamentary structure. Based on metallicity and velocity arguments, we conclude that the neutral gas traced in absorption is only partly related to these emitting galaxies while a larger fraction is likely the signature of gas with surface brightness almost four orders of magnitude fainter that current detection limits. Together, these findings challenge a picture where strong-HI quasar absorbers are associated with a single bright galaxy and favour a scenario where the HI gas probed in absorption is related to far more complex galaxy structures.
We explore the survival of cool clouds in multi-phase circum-galactic media. We revisit the cloud crushing problem in a large survey of simulations including radiative cooling, self-shielding, self-gravity, magnetic fields, and anisotropic Braginskii conduction and viscosity (with saturation). We explore a wide range of parameters including cloud size, velocity, ambient temperature and density, as well as a variety of magnetic field configurations and cloud turbulence. We find that realistic magnetic fields and turbulence have weaker effects on cloud survival; the most important physics is radiative cooling and conduction. Self-gravity and self-shielding are important for clouds which are initially Jeans-unstable, but largely irrelevant otherwise. Non-self-gravitating, realistically magnetized clouds separate into four regimes: (1) At low column densities, clouds evaporate rapidly via conduction. (2) A failed pressure confinement regime, where the ambient hot gas cools too rapidly to provide pressure confinement for the cloud. (3) An infinitely long-lived regime, in which the cloud lifetime becomes longer than the cooling time of gas swept up in the leading bow shock, so the cloud begins to accrete and grow. (4) A classical cloud destruction regime, where clouds are eventually destroyed by instabilities. In the final regime, the cloud lifetime can exceed the naive cloud-crushing time owing to conduction-induced compression. However, small and/or slow-moving clouds can also evaporate more rapidly than the cloud-crushing time. We develop simple analytic models that explain the simulated cloud destruction times in this regime.
Gas flows in and out of galaxies through their circumgalactic medium (CGM) are poorly constrained and direct observations of this faint, diffuse medium remain challenging. We use a sample of five $z$ $sim$ 1-2 galaxy counterparts to Damped Lyman-$alpha$ Absorbers (DLAs) to combine data on cold gas, metals and stellar content of the same galaxies. We present new HST/WFC3 imaging of these fields in 3-5 broadband filters and characterise the stellar properties of the host galaxies. By fitting the spectral energy distribution, we measure their stellar masses to be in the range of log($M_*$/$text{M}_{odot}$) $sim$ 9.1$-$10.7. Combining these with IFU observations, we find a large spread of baryon fractions inside the host galaxies, between 7 and 100 percent. Similarly, we find gas fractions between 3 and 56 percent. Given their star formation rates, these objects lie on the expected main sequence of galaxies. Emission line metallicities indicate they are consistent with the mass-metallicity relation for DLAs. We also report an apparent anti-correlation between the stellar masses and $N$(HI), which could be due to a dust bias effect or lower column density systems tracing more massive galaxies. We present new ALMA observations of one of the targets leading to a molecular gas mass of log($M_{rm mol}$/$text{M}_{odot}$) < 9.89. We also investigate the morphology of the DLA counterparts and find that most of the galaxies show a clumpy structure and suggest ongoing tidal interaction. Thanks to our high spatial resolution HST data, we gain new insights in the structural complexity of the CGM.
We present a model to self-consistently describe the joint evolution of starburst galaxies and the galactic wind resulting from this evolution. We combine the population synthesis code Starburst99 with a semi-analytical model of galactic outflows and a model for the distribution and abundances of chemical elements inside the outflows. Starting with a galaxy mass, formation redshift, and adopting a particular form for the star formation rate, we describe the evolution of the stellar populations in the galaxy, the evolution of the metallicity and chemical composition of the interstellar medium (ISM), the propagation of the galactic wind, and the metal-enrichment of the intergalactic medium (IGM). In this paper, we study the properties of the model, by varying the mass of the galaxy, the star formation rate, and the efficiency of star formation. Our main results are the following: (1) For a given star formation efficiency f*, a more extended period of active star formation tends to produce a galactic wind that reaches a larger extent. If f* is sufficiently large, the energy deposited by the stars completely expels the ISM. Eventually, the ISM is being replenished by mass loss from supernovae and stellar winds. (2) For galaxies with masses above 10^11 Msun, the material ejected in the IGM always falls back onto the galaxy. Hence lower-mass galaxies are the ones responsible for enriching the IGM. (3) Stellar winds play a minor role in the dynamical evolution of the galactic wind, because their energy input is small compared to supernovae. However, they contribute significantly to the chemical composition of the galactic wind. We conclude that the history of the ISM enrichment plays a determinant role in the chemical composition and extent of the galactic wind, and therefore its ability to enrich the IGM.
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