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Register a new userAbsorption systems at $zsim 2$ as a probe of the circum galactic medium: a probabilistic approach
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We characterize the properties of the intergalactic medium (IGM) around a sample of galaxies extracted from state-of-the-art hydrodynamical simulations of structure formation in a cosmological volume of 25 Mpc comoving at $zsim 2$. The simulations are based on two different subresolution schemes for star formation and supernova feedback: the MUlti-Phase Particle Integrator (MUPPI) scheme and the Effective Model. We develop a quantitative and probabilistic analysis based on the apparent optical depth method of the properties of the absorbers as a function of impact parameter from their nearby galaxies: in such a way we probe different environments from circumgalactic medium to low-density filaments. Absorbers properties are then compared with a spectroscopic observational data set obtained from high-resolution quasar spectra. Our main focus is on the N$_{rm{ CIV}}$-N$_{rm { HI}}$ relation around simulated galaxies: the results obtained with MUPPI and the Effective model are remarkably similar, with small differences only confined to regions at impact parameters $b = [1-3] times r_{rm {vir}}$. Using $mbox{C IV}$ as a tracer of the metallicity, we obtain evidence that the observed metal absorption systems have the highest probability to be confined in a region of 150-400 kpc around galaxies. Near-filament environments have instead metallicities too low to be probed by present-day telescopes, but could be probed by future spectroscopical studies. Finally we compute $mbox{C IV}$ covering fractions which are in agreement with observational data.
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Observed reddening in the circum-galactic medium (CGM) indicates a significant abundance of small grains, of which the origin is still to be clarified. We examine a possible path of small-grain production through shattering of pre-existing large grains in the CGM. Possible sites where shattering occurs on a reasonable time-scale are cool clumps with hydrogen number density $n_mathrm{H}sim 0.1$ cm$^{-3}$ and gas temperature $T_mathrm{gas}sim 10^4$ K, which are shown to exist through observations of Mg II absorbers. We calculate the evolution of grain size distribution in physical conditions appropriate for cool clumps in the CGM, starting from a large-grain-dominated distribution suggested from theoretical studies. With an appropriate gas turbulence model expected from the physical condition of cold clumps (maximum eddy size and velocity of $sim$100 pc and 10 km s$^{-1}$, respectively), together with the above gas density and temperature and the dust-to-gas mass ratio inferred from observations (0.006), we find that small-grain production occurs on a time-scale (a few $times 10^8$ yr) comparable to the lifetime of cool clumps derived in the literature. Thus, the physical conditions of the cool clouds are favrourable for small-grain production. We also confirm that the reddening becomes significant on the above time-scale. Therefore, we conclude that small-grain production by shattering is a probable cause for the observed reddening in the CGM. We also mention the effect of grain materials (or their mixtures) on the reddening at different redshifts (1 and 2).
We investigate the properties of HI-rich galaxies detected in blind radio surveys within the hierarchical structure formation scenario using a semi-analytic model of galaxy formation. By drawing a detailed comparison between the properties of HI-selected galaxies and HI absorption systems, we argue a link between the local galaxy population and quasar absorption systems, particularly for Damped Ly-alpha absorption (DLA) systems and sub-DLA systems. First, we evaluate how many HI-selected galaxies exhibit HI column densities as high as those of DLA systems. We find that HI-selected galaxies with HI masses M(HI) > 10^8 solar masses have gaseous disks that produce HI column densities comparable to those of DLA systems. We conclude that DLA galaxies where the HI column densities are as high as those of DLA systems, contribute significantly to the population of HI-selected galaxies at M(HI) > 10^8 solar masses. Second, we find that star formation rates (SFRs) correlate tightly with HI masses rather than B- (and J-) band luminosities. In the low-mass range M(HI) < 10^8 solar masses, sub-DLA galaxies replace DLA galaxies as the dominant population. The number fraction of sub-DLA galaxies relative to galaxies reaches 40%-60% at HI masses 10^8 solar masses and 30%-80% at 10^7 solar masses. The HI-selected galaxies at 10^7 solar masses are a strong probe of sub-DLA systems that place stringent constraints on galaxy formation and evolution.
We present simulations of isolated disc galaxies in a realistic environment performed with the Tree-SPMHD-Code Gadget-3. Our simulations include a spherical circum-galactic medium (CGM) surrounding the galactic disc, motivated by observations and the results of cosmological simulations. We present three galactic models with different halo masses between 10e10 Msol and 10e12 Msol, and for each we use two different approaches to seed the magnetic field, as well as a control simulation without a magnetic field. We find that the amplification of the magnetic field in the centre of the disc leads to a biconical magnetic outflow of gas that magnetizes the CGM. This biconical magnetic outflow reduces the star formation rate (SFR) of the galaxy by roughly 40 percent compared to the simulations without magnetic fields. As the key aspect of our simulations, we find that small scale turbulent motion of the gas in the disc leads to the amplification of the magnetic field up to tens of 10e-6 G, as long as the magnetic field strength is low. For stronger magnetic fields turbulent motion does not lead to significant amplification but is replaced by an alpha-omega dynamo. The occurance of a small scale turbulent dynamo becomes apparent through the magnetic power spectrum and analysis of the field lines curvature. In accordance with recent observations we find an anti-correlation between the spiral structure in the gas density and in the magnetic field due to a diffusion term added to the induction equation.
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.
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