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Register a new userThe circum-galactic gas around cosmologically simulated disks
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Added by
St\\'ephanie Courty
Publication date
2010
fields
Physics
and research's language is
English
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We analyze the physical properties and infall rates of the circum-galactic gas around disks obtained in multi-resolved, cosmological, AMR simulations. At intermediate and low redshifts, disks are embedded into an extended, hot, tenuous corona that contributes largely in fueling the disk with non-enriched gas whereas the accretion of enriched gas from tidal streams occurs throughout episodic events. We derive an infall rate close to the disk of the same value as the one of the star formation rate in the disk and its temporal evolution as a function of galacto-centric radius nicely shows that the growth of galactic disks proceeds according to an inside-out formation scenario.
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We use cosmological SPH simulations to study the kinematic signatures of cool gas accretion onto a pair of well-resolved galaxy halos. Cold-flow streams and gas-rich mergers produce a circum-galactic component of cool gas that generally orbits with high angular momentum about the galaxy halo before falling in to build the disk. This signature of cosmological accretion should be observable using background-object absorption line studies as features that are offset from the galaxys systemic velocity by ~100 km/s. Accreted gas typically co-rotates with the central disk in the form of a warped, extended cold flow disk, such that the observed velocity offset is in the same direction as galaxy rotation, appearing in sight lines that avoid the galactic poles. This prediction provides a means to observationally distinguish accreted gas from outflow gas: the accreted gas will show large one-sided velocity offsets in absorption line studies while radial/bi-conical outflows will not (except possibly in special polar projections). This rotation signature has already been seen in studies of intermediate redshift galaxy-absorber pairs; we suggest that these observations may be among the first to provide indirect observational evidence for cold accretion onto galactic halos. Cold mode halo gas typically has ~3-5 times more specific angular momentum than the dark matter. The associated cold mode disk configurations are likely related to extended HI/XUV disks seen around galaxies in the local universe. The fraction of galaxies with extended cold flow disks and associated offset absorption-line gas should decrease around bright galaxies at low redshift, as cold mode accretion dies out.
We present a detailed analysis of the local evolution of 206 Lagrangian Volumes (LVs) selected at high redshift around galaxy seeds, identified in a large-volume $Lambda$ cold dark matter ($Lambda$CDM) hydrodynamical simulation. The LVs have a mass range of $1 - 1500 times 10^{10} M_odot$. We follow the dynamical evolution of the density field inside these initially spherical LVs from $z=10$ up to $z_{rm low} = 0.05$, witnessing highly non-linear, anisotropic mass rearrangements within them, leading to the emergence of the local cosmic web (CW). These mass arrangements have been analysed in terms of the reduced inertia tensor $I_{ij}^r$, focusing on the evolution of the principal axes of inertia and their corresponding eigendirections, and paying particular attention to the times when the evolution of these two structural elements declines. In addition, mass and component effects along this process have also been investigated. We have found that deformations are led by dark matter dynamics and they transform most of the initially spherical LVs into prolate shapes, i.e. filamentary structures. An analysis of the individual freezing-out time distributions for shapes and eigendirections shows that first most of the LVs fix their three axes of symmetry (like a skeleton) early on, while accretion flows towards them still continue. Very remarkably, we have found that more massive LVs fix their skeleton earlier on than less massive ones. We briefly discuss the astrophysical implications our findings could have, including the galaxy mass-morphology relation and the effects on the galaxy-galaxy merger parameter space, among others.
Active galactic nuclei (AGN) are thought to play a critical role in shaping galaxies, but their effect on the circumgalactic medium (CGM) is not well studied. We present results from the COS-AGN survey: 19 quasar sightlines that probe the CGM of 20 optically-selected AGN host galaxies with impact parameters $80 < rho_{imp} < 300$ kpc. Absorption lines from a variety of species are measured and compared to a stellar mass and impact parameter matched sample of sightlines through non-AGN galaxies. Amongst the observed species in the COS-AGN sample (HI, CII, SiII, SiIII, CIV, SiIV, NV), only Ly$alpha$ shows a high covering fraction ($94^{+6}_{-23}$% for rest-frame equivalent widths EW $> 124$ mAA) whilst many of the metal ions are not detected in individual sightlines. A sightline-by-sightline comparison between COS-AGN and the control sample yields no significant difference in EW distribution. However, stacked spectra of the COS-AGN and control samples show significant (> 3 sigma) enhancements in the EW of both Ly$alpha$ and SiIII at impact parameters $> 164$ kpc by a factor of $+0.45pm0.05$ dex and $> +0.75$ dex respectively. The lack of detections of both high-ionization species near the AGN and strong kinematic offsets between the absorption systemic galaxy redshifts indicates that neither the AGNs ionization nor its outflows are the origin of these differences. Instead, we suggest the observed differences could result from either AGN hosts residing in haloes with intrinsically distinct gas properties, or that their CGM has been affected by a previous event, such as a starburst, which may also have fuelled the nuclear activity.
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.
In the context of the FLASHLIGHT survey, we obtained deep narrow band images of 15 $zsim2$ quasars with GMOS on Gemini-South in an effort to measure Ly$alpha$ emission from circum- and inter-galactic gas on scales of hundreds of kpc from the central quasar. We do not detect bright giant Ly$alpha$ nebulae (SB~10$^{-17}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$ at distances >50 kpc) around any of our sources, although we routinely ($simeq47$%) detect smaller scale <50 kpc Ly$alpha$ emission at this SB level emerging from either the extended narrow emission line regions powered by the quasars or by star-formation in their host galaxies. We stack our 15 deep images to study the average extended Ly$alpha$ surface brightness profile around $zsim2$ quasars, carefully PSF-subtracting the unresolved emission component and paying close attention to sources of systematic error. Our analysis, which achieves an unprecedented depth, reveals a surface brightness of SB$_{rm Lyalpha}sim10^{-19}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$ at $sim200$ kpc, with a $2.3sigma$ detection of Ly$alpha$ emission at SB$_{rm Lyalpha}=(5.5pm3.1)times10^{-20}$ erg s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$ within an annulus spanning 50 kpc <R< 500 kpc from the quasars. Assuming this Ly$alpha$ emission is powered by fluorescence from highly ionized gas illuminated by the bright central quasar, we deduce an average volume density of $n_{rm H}=0.6times10^{-2}$ cm$^{-3}$ on these large scales. Our results are in broad agreement with the densities suggested by cosmological hydrodynamical simulations of massive ($Msimeq10^{12.5}M_odot$) quasar hosts, however they indicate that the typical quasars at these redshifts are surrounded by gas that is a factor of ~100 times less dense than the (~1 cm$^{-3}$) gas responsible for the giant bright Ly$alpha$ nebulae around quasars recently discovered by our group.
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