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Register a new userSimulating the Gaseous Halos of Galaxies
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Tobias Kaufmann
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Observations of local X-ray absorbers, high-velocity clouds, and distant quasar absorption line systems suggest that a significant fraction of baryons may reside in multi-phase, low-density, extended, ~100 kpc, gaseous halos around normal galaxies. We present a pair of high-resolution SPH (smoothed particle hydrodynamics) simulations that explore the nature of cool gas infall into galaxies, and the physical conditions necessary to support the type of gaseous halos that seem to be required by observations. The two simulations are identical other than their initial gas density distributions: one is initialized with a standard hot gas halo that traces the cuspy profile of the dark matter, and the other is initialized with a cored hot halo with a high central entropy, as might be expected in models with early pre-heating feedback. Galaxy formation proceeds in dramatically different fashions in these two cases. While the standard cuspy halo cools rapidly, primarily from the central region, the cored halo is quasi-stable for ~4 Gyr and eventually cools via the fragmentation and infall of clouds from ~100 kpc distances. After 10 Gyr of cooling, the standard halos X-ray luminosity is ~100 times current limits and the resultant disk galaxy is twice as massive as the Milky Way. In contrast, the cored halo has an X-ray luminosity that is in line with observations, an extended cloud population reminiscent of the high-velocity cloud population of the Milky Way, and a disk galaxy with half the mass and ~50% more specific angular momentum than the disk formed in the low-entropy simulation. These results suggest that the distribution and character of halo gas provides an important testing ground for galaxy formation models and may be used to constrain the physics of galaxy formation.
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Gaseous halos play a key role for understanding inflow, feedback and the overall baryon budget in galaxies. Literature models predict transitions of the state of the gaseous halo between cold and hot accretion, winds, fountains and hydrostatic halos at certain galaxy masses. Since luminosities of radio AGN are sensitive to halo densities, any significant transition would be expected to show up in the radio luminosities of large samples of galaxies. The Low Frequency Array (LOFAR) Two Metre Sky Survey (LoTSS) has indeed identified a galaxy stellar mass scale, $10^{11} M_odot$ , above which the radio luminosities increase disproportionately. Here, we investigate, if radio luminosities of galaxies, especially the marked rise at galaxy masses around $10^{11} M_odot$, can be explained with standard assumptions on jet powers, scaling between black hole-mass and galaxy mass and gaseous halos. We developed models for the radio luminosity of radio AGN in halos under infall, galactic wind and hydrostatic conditions based on observational data and theoretical constraints, and compared it to LoTSS data for a large sample of galaxies in the mass rangebetween $10^{8.5} M_odot$ and $10^{12} M_odot$. Assuming the same characteristic upper limit to jet powers as is known from high galaxy masses to hold at all masses, we find that the maximum radio luminosities for the hydrostatic gas halos fit well with the upper envelope of the distribution of the LOFAR data. The marked rise in radio luminosity at $10^{11} M_odot$ is matched in our model, and is related to significant change in halo gas density around this galaxy mass, which is a consequence of the lower cooling rates at higher virial temperature. Wind and infall models overpredict the radio luminosities at small galaxy masses and have no particular steepening of the run of the radio luminosities predicted at any galaxy mass. [...]
This study presents first results from an X-ray mini-survey carried out with XMM-Newton to investigate the diffuse Hot Ionized Medium in the halos of nine nearby star-forming edge-on spiral galaxies. Diffuse gaseous X-ray halos are detected in eight of our targets, covering a wide range of star formation rates from quiescent to starburst cases. For four edge-on spiral galaxies, namely NGC3044, NGC3221, NGC4634, and NGC5775, we present the first published high resolution/sensitivity detections of extended soft X-ray halos. EPIC X-ray contour maps overlaid onto Halpha imaging data reveals that in all cases the presence of X-ray halos is correlated with extraplanar Diffuse Ionized Gas. Moreover, these halos are also associated with non-thermal cosmic ray halos, as evidenced by radio continuum observations. Supplemental UV-data obtained with the OM-telescope at 210nm show Diffuse Ionized Gas to be well associated with UV emission originating in the underlying disk. Beside NGC891, NGC4634 is the second non-starburst galaxy with a diffuse soft X-ray halo (|z|<4kpc). In case of NGC3877, for which we also present the first high resolution X-ray imaging data, no halo emission is detectable. EPIC pn spectra (0.3-12keV) of the diffuse X-ray emission are extracted at different offset positions from the disk, giving evidence to a significant decrease of gas temperatures, electron densities, and gas masses with increasing distance to the plane. A comparison between dynamical and radiative cooling time scales implies that the outflow in all targets is likely to be sustained. We find very strong indications that spatially correlated multi-phase gaseous halos are created by star forming activity in the disk plane.
A cosmological zoom-in simulation which develops into a Milky Way-like halo is started at redshift 7. The initial dark matter distribution is seeded with dense star clusters, median mass $5times 10^5 M_sun$, placed in the largest sub-halos present, which have a median peak circular velocity of 25 kms. Three simulations are initialized using the same dark matter distribution, with the star clusters started on approximately circular orbits having initial median radii 6.8 kpc, 0.14 kpc, and, at the exact center of the sub-halos. The simulations are evolved to the current epoch at which time the median galactic orbital radii of the three sets of clusters are 30, 5 and 16 kpc, with the clusters losing about 2, 50 and 15% of their mass, respectively. Clusters started at small orbital radii have so much tidal forcing that they are often not in equilibrium. Clusters started at larger sub-halo radii have a velocity dispersion that declines smoothly to $simeq$20% of the central value at $simeq$20 half mass radii. The clusters started at the sub-halo centers can show a rise in velocity dispersion beyond 3-5 half mass radii. That is, the clusters formed without local dark matter always have stellar mass dominated kinematics at all radii, whereas about 25% of the clusters started at sub-halo centers have remnant local dark matter.
Recent progress is summarized on the determination of the density distributions of stars and dark matter, stellar kinematics, and stellar population properties, in the extended, low surface brightness halo regions of elliptical galaxies. With integral field absorption spectroscopy and with planetary nebulae as tracers, velocity dispersion and rotation profiles have been followed to ~4 and ~5-8 effective radii, respectively, and in M87 to the outer edge at ~150 kpc. The results are generally consistent with the known dichotomy of elliptical galaxy types, but some galaxies show more complex rotation profiles in their halos and there is a higher incidence of misalignments, indicating triaxiality. Dynamical models have shown a range of slopes for the total mass profiles, and that the inner dark matter densities in ellipticals are higher than in spiral galaxies, indicating earlier assembly redshifts. Analysis of the hot X-ray emitting gas in X-ray bright ellipticals and comparison with dynamical mass determinations indicates that non-thermal components to the pressure may be important in the inner ~10 kpc, and that the properties of these systems are closely related to their group environments. First results on the outer halo stellar population properties do not yet give a clear picture. In the halo of one bright galaxy, lower [alpha/Fe] abundances indicate longer star formation histories pointing towards late accretion of the halo. This is consistent with independent evidence for on-going accretion, and suggests a connection to the observed size evolution of elliptical galaxies with redshift.
How the Milky Way has accumulated its mass over the Hubble time, whether significant amounts of gas and stars were accreted from satellite galaxies, or whether the Milky Way has experienced an initial gas assembly and then evolved more-or-less in isolation is one of the burning questions in modern astronomy, because it has consequences for our understanding of galaxy formation in the cosmological context. Here we present the evolutionary model of a Milky Way-type satellite system zoomed into a cosmological large-scale simulation. Embedded into Dark Matter halos and allowing for baryonic processes these chemo-dynamical simulations aim at studying the gas and stellar loss from the satellites to feed the Milky Way halo and the stellar chemical abundances in the halo and the satellite galaxies.
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