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Register a new userChandra Observations of ULIRGs: Extended Hot Gas Halos in Merging Galaxies
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Z.Y. Huo
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We study the properties of hot gaseous halos in 10 nearby ultraluminous IRAS galaxies observed with the ACIS instrument on board Chandra. For all sample galaxies, diffuse soft X-ray emissions are found within ~10 kpc of the central region; their spectra are well fitted by a MEKAL model plus emission lines from alpha-elements and other ions. The temperature of the hot gas is about 0.7 keV and metallicity is about 1 solar. Outside the central region, extended hot gaseous halos are found for nine out of the ten ULIRGs. Most spectra of these extended halos can be fitted with a MEKAL model with a temperature of about 0.6 keV and a low metallicity (~ 0.1 solar). We discuss the implications of our results on the origin of X-ray halos in elliptical galaxies and the feedback processes associated with starbursts.
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Recently, the temperature T and luminosity L_X of the hot gas halos of early type galaxies have been derived with unprecedented accuracy from Chandra data, for 30 galaxies covering a wider range of galactic luminosity (and central velocity dispersion sigma_c) than before. This work investigates the origin of the observed temperatures, by examining the relationship between them and the galaxy structure, the gas heating due to Type Ia supernovae (SNIas) and the gravitational potential, and the dynamical status of the gas flow. In galaxies with sigma_c<200 km/s, the Ts are close to a fiducial average temperature for the gas when in outflow; at 200<sigma_c (km/s)<250, the Ts are generally lower than this, and unrelated with sigma_c, which requires a more complex gas flow status; at larger sigma_c, the Ts may increase as sigma_c^2, as expected for infall heating, though heating from SNIas, independent of sigma_c, should be dominant. All observed Ts are larger than the virial temperature, by up to ~0.5 keV. This additional heating can be provided in the X-ray brightest galaxies by SNIas and infall heating, with a SNIas energy input even lower than in standard assumptions; in the X-ray fainter ones it can be provided by SNIas, whose energy input would be required close to the full standard value at the largest sigma_c. This same energy input, though, would produce temperatures larger than observed at low sigma_c, if entirely thermalized. The values of the observed Ts increase from outflows to inflows; the gas is relatively hotter in outflows, though, if the Ts are rescaled by the virial temperature. For 200<sigma_c(km/s)<250, lower L_X values tend to correspond to lower Ts, which deserves further investigation.
Recent X-ray observations of galaxy clusters have shown that there is substructure present in the intracluster medium (ICM), even in clusters that are seemingly relaxed. This substructure is sometimes a result of sloshing of the ICM, which occurs in cool core clusters that have been disturbed by an off-axis merger with a sub-cluster or group. We present deep Chandra observations of the cool core cluster Abell 2029, which has a sloshing spiral extending radially outward from the center of the cluster to approximately 400 kpc at its fullest extent---the largest continuous spiral observed to date. We find a surface brightness excess, a temperature decrement, a density enhancement, an elemental abundance enhancement, and a smooth pressure profile in the area of the spiral. The sloshing gas seems to be interacting with the southern lobe of the central radio galaxy, causing it to bend and giving the radio source a wide-angle tail (WAT) morphology. This shows that WATs can be produced in clusters that are relatively relaxed on large scales. We explore the interaction between heating and cooling in the central region of the cluster. Energy injection from the active galactic nucleus (AGN) is likely insufficient to offset the cooling, and sloshing may be an important additional mechanism in preventing large amounts of gas from cooling to very low temperatures.
Ongoing accretion onto galactic disks has been recently theorized to progress via the unstable cooling of the baryonic halo into condensed clouds. These clouds have been identified as analogous to the High-Velocity Clouds (HVCs) observed in HI in our Galaxy. Here we compare the distribution of HVCs observed around our own Galaxy and extra-planar gas around the Andromeda galaxy to these possible HVC analogs in a simulation of galaxy formation that naturally generates these condensed clouds. We find a very good correspondence between these observations and the simulation, in terms of number, angular size, velocity distribution, overall flux and flux distribution of the clouds. We show that condensed cloud accretion only accounts for ~ 0.2 M_solar / year of the current overall Galactic accretion in the simulations. We also find that the simulated halo clouds accelerate and become more massive as they fall toward the disk. The parameter space of the simulated clouds is consistent with all of the observed HVC complexes that have distance constraints, except the Magellanic Stream which is known to have a different origin. We also find that nearly half of these simulated halo clouds would be indistinguishable from lower-velocity gas and that this effect is strongest further from the disk of the galaxy, thus indicating a possible missing population of HVCs. These results indicate that the majority of HVCs are consistent with being infalling, condensed clouds that are a remnant of Galaxy formation.
Most of the baryons in L* galaxies are unaccounted for and are predicted to lie in hot gaseous halos (T ~ 3E6 K) that may extend beyond R200. A hot gaseous halo will produce a thermal Sunyaev-Zeldovich signal that is proportional to the product of the gas mass and the mass-weighted temperature. To best detect this signal, we used a Needlet Independent Linear Combination all-sky Planck map that we produced from the most recent Planck data release, also incorporating WMAP data. The sample is 12 L* spiral galaxies with distances of 3-10 Mpc, which are spatially resolved so that contamination from the optical galaxy can be excluded. One galaxy, NGC 891, has a particularly strong SZ signal, and when excluding it, the stack of 11 galaxies is detected at about 4sigma (declining with radius) and is extended to at least 250 kpc (~R_{200}) at > 99% confidence. The gas mass within a spherical volume to a radius of 250 kpc is 9.8 +/- 2.8 E10 Msun, for Tavg = 3E6 K. This is about 30% of the cosmic baryon content of the average galaxy (3.1E11 Msun), and about equal to the mass of stars, disk gas, and warm halo gas. The remaining missing baryons (~ 1.4E11 Msun, 40-50% of the total baryon content) are likely to be hot and extend to the 400-500 kpc volume, if not beyond. The result is higher than predictions, but within the uncertainties.
Models of disk galaxy formation commonly predict the existence of an extended reservoir of hot gas surrounding massive spirals at low redshift. As a test of these models, we have obtained X-ray and optical data of the two massive edge-on spirals NGC 5746 and NGC 5170, in order to investigate the amount and origin of hot gas in their disks and halos. Chandra observations of NGC 5746 reveal evidence for diffuse X-ray emission with a total luminosity of ~7 x 10^39 erg/s surrounding this galaxy out to at least ~20 kpc from the disk, whereas an identical study of the less massive NGC 5170 fails to detect any extraplanar X-ray emission. Unlike the case for other disk galaxies with detected X-ray halos, the halo emission around NGC 5746 is not accompanied by extraplanar H-alpha or radio emission, and there is no evidence for significant nuclear or starburst activity in the disk. In contrast to these other cases, the emission around NGC 5746 therefore appears to arise from the cooling of externally accreted material rather than from disk outflows. To verify this idea, we present results of cosmological simulations of galaxy formation and evolution, showing our observations to be in good agreement with expectations for cosmological accretion, while also confirming that the X-ray halos of other spirals do not fit well into an accretion scenario. We find that the estimated cooling rate of hot halo gas around NGC 5746 would provide sufficient material for star formation in the disk to proceed at its present rate. This lends support to the idea that a supply of hot ambient gas is potentially available as fuel for star formation in massive, nearby spirals, and suggests that accretion of hot gas could be important for maintaining the stellar disks of such galaxies.
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