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Register a new userA high spatial resolution X-ray and H-alpha study of hot gas in the halos of star-forming disk galaxies. I. Spatial and spectral properties of the diffuse X-ray emission
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We present arcsecond resolution Chandra X-ray and ground-based optical H-alpha imaging of a sample of ten edge-on star-forming disk galaxies (seven starburst and three ``normal spiral galaxies), a sample which covers the full range of star-formation intensity found in disk galaxies. We use the unprecedented spatial resolution of the Chandra X-ray observatory to robustly remove point sources, and hence obtain the X-ray properties of the diffuse thermal emission alone. The X-ray observations are combined with comparable-resolution H-alpha and R-band imaging, and presented as a mini-atlas of images on a common spatial and surface brightness scale. The vertical distribution of the halo-region X-ray surface brightness is best described as an exponential, with the observed scale heights lying in the range H_eff = 2 -- 4 kpc. The ACIS X-ray spectra of extra-planar emission from all these galaxies can be fit with a common two-temperature spectral model with an enhanced alpha-to-iron element ratio. This is consistent with the origin of the X-ray emitting gas being either metal-enriched merged SN ejecta or shock-heated ambient halo or disk material with moderate levels of metal depletion onto dust. The thermal X-ray emission observed in the halos of the starburst galaxies is either this pre-existing halo medium, which has been swept-up and shock heated by the starburst-driven wind, or wind material compressed near the walls of the outflow by reverse shocks within the wind. In either case the X-ray emission provides us with a powerful probe of the properties of gaseous halos around star-forming disk galaxies.
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We investigate how the empirical properties of hot X-ray-emitting gas in a sample of seven starburst and three normal edge-on spiral galaxies (a sample which covers the full range of star-formation intensity found in disk galaxies) correlate with the size, mass, star formation rate and star formation intensity in the host galaxies. Intriguingly, the diffuse X-ray properties of the normal spirals (both in their disks and halos) fall where extrapolation of the trends from the starburst galaxies with superwinds would predict. We demonstrate that the luminosity of diffuse X-ray emission in both disk and halo is directly proportional to the rate of mechanical energy feedback from massive stars. Nevertheless, with only three non-starburst normal spiral galaxies it is hard to exclude an accretion-based origin for extra-planar diffuse X-ray emission around normal star-forming galaxies. Larger galaxies have more extended X-ray-emitting halos, but galaxy mass appears to play no role in determining the properties of the disk or extra-planar X-ray emitting plasma. The combination of these luminosity and size correlations leads to a correlation between the surface brightness of the diffuse X-ray emission and the mean star formation rate per unit area in the disk (L_FIR/D_25^2). We argue that the crucial spatial region around a galaxy that controls whether gas in starburst-driven superwinds will escape into the IGM is not the outer halo ~100 kpc from the host galaxy, but the inner few halo scale heights, within ~20 kpc of the galaxy plane. Given the properties of the gaseous halos we observe, superwind outflows from disk galaxies of mass M ~ 10^10 -- 10^11 Msun should still eject some fraction of their material into the IGM. (abstract abridged)
We present an analysis of the properties of the hot interstellar medium (ISM) in the merging pair of galaxies known as The Antennae (NGC 4038/39), performed using the deep, coadded ~411 ks Chandra ACIS-S data set. These deep X-ray observations and Chandras high angular resolution allow us to investigate the properties of the hot ISM with unprecedented spatial and spectral resolution. Through a spatially resolved spectral analysis, we find a variety of temperatures (from 0.2 to 0.7 keV) and Nh (from Galactic to 2x10^21 cm^-2). Metal abundances for Ne, Mg, Si, and Fe vary dramatically throughout the ISM from sub-solar values (~0.2) up to several times solar.
We study the diffuse X-ray luminosity ($L_X$) of star forming galaxies using 2-D axisymmetric hydrodynamical simulations and analytical considerations of supernovae (SNe) driven galactic outflows. We find that the mass loading of the outflows, a crucial parameter for determining the X-ray luminosity, is constrained by the availability of gas in the central star forming region, and a competition between cooling and expansion. We show that the allowed range of the mass loading factor can explain the observed scaling of $L_X$ with star formation rate (SFR) as $L_X propto$ SFR$^2$ for SFR $gtrsim 1$ M$_odot$yr$^{-1}$, and a flatter relation at low SFRs. We also show that the emission from the hot circumgalactic medium (CGM) in the halo of massive galaxies can explain the sub-linear behaviour of the $L_X-$SFR relation as well as a large scatter in the diffuse X-ray emission for low SFRs ($lesssim$ few M$_odot$yr$^{-1}$). Our results point out that galaxies with small SFRs and large diffuse X-ray luminosities are excellent candidates for detection of the elusive CGM.
Based on a homogeneous set of X-ray, infrared and ultraviolet observations from Chandra, Spitzer, GALEX and 2MASS archives, we study populations of high-mass X-ray binaries (HMXBs) in a sample of 29 nearby star-forming galaxies and their relation with the star formation rate (SFR). In agreement with previous results, we find that HMXBs are a good tracer of the recent star formation activity in the host galaxy and their collective luminosity and number scale with the SFR, in particular, Lx~2.6 10^{39} SFR. However, the scaling relations still bear a rather large dispersion of ~0.4 dex, which we believe is of a physical origin. We present the catalog of 1057 X-ray sources detected within the $D25$ ellipse for galaxies of our sample and construct the average X-ray luminosity function (XLF) of HMXBs with substantially improved statistical accuracy and better control of systematic effects than achieved in previous studies. The XLF follows a power law with slope of 1.6 in the logLx~35-40 luminosity range with a moderately significant evidence for a break or cut-off at Lx~10^{40} erg/s. As before, we did not find any features at the Eddington limit for a neutron star or a stellar mass black hole. We discuss implications of our results for the theory of binary evolution. In particular we estimate the fraction of compact objects that once upon their lifetime experienced an X-ray active phase powered by accretion from a high mass companion and obtain a rather large number, fx~0.2 (0.1 Myr/tau_x) (tau_x is the life time of the X-ray active phase). This is ~4 orders of magnitude more frequent than in LMXBs. We also derive constrains on the mass distribution of the secondary star in HMXBs.
The luminous unstable star (star system) eta Carinae is surrounded by an optically bright bipolar nebula, the Homunculus and a fainter but much larger nebula, the so-called outer ejecta. As images from the EINSTEIN and ROSAT satellites have shown, the outer ejecta is also visible in soft X-rays, while the central source is present in the harder X-ray bands. With our CHANDRA observations we show that the morphology and properties of the X-ray nebula are the result of shocks from fast clumps in the outer ejecta moving into a pre-existing denser circumstellar medium. An additional contribution to the soft X-ray flux results from mutual interactions of clumps within the ejecta. Spectra extracted from the CHANDRA data yield gas temperatures kT of 0.6-0.76 keV. The implied pre-shock velocities of 670-760 km/s are within the scatter of the velocities we measure for the majority of the clumps in the corresponding regions. Significant nitrogen enhancements over solar abundances are needed for acceptable fits in all parts of the outer ejecta, consistent with CNO processed material and non-uniform enhancement. The presence of a diffuse spot of hard X-ray emission at the S condensation shows some contribution of the highest velocity clumps and further underlines the multicomponent, non-equilibrium nature of the X-ray nebula. The detection of an X-ray ``bridge between the northern and southern part of the X-ray nebula and an X-ray shadow at the position of the NN bow can be attributed to a large expanding disk, which would appear as an extension of the equatorial disk. No soft emission is seen from the Homunculus, or from the NN bow or the ``strings.
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