A High Spatial Resolution X‐Ray and Hα Study of Hot Gas in the Halos of Star‐forming Disk Galaxies. I. Spatial and Spectral Properties of the Diffuse X‐Ray Emission (original) (raw)
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Bulletin of the American Astronomical Society, 2002
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. From this analysis we investigate various aspects of mechanical energy "feedback"-the return of energy to the ISM from massive star supernovae and stellar winds-on galactic scales. The X-ray observations make use of the unprecedented spatial resolution of the Chandra X-ray observatory to remove X-ray emission from point sources more accurately than in any previous study, and hence obtain the X-ray properties of the diffuse thermal emission alone. 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, using a variety of multi-wavelength star formation rate and intensity indicators, that the luminosity of diffuse X-ray emission in the disk (and where detected, in the halo) is directly proportional to the rate of mechanical energy feedback from massive stars in the host galaxies. Accretion of gas from the IGM does not appear to be a significant contributor to the diffuse X-ray emission in this sample. Nevertheless, with only three nonstarburst 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 1 Chandra Fellow.
Diffuse X-ray emission from star forming galaxies
2016
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 ∝ SFR^2 for SFR ≳ 1 M_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 (≲ few M_yr^-1). Our results point out that galaxies with small SFRs and large diffuse X-ray luminosities are excellent candidates for...
Extraplanar X-ray emission from disc-wide outflows in spiral galaxies
Monthly Notices of the Royal Astronomical Society
We study the effects of mass and energy injection due to OB associations spread across the rotating disc of a Milky Way-type galaxy, with the help of three-dimensional (3D) hydrodynamic simulations. We compare the resulting X-ray emission with that produced from the injection of mass and energy from a central region. We find that the predicted X-ray image shows a filamentary structure that arises even in the absence of disc gas inhomogeneity. This structure stems from warm clumps made of disc material being lifted by the injected gas. We show that as much as half of the total X-ray emission comes from regions surrounding warm clumps that are made of a mix of disc and injected gas. This scenario has the potential to explain the origin of the observed extraplanar X-ray emission around star-forming galaxies and can be used to understand the observed sub-linear relation between the L X , the total X-ray luminosity, and star formation rate (SFR). We quantify the mass contained in these 'bowshock' regions. We also show that the topmost region of the outer shock above the central area emits harder X-rays than the rest. Further, we find that the mass distribution in different temperature ranges is bimodal, peaking at 10 4-10 5 K (in warm clumps) and 10 6-10 7 K (X-ray emitting gas). The mass-loading factor is found to decrease with increasing SFR, consistent with previous theoretical estimates and simulations.
A Mini-survey of X-ray Point Sources in Starburst and Non-Starburst Galaxies
2002
We present a comparison of X-ray point source luminosity functions of 3 starburst galaxies (the Antennae, M82, and NGC 253) and 4 non-starburst spiral galaxies (NGC 3184, NGC 1291, M83, and IC 5332). We find that the luminosity functions of the starbursts are flatter than those of the spiral galaxies; the starbursts have relatively more sources at high luminosities. This trend extends to early-type galaxies which have steeper luminosity functions than spirals. We show that the luminosity function slope is correlated with 60µm luminosity, a measure of star formation. We suggest that the difference in luminosity functions is related to the age of the X-ray binary populations and present a simple model which highlights how the shape of the luminosity distribution is affected by the age of the underlying X-ray binary population.
The Astrophysical Journal, 2002
We present a detailed case study of the diffuse X-ray and Hα emission in the halo of NGC 253, a nearby edge-on starburst galaxy driving a galactic superwind. The arcsecond spatial resolution of the ACIS imaging spectroscope on the Chandra X-ray Observatory allows us to study the spatial and spectral properties of the diffuse X-ray emitting plasma, at a height of between 3 and 9 kpc above the disk in the northern halo of NGC 253, with greatly superior spatial and spectral resolution compared to previous X-ray instruments. We find statistically significant structure within the diffuse emission on angular scales down to ∼ 10 ′′ (∼ 130 pc), and place limits on the luminosity of any X-ray-emitting "clouds" on smaller scales. There is no statistically significant evidence for any spatial variation in the spectral properties of the diffuse emission over scales from several ∼ 400 pc to ∼ 3 kpc. The spectrum of the diffuse X-ray emission is clearly thermal, although with the higher spectral resolution and sensitivity of Chandra it is clear that current simple spectral models do not provide a physically meaningful description of the spectrum. In particular, the fitted metal abundances are unphysically low. There is no convincing evidence for diffuse X-ray emission at energies above 2 keV in the halo. We show that the X-shaped soft X-ray morphology of the superwind previously revealed by ROSAT is matched by very similar X-shaped Hα emission, extending at least 8 kpc above the plane of the galaxy. In the northern halo the X-ray emission appears to lie slightly interior to the boundary marked by the Hα emission. The total 0.3-2.0 keV energy band X-ray luminosity of the northern halo L X ∼ 5 × 10 38 erg s −1 , is very similar to the halo Hα luminosity of L Hα ∼ 4 × 10 38 erg s −1 , both of which are a small fraction of the estimated wind energy injection rate of ∼ 10 42 erg s −1 from supernovae in the starburst. We show that there are a variety of models that can simultaneously explain spatially-correlated X-ray and Hα emission in the halos of starburst galaxies, although the physical origin of the various emission components can be very different in different models. These findings indicate that the physical origin of the X-ray-emitting milliondegree plasma in superwinds is closely linked to the presence of much cooler and denser T ∼ 10 4 gas, not only within the central kpc regions of starbursts, but also on ∼ 10 kpc-scales within the halos of these galaxies.
X-ray haloes and star formation in early-type galaxies
Monthly Notices of the Royal Astronomical Society
High-resolution 2D hydrodynamical simulations describing the evolution of the hot interstellar medium (ISM) in axisymmetric two-component models of early-type galaxies well reproduced the observed trends of the X-ray luminosity (L X) and temperature (T X) with galaxy shape and rotation, however they also revealed the formation of an exceedingly massive cooled gas disc in rotating systems. In a follow-up of this study, here we investigate the effects of star formation in the disc, including the consequent injection of mass, momentum and energy in the pre-existing ISM. It is found that subsequent generations of stars originate one after the other in the equatorial region; the mean age of the new stars is >5 Gyr, and the adopted recipe for star formation can reproduce the empirical Kennicutt-Schmidt relation. The results of the previous investigation without star formation, concerning L X and T X of the hot gas, and their trends with galactic shape and rotation, are confirmed. At the same time, the consumption of most of the cold gas disc into new stars leads to more realistic final systems, whose cold gas mass and star formation rate agree well with those observed in the local Universe. In particular, our models could explain the observation of kinematically aligned gas in massive, fast-rotating early-type galaxies.
The effects of stellar dynamics on the X-ray emission of flat early-type galaxies
Monthly Notices of the Royal Astronomical Society, 2014
Past observational and numerical studies indicated that the hot gaseous haloes of early-type galaxies may be sensitive to the stellar kinematics. With high resolution ZEUS 2D hydro simulations we study the hot gas evolution in flat early-type galaxies of fixed (stellar plus dark) mass distribution, but with variable amounts of azimuthal velocity dispersion and rotational support, including the possibility of a (counter)rotating inner disc. The hot gas is fed by stellar mass losses, and heated by supernova explosions and thermalization of stellar motions. The simulations provide γ th , the ratio between the heating due to the relative velocity between the stellar streaming and the ISM bulk flow, and the heating attainable by complete thermalization of the stellar streaming. We find that 1) X-ray emission weighted temperatures and luminosities match observed values, and are larger in fully velocity dispersion supported systems; X-ray isophotes are boxy where rotation is significant; 2) γ th 0.1 − 0.2 for isotropic rotators; 3) γ th 1 for systems with an inner (counter)rotating disc. The lower X-ray luminosities of isotropic rotators are not explained just by their low γ th , but by a complicated flow structure and evolution, consequence of the angular momentum stored at large radii. Rotation is therefore important to explain the lower average X-ray emission and temperature observed in flat and more rotationally supported galaxies.
Diffuse X‐Rays from the Inner 3 Parsecs of the Galaxy
The Astrophysical Journal, 2004
Recent observations with the Chandra X-ray Observatory have provided us with the capability to discriminate point sources, such as the supermassive black hole Sgr A*, from the diffuse emission within the inner 10 ′′ of the Galaxy. The hot plasma producing the diffuse X-radiation, estimated at ≈ 7.6 × 10 31 ergs s −1 arcsec −2 in the 2-10 keV band, has a RMS electron density ≈ 26 cm −3 and a temperature kT ≈ 1.3 keV, with a total inferred mass of ≈ 0.1 M ⊙ . At least some of this gas must be injected into the ISM via stellar winds. In the most recent census, about 25 bright, young stars have been identified as the dominant sources of the overall mass efflux from the Galactic center. In this paper, we use detailed 3-dimensional SPH simulations to study the wind-wind interactions occurring in the inner 3 parsecs of the Galaxy, with a goal of understanding what fraction, if any, of the diffuse X-ray flux measured by Chandra results from the ensuing shock heating of the ambient medium. We conclude that this process alone can account for the entire X-ray flux observed by Chandra in the inner 10 ′′ of the Galaxy. Understanding the X-ray morphology of the environment surrounding Sgr A* will ultimately provide us with a greater precision in modeling the accretion of gas onto this object, which appears to be relatively underluminous compared to its brethren in the nuclei of other galaxies.
New Astronomy, 2006
Hot gaseous haloes surrounding galaxies and extending well beyond the distribution of stars are a ubiquitous prediction of galaxy formation scenarios. The haloes are believed to consist of gravitationally trapped gas with a temperature of millions of Kelvin. The existence of such hot haloes around massive elliptical galaxies has been established through their X-ray emission. While gas out-flowing from starburst spiral galaxies has been detected, searches for hot haloes around normal, quiescent spiral galaxies have so far failed, casting doubts on the fundamental physics in galaxy formation models. Here we present the first detection of a hot, large-scale gaseous halo surrounding a normal, quiescent spiral galaxy, NGC 5746, alleviating a long-standing problem for galaxy formation models. In contrast to starburst galaxies, where the X-ray halo can be powered by the supernova energy, there is no such power source in NGC 5746. The only compelling explanation is that we are here witnessing a galaxy forming from gradually in-flowing hot and dilute halo gas.