X-Ray Emission from Hot Gas in Galaxy Mergers (original) (raw)
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Generating Hot Gas in Simulations of Disk-Galaxy Major Mergers
Astrophysical Journal, 2004
We report on the merger-induced generation of a shock-heated gas wind and formation of a remnant gas halo in simulations of colliding disk galaxies. The simulations use cosmologically motivated initial conditions and include the effects of radiative cooling, star formation, stellar feedback and the non-adiabatic heating of gas. The non-adiabatic heating, i.e. shocks, generated in the final merger forces gas out of the central region of the merger remnant and into the dark-matter halo. We demonstrate that the amount of heating depends on the size of the progenitor disk galaxy as well as the initial orbit the galaxies are placed on. Based upon these dependencies, we motivate a possible recipe for including this effect in semi-analytic models of galaxy formation.
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.
2011
Disk galaxies at high redshift (z ∼ 2) are characterized by high fractions of cold gas, strong turbulence, and giant star-forming clumps. Major mergers of disk galaxies at high redshift should then generally involve such turbulent clumpy disks. Merger simulations, however, model the ISM as a stable, homogeneous, and thermally pressurized medium. We present the first merger simulations with high fractions of cold, turbulent, and clumpy gas. We discuss the major new features of these models compared to models where the gas is artificially stabilized and warmed. Gas turbulence, which is already strong in high-redshift disks, is further enhanced in mergers. Some phases are dispersion-dominated, with most of the gas kinetic energy in the form of velocity dispersion and very chaotic velocity fields, unlike merger models using a thermally stabilized gas. These mergers can reach very high star formation rates, and have multi-component gas spectra consistent with SubMillimeter Galaxies. Major mergers with high fractions of cold turbulent gas are also characterized by highly dissipative gas collapse to the center of mass, with the stellar component following in a global contraction. The final galaxies are early-type with relatively small radii and high Sersic indices, like high-redshift compact spheroids. The mass fraction in a disk component that survives or reforms after a merger is severely reduced compared to models with stabilized gas, and the formation of a massive disk component would require significant accretion of external baryons afterwards. Mergers thus appear to destroy extended disks even when the gas fraction is high, and this lends further support to smooth infall as the main formation mechanism for massive disk galaxies.
The Astrophysical Journal, 2014
We have obtained a deep, sub-arcsecond resolution X-ray image of the nuclear region of the luminous galaxy merger NGC 6240 with Chandra, which resolves the X-ray emission from the pair of active nuclei and the diffuse hot gas in great detail. We detect extended hard X-ray emission from kT ∼ 6 keV (∼70 million K) hot gas over a spatial scale of 5 kpc, indicating the presence of fast shocks with velocity of ∼2200 km s −1. For the first time we obtain the spatial distribution of this highly ionized gas emitting Fe XXV, which shows a remarkable correspondence to the large scale morphology of H 2 (1-0) S(1) line emission and Hα filaments. Propagation of fast shocks originated in the starburst driven wind into the ambient dense gas can account for this morphological correspondence. With an observed L 0.5−8keV = 5.3 × 10 41 erg s −1 , the diffuse hard X-ray emission is ∼100 times more luminous than that observed in the classic starburst galaxy
Disk galaxies at high redshift (z ∼ 2) are characterized by high fractions of cold gas, strong turbulence, and giant star-forming clumps. Major mergers of disk galaxies at high redshift should then generally involve such turbulent clumpy disks. Merger simulations, however, model the interstellar medium as a stable, homogeneous, and thermally pressurized medium. We present the first merger simulations with high fractions of cold, turbulent, and clumpy gas. We discuss the major new features of these models compared to models where the gas is artificially stabilized and warmed. Gas turbulence, which is already strong in high-redshift disks, is further enhanced in mergers. Some phases are dispersion dominated, with most of the gas kinetic energy in the form of velocity dispersion and very chaotic velocity fields, unlike merger models using a thermally stabilized gas. These mergers can reach very high star formation rates, and have multi-component gas spectra consistent with SubMillimeter Galaxies. Major mergers with high fractions of cold turbulent gas are also characterized by highly dissipative gas collapse to the center of mass, with the stellar component following in a global contraction. The final galaxies are early type with relatively small radii and high Sersic indices, like high-redshift compact spheroids. The mass fraction in a disk component that survives or re-forms after a merger is severely reduced compared to models with stabilized gas, and the formation of a massive disk component would require significant accretion of external baryons afterwards. Mergers thus appear to destroy extended disks even when the gas fraction is high, and this lends further support to smooth infall as the main formation mechanism for massive disk galaxies.
The Astrophysical Journal Supplement Series, 2004
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.
An X‐Ray and Optical Investigation of the Starburst‐driven Superwind in the Galaxy Merger Arp 299
The Astrophysical Journal, 1999
We present a detailed investigation of the X-ray and optical properties of the starburst-merger system Arp299 (NGC 3690, Mrk 171), with an emphasis on its spectacular gaseous nebula. We analyse ROSAT and ASCA X-ray data and optical spectra and narrow-band images. The X-ray nebula in Arp 299 has a diameter of ≈45 kpc, is elongated roughly along the HI minor axis of the merging system, has an outer (inner) temperature of 2.3 (9) million K, a luminosity of ∼ 2 × 10 41 erg s −1 , a gas mass of 7×10 9 M ⊙ , and a thermal energy content of ∼10 58 ergs. An additional hard X-ray component is present with a luminosity in the 0.1 to 10 keV band of ∼ 4 × 10 41 erg s −1 most likely due to X-ray binaries or possibly inverse Compton scattering. The overall X-ray spectrum is consistent with that of other starbursts (Dahlem, Weaver & Heckman). Compared to the X-ray nebula, the optical emission-line nebula is smaller (20 kpc) but much more luminous (∼ 10 44 erg s −1 ≈ 4% L bol for Arp 299). The kinematics of the emission-line nebula are complex, with the line-widths increasing systematically with decreasing Hα surface-brightness to values of 500 to 700 km s −1 in the faint outer filaments. The relative strengths of the low-ionization forbidden lines ([SII],[NII], and [OI]) also increase systematically as the gas surface-brightness decreases and as the line widths increase. We measure high gas pressures in the inner emission-line nebula (P/k ∼ few × 10 6 K cm −3) that decline systematically with increasing radius. We suggest that the ongoing galaxy collision has tidally-redistributed the ISM of the merging galaxies. The optical emission-line nebula results as this gas is photoionized by radiation that escapes from the starburst, and is shock-heated, accelerated, and pressurized by a 'superwind' driven by the collective effect of the starburst supernovae and stellar winds. Since empirically only those galaxy mergers that that contain luminous starbursts have bright X-ray nebulae, this implies that the luminous X-ray nebula in Arp 299 is also powered by the starburst outflow (rather than by the collisions of gas clouds during the merger). This outflow can be most directly traced by its X-ray emission, which is plausibly a mass-loaded flow (∼ 10 2 M ⊙ yr −1) of adiabatically-cooling gas that carries out a substantial fraction of the energy and metals injected by the starburst at a speed close to the escape velocity from Arp 299. The mass outflow rate exceeds the star-formation rate in this system. We conclude that powerful starbursts are able to heat (and possibly eject) a significant fraction of the interstellar medium in galaxies that are the products of mergers.
HOT GAS HALOS AROUND DISK GALAXIES: CONFRONTING COSMOLOGICAL SIMULATIONS WITH OBSERVATIONS
The Astrophysical Journal, 2009
Models of disk galaxy formation commonly predict the existence of an extended reservoir of accreted hot gas surrounding massive spirals at low redshift. As a test of these models, we use X-ray and Hα data of the two massive, quiescent edge-on spirals NGC 5746 and NGC 5170 to investigate the amount and origin of any hot gas in their halos. Contrary to our earlier claim, the Chandra analysis of NGC 5746, employing more recent calibration data, does not reveal any significant evidence for diffuse X-ray emission outside the optical disk, with a 3σ upper limit to the halo X-ray luminosity of 4×10 39 erg s −1 . An identical study of the less massive NGC 5170 also fails to detect any extraplanar Xray emission. By extracting hot halo properties of disk galaxies formed in cosmological hydrodynamical simulations, we compare these results to expectations for cosmological accretion of hot gas by spirals. For Milky Way-sized galaxies, these high-resolution simulations predict hot halo X-ray luminosities which are lower by a factor of ∼ 2 compared to our earlier results reported by . We find the new simulation predictions to be consistent with our observational constraints for both NGC 5746 and NGC 5170, while also confirming that the hot gas detected so far around more actively star-forming spirals is in general probably associated with stellar activity in the disk. Observational results on quiescent disk galaxies at the high-mass end are nevertheless providing powerful constraints on theoretical predictions, and hence on the assumed input physics in numerical studies of disk galaxy formation and evolution.
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 driving mechanism of starbursts in galaxy mergers
2010
We present hydrodynamic simulations of a major merger of disk galaxies, and study the ISM dynamics and star formation properties. High spatial and mass resolutions of 12 pc and 4 × 10 4 M ⊙ allow to resolve cold and turbulent gas clouds embedded in a warmer diffuse phase. We compare to lower resolution models, where the multiphase ISM is not resolved and is modeled as a relatively homogeneous and stable medium. While merger-driven bursts of star formation are generally attributed to large-scale gas inflows towards the nuclear regions, we show that once a realistic ISM is resolved, the dominant process is actually gas fragmentation into massive and dense clouds and rapid star formation therein. As a consequence, star formation is more efficient by a factor of up to ∼ 10 and is also somewhat more extended, while the gas density probability distribution function (PDF) rapidly evolves towards very high densities. We thus propose that the actual mechanism of starburst triggering in galaxy collisions can only be captured at high spatial resolution and when the cooling of gas is modeled down to less than 10 3 K. Not only does our model reproduce the properties of the Antennae system, but it also explains the "starburst mode" revealed recently in high-redshift mergers compared to quiescent disks.