Chandra Observations of NGC 253. II. On the Origin of Diffuse X-Ray Emission in the Halos of Starburst Galaxies

By D. Strickland, T. Heckman, K. Weaver, C. Hoopes, M. Dahlem. 2001

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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 ~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, LX ~ 5 × 1038 ergs s-1, is very similar to the halo Hα luminosity of LHα ~ 4 × 1038 ergs s-1, both of which are a small fraction of the estimated wind energy injection rate of ~1042 ergs 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 million-degree plasma in superwinds is closely linked to the presence of much cooler and denser T ~ 104K gas, not only within the central kiloparsec regions of starbursts but also on ~10 kpc scales within the halos of these galaxies.

Subfields: Astronomy and Astrophysics, Instrumentation

View Paper: https://www.semanticscholar.org/paper/20ba1b5a31f236336eabf935adfdabb1f3e19481

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Citations in the corpus

Magnetic Fields in Starburst Galaxies and the Origin of the FIR-Radio Correlation (2006, background): “…much of the hard X-ray emission has been resolved into point sources withChandra, although IC emission may still contribute to the hard X-rays from the very central nucleus of the starburst (e.g., Lira et al. 2002; Strickland et al. 2002; D. K. Strickland & T. Heckman 2006, private communication).”
An origin for multiphase gas in galactic winds and haloes (2015, methodology): “In accord with this view, the soft extraplanar X-ray emission from wind galaxies is often interpreted as shocked interfaces between the cool clouds and the surrounding hot medium (e.g., Strickland et al. 2000, 2002).”
The Large-Scale Bipolar Wind in the Galactic Center (2002, methodology): “This has been investigated by sophisticated numerical simulations (Suchkov et al. 1994; Strickland & Stevens 2000; Strickland et al. 2002).”
Mass, metal, and energy feedback in cosmological simulations (2007, background): “Mass outflow rates are difficult to measure owing to the multiphase nature of galactic outflows (Strickland et al. 2002; Martin 2005b), but at least at high-z there are suggestions that the mass outflow rate is of the order of the star formation rate in Lyman break galaxies (Erb et al. 2006).”
Three-Dimensional Simulations of a Starburst-driven Galactic Wind (2008, background): “One of the most striking results of these observations is the close spatial relationship with the H -emitting gas (e.g., Strickland et al. 2000, 2002, 2004a; Cecil et al. 2002; Martin et al. 2002; Grimes et al. 2005; Ott et al. 2005), suggesting a close physical connection.”
A High Spatial Resolution X-Ray and Hα Study of Hot Gas in the Halos of Star-forming Disk Galaxies. II. Quantifying Supernova Feedback (2003, methodology): “Depending on the exact model assumed, the mass of gas in the halo of a Milky Way–like galaxy prior to any starburst ranges from a few times 107 to a few times 108M (see models 4 and 5 in Strickland et al. 2002).”
Mapping Large-Scale Gaseous Outflows in Ultraluminous Galaxies with Keck II ESI Spectra: Variations in Outflow Velocity with Galactic Mass (2004, background): “Material evaporated from these clouds could be the source of the mass-loading required to explain the large X-ray surface brightnesses of galactic winds (e.g. Suchkov et al. 1994; Suchkov et al. 1996; Strickland et al. 2002; Martin, Kobulnicky, & Heckman 2002).”
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 (2003, methodology): “The assumption that there is negligible diffuse hard X-ray emission in the halos of these galaxies is justified on both an empirical and theoretical basis (e.g., Dahlem et al. 1998; Strickland & Stevens 2000; Strickland et al. 2002).”
The Origin of the Mass-Metallicity Relation: Insights from 53,000 Star-forming Galaxies in the Sloan Digital Sky Survey (2004, background): “However, detailed studies of winds in nearby starbursts have shown them to be a complex, multiphase, hydrodynamical phenomenon with the majority of the energy and newly synthesized metals existing in the hard-to-observe coronal and hot phases (T = 105 − 107 K) (e.g. Strickland et al. 2002).”