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Register a new userThe Diffuse and Compact X-ray Components of the Starburst Galaxy Henize~2-10
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Henry A. (Chip) Kobulnicky
Publication date
2010
fields
Physics
and research's language is
English
Authors
Henry A. Kobulnicky
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Chandra X-ray imaging spectroscopy of the starburst galaxy Henize 2-10 reveals a strong nuclear point source and at least two fainter compact sources embedded within a more luminous diffuse thermal component. Spectral fits to the nuclear X-ray source imply an unabsorbed X-ray luminosity L_x >10^40 erg/s for reasonable power law or blackbody models, consistent with accretion onto a >50 solar mass black hole behind a foreground absorbing column of N_H>10^23 /cm^2. Two of these point sources have L_x=2-5 x 10^38 erg/s, comparable to luminous X-ray binaries. These compact sources constitute a small fraction (<16%) of the total X-ray flux from He~2-10 in the 0.3--6.0 keV band and just 31% of the X-rays in the hard 1.1--6.0 keV band which is dominated by diffuse emission. Two-temperature solar-composition plasmas (kT~0.2 keV and kT~0.7 keV) fit the diffuse X-ray component as well as single-temperature plasmas with enhanced alpha/Fe ratios. Since the observed radial gradient of the X-ray surface brightness closely follows that of the Halpha emission, the composition of the X-ray plasma likely reflects mixing of the ambient cool/warm ISM with an even hotter, low emission measure plasma, thereby explaining the ~solar ISM composition. Aperture synthesis 21-cm maps show an extended neutral medium to radii of 60 so that the warm and hot phases of the ISM, which extend to ~30, are enveloped within the 8x10^20 /cm^2 contour of the cool neutral medium. This extended neutral halo may serve to inhibit a starburst-driven outflow unless it is predominantly along the line of sight. The high areal density of star formation can also be reconciled with the lack of prominent outflow signatures if Henize 2-10 is in the very early stages of developing a galactic wind.
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We present follow-up X-ray observations of the candidate massive black hole (BH) in the nucleus of the low-mass, compact starburst galaxy Henize 2-10. Using new high-resolution observations from the Chandra X-ray Observatory totaling 200 ks in duration, as well as archival Chandra observations from 2001, we demonstrate the presence of a previously unidentified X-ray point source that is spatially coincident with the known nuclear radio source in Henize 2-10 (i.e., the massive BH). We show that the hard X-ray emission previously identified in the 2001 observation is dominated by a source that is distinct from the nucleus, with the properties expected for a high-mass X-ray binary. The X-ray luminosity of the nuclear source suggests the massive BH is radiating significantly below its Eddington limit (~10^-6 L_Edd), and the soft spectrum resembles other weakly accreting massive BHs including Sagittarius A*. Analysis of the X-ray light curve of the nucleus reveals the tentative detection of a ~9-hour periodicity, although additional observations are required to confirm this result. Our study highlights the need for sensitive high-resolution X-ray observations to probe low-level accretion, which is the dominant mode of BH activity throughout the Universe.
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