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The 0.3-30 keV Spectra of Powerful Starburst Galaxies: NuSTAR and Chandra Observations of NGC 3256 and NGC 3310

Lehmer, B. D. and Tyler, J. B. and Hornschemeier, A. E. and Wik, D. R. and Yukita, M. and Antoniou, V. and Boggs, S. and Christensen, F. E. and Craig, W. W. and Hailey, C. J. and Harrison, F. A. and Maccarone, T. J. and Ptak, A. and Stern, D. and Zezas, A. and Zhang, W. W. (2015) The 0.3-30 keV Spectra of Powerful Starburst Galaxies: NuSTAR and Chandra Observations of NGC 3256 and NGC 3310. Astrophysical Journal, 806 (1). Art. No. 126. ISSN 0004-637X. http://resolver.caltech.edu/CaltechAUTHORS:20150722-123446121

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Abstract

We present nearly simultaneous Chandra and NuSTAR observations of two actively star-forming galaxies within 50 Mpc: NGC 3256 and NGC 3310. Both galaxies are significantly detected by both Chandra and NuSTAR, which together provide the first-ever spectra of these two galaxies spanning 0.3–30 keV. The X-ray emission from both galaxies is spatially resolved by Chandra; we find that hot gas dominates the E < 1–3 keV emission while ultraluminous X-ray sources (ULXs) provide majority contributions to the emission at E > 1–3 keV. The NuSTAR galaxy-wide spectra of both galaxies follow steep power-law distributions with Γ ≈ 2.6 at E > 5–7 keV. Using new and archival Chandra data, we search for signatures of heavily obscured or low luminosity active galactic nuclei (AGNs). We find that both NGC 3256 and NGC 3310 have X-ray detected sources coincident with nuclear regions; however, the steep NuSTAR spectra of both galaxies restricts these sources to be either low luminosity AGNs (L_(2−10 keV)/L_(Edd) 10^(−5)) or non-AGNs in nature (e.g., ULXs or crowded X-ray sources that reach L_(2−10 keV) ~ 10^(40) erg s^(−1) cannot be ruled out). Combining our constraints on the 0.3–30 keV spectra of NGC 3256 and NGC 3310 with equivalent measurements for nearby star-forming galaxies M83 and NGC 253, we analyze the star formation rate (SFR) normalized spectra of these starburst galaxies. The spectra of all four galaxies show sharply declining power-law slopes at energies above 3–6 keV primarily due to ULX populations. Our observations therefore constrain the average spectral shape of galaxy-wide populations of luminous accreting binaries (i.e., ULXs). Interestingly, despite a completely different galaxy sample selection, emphasizing here a range of SFRs and stellar masses, these properties are similar to those of super-Eddington accreting ULXs that have been studied individually in a targeted NuSTAR ULX program. We also find that NGC 3310 exhibits a factor of ≈3–10 elevation of X-ray emission over the other star-forming galaxies due to a corresponding overabundance of ULXs. We argue that the excess of ULXs in NGC 3310 is most likely explained by the relatively low metallicity of the young stellar population in this galaxy, a property that is expected to produce an excess of luminous X-ray binaries for a given SFR.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1088/0004-637X/806/1/126DOIArticle
http://iopscience.iop.org/0004-637X/806/1/126/PublisherArticle
http://arxiv.org/abs/1505.00789arXivDiscussion Paper
ORCID:
AuthorORCID
Boggs, S.0000-0001-9567-4224
Harrison, F. A.0000-0003-2992-8024
Ptak, A.0000-0001-5655-1440
Stern, D.0000-0003-2686-9241
Additional Information:© 2015 The American Astronomical Society. Received 2014 December 22; accepted 2015 May 3; published 2015 June 12. We thank the anonymous referee for helpful comments, which have improved the quality of this paper. We gratefully acknowledge financial support from Chandra X-ray Center grant GO4-15086 Z (B.D.L., J.B.T.) and NASA ADAP grant NNX13AI48G (B.D.L.). A.Z. acknowledges funding from the European Research Council under the European Unionʼs Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement n. 617001. This work was supported under NASA Contract No. NNG08FD60C, and made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (Italy) and the California Institute of Technology (USA). Facilities: Chandra, NuSTAR
Group:NuSTAR, Space Radiation Laboratory
Funders:
Funding AgencyGrant Number
Chandra X-ray CenterGO4-15086 Z
NASANNX13AI48G
European Research Council (ERC)617001
NASANNG08FD60C
Subject Keywords:galaxies: active; galaxies: individual (NGC 3256 and NGC 3310); galaxies: starburst; galaxies: star formation; X-rays: galaxies
Other Numbering System:
Other Numbering System NameOther Numbering System ID
Space Radiation Laboratory2015-57
Record Number:CaltechAUTHORS:20150722-123446121
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20150722-123446121
Official Citation:The 0.3-30 keV Spectra of Powerful Starburst Galaxies: NuSTAR and Chandra Observations of NGC 3256 and NGC 3310 B. D. Lehmer et al. 2015 ApJ 806 126
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:58984
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:22 Jul 2015 19:51
Last Modified:02 Oct 2017 23:58

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