TY - JOUR
T1 - High-mass X-ray binaries in nearby metal-poor galaxies
T2 - On the contribution to nebular He II emission
AU - Senchyna, Peter
AU - Stark, Daniel P.
AU - Mirocha, Jordan
AU - Reines, Amy E.
AU - Charlot, Stéphane
AU - Jones, Tucker
AU - Mulchaey, John S.
N1 - Funding Information:
The authors would like to thank Kevin Hainline and Juna Kollmeier for helpful conversations and comments. This research has made use of data obtained from the Chandra Data Archive and the Chandra Source Catalog, and software provided by the Chandra X-ray Center (CXC) in the application package CIAO. Some of the observations reported here were obtained at the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution. Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.
Funding Information:
The authors would like to thank Kevin Hainline and Juna Kollmeier for helpful conversations and comments. This research has made use of data obtained from the Chandra Data Archive and the Chandra Source Catalog, and software provided by the Chandra X-ray Center (CXC) in the application package CIAO. Some of the observations reported here were obtained at the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution. Some of the data presented herein were obtained at the W.M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. DPS acknowledges support from the National Science Foundation through the grant AST-1410155. JM acknowledges support from a CITA National Fellowship. An allocation of computer time from the UA Research Computing High Performance Computing (HPC) at the University of Arizona is gratefully acknowledged. This research made use of ASTROPY, a community-developed core PYTHON package for Astronomy (Astropy Collaboration et al. 2013); MATPLOTLIB (Hunter 2007); NUMPY and SCIPY (Jones et al. 2001); the SIMBAD database, operated at CDS, Strasbourg, France; and NASA's Astrophysics Data System.
Funding Information:
DPS acknowledges support from the National Science Foundation through the grant AST-1410155. JM acknowledges support from a CITA National Fellowship.
Publisher Copyright:
© 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society
PY - 2020/5/1
Y1 - 2020/5/1
N2 - Despite significant progress both observationally and theoretically, the origin of high-ionization nebular He II emission in galaxies dominated by stellar photoionization remains unclear. Accretion-powered radiation from high-mass X-ray binaries (HMXBs) is still one of the leading proposed explanations for the missing He+-ionizing photons, but this scenario has yet to be conclusively tested. In this paper, we present nebular line predictions from a grid of photoionization models with input spectral energy distributions containing the joint contribution of both stellar atmospheres and a multicolour disc model for HMXBs. This grid demonstrates that HMXBs are inefficient producers of the photons necessary to power He II, and can only boost this line substantially in galaxies with HMXB populations large enough to power X-ray luminosities of 1042 erg s−1 per unit star formation rate (SFR). To test this, we assemble a sample of 11 low-redshift star-forming galaxies with high-quality constraints on both X-ray emission from Chandra and He II emission from deep optical spectra, including new observations with the MMT. These data reveal that the HMXB populations of these nearby systems are insufficient to account for the observed He II strengths, with typical X-ray luminosities or upper limits thereon of only 1040-1041 erg s−1 per SFR. This indicates that HMXBs are not the dominant source of He+ ionization in these metal-poor star-forming galaxies. We suggest that the solution may instead reside in revisions to stellar wind predictions, softer X-ray sources, or very hot products of binary evolution at low metallicity.
AB - Despite significant progress both observationally and theoretically, the origin of high-ionization nebular He II emission in galaxies dominated by stellar photoionization remains unclear. Accretion-powered radiation from high-mass X-ray binaries (HMXBs) is still one of the leading proposed explanations for the missing He+-ionizing photons, but this scenario has yet to be conclusively tested. In this paper, we present nebular line predictions from a grid of photoionization models with input spectral energy distributions containing the joint contribution of both stellar atmospheres and a multicolour disc model for HMXBs. This grid demonstrates that HMXBs are inefficient producers of the photons necessary to power He II, and can only boost this line substantially in galaxies with HMXB populations large enough to power X-ray luminosities of 1042 erg s−1 per unit star formation rate (SFR). To test this, we assemble a sample of 11 low-redshift star-forming galaxies with high-quality constraints on both X-ray emission from Chandra and He II emission from deep optical spectra, including new observations with the MMT. These data reveal that the HMXB populations of these nearby systems are insufficient to account for the observed He II strengths, with typical X-ray luminosities or upper limits thereon of only 1040-1041 erg s−1 per SFR. This indicates that HMXBs are not the dominant source of He+ ionization in these metal-poor star-forming galaxies. We suggest that the solution may instead reside in revisions to stellar wind predictions, softer X-ray sources, or very hot products of binary evolution at low metallicity.
KW - Galaxies: stellar content
KW - X-rays: binaries
KW - X-rays: galaxies
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U2 - 10.1093/mnras/staa586
DO - 10.1093/mnras/staa586
M3 - Article
AN - SCOPUS:85083506317
VL - 494
SP - 941
EP - 957
JO - Monthly Notices of the Royal Astronomical Society
JF - Monthly Notices of the Royal Astronomical Society
SN - 0035-8711
IS - 1
ER -