Query : 2021A&A...656A.127S

2021A&A...656A.127S - Astronomy and Astrophysics, volume 656A, 127-127 (2021/12-1)

Can nebular He II emission be explained by ultra-luminous X-ray sources?


Abstract (from CDS):

Context. The shape of the ionising spectra of galaxies is a key ingredient to reveal their physical properties and for our understanding of the ionising background radiation. A long-standing unsolved problem is the presence of HeII nebular emission in many low-metallicity star-forming galaxies. This emission requires ionising photons with an energy > 54 eV; it is important to note that these particular photons are not produced in sufficient amounts by normal stellar populations.
Aims. To examine if high mass X-ray binaries and ultra-luminous X-ray sources (HMXBs and ULXs) can explain the observed HeII nebular emission and how their presence alters other emission lines, we computed photoionisation models of galaxies including such sources.
Methods. We combined spectral energy distributions (SEDs) of integrated stellar populations with constrained SEDs of ULXs to obtain composite spectra with varying amounts of X-ray luminosity, parameterised by LX/SFR. With these, we computed photoionisation models to predict the emission line fluxes of the optical recombination lines of H and He+, as well as the main metal lines of [OIII], [OII], [OI], and [NII]. The predictions were then compared to a large sample of low-metallicity galaxies.
Results. We find that it is possible to reproduce the nebular HeII λ4686 and other line observations with our spectra and with amounts of LX/SFR compatible with the observations. Our work suggests that HMBXs and ULXs could be responsible for the observed nebular HeII emission. However, the strengths of the high and low ionisation lines, such as HeII and [OI] λ6300, strongly depend on the X-ray contribution and on the assumed SEDs of the high energy source(s), the latter being poorly known.

Abstract Copyright: © ESO 2021

Journal keyword(s): galaxies: ISM - galaxies: high-redshift - X-rays: binaries - radiation mechanisms: general

Simbad objects: 5

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Number of rows : 5
N Identifier Otype ICRS (J2000)
ICRS (J2000)
Mag U Mag B Mag V Mag R Mag I Sp type #ref
1850 - 2023
1 UGC 4305 IG 08 19 04.9864630512 +70 43 13.086910452 12.06 11.72 11.39 11.11   ~ 743 2
2 [SST2011] J081929.00+704219.3 ULX 08 19 28.99 +70 42 19.4           ~ 180 2
3 Mrk 116 PaG 09 34 02.1 +55 14 25           ~ 1074 1
4 [FK2005] 25 ULX 14 03 19.63 -41 22 58.7           ~ 219 0
5 NAME NGC 6946 SNR SNR 20 35 00.74 +60 11 30.6           ~ 98 0

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