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Register a new userOn the contribution of the X-ray source to the extended nebular HeII emission in IZw18
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Nebular HeII emission implies the presence of energetic photons (E$ge$54 eV). Despite the great deal of effort dedicated to understanding HeII ionization, its origin has remained mysterious, particularly in metal-deficient star-forming (SF) galaxies. Unfolding HeII-emitting, metal-poor starbursts at z ~ 0 can yield insight into the powerful ionization processes occurring in the primordial universe. Here we present a new study on the effects that X-ray sources have on the HeII ionization in the extremely metal-poor galaxy IZw18 (Z ~ 3 % Zsolar), whose X-ray emission is dominated by a single high-mass X-ray binary (HMXB). This study uses optical integral field spectroscopy, archival Hubble Space Telescope observations, and all of the X-ray data sets publicly available for IZw18. We investigate the time-variability of the IZw18 HMXB for the first time; its emission shows small variations on timescales from days to decades. The best-fit models for the HMXB X-ray spectra cannot reproduce the observed HeII ionization budget of IZw18, nor can recent photoionization models that combine the spectra of both very low metallicity massive stars and the emission from HMXB. We also find that the IZw18 HMXB and the HeII-emission peak are spatially displaced at a projected distance of $simeq$ 200 pc. These results reduce the relevance of X-ray photons as the dominant HeII ionizing mode in IZw18, which leaves uncertain what process is responsible for the bulk of its HeII ionization. This is in line with recent work discarding X-ray binaries as the main source responsible for HeII ionization in SF galaxies.
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The origin of nebular HeII emission, which is frequently observed in low-metallicity (O/H) star-forming galaxies, remains largely an unsolved question. Using the observed anticorrelation of the integrated X-ray luminosity per unit of star formation rate ($L_X/{rm SFR}$) of an X-ray binary population with metallicity and other empirical data from the well-studied galaxy I Zw 18, we show that the observed HeII 4686 intensity and its trend with metallicity is naturally reproduced if the bulk of He$^+$ ionizing photons are emitted by the X-ray sources. We also show that a combination of X-ray binary population models with normal single and/or binary stellar models reproduces the observed $I(4686)/I(Hbeta)$ intensities and its dependency on metallicity and age. We conclude that both empirical data and theoretical models suggest that high-mass X-ray binaries are the main source of nebular HeII emission in low-metallicity star-forming galaxies.
The shape of the ionising spectra of galaxies is a key ingredient to reveal their physical properties and to 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 energy >54 eV, which are not produced in sufficient amounts by normal stellar populations. To examine if high mass X-ray binaries and ultra-luminous X-ray sources (HMXB/ULX) can explain the observed HeII nebular emission and how their presence alters other emission lines, we compute photoionisation models of galaxies including such sources. We combine 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 compute photoionisation models to predict the emission line fluxes of the optical recombination lines of H and He+, and the main metal lines of OIII, OII, OI, and NII. The predictions are then compared to a large sample of low-metallicity galaxies. We find that it is possible to reproduce the nebular HeII and other line observations with our spectra and with amounts of Lx/SFR compatible with the observations. Our work suggests that HMBX/ULX could be responsible for the observed nebular HeII emission. However, the strengths of the high and low ionisation lines, such as HeII and OI, depend strongly on the X-ray contribution and on the assumed SEDs of the high energy source(s); the latter are poorly known.
Context. The diffuse X-ray emission surrounding radio galaxies is generally interpreted either as due to inverse Compton scattering of non-thermal radio-emitting electrons on the Cosmic Microwave Background (IC/CMB), or as the thermal emission arising from the hot gas of the intergalactic medium (IGM) permeating galaxy clusters hosting such galaxies, or as a combination of both. In this work we present an imaging and spectral analysis of Chandra observations for the radio galaxy 3C 187 to investigate its diffuse X-ray emission and constrain the contribution of these different physical mechanisms. Aims. The main goals of this work are: (i) to evaluate the extension of the diffuse X-ray emission from this source, (ii) to investigate the two main processes that can account for its origin - IC/CMB and thermal emission from the IGM - and (iii) to test the possibility for 3C 187 to belong to a cluster of galaxies, that can account for the observed diffuse X-ray emission. Methods. To evaluate the extension of the X-ray emission around 3C 187 we extracted surface flux profiles along and across the radio axis. We also extracted X-ray spectra in the region of the radio lobes and in the cross-cone region to estimate the contribution of the non-thermal (IC/CMB) and thermal (IGM) processes to the observed emission, making use of radio (VLA and GMRT) data to investigate the multi-wavelength emission arising from the lobes. We collected Pan-STARRS photometric data to investigate the presence of a galaxy cluster hosting 3C 187, looking for the presence of a red sequence in the source field in the form of a tight clustering of the galaxies in the color space...
The morphological, spectroscopic and kinematical properties of the warm interstellar medium (wim) in early-type galaxies (ETGs) hold key observational constraints to nuclear activity and the buildup history of these massive quiescent systems. High-quality integral field spectroscopy (IFS) data with a wide spectral and spatial coverage, such as those from the CALIFA survey, offer a precious opportunity for advancing our understanding in this respect. We use deep IFS data from CALIFA (califa.caha.es) to study the wim over the entire extent and optical spectral range of 32 nearby ETGs. We find that all ETGs in our sample show faint (Halpha equivalent width EW~0.5...2 {AA}) extranuclear nebular emission extending out to >= 2 Petrosian_50 radii. Confirming and strengthening our conclusions in Papaderos et al. (2013) we argue that ETGs span a broad continuous sequence with regard to the properties of their wim, and they can be roughly subdivided into two characteristic classes. The first one (type i) comprises ETGs with a nearly constant EW~1-3 {AA} in their extranuclear component, in quantitative agreement with (even though, no proof for) the hypothesis of photoionization by pAGB stars. The second class (type ii) consists of virtually wim-evacuated ETGs with a large Lyman continuum (Lyc) photon escape fraction and a very low (<= 0.5 {AA}) EW in their nuclear zone. These two classes appear indistinguishable from one another by their LINER-specific emission-line ratios. Additionally, here we extend the classification by the class i+ which stands for a subset of type i ETGs with low-level star-fomation in contiguous spiral-arm like features in their outermost periphery. These faint features, together with traces of localized star formation in several type i&i+ systems point to a non-negligible contribution from young massive stars to the global ionizing photon budget in ETGs.
SBS0335-052E, one of the most metal-poor (Z ~ 3-4% Z$_{odot}$) HeII-emitter starbursts known in the nearby universe, is studied using optical VLT/MUSE spectroscopic and Chandra X-ray observations. We spatially resolved the spectral map of the nebular HeII$lambda$4686 emission from which we derived for the first time the total HeII-ionizing energy budget of SBS0335-052E. The nebular HeII line is indicative of a quite hard ionizing spectrum with photon energies > 4 Ryd, and is observed to be more common at high-z than locally. Our study rules out a significant contribution from X-ray sources and shocks to the HeII photoionization budget, indicating that the He$^{+}$ excitation is mainly due to hot stellar continua. We discovered a new WR knot, but we also discard single WR stars as the main responsible for the HeII ionization. By comparing observations with current models, we found that the HeII-ionization budget of SBS0335-052E can only be produced by either single, rotating metal-free stars or a binary population with Z ~ 10$^{-5}$ and a top-heavy IMF. This discrepancy between the metallicity of such stars and that of the HII regions in SBS0335-052E is similar to results obtained by Kehrig et al. (2015) for the very metal-deficient HeII-emitting galaxy IZw18. These results suggest that the HeII ionization is still beyond the capabilities of state-of-the-art models. Extremely metal-poor, high-ionizing starbursts in the local universe, like SBS0335-052E, provide unique laboratories for exploring in detail the extreme conditions likely prevailing in the reionization era.
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