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Register a new userThe extended HeII$lambda$4686 emission in the extremely metal-poor galaxy SBS0335-052E seen with MUSE
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SBS0335-052E, one of the most metal-poor (Z ~ 3-4% Z$_{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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Carbon monoxide (CO) is one of the primary coolants of gas and an easily accessible tracer of molecular gas in spiral galaxies but it is unclear if CO plays a similar role in metal poor dwarfs. We carried out a deep observation with IRAM 30 m to search for CO emission by targeting the brightest far-IR peak in a nearby extremely metal poor galaxy, Sextans A, with 7% Solar metallicity. A weak CO J=1-0 emission is seen, which is already faint enough to place a strong constraint on the conversion factor (a_CO) from the CO luminosity to the molecular gas mass that is derived from the spatially resolved dust mass map. The a_CO is at least seven hundred times the Milky Way value. This indicates that CO emission is exceedingly weak in extremely metal poor galaxies, challenging its role as a coolant in these galaxies.
Among the nearest most metal-poor starburst-dwarf galaxies known, SBS 0335-052E is the most luminous in integrated nebular He II {lambda}4686 emission. This makes it a unique target to test spectral synthesis models and spectral interpretation tools of the kind that will be used to interpret future rest-frame UV observations of primeval galaxies. Previous attempts to reproduce its He II {lambda}4686 emission luminosity found that X-ray sources, shocks, and single Wolf-Rayet stars are not main contributors to the He II-ionizing budget; and that only metal-free single rotating stars or binary stars with a top-heavy IMF and an unphysically-low metallicity can reproduce it. We present new UV (COS) and optical (MUSE) spectra which integrate the light of four super star clusters in SBS 0335-052E. Nebular He II, [C III], C III], C IV, and O III] UV emission lines with equivalent widths between 1.7 and 5 {AA}, and a C IV {lambda}{lambda}1548, 1551 P-Cygni like profile are detected. Recent extremely-metal poor shock + precursor models and binary models fail to reproduce the observed optical emission-line ratios. We use different sets of UV and optical observables to test models of constant star formation with single non-rotating stars which account for very massive stars, as blueshifted O V {lambda}1371 absorption is present. Simultaneously fitting the fluxes of all high-ionization UV lines requires an unphysically-low metallicity. Fitting the P-Cygni like + nebular components of C IV {lambda}{lambda}1548, 1551 does not constrain the stellar metallicity and time since the beginning of star formation. We obtain 12+log(O/H)=7.45pm0.04 and log(C/O)=-0.45(+0.03)(-0.04) for the galaxy. Model-testing would benefit from higher spatial resolution UV and optical spectroscopy of the galaxy.
ABRIDGED: Diffuse interstellar bands (DIBs) are faint spectral absorption features of unknown origin. Research on DIBs beyond the Local Group (LG) will surely blossom in the era of the ELTs. A possibility that needs to be explored is the use of integral field spectrographs. We do so by using MUSE data for the Antennae Galaxy, the closest major galaxy merger. High S-to-N spectra were created by co-adding the signal of many spatial elements. The emission of the underlying stellar population was modeled using STARLIGHT. To our knowledge, we have derived the first maps for the DIBs at l5780 and l5797 in galaxies outside the LG. The l5780 DIB was detected in an area of ~0.6 arcmin2, corresponding to a linear scale of ~25 kpc2. This region was sampled using >200 independent lines of sight. The DIB l5797 was detected in >100 independent lines of sight. Both DIBs are associated with a region with high emission in the HI 21 cm line, implying a connection between atomic gas and DIBs, as the correlations for the Milky Way also suggest. Conversely, there is mild spatial association between the two DIBs and the molecular gas, in agreement with results for our Galaxy that indicate a lack of correlation between DIBs and molecular gas. The overall structure for the DIB strength distribution and extinction are comparable. Within the system, the l5780 DIB clearly correlates with the extinction. Both DIBs follow the relationship between equivalent width and reddening when data for several galaxies are considered. Unidentified Infrared emission Bands (UIBs, likely caused by PAHs) and the l5780 and l5797 DIBs show similar but not identical spatial distributions. We attribute the differences to extinction effects without necessarily implying a radically different nature of the respective carriers. The results illustrate the enormous potential of integral field spectrographs for extragalactic DIB research.
Extremely metal-poor galaxies with metallicity below 10% of the solar value in the local universe are the best analogues to investigating the interstellar medium at a quasi-primitive environment in the early universe. In spite of the ongoing formation of stars in these galaxies, the presence of molecular gas (which is known to provide the material reservoir for star formation in galaxies, such as our Milky Way) remains unclear. Here, we report the detection of carbon monoxide (CO), the primary tracer of molecular gas, in a galaxy with 7% solar metallicity, with additional detections in two galaxies at higher metallicities. Such detections offer direct evidence for the existence of molecular gas in these galaxies that contain few metals. Using archived infrared data, it is shown that the molecular gas mass per CO luminosity at extremely low metallicity is approximately one-thousand times the Milky Way value.
We present infrared (IR) spectral energy distributions (SEDs) of individual star-forming regions in four extremely metal poor (EMP) galaxies with metallicity Z around Zsun/10 as observed by the Herschel Space Observatory. With the good wavelength coverage of the SED, it is found that these EMP star-forming regions show distinct SED shapes as compared to those of grand design Spirals and higher metallicity dwarfs: they have on average much higher f70um/f160um ratios at a given f160um/f250um ratio; single modified black-body (MBB) fittings to the SED at lambda >= 100 um still reveal higher dust temperatures and lower emissivity indices compared to that of Spirals, while two MBB fittings to the full SED with a fixed emissivity index (beta = 2) show that even at 100 um about half of the emission comes from warm (50 K) dust, in contrast to the cold (~20 K) dust component. Our spatially resolved images further reveal that the far-IR colors including f70um/f160um, f160um/f250um and f250um/f350um are all related to the surface densities of young stars as traced by far-UV, 24 um and SFRs, but not to the stellar mass surface densities. This suggests that the dust emitting at wavelengths from 70 um to 350 um is primarily heated by radiation from young stars.
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