Detection of [OI] 6300 and Other Diagnostic Emission Lines in the Diffuse Ionized Gas of M33 with Gemini-North

We present spectroscopic observations of diffuse ionized gas (DIG) in M33 near the HII region NGC 604. We present the first detection of [OI] 6300 in the DIG of M33, one of the critical lines for distinguishing photo- from shock ionization models. We measure [OI]/Ha in the range of 0.04 to 0.10 and an increase in this ratio with decreasing emission measure. Our measurements of [SII]/Ha and [NII]/Ha also rise with decreasing emission measure, while our [OIII]/Hb measurements remain fairly constant. We have one tentative detection of He I in the region of brightest emission measure, with a ratio of He I/Ha = 0.033 +- 0.019, indicating that the helium is at least partially ionized. We compare our observed emission line ratios to photoionization models and find that field star ionization models do not fit our data well. Leaky HII region models are consistent with our data, without the need to invoke additional ionization mechanisms to fit our [OI] or [OIII] measurements. The closest large HII region is NGC 604 and is therefore a likely candidate for the source of the ionizing photons for the gas in this region.Comment: 12 pages, 4 figures, accepted by ApJ

HeI in the central Giant HII Region of NGC 5253. A 2D observational approach to collisional and radiative transfer effects

ABRIDGED: NGC5253 is an ideal laboratory for detailed studies of starburst galaxies. We present for the first time in a starburst galaxy a 2D study of the spatial behavior of collisional and radiative transfer effects in He^+. The HeI lines are analysed based on data obtained with FLAMES and GMOS. Collisional effects are negligible for transitions in the singlet cascade while relatively important for those in the triplet cascade. In particular, they can contribute up to 20% of the flux in the HeIl7065 line. Radiative transfer effects are important over an extended and circular area of 30pc in diameter centered at the Super Star Clusters. HeI abundance, y^+, has been mapped using extinction corrected fluxes of six HeI lines, realistic assumptions for T_e, n_e, and the stellar absorption equivalent width as well as the most recent emissivities. We found a mean of 10^3 y^+ ~80.3 over the mapped area. The relation between the excitation and the total helium abundance, y_tot, is consistent with no abundance gradient. Uncertainties in the derivation of He abundances are dominated by the adopted assumptions. We illustrated the difficulty of detecting a putative He enrichment due to the presence of Wolf-Rayet stars in the main GHIIR. Data are marginally consistent with an excess in the N/He ratio in the N enriched area of the order of both, the atmospheric N/He ratios in WR stars and the uncertainties estimated for the N/He ratios.Comment: Accepted in Astronomy and Astrophysics; the emissivities presented in the Corrigendum, Porter et al. 2013, arXiv:1303.5115, have been include

O stars effective temperature and HII regions ionization parameter gradients in the Galaxy

Extensive photoionization model grids are computed for single star HII regions using stellar atmosphere models from the WM-basic code. Mid-IR emission line intensities are predicted and diagnostic diagrams of [NeIII]/[NeII] and [SIV]/[SIII] excitation ratio are build, taking into account the metallicities of both the star and the HII region. The diagrams are used in conjunction with galactic HII region observations obtained with the ISO Observatory to determine the effective temperature Teff of the exciting O stars and the mean ionization parameter U. Teff and U are found to increase and decrease, respectively, with the metallicity of the HII region represented by the [Ne/Ne_sol] ratio. No evidence is found for gradients of Teff or U with galactocentric distance Rgal. The observed excitation sequence with Rgal is mainly due to the effect of the metallicity gradient on the spectral ionizing shape, upon which the effect of an increase in Teff with Z is superimposed. We show that not taking properly into account the effect of metallicity on the ionizing shape of the stellar atmosphere would lead to an apparent decrease of Teff with Z and an increase of Teff with Rgal.Comment: Accepted in Ap

Is There a Fundamental Line for Disk Galaxies?

We show that there are strong local correlations between metallicity, surface brightness, and dynamical mass-to-light ratio within M33, analogous to the fundamental line of dwarf galaxies identified by Prada & Burkert (2002). Using near-infrared imaging from 2MASS, the published rotation curve of M33, and literature measurements of the metallicities of HII regions and supergiant stars, we demonstrate that these correlations hold for points at radial distances between 140 pc and 6.2 kpc from the center of the galaxy. At a given metallicity or surface brightness, M33 has a mass-to-light ratio approximately four times as large as the Local Group dwarf galaxies; other than this constant offset, we see broad agreement between the M33 and dwarf galaxy data. We use analytical arguments to show that at least two of the three fundamental line correlations are basic properties of disk galaxies that can be derived from very general assumptions. We investigate the effect of supernova feedback on the fundamental line with numerical models and conclude that while feedback clearly controls the scatter in the fundamental line, it is not needed to create the fundamental line itself, in agreement with our analytical calculations. We also compare the M33 data with measurements of a simulated disk galaxy, finding that the simulation reproduces the trends in the data correctly and matches the fundamental line, although the metallicity of the simulated galaxy is too high, and the surface brightness is lower than that of M33.Comment: 14 pages, 14 figures (5 in color). Accepted for publication in Ap

The extended HeII4686-emitting region in IZw18 unveiled: clues for peculiar ionizing sources

New integral field spectroscopy has been obtained for IZw18, the nearby lowest-metallicity galaxy considered our best local analog of systems forming at high-z. Here we report the spatially resolved spectral map of the nebular HeII4686 emission in IZw18, from which we derived for the first time its total HeII-ionizing flux. Nebular HeII emission implies the existence of a hard radiation field. HeII-emitters are observed to be more frequent among high-z galaxies than for local objects. So investigating the HeII-ionizing source(s) in IZw18 may reveal the ionization processes at high-z. HeII emission in star-forming galaxies, has been suggested to be mainly associated with Wolf-Rayet stars (WRs), but WRs cannot satisfactorily explain the HeII-ionization at all times, in particular at lowest metallicities. Shocks from supernova remnants, or X-ray binaries, have been proposed as additional potential sources of HeII-ionizing photons. Our data indicate that conventional HeII-ionizing sources (WRs, shocks, X-ray binaries) are not sufficient to explain the observed nebular HeII4686 emission in IZw18. We find that the HeII-ionizing radiation expected from models for either low-metallicity super-massive O stars or rotating metal-free stars could account for the HeII-ionization budget measured, while only the latter models could explain the highest values of HeII4686/Hbeta observed. The presence of such peculiar stars in IZw18 is suggestive and further investigation in this regard is needed. This letter highlights that some of the clues of the early Universe can be found here in our cosmic backyard.Comment: 6 pages, 3 figures. Accepted for publication in ApJ Letter