421 research outputs found
Excitation properties of galaxies with the highest [OIII]/[OII] ratios: No evidence for massive escape of ionizing photons
The possibility that star-forming galaxies may leak ionizing photons is at
the heart of many present-day studies that investigate the reionization of the
Universe. We test this hypothesis on local blue compact dwarf galaxies of very
high excitation. We assembled a sample of such galaxies by examining the
spectra from Data Releases 7 and 10 of the Sloan Digital Sky Survey. We argue
that reliable conclusions cannot be based on strong lines alone, and adopt a
strategy that includes important weak lines such as [OI] and the
high-excitation HeII and [ArIV] lines. Our analysis is based on purely
observational diagrams and on a comparison of photoionization models with
well-chosen emission-line ratio diagrams. We show that spectral energy
distributions from current stellar population synthesis models cannot account
for all the observational constraints, which led us to mimick several scenarios
that could explain the data. These include the additional presence of hard
X-rays or of shocks. We find that only ionization-bounded models (or models
with an escape fraction of ionizing photons lower than 10%) are able to
simultaneously explain all the observational constraints.Comment: accepted in Astronomy & Astrophysic
BOND: Bayesian Oxygen and Nitrogen abundance Determinations in giant H II regions using strong and semi-strong lines
We present BOND, a Bayesian code to simultaneously derive oxygen and nitrogen
abundances in giant H II regions. It compares observed emission lines to a grid
of photoionization models without assuming any relation between O/H and N/O.
Our grid spans a wide range in O/H, N/O and ionization parameter U, and covers
different starburst ages and nebular geometries. Varying starburst ages
accounts for variations in the ionizing radiation field hardness, which arise
due to the ageing of H II regions or the stochastic sampling of the initial
mass function. All previous approaches assume a strict relation between the
ionizing field and metallicity. The other novelty is extracting information on
the nebular physics from semi-strong emission lines. While strong lines ratios
alone ([O III]/Hbeta, [O II]/Hbeta and [N II]/Hbeta) lead to multiple O/H
solutions, the simultaneous use of [Ar III]/[Ne III] allows one to decide
whether an H II region is of high or low metallicity. Adding He I/Hbeta pins
down the hardness of the radiation field. We apply our method to H II regions
and blue compact dwarf galaxies, and find that the resulting N/O vs O/H
relation is as scattered as the one obtained from the temperature-based method.
As in previous strong-line methods calibrated on photoionization models, the
BOND O/H values are generally higher than temperature-based ones, which might
indicate the presence of temperature fluctuations or kappa distributions in
real nebulae, or a too soft ionizing radiation field in the models.Comment: MNRAS in press; 21 pages, 22 figures, 2 tables; code, data and
results available at http://bond.ufsc.b
Ionization of the diffuse gas in galaxies: Hot low-mass evolved stars at work
We revisit the question of the ionization of the diffuse medium in late type
galaxies, by studying NGC 891, the prototype of edge-on spiral galaxies. The
most important challenge for the models considered so far was the observed
increase of [OIII]/Hbeta, [OII]/Hbeta, and [NII]/Halpha with increasing
distance to the galactic plane. We propose a scenario based on the expected
population of massive OB stars and hot low-mass evolved stars (HOLMES) in this
galaxy to explain this observational fact. In the framework of this scenario we
construct a finely meshed grid of photoionization models. For each value of the
galactic latitude z we look for the models which simultaneously fit the
observed values of the [OIII]/Hbeta, [OII]/Hbeta, and [NII]/Halpha ratios. For
each value of z we find a range of solutions which depends on the value of the
oxygen abundance. The models which fit the observations indicate a systematic
decrease of the electron density with increasing z. They become dominated by
the HOLMES with increasing z only when restricting to solar oxygen abundance
models, which argues that the metallicity above the galactic plane should be
close to solar. They also indicate that N/O increases with increasing z.Comment: Accepted for publication in MNRA
Improved collision strengths and line ratios for forbidden [O III] far-infrared and optical lines
Far-infrared and optical [O III] lines are useful temeprature-density
diagnostics of nebular as well as dust obscured astrophysical sources. Fine
structure transitions among the ground state levels 1s^22s^22p^3 \ ^3P_{0,1,2}
give rise to the 52 and 88 micron lines, whereas transitions among the
levels yield the well-known optical lines 4363,
4959 and 5007 Angstroms. These lines are excited primarily by electron impact
excitation. But despite their importance in nebular diagnostics collision
strengths for the associated fine structure transitions have not been computed
taking full account of relativistic effects. We present Breit-Pauli R-matrix
calculations for the collision strengths with highly resolved resonance
structures. We find significant differences of up to 20% in the Maxwellian
averaged rate coefficients from previous works. We also tabulate these to lower
temperatures down to 100 K to enable determination of physical conditions in
cold dusty environments such photo-dissociation regions and ultra-luminous
infrared galaxies observed with the Herschel space observatory. We also examine
the effect of improved collision strengths on temperature and density sensitive
line ratios.Comment: Letter in press, Monthly Notices of Royal Astronomical Society, 5
pages, 6 figure
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