91 research outputs found
Near-infrared spectroscopy of a large sample of low-metallicity blue compact dwarf galaxies
We present near-infrared (NIR) spectroscopic observations in the wavelength
range 0.90-2.40mum of eighteen low-metallicity blue compact dwarf (BCD)
galaxies and six HII regions in spiral and interacting galaxies. Hydrogen and
helium emission lines are detected in all spectra, while H2 and iron emission
lines are detected in most spectra. The NIR data for all objects have been
supplemented by optical spectra. In all objects, except perhaps for the highest
metallicity ones, we find that the extinctions A(V) in the optical and NIR
ranges are similar, implying that the NIR hydrogen emission lines in
low-metallicity BCDs do not reveal more star formation than seen in the
optical. We conclude that emission-line spectra of low-metallicity BCDs in the
0.36-2.40mum wavelength range are emitted by a relatively transparent ionized
gas. The H2 emission line fluxes can be accounted for by fluorescence in most
of the observed galaxies. We find a decrease of the H2 2.122mum emission line
relative to the Brgamma line with increasing ionization parameter. This
indicates an efficient destruction of H2 by the stellar UV radiation. The
intensities of the [FeII] 1.257mum and 1.644mum emission lines in the spectra
of all galaxies, but one, are consistent with the predictions of Cloudy stellar
photoinization models. There is thus no need to invoke shock excitation for
these lines, and they are not necessarily shock indicators in low-metallicity
high-excitation BCDs. The intensity of the HeI 2.058mum emission line is lower
in high-excitation BCDs with lower neutral gas column densities and higher
turbulent motions.Comment: 11 pages, 6 figures, accepted for publication in MNRAS. arXiv admin
note: text overlap with arXiv:1104.081
A new determination of the primordial He abundance using the HeI 10830A emission line: cosmological implications
We present near-infrared spectroscopic observations of the high-intensity HeI
10830 emission line in 45 low-metallicity HII regions. We combined these NIR
data with spectroscopic data in the optical range to derive the primordial He
abundance. The use of the HeI 10830A line, the intensity of which is very
sensitive to the density of the HII region, greatly improves the determination
of the physical conditions in the He^+ zone. This results in a considerably
tighter Y - O/H linear regression compared to all previous studies. We
extracted a final sample of 28 HII regions with Hbeta equivalent width
EW(Hbeta)>150A, excitation parameter O^2+/O>0.8, and with helium mass fraction
Y derived with an accuracy better than 3%. With this final sample we derived a
primordial He mass fraction Yp = 0.2551+/-0.0022. The derived value of Yp is
higher than the one predicted by the standard big bang nucleosynthesis (SBBN)
model. Using our derived Yp together with D/H = (2.53+/-0.04)x10^-5, and the
chi^2 technique, we found that the best agreement between these light element
abundances is achieved in a cosmological model with a baryon mass density
Omega_b h^2 = 0.0240+/-0.0017 (68% CL), +/-0.0028 (95.4% CL), +/-0.0034 (99%
CL) and an effective number of neutrino species Neff = 3.58+/-0.25 (68% CL),
+/-0.40 (95.4% CL), +/-0.50 (99% CL). A non-standard value of Neff is preferred
at the 99% CL, implying the possible existence of additional types of neutrino
species.Comment: 18 pages, 11 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Society. arXiv admin note: text overlap with
arXiv:1308.210
LBT observations of compact star-forming galaxies with extremely high [OIII]/[OII] flux ratios: HeI emission-line ratios as diagnostics of Lyman continuum leakage
We present Large Binocular Telescope spectrophotometric observations of five
low-redshift (z<0.070) compact star-forming galaxies (CSFGs) with extremely
high emission-line ratios O32 = [OIII]5007/[OII]3727, ranging from 23 to 43.
Galaxies with such high O32 are thought to be promising candidates for leaking
large amounts of Lyman continuum (LyC) radiation and, at high redshifts, for
contributing to the reionization of the Universe. The equivalent widths
EW(Hbeta) of the Hbeta emission line in the studied galaxies are very high,
~350-520A, indicating very young ages for the star formation bursts, <3 Myr.
All galaxies are characterized by low oxygen abundances 12+logO/H = 7.46 - 7.79
and low masses Mstar~10^6-10^7 Msun, much lower than the Mstar for known
low-redshift LyC leaking galaxies, but probably more typical of the
hypothetical population of low-luminosity dwarf LyC leakers at high redshifts.
A broad Halpha emission line is detected in the spectra of all CSFGs, possibly
related to expansion motions of supernova remnants. Such rapid ionized gas
motions would facilitate the escape of the resonant Ly emission from
the galaxy. We show that high O32 may not be a sufficient condition for LyC
leakage and propose new diagnostics based on the HeI 3889/6678 and 7065/6678
emission-line flux ratios. Using these diagnostics we find that three CSFGs in
our sample are likely to have density-bounded HII regions and are thus leaking
large amounts of LyC radiation. The amount of leaking LyC radiation is probably
much lower in the other two CSFGs.Comment: 16 pages, 8 figures, accepted for publication in Monthly Notices of
the Royal Astronomical Societ
The X-ray Properties of the Cometary Blue Compact Dwarf galaxies Mrk 59 and Mrk 71
We present XMM-Newton and Chandra observations of two low-metallicity
cometary blue compact dwarf (BCD) galaxies, Mrk 59 and Mrk 71. The first BCD,
Mrk 59, contains two ultraluminous X-ray (ULX) sources, IXO 72 and IXO 73, both
associated with bright massive stars and H II complexes, as well as one fainter
extended source associated with a massive H II complex at the head of the
cometary structure. The low-metallicity of Mrk 59 appears to be responsible for
the presence of the two ULXs. IXO 72 has varied little over the last 10 yr,
while IXO 73 has demonstrated a variability factor of ~4 over the same period.
The second BCD, Mrk 71, contains two faint X-ray point sources and two faint
extended sources. One point source is likely a background AGN, while the other
appears to be coincident with a very luminous star and a compact H II region at
the "head" of the cometary structure. The two faint extended sources are also
associated with massive H II complexes. Although both BCDs have the same
metallicity, the three sources in Mrk 71 have X-ray luminosities ~1-2 orders of
magnitude fainter than those in Mrk 59. The age of the starburst may play a
role.Comment: 14 pages, 7 figures, accepted for publication in MNRA
Two extremely metal-poor emission-line galaxies in the Sloan Digital Sky Survey
We present spectroscopic observations with the 3.6m ESO telescope of two
emission-line galaxies, J2104-0035 and J0113+0052, selected from the Data
Release 4 (DR4) of the Sloan Digital Sky Survey (SDSS). From our data we
determine the oxygen abundance of these systems to be respectively 12+logO/H =
7.26+/-0.03 and 7.17+/-0.09, making them the two most metal-deficient galaxies
found thus far in the SDSS and placing them among the five most metal-deficient
emission-line galaxies ever discovered. Their oxygen abundances are close to
those of the two most metal-deficient emission-line galaxies known,
SBS0335-052W with 12+logO/H = 7.12+/-0.03 and I Zw 18 with 12+logO/H =
7.17+/-0.01.Comment: 5 pages, 3 figures. Accepted for publication in Astronomy and
Astrophysic
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