1,998 research outputs found
SBS 0335-052E+W: deep VLT/FORS+UVES spectroscopy of the pair of the lowest-metallicity blue compact dwarf galaxies
(abridged) We present deep archival VLT/FORS1+UVES spectroscopic observations
of the system of two blue compact dwarf (BCD) galaxies SBS 0335-052E and SBS
0335-052W. Our aim is to derive element abundances in different HII regions of
this unique system of galaxies and to study spatial abundance variations. We
determine abundances of helium, nitrogen, oxygen, neon, sulfur, chlorine, argon
and iron. The oxygen abundance in the brighter eastern galaxy varies in the
range 7.11 to 7.32 in different HII regions supporting previous findings and
suggesting the presence of oxygen abundance variations on spatial scales of
~1-2 kpc. The oxygen abundance in the brightest region No.1 of SBS 0335-052W is
7.22+/-0.07, consistent with previous determinations.Three other HII regions
are much more metal-poor with an unprecedently low oxygen abundance of
12+logO/H=7.01+/-0.07 (region No.2), 6.98+/-0.06 (region No.3), and 6.86+/-0.14
(region No.4). These are the lowest oxygen abundances ever derived in
emission-line galaxies. Helium abundances derived for the brightest HII regions
of both galaxies are mutually consistent. We derive weighted mean He mass
fractions of 0.2485+/-0.0012 and 0.2514+/-0.0012 for two different sets of HeI
emissivities. The N/O abundance ratio in both galaxies is slightly higher than
that derived for other BCDs with 12+logO/H<7.6. This implies that the N/O in
extremely metal-deficient galaxies could increase with decreasing metallicity.Comment: 20 pages, 11 figures, accepted for pulication in Astronomy and
Astrophysic
On the universality of luminosity-metallicity and mass-metallicity relations for compact star-forming galaxies at redshifts 0 < z < 3
We study relations between global characteristics of low-redshift (0 < z < 1)
compact star-forming galaxies, including absolute optical magnitudes, Hbeta
emission-line luminosities (or equivalently star-formation rates), stellar
masses, and oxygen abundances. The sample consists of 5182 galaxies with
high-excitation HII regions selected from the SDSS DR7 and SDSS/BOSS DR10
surveys adopting a criterion [OIII]4959/Hbeta > 1. These data were combined
with the corresponding data for high-redshift (2 < z < 3) star-forming
galaxies. We find that in all diagrams low-z and high-z star-forming galaxies
are closely related indicating a very weak dependence of metallicity on stellar
mass, redshift, and star-formation rate. This finding argues in favour of the
universal character of the global relations for compact star-forming galaxies
with high-excitation HII regions over redshifts 0 < z < 3.Comment: 13 pages, 10 figures, accepted for publication in MNRA
Searching for metal-deficient emission-line galaxy candidates: the final sample of the SDSS DR12 galaxies
We present a spectroscopic study of metal-deficient dwarf galaxy candidates,
selected from the SDSS DR12. The oxygen abundances were derived using the
direct method in galaxies with the electron temperature-sensitive emission line
[OIII]4363A measured with an accuracy better than 30%. The oxygen abundances
for the remaining galaxies with larger uncertainties of the [OIII]4363A line
fluxes were calculated using a strong-line semi-empirical method by Izotov and
Thuan. The resulting sample consists of 287 low-metallicity candidates with
oxygen abundances below 12+logO/H=7.65 including 23 extremely metal-deficient
(XMD) candidates with 12+log O/H<7.35. Ten out of sixteen XMDs known so far (or
~60%) have been discovered by our team using the direct method. Three XMDs were
found in the present study. We study relations between global parameters of
low-metallicity galaxies, including absolute optical magnitudes, Hbeta
luminosities (or equivalently star formation rates), stellar masses,
mid-infrared colours, and oxygen abundances. Low-metallicity and XMD galaxies
strongly deviate to lower metallicities in L-Z, L(Hbeta)-Z and Mstar-Z diagrams
than in relations obtained for large samples of low-redshift, star-forming
galaxies with non-restricted metallicities. These less chemically evolved
galaxies with stellar masses ~10^6-10^8Msun, Hbeta luminosities ~10^38-10^41
erg/s, SFR~0.01-1.0Msun/yr, and sSFR~50 Gyr^-1 have physical conditions which
may be characteristic of high-redshift low-mass star-forming galaxies which are
still awaiting discovery.Comment: 13 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
New candidates for extremely metal-poor emission-line galaxies in the SDSS/BOSS DR10
We present a spectroscopic study of eight extremely low-metallicity candidate
emission-line galaxies with oxygen abundances possibly below 12 +log O/H =
7.35. These galaxies were selected from Data Release 10 of the Sloan Digital
Sky Survey/Baryon Oscillation Spectroscopic Survey (SDSS/BOSS DR10). We will
call these extremely metal-deficient galaxies XMD galaxies. The electron
temperature-sensitive emission line [O~{\sc iii}] 4363 is detected in
three galaxies and marginally detected in two galaxies, allowing for abundance
determination by a "direct" method. Because of large uncertainties in the [O
{\sc iii}]4363\AA\ line fluxes, we also calculated oxygen abundance in
these galaxies together with the remaining three galaxies using a strong-line
semi-empirical method. This method gives oxygen abundances higher than 7.35 for
three galaxies with detected [O {\sc iii}]4363\AA\ line and lower than
7.35 for the remaining five objects of the sample. The newly-discovered
galaxies represent excellent targets for follow-up spectroscopic observations
with the largest telescopes to improve the oxygen abundance determination and
to increase the number of these very rare low-metallicity objects. The extreme
location of the most massive and luminous XMD galaxies and XMD candidates in
the stellar mass-metallicity diagram implies that these galaxies may be genuine
young objects.
With stellar masses of up to 10 - 10, the galaxies
are not chemically enriched and strongly deviate to lower metallicity as
compared to the relation obtained for a large sample of low-redshift,
star-forming galaxies.Comment: 10 pages, 5 figures, 4 tables, accepted in Astronomy and Astrophysic
VLT/X-shooter observations of blue compact galaxies Haro 11 and ESO 338-IG 004
(abridged) Strongly star-forming galaxies of subsolar metallicities are
typical of the high-redshift universe. Here we therefore provide accurate data
for two low-z analogs, the well-known low-metallicity emission-line galaxies
Haro 11 and ESO 338-IG 004. On the basis of Very Large Telescope/X-shooter
spectroscopic observations in the wavelength range 3000-24000\AA, we use
standard direct methods to derive physical conditions and element abundances.
Furthermore, we use X-shooter data together with Spitzer observations in the
mid-infrared range to attempt to find hidden star formation. We derive
interstellar oxygen abundances of 12 + log O/H = 8.33+/-0.01, 8.10+/-0.04, and
7.89+/-0.01 in the two HII regions B and C of Haro 11 and in ESO 338-IG 004,
respectively. The observed fluxes of the hydrogen lines correspond to the
theoretical recombination values after correction for extinction with a single
value of the extinction coefficient C(Hbeta) across the entire wavelength range
from the near-ultraviolet to the NIR and mid-infrared for each of the studied
HII regions. Therefore there are no emission-line regions contributing to the
line emission in the NIR range, which are hidden in the optical range. The
agreement between the extinction-corrected and CLOUDY-predicted fluxes implies
that a HII region model including only stellar photoionisation is able to
account for the observed fluxes, in both the optical and NIR ranges. All
observed spectral energy distributions (SEDs) can be reproduced quite well
across the whole wavelength range by model SEDs except for Haro 11B, where
there is a continuum flux excess at wavelengths >1.6mum. It is possible that
one or more red supergiant stars are responsible for the NIR flux excess in
Haro 11B. We find evidence of a luminous blue variable (LBV) star in Haro 11C.Comment: 18 pages, 6 figures. Accepted for publication in Astronomy and
Astrophysic
The Mg II 2797, 2803 emission in low-metallicity star-forming galaxies from the SDSS
We present 65 Sloan Digital Sky Survey (SDSS) spectra of 62 star-forming
galaxies with oxygen abundances 12 + logO/H ~ 7.5-8.4. Redshifts of selected
galaxies are in the range z~0.36-0.70. This allows us to detect the redshifted
MgII 2797,2803 emission lines. Our aim is to use these lines for the magnesium
abundance determination. The MgII emission was detected in ~2/3 of the
galaxies. We find that the MgII 2797 emission-line intensity follows a trend
with the excitation parameter x= O^{2+}/O that is similar to that predicted by
CLOUDY photoionised HII region models, suggesting a nebular origin of MgII
emission. The Mg/O abundance ratio is lower by a factor ~2 than the solar
ratio. This is probably the combined effect of interstellar MgII absorption and
depletion of Mg onto dust. However, the effect of dust depletion in selected
galaxies, if present, is small, by a factor of ~2 lower than that of iron.Comment: Accepted for publication in Astronomy and Astrophysics, 14 pages, 8
figure
Dust emission in star-forming dwarf galaxies: General properties and the nature of the sub-mm excess
We studied the global characteristics of dust emission in a large sample of
emission-line star-forming galaxies. The sample consists of two subsamples. One
subsample (SDSS sample) includes ~4000 compact star-forming galaxies from the
SDSS, which were also detected in all four bands at 3.4, 4.6, 12, and 22 mum of
the WISE all-sky survey. The second subsample (Herschel sample) is a sample of
28 compact star-forming galaxies observed with Herschel in the FIR range. Data
of the Herschel sample were supplemented by the photometric data from the
Spitzer observations, GALEX, SDSS, WISE, 2MASS, NVSS, and FIRST surveys, as
well as optical and Spitzer spectra and data in sub-mm and radio ranges. It is
found that warm dust luminosities of galaxies from the SDSS sample and cold and
warm dust luminosities of galaxies from the Herschel sample are strongly
correlated with Hbeta luminosities, which implies that one of the main sources
of dust heating in star-forming galaxies is ionising UV radiation of young
stars. Using the relation between warm and cold dust masses for estimating the
total dust mass in star-forming galaxies with an accuracy better than ~0.5 dex
is proposed. On the other hand, it is shown for both samples that dust
temperatures do not depend on the metallicities. The dust-to-neutral gas mass
ratio strongly declines with decreasing metallicity, similar to that found in
other studies of local emission-line galaxies, high-redshift GRB hosts, and
DLAs. On the other hand, the dust-to-ionised gas mass ratio is about one
hundred times as high implying that most of dust is located in the neutral gas.
It is found that thermal free-free emission of ionised gas in compact
star-forming galaxies might be responsible for the sub-mm emission excess. This
effect is stronger in galaxies with lower metallicities and is also positively
affected by an increased star-formation rate.Comment: 22 pages, 15 figures, accepted for publication in Astronomy and
Astrophysic
J1154+2443: a low-redshift compact star-forming galaxy with a 46 per cent leakage of Lyman continuum photons
We report the detection of the Lyman continuum (LyC) radiation of the compact
star-forming galaxy (SFG) J1154+2443 observed with the Cosmic Origins
Spectrograph (COS) onboard the Hubble Space Telescope. This galaxy, at a
redshift of z=0.3690, is characterized by a high emission-line flux ratio
O32=[OIII]5007/[OII]3727=11.5. The escape fraction of the LyC radiation
fesc(LyC) in this galaxy is 46 per cent, the highest value found so far in
low-redshift SFGs and one of the highest values found in galaxies at any
redshift. The narrow double-peaked Lya emission line is detected in the
spectrum of J1154+2443 with a separation between the peaks Vsep of 199 km/s,
one of the lowest known for Lya-emitting galaxies, implying a high fesc(Lya).
Comparing the extinction-corrected Lya/Hb flux ratio with the case B value we
find fesc(Lya) = 98 per cent. Our observations, combined with previous
detections in the literature, reveal an increase of O32 with increasing
fesc(LyC). We also find a tight anticorrelation between fesc(LyC) and Vsep. The
surface brightness profile derived from the COS acquisition image reveals a
bright star-forming region in the centre and an exponential disc in the
outskirts with a disc scale length alpha=1.09 kpc. J1154+2443, compared to
other known low-redshift LyC leakers, is characterized by the lowest
metallicity, 12+logO/H=7.65+/-0.01, the lowest stellar mass M*=10^8.20 Msun, a
similar star formation rate SFR=18.9 Msun/yr and a high specific SFR of
1.2x10^-7 yr^-1.Comment: 16 pages, 8 figures, accepted for publication in MNRAS. arXiv admin
note: text overlap with arXiv:1605.0516
- …