234 research outputs found
HST observations of the blue compact dwarf SBS 0335-052: a probable young galaxy
We present HST WFPC2 V and I images and GHRS UV spectrophotometry of the
spectral regions around Ly and OI 1302 of the extremely metal-deficient
(Z~Zsun/41) blue compact dwarf (BCD) galaxy SBS 0335-052. All the star
formation in the BCD occurs in six super-star clusters (SSC) with ages =< 3-4
Myr. Dust is clearly present and mixed spatially with the SSCs. There is a
supershell of radius ~380 pc, delineating a large supernova cavity. The
instantaneous star formation rate is ~0.4 Msun yr^-1. Strong narrow Ly
emission is not observed. Rather there is low intensity broad (FWZI = 20 A)
Ly emission superposed on even broader Ly absorption by the HI
envelope. This broad low-intensity emission is probably caused by resonant
scattering of Ly photons. The BCD appears to be a young galaxy,
undergoing its very first burst of star formation. This conclusion is based on
the following evidence: 1) the underlying extended low-surface-brightness
component is very irregular and filamentary, suggesting that a significant part
of the emission comes from ionized gas; 2) it has very blue colors (-0.34 =<
(V-I) =< 0.16), consistent with gaseous emission colors; 3) the OI 1302
line is not detected in absorption in the GHRS spectrum, setting an upper limit
for N(O)/N(H) in the HI envelope of the BCD of more than 3000 times smaller
than the value in Orion.Comment: 20 pages and 6 Postscript figures. Submitted to Astrophysical Journa
Close pairs of galaxies with different activity levels
We selected and studied 180 pairs with dV < 800 km/s and Dp < 60 kpc
containing Markarian (MRK) galaxies to investigate the dependence of galaxies'
integral parameters, star-formation (SF) and active galactic nuclei (AGN)
properties on kinematics of pairs, their structure and large-scale
environments. Projected radial separation Dp and perturbation level P are
better measures of interaction strength than dV. The latter correlates with the
density of large-scale environment and with the morphologies of galaxies. Both
galaxies in a pair are of the same nature, the only difference is that MRK
galaxies are usually righter than their neighbors. Specific star formation
rates (SSFR) of galaxies in pairs with smaller Dp or dV is in average 0.5 dex
higher than that of galaxies in pairs with larger Dp or dV. Closeness of a
neighbor with the same and later morphological type increases the SSFR, while
earlier-type neighbors do not increase SSFR. Major interactions/mergers trigger
SF and AGN more effectively than minor ones. The fraction of AGNs is higher in
more perturbed pairs and pairs with smaller Dp. AGNs typically are in stronger
interacting systems than star-forming and passive galaxies. There are
correlations of both SSFRs and spectral properties of nuclei between pair
members.Comment: 4 pages, 3 figures. arXiv admin note: substantial text overlap with
arXiv:1310.024
Close neighbors of Markarian galaxies. II. Statistics and discussions
According to the database from the first paper, we select 180 pairs with dV <
800 km/s and Dp < 60 kpc containing Markarian (MRK) galaxies. We study the
dependence of galaxies integral parameters, star-formation (SF) and active
galactic nuclei (AGN) properties on kinematics of pairs, their structure and
large-scale environments. Following main results were obtained: projected
radial separation Dp between galaxies correlates with the perturbation level P
of the pairs. Both parameters do not correlate with line-of-sight velocity
difference dV of galaxies. Dp and P are better measures of interaction strength
than dV. The latter correlates with the density of large-scale environment and
with the morphologies of galaxies. Both galaxies in a pair are of the same
nature, the only difference is that MRK galaxies are usually brighter than
their neighbors in average by 0.9 mag. Specific star formation rates (SSFR) of
galaxies in pairs with smaller Dp or dV is in average 0.5 dex higher than that
of galaxies in pairs with larger Dp or dV. Closeness of a neighbor with the
same and later morphological type increases the SSFR, while earlier-type
neighbors do not increase SSFR. Major interactions/mergers trigger SF and AGN
more effectively than minor ones. The fraction of AGNs is higher in more
perturbed pairs and pairs with smaller Dp. AGNs typically are in stronger
interacting systems than star-forming and passive galaxies. There are
correlations of both SSFRs and spectral properties of nuclei between pair
members.Comment: 13 pages, 8 figures, 2 table
A Bayesian Estimate of the Primordial Helium Abundance
We introduce a new statistical method to estimate the primordial helium
abundance, Y_p from observed abundances in a sample of galaxies which have
experienced stellar helium enrichment. Rather than using linear regression on
metal abundance we construct a likelihood function using a Bayesian prior,
where the key assumption is that the true helium abundance must always exceed
the primordial value. Using a sample of measurements compiled from the
literature we find estimates of Y_p between 0.221 and 0.236, depending on the
specific subsample and prior adopted, consistent with previous estimates either
from a linear extrapolation of the helium abundance with respect to
metallicity, or from the helium abundance of the lowest metallicity HII region,
I Zw 18. We also find an upper limit which is insensitive to the specific
subsample or prior, and estimate a model-independent bound Y_p < 0.243 at 95%
confidence, favoring a low cosmic baryon density and a high primordial
deuterium abundance. The main uncertainty is not the model of stellar
enrichment but possible common systematic biases in the estimate of Y in each
individual HII region.Comment: 14 pages, latex, 3 ps figure
Systematic effects and a new determination of the primordial abundance of 4He and dY/dZ from observations of blue compact galaxies
We use spectroscopic observations of a sample of 82 HII regions in 76 blue
compact galaxies to determine the primordial helium abundance Yp and the slope
dY/dZ from the Y-O/H linear regression. To improve the accuracy of the dY/dZ
measurement, we have included new spectrophotometric observations of 33 HII
regions which span a large metallicity range, with oxygen abundance 12+log(O/H)
varying between 7.43 and 8.30 (Zsun/30<Z<Zsun/4). For a subsample of 7 HII
regions, we derive the He mass fraction taking into account known systematic
effects, including collisional and fluorescent enhancements of HeI emission
lines, collisional excitation of hydrogen emission, underlying stellar HeI
absorption and the difference between the temperatures Te(HeII) in the He^+
zone and Te(OIII) derived from the collisionally excited [OIII] lines. We find
that the net result of all the systematic effects combined is small, changing
the He mass fraction by less than 0.6%. By extrapolating the Y vs. O/H linear
regression to O/H=0 for 7 HII regions of this subsample, we obtain
Yp=0.2421+/-0.0021 and dY/dO=5.7+/-1.8, which corresponds to dY/dZ=3.7+/-1.2,
assuming the oxygen mass fraction to be O=0.66Z. In the framework of the
standard Big Bang nucleosynthesis theory, this Yp corresponds to Omega_b h^2 =
0.012^+0.003_-0.002, where h is the Hubble constant in units of 100 km/s/Mpc.
This is smaller at the 2sigma level than the value obtained from recent
deuterium abundance and microwave background radiation measurements. The linear
regression slope dY/dO=4.3+/-0.7 (corresponding to dY/dZ=2.8+/-0.5) for the
whole sample of 82 HII regions is similar to that derived for the subsample of
7 HII regions, although it has a considerably smaller uncertainty.Comment: 53 pages, 3 Postscript figures, accepted for publication in the
Astrophysical Journa
Constraints on the Lyman continuum radiation from galaxies: first results with FUSE on Mrk 54
We present Far Ultraviolet Spectroscopic Explorer observations of the
star-forming galaxy Mrk 54 at z = 0.0448. The Lyman continuum radiation is not
detected above the HI absorption edge in our Galaxy. An upper limit is
evaluated by comparison with the background measured in regions of the detector
adjacent to the observed spectrum. A spectral window of 16 A, reasonably free
of additional HI Lyman series line absorption is used. No correction is needed
for molecular hydrogen absorption in our Galaxy but a foreground extinction of
0.29 mag is accounted for. An upper limit of 6.15 10^{-16} erg/cm^2/s/A is
obtained for the flux at ~ 900 A in the rest frame of Mrk 54. By comparison
with the number of ionizing photons derived from the H-alpha flux, this limit
translates into an upper limit of f_esc < 0.062 for the fraction of Lyman
continuum photons that escape the galaxy without being absorbed by interstellar
material. This limit compares with the limits obtained in three other nearby
galaxies and is compatible with the escape fractions predicted by models.
The upper limits obtained in nearby galaxies contrasts with the detection of
Lyman continuum flux in the composite spectrum of Lyman-break galaxies at z ~
3.4. The difficulties and implications of a comparison are discussed.Comment: 9 pages, 3 figures, accepted for publication in A&A include aa.cls
v5.0
The Star Formation History of IZw18
The star formation history in IZw18 has been inferred from HST/WFPC2 archival
data. This is done by comparing the derived V, B-V and V, V-I color-magnitude
diagrams and luminosity functions with synthetic ones, based on various sets of
stellar evolutionary tracks. At a distance of 10 Mpc, the stars resolved in the
field of IZw18 allow for a lookback time up to 1 Gyr. We find that the main
body is not experiencing its first episode of star formation. Instead, it has
been forming stars over the last 0.5-1 Gyr, at a rate of ~ 1-2 * 10**(-2) Msol
per year per kpc**2. A more intense activity of 6-16 * 10**(-2) Msol per year
per kpc**2 has taken place between 15 and 20 Myr ago. For the secondary body,
the lookback time is 0.2 Gyr at most and the uncertainty is much higher, due to
the shallower diagrams and the small number of resolved stars. The derived
range of star formation rate is 3-10 * 10**(-3) Msol per year per kpc**2. The
IMF providing the best fit to the observed stellar populations in the main body
has a slope 1.5, much flatter than in any similar galaxy analyzed with the same
method. In the secondary body, it is peaked at 2.2, closer to Salpeter's slope
(2.35).Comment: 70 pages including 18 figures, to be published in The Astronomical
Journa
The Rest-Frame Optical Properties of z~3 Galaxies
We present the results of a near-infrared imaging survey of z~3 Lyman Break
Galaxies (LBGs). The survey covers a total of 30 arcmin^2 and includes 118
photometrically selected LBGs with K_s band measurements, 63 of which also have
J band measurements, and 81 of which have spectroscopic redshifts. Using the
distribution of optical {\cal R} magnitudes from previous work and {\cal R}-K_s
colors for this sub-sample, we compute the rest-frame optical luminosity
function of LBGs. At the brightest magnitudes, where it is fairly well
constrained, this luminosity function strikingly exceeds locally determined
optical luminosity functions. The V-band luminosity density of only the
observed bright end of the z~3 LBG luminosity function already approaches that
of all stars in the local universe. For the 81 galaxies with measured
redshifts, we investigate the range of LBG stellar populations implied by the
photometry which generally spans the range 900--5500 AA in the rest-frame.
While there are only weak constraints on the parameters for most of the
individual galaxies, there are strong trends in the sample as a whole. A
unified scenario which accounts for the observed trends in bright LBGs is one
in which a relatively short period of very rapid star-formation (hundreds of
M_sun/yr) lasts for roughly 50--100 Myr, after which both the extinction and
star-formation rate are considerably reduced and stars are formed at a more
quiescent, but still rapid, rate for at least a few hundred Myr. In our sample,
a considerable fraction (~20%) of the LBGs have best-fit star-formation ages ~>
1 Gyr, implied stellar masses of ~> 10^10 M_sun, and are still forming stars at
\~30 M_sun/yr.Comment: 61 pages including 19 figures. Accepted for publication in Ap
POX 186: A Dwarf Galaxy in the Process of Formation?
We present deep U, V and I band images of the "ultracompact" blue dwarf
galaxy POX 186 obtained with the Planetary Camera 2 of the Hubble Space
Telescope. We have also obtained a near-ultraviolet spectrum of the object with
the Space Telescope Imaging Spectrograph, and combine this with a new
ground-based optical spectrum. The images confirm the galaxy to be extremely
small, with a maximum extent of only 300 pc, a luminosity ~ 10^-4 L*, and an
estimated mass ~ 10^7 M(sun). Its morphology is highly asymmetric, with a tail
of material on its western side that may be tidal in origin. The U-band image
shows this tail to be part of a stream of material in which stars have recently
formed. Most of the star formation in the galaxy is however concentrated in a
central, compact (d ~ 10 - 15 pc) star cluster. The outer regions of the galaxy
are significantly redder than the cluster, with V - I colors consistent with a
population dominated by K and M stars. While these results rule out earlier
speculation that POX 186 is a protogalaxy, its morphology, mass and active star
formation suggest that it represents a recent (within ~ 10^8 yr) collision
between two clumps of stars of sub-galactic size (~ 100 pc). POX 186 may thus
be a very small dwarf galaxy that, dynamically speaking, is still in the
process of formation. This interpretation is supported by the fact that it
resides in a void, so its morphology cannot be explained as the result of an
encounter with a more massive galaxy. Clumps of stars this small may represent
the building blocks required by hierarchical models of galaxy formation, and
these results also support the recent "downsizing" picture of galaxy formation
in which the least massive objects are the last to form.Comment: accepted for publication in ApJ; 23 pages, 5 figure
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