416 research outputs found
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
High Carbon in I Zwicky 18: New Results from Hubble Space Telescope Spectroscopy
We present new measurements of the gas-phase C/O abundance ratio in both the
NW and SE components of the extremely metal-poor dwarf irregular galaxy I Zw
18, based on ultraviolet spectroscopy of the two H II regions using the Faint
Object Spectrograph on the Hubble Space Telescope. We determine values of log
C/O = -0.63 +/- 0.10 for the NW component and log C/O = -0.56 +/- 0.09 for the
SE component. In comparison, log C/O = -0.37 in the sun, while log C/O = -0.85
+/- 0.07 in the three most metal-poor irregular galaxies measured by Garnett et
al. (1995a). Our measurements show that C/O in I Zw 18 is significantly higher
than in other comparably metal-poor irregular galaxies, and above predictions
for the expected C/O from massive star nucleosynthesis. These results suggest
that carbon in I Zw 18 has been enhanced by an earlier population of lower-mass
carbon producing stars; this idea is supported by stellar photometry of I Zw 18
and its companion, which demonstrate that the current bursts of massive stars
were not the first. Despite its very low metallicity, it is likely that I Zw 18
is not a ``primeval'' galaxy.Comment: 14 pages including 4 figures; uses aaspp4.sty. Accepted for
publication in ApJ. Postscript version also available by e-mail request to
author at [email protected]
FUSE observations of the HI interstellar gas of IZw18
We present the analysis of FUSE observations of the metal-deficient dwarf
galaxy IZw18. We measured column densities of HI, NI, OI, ArI, SiII, and FeII.
The OI/HI ratio (log(OI/HI)=-4.7^{+0.8}_{-0.6}) is consistent with the O/H
ratio observed in the HII regions (all uncertainties are 2-sigma). If the
oxygen is depleted in the HI region compared to the HII regions, the depletion
is at most 0.5dex. This is also consistent with the log(O/H) ratios ~-5
measured with FUSE in the HI regions of other blue compact dwarf galaxies. With
log(NI/OI)=-2.4^{+0.6}_{-0.8}, the measured NI/OI ratio is lower than expected
for primary nitrogen. The determination of the NII column density is needed to
discriminate between a large ionization of NI or a possible nitrogen
deficiency. The neutral argon is also apparently underabundant, indicating that
ionization into ArII is likely important. The column densities of the other
alpha-chain elements SiII and ArI favor the lower edge of the permitted range
of OI column density, log(N(OI))~16.3.Comment: Accepted for publication in A&
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
Rest-Frame Ultraviolet Spectra of z~3 Lyman Break Galaxies
We present the results of a systematic study of the rest-frame UV
spectroscopic properties of Lyman Break Galaxies (LBGs). The database of almost
1000 LBG spectra proves useful for constructing high S/N composite spectra. The
composite spectrum of the entire sample reveals a wealth of features
attributable to hot stars, HII regions, dust, and outflowing neutral and
ionized gas. By grouping the database according to galaxy parameters such as
Lyman-alpha equivalent width, UV spectral slope, and interstellar kinematics,
we isolate some of the major trends in LBG spectra which are least compromised
by selection effects. We find that LBGs with stronger Lyman-alpha emission have
bluer UV continua, weaker low-ionization interstellar absorption lines, smaller
kinematic offsets between Lyman-alpha and the interstellar absorption lines,
and lower star-formation rates. There is a decoupling between the dependence of
low- and high-ionization outflow features on other spectral properties. Most of
the above trends can be explained in terms of the properties of the large-scale
outflows seen in LBGs. According to this scenario, the appearance of LBG
spectra is determined by a combination of the covering fraction of outflowing
neutral gas which contains dust, and the range of velocities over which this
gas is absorbing. Higher sensitivity and spectral resolution observations are
still required for a full understanding of the covering fraction and velocity
dispersion of the outflowing neutral gas in LBGs, and its relationship to the
escape fraction of Lyman continuum radiation in galaxies at z~3.Comment: 28 pages including 17 figures. Accepted for publication in Ap
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
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
On the Nature of Unconfirmed Supernovae
We study the nature of 39 unconfirmed supernovae (SNe) from the sky area
covered by the Sloan Digital Sky Survey (SDSS) Data Release 8 (DR8), using
available photometric and imaging data and intensive literature search. We
confirm that 21 objects are real SNe, 2 are Galactic stars, 4 are probable SNe,
and 12 remain unconfirmed events. The probable types for 4 objects are
suggested: 3 SNe are of probable type Ia, and SN 1953H is probable type II SN.
In addition, we identify the host galaxy of SN 1976N and correct the
offsets/coordinates of SNe 1958E, 1972F, and 1976N.Comment: 9 pages, 4 figures, 5 tables, published in Astrophysics (English
translation of Astrofizika
Supernovae in paired galaxies
We investigate the influence of close neighbor galaxies on the properties of
supernovae (SNe) and their host galaxies using 56 SNe located in pairs of
galaxies with different levels of star formation (SF) and nuclear activity. The
mean distance of type II SNe from nuclei of hosts is greater by about a factor
of 2 than that of type Ibc SNe. The distributions and mean distances of SNe are
consistent with previous results compiled with the larger sample. For the first
time it is shown that SNe Ibc are located in pairs with significantly smaller
difference of radial velocities between components than pairs containing SNe Ia
and II. We consider this as a result of higher star formation rate (SFR) of
these closer systems of galaxies.Comment: 2 pages. arXiv admin note: text overlap with arXiv:1312.494
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