30 research outputs found
HI and star formation in the most metal-deficient galaxies
We present Giant Metrewave Radio Telescope (GMRT) observations for three
(viz., DDO 68, SDSS J2104-0035 and UGC 772) of the six most metal-deficient
actively star-forming galaxies known. Although there is a debate as to whether
these galaxies are undergoing their first episode of star formation or not,
they are `young' in the sense that their ISM is chemically unevolved. In this
regard, they are the nearest equivalents of young galaxies in the early
Universe. All three galaxies, that we have observed, have irregular HI
morphologies and kinematics, which we interpret as either due to tidal
interaction with neighbouring galaxies, or the consequences of a recent merger.
The remaining three of the six most metal-deficient galaxies are also known to
have highly disturbed HI distributions and are interacting. It is interesting
because these galaxies were chosen solely on the basis of their metallicity and
not for any particular signs of interaction. In this sense (i.e., their gas has
not yet had time to settle into a regular disc), one could regard these
extremely metal deficient (XMD) galaxies as `young'. The current star formation
episode is likely to have been triggered by interaction/merger. It is also
possible that the tidal interaction has lead to enhanced mixing with metal-poor
gas in outer disc, and hence to a low gas-phase metallicity in the central
star-forming regions. We also find that in general these galaxies do not show a
one-to-one correspondence between regions of high HI column density and regions
with current star formation. However, to the extent that one can define a
threshold density, its value (~10^{21} atoms cm^{-2}) is similar to that in
galaxies with much higher metallicity.Comment: 11 pages, 11 figures. Accepted for publication in MNRA
HI in very metal-poor galaxies: the SBS 0335-052 system
We present Giant Metrewave Radio Telescope (GMRT), HI 21cm observations of
SBS 0335-052E and SBS 0335-052W, a close pair of dwarf galaxies, which are
further unusual in being the most metal-poor star-forming galaxies known. We
present images at several angular resolutions, ranging from ~40 to 4 arcsec.
These images show that SBS 0335-052 is a strongly interacting system, with a
faint diffuse HI bridge seen at low resolution, and elongated tails seen at the
higher resolutions. The overall morphology suggests that the pair represents a
major merger of extremely gas-rich galaxies. The low-resolution velocity field
is dominated by the velocity difference between the two galaxies and the
velocity gradient along the tidal features. However, for SBS 0335-052W at
least, at high angular resolution, one sees a central velocity field that could
be associated with the spin of the original undisturbed disc. The highest
angular resolution HI images show that the ionized superbubble, identified by
Thuan, Izotov & Lipovetsky (1997), in the Hubble Space Telescope (HST) images
of SBS 0335-052E, is extended along one of the diffuse tidal features, and that
there is a high-density HI clump at the other end of the superbubble. The star
formation in SBS 0335-052E occurs mainly in a group of superstar clusters
(SSCs) with a clear age gradient; the age decreases as one approaches the dense
HI clump. We suggest that this propagating star formation is driven by the
superbubble expanding into a medium with a tidally-produced density gradient.
The high pressures associated with the compressed material would also naturally
explain why current star formation is mainly concentrated in superstar
clusters.Comment: Accepted for publication in MNRA
Spectroscopic analysis of finite size effects around a Kondo quantum dot
We consider a simple setup in which a small quantum dot is strongly connected
to a finite size box. This box can be either a metallic box or a finite size
quantum wire.The formation of the Kondo screening cloud in the box strongly
depends on the ratio between the Kondo temperature and the box level spacing.
By weakly connecting two metallic reservoirs to the quantum dot, a detailed
spectroscopic analysis can be performed. Since the transport channels and the
screening channels are almost decoupled, such a setup allows an easier access
to the measure of finite-size effects associated with the finite extension of
the Kondo cloud.Comment: contribution to Les Houches proceeding, ``Quantum magnetism'' 200
Low-Surface-Brightness Galaxies in the Sloan Digital Sky Survey. I. Search Method and Test Sample
In this paper we present results of a pilot study to use imaging data from
the Sloan Digital Sky Survey (SDSS) to search for low-surface-brightness (LSB)
galaxies. For our pilot study we use a test sample of 92 galaxies from the
catalog of Impey et al. (1996) distributed over 93 SDSS fields of the Early
Data Release (EDR). Many galaxies from the test sample are either LSB or dwarf
galaxies. To deal with the SDSS data most effectively a new photometry software
was created, which is described in this paper. We present the results of the
selection algorithms applied to these 93 EDR fields. Two galaxies from the
Impey et al. test sample are very likely artifacts, as confirmed by follow-up
imaging. With our algorithms, we were able to recover 87 of the 90 remaining
test sample galaxies, implying a detection rate of 96.5%. The three
missed galaxies fall too close to very bright stars or galaxies. In addition,
42 new galaxies with parameters similar to the test sample objects were found
in these EDR fields (i.e., 47% additional galaxies). We present the main
photometric parameters of all identified galaxies and carry out first
statistical comparisons. We tested the quality of our photometry by comparing
the magnitudes for our test sample galaxies and other bright galaxies with
values from the literature. All these tests yielded consistent results. We
briefly discuss a few unusual galaxies found in our pilot study, including an
LSB galaxy with a two-component disk and ten new giant LSB galaxies.Comment: 36 pages, 16 figures, accepted for publication by AJ, some figures
were bitmapped to reduce the siz
On the Nature of the Apparent Ring Galaxy SDSS J075234.33+292049.8
(abridged) An object classified as a galaxy in on-line data bases and
revealed on sky survey images as a distant ring galaxy is a rare case of polar
ring galaxy where the ring is only slightly inclined to the equatorial plane of
the central body. SDSS imaging indicates that the diameter of the ring is about
36 kpc. The SDSS data was combined with long-slit spectroscopic observations
and with Fabry-Perot Interferometer H-beta mapping obtained at the Russian
Academy of Sciences 6-m telescope. We derived the complex morphologies of this
presumed ring galaxy from a combination of SDSS images and from the kinematical
behaviour of the central body and of the ring, and determined the stellar
population compositions of the two components from SDSS colours, spectroscopy,
and evolutionary stellar synthesis models. The ring metallicity is slightly
under-abundant. The total luminosity and the total mass of the system are not
extreme, but the rather high M/L~20 indicates the presence of large amounts of
dark matter. Two alternative explanations of this object are proposed (1) a
ring formed by two semi-circular and tight spiral arms at the end of a central
bar with a warp or precession of the ring material. The object could,
therefore, be explained as an extreme SBa(R) galaxy, or (2) a Polar Ring Galaxy
where the inner object is an S0 and the ring is significantly more luminous
than the central object. The compound object would then be similar to the NGC
4650A galaxy, being a rare object with a polar component only modestly inclined
to the equatorial plane of the central body. Arguments for (and against) both
explanations are given and discussed, with the second alternative being more
acceptable.Comment: 16 pages, 11 figures, MNRAS in pres
[CII] 158m Emission and Metallicity in PDRs
We study the effects of a metallicity variation on the thermal balance and
[CII] fine-structure line strengths in interstellar photon dominated regions
(PDRs). We find that a reduction in the dust-to-gas ratio and the abundance of
heavy elements in the gas phase changes the heat balance of the gas in PDRs.
The surface temperature of PDRs decreases as the metallicity decreases except
for high density ( cm) clouds exposed to weak () FUV
fields where vibrational H-deexcitation heating dominates over
photoelectric heating of the gas. We incorporate the metallicity dependence in
our KOSMA- PDR model to study the metallicity dependence of [CII]/CO line
ratios in low metallicity galaxies. We find that the main trend in the
variation of the observed CII/CO ratio with metallicity is well reproduced by a
single spherical clump, and does not necessarily require an ensemble of clumps
as in the semi-analytical model presented by Bolatto et al. (1999).Comment: 16 pages, 14 figures, accepted by A&