6,562 research outputs found
Metallicity inhomogeneities in local star-forming galaxies as sign of recent metal-poor gas accretion
We measure the oxygen metallicity of the ionized gas along the major axis of
seven dwarf star-forming galaxies. Two of them, SDSSJ1647+21 and SDSSJ2238+14,
show 0.5 dex metallicity decrements in inner regions with enhanced
star-formation activity. This behavior is similar to the metallicity drop
observed in a number of local tadpole galaxies by Sanchez Almeida et al. (2013)
and interpreted as showing early stages of assembling in disk galaxies, with
the star formation sustained by external metal-poor gas accretion. The
agreement with tadpoles has several implications: (1) it proves that galaxies
other than the local tadpoles present the same unusual metallicity pattern. (2)
Our metallicity inhomogeneities were inferred using the direct method, thus
discarding systematic errors usually attributed to other methods. (3) Taken
together with the tadpole data, our findings suggest a threshold around one
tenth the solar value for the metallicity drops to show up. Although galaxies
with clear metallicity drops are rare, the physical mechanism responsible for
them may sustain a significant part of the star-formation activity in the local
Universe. We argue that the star-formation dependence of the mass-metallicity
relationship, as well as other general properties followed by most local disk
galaxies, are naturally interpreted as side effects of pristine gas infall.
Alternatives to the metal poor gas accretion are examined too.Comment: Accepted for publication in ApJ. 10 pages. 5 Fig
Temperature constraints on the coldest brown dwarf known WISE 0855-0714
Context. Nearby isolated planetary mass objects are beginning to be
discovered, but their individual properties are poorly constrained because
their low surface temperatures and strong molecular self-absorption make them
extremely faint.
Aims. We aimed to detect the near infrared emission of the coldest brown
dwarf (BD) found so far, WISE08550714, located 2.2 pc away, and to
improve its temperature estimate (T= 225-260 K) from a comparison
with state-of-the-art models of BD atmospheres.
Methods. We observed the field containing WISE0855-0714 with HAWK-I at the
VLT in the band. For BDs with T500\,K theoretical models
predict strong signal (or rather less molecular absorption) in this band.
Results. WISE0855-0714 was not detected in our Y-band images, thus placing an
upper limit on its brightness to Y>24.4 mag at 3- level, leading to
Y-[4.5]>10.5. Combining this limit with previous detections and upper limits at
other wavelengths, WISE08550714 is confirmed as the reddest BD detected,
further supporting its status as the coldest known brown dwarf. We applied
spectral energy distribution fitting with collections of models from two
independent groups for extremely cool BD atmospheres leading to an effective
temperature of T250\,K,.Comment: 4 pages, 4 figures. A&A letter Accepte
Shot Noise Enhancement in Resonant Tunneling Structures in a Magnetic Field
We have observed that the shot noise of tunnel current, I, in
GaSb-AlSb-InAs-AlSb-GaSb double-barrier structure under a magnetic field can
exceed 2qI. The measurements were done at T=4K in fields up to 5T parallel to
the current. The noise enhancement occurred at each of the several
negative-differential conductance regions induced by the tunneling of holes
through Landau levels in the InAs quantum well. The amount of the enhancement
increased with the strength of the negative conductance and reached values up
to 8qI. These results are explained qualitatively by fluctuations of the
density of states in the well, but point out the need for a detailed theory of
shot noise enhancement in resonant tunneling devices.Comment: 4 pages, RevTex, 3 figure
Young stars in the periphery of the Large Magellanic Cloud
Despite their close proximity, the complex interplay between the two
Magellanic Clouds, the Milky Way, and the resulting tidal features, is still
poorly understood. Recent studies have shown that the Large Magellanic Cloud
(LMC) has a very extended disk strikingly perturbed in its outskirts. We search
for recent star formation in the far outskirts of the LMC, out to ~30 degrees
from its center. We have collected intermediate-resolution spectra of
thirty-one young star candidates in the periphery of the LMC and measured their
radial velocity, stellar parameters, distance and age. Our measurements confirm
membership to the LMC of six targets, for which the radial velocity and
distance values match well those of the Cloud. These objects are all young
(10-50 Myr), main-sequence stars projected between 7 and 13 degrees from the
center of the parent galaxy. We compare the velocities of our stars with those
of a disk model, and find that our stars have low to moderate velocity
differences with the disk model predictions, indicating that they were formed
in situ. Our study demonstrates that recent star formation occurred in the far
periphery of the LMC, where thus far only old objects were known. The spatial
configuration of these newly-formed stars appears ring-like with a radius of 12
kpc, and a displacement of 2.6 kpc from the LMC's center. This structure, if
real, would be suggestive of a star-formation episode triggered by an
off-center collision between the Small Magellanic Cloud and the LMC's disk.Comment: Accepted for publication in MNRA
- …