150 research outputs found
The incidence of mid-infrared excesses in G and K giants
Using photometric data from the 2MASS and GLIMPSE catalogues, I investigate
the incidence of mid-infrared excesses (~10 microns) of G and K stars of
luminosity class III. In order to obtain a large sample size, stars are
selected using a near-IR colour-magnitude diagram. Sources which are candidates
for showing mid-IR excess are carefully examined and modelled to determined
whether they are likely to be G/K giants. It is found that mid-IR excesses are
present at a level of (1.8 +/- 0.4) x 10^-3. While the origin of these excesses
remains uncertain, it is plausible that they arise from debris discs around
these stars. I note that the measured incidence is consistent with a scenario
in which dust lifetimes in debris discs are determined by Poynting-Robertson
drag rather than by collisions.Comment: Accepted for publication in MNRAS. 13 pages, 5 figures, 2 tables (1
landscape table
Filtering of Signal Dependent Noise Applied to MIPS Data
Linear filtering and usual nonlinear median filtering are not effective for signal-dependent noise removal. We apply here an approximate decoupling of signal and noise by means of a nonlinear transform. The transform is followed by a linear filter and the corresponding inverse transform. This procedure allows us to mitigate the signal-dependent noise in the images obtained by the Multiband Imaging Photometer for Spitzer (MIPS), 70μm imaging band
A new twist to an old story: HE 0450-2958, and the ULIRG(optically bright QSO) transition hypothesis
We report on interferometric imaging of the CO J=1--0 and J=3--2 line
emission from the controversial QSO/galaxy pair HE 0450--2958. {\it The
detected CO J=1--0 line emission is found associated with the disturbed
companion galaxy not the luminous QSO,} and implies , which is \ga 30% of the dynamical mass in
its CO-luminous region. Fueled by this large gas reservoir this galaxy is the
site of an intense starburst with , placing
it firmly on the upper gas-rich/star-forming end of Ultra Luminous Infrared
Galaxies (ULIRGs, ). This makes HE 0450--2958 the
first case of extreme starburst and powerful QSO activity, intimately linked
(triggered by a strong interaction) but not coincident. The lack of CO emission
towards the QSO itself renews the controversy regarding its host galaxy by
making a gas-rich spiral (the typical host of Narrow Line Seyfert~1 AGNs) less
likely. Finally, given that HE 0450--2958 and similar IR-warm QSOs are
considered typical ULIRG(optically bright QSO) transition candidates, our
results raise the possibility that some may simply be {\it gas-rich/gas-poor
(e.g. spiral/elliptical) galaxy interactions} which ``activate'' an optically
bright unobscured QSO in the gas-poor galaxy, and a starburst in the gas-rich
one. We argue that such interactions may have gone largely unnoticed even in
the local Universe because the combination of tools necessary to disentagle the
progenitors (high resolution and S/N optical {\it and} CO imaging) became
available only recently.Comment: 25 pages, 5 figures, accepted for publication by The Astrophysical
Journa
An Integrated Spectrophotometric Survey of Nearby Star-Forming Galaxies
We present integrated optical spectrophotometry for a sample of 417 nearby
galaxies. Our observations consist of spatially integrated, S/N=10-100
spectroscopy between 3600 and 6900 Angstroms at ~8 Angstroms FWHM resolution.
In addition, we present nuclear (2.5"x2.5") spectroscopy for 153 of these
objects. Our sample targets a diverse range of galaxy types, including
starbursts, peculiar galaxies, interacting/merging systems, dusty,
infrared-luminous galaxies, and a significant number of normal galaxies. We use
population synthesis to model and subtract the stellar continuum underlying the
nebular emission lines. This technique results in emission-line measurements
reliably corrected for stellar absorption. Here, we present the integrated and
nuclear spectra, the nebular emission-line fluxes and equivalent widths, and a
comprehensive compilation of ancillary data available in the literature for our
sample. In a series of subsequent papers we use these data to study optical
star-formation rate indicators, nebular abundance diagnostics, the
luminosity-metallicity relation, the dust properties of normal and starburst
galaxies, and the star-formation histories of infrared-luminous galaxies.Comment: Accepted for publication in ApJ
Thermal Emission from HII Galaxies: Discovering the Youngest Systems
We studied the radio properties of very young massive regions of star
formation in HII galaxies, with the aim of detecting episodes of recent star
formation in an early phase of evolution where the first supernovae start to
appear. Our sample consists of 31 HII galaxies, characterized by strong
Hydrogen emission lines, for which low resolution VLA 3.5cm and 6cm
observations were obtained. The radio spectral energy distribution has a range
of behaviours; 1) there are galaxies where the SED is characterized by a
synchrotron-type slope, 2) galaxies with a thermal slope, and, 3) galaxies with
possible free-free absorption at long wavelengths. The latter SEDs were found
in a few galaxies and represent a signature of heavily embedded massive star
clusters closely related to the early stages of massive star formation. Based
on the comparison of the star formation rates determined from the recombination
lines and those determined from the radio emission we find that SFR(Ha) is on
average five times higher than SFR(1.4GHz). We confirm this tendency by
comparing the ratio between the observed flux at 20 cm and the expected one,
calculated based on the Ha star formation rates, both for the galaxies in our
sample and for normal ones. This analysis shows that this ratio is a factor of
2 smaller in our galaxies than in normal ones, indicating that they fall below
the FIR/radio correlation. These results suggest that the emission of these
galaxies is dominated by a recent and massive star formation event in which the
first supernovae (SN) just started to explode. We conclude that the systematic
lack of synchrotron emission in those systems with the largest equivalent width
of Hb can only be explained if those are young starbursts of less than 3.5Myr
of age.Comment: Accepted for publication in Ap
The molecular polar disc in NGC 2768
We present CO(1-0) and CO(2-1) maps of the molecular polar disc in the
elliptical galaxy NGC 2768 obtained at the IRAM Plateau de Bure Interferometer.
The maps have a resolution of 2.6" x 2.3" and 1.2" x 1.2" for the CO(1-0) and
CO(2-1) lines, respectively. The CO maps complete the unique picture of the
interstellar medium (ISM) of NGC 2768; the dust, molecular gas, ionised gas and
neutral hydrogen (HI) trace the recent acquisition of cold and cool gas over
two orders of magnitude in radii (and much more in density). In agreement with
the other ISM components, the CO distribution extends nearly perpendicularly to
the photometric major axis of the galaxy. Velocity maps of the CO show a
rotating polar disc or ring in the inner kiloparsec. This cool gas could lead
to kinematic substructure formation within NGC 2768. However, the stellar
velocity field and H-beta absorption linestrength maps from the optical
integral-field spectrograph SAURON give no indication of a young and
dynamically cold stellar population coincident with the molecular polar disc.
Very recent or weak star formation, undetectable in linestrengths, nevertheless
remains a possibility and could be at the origin of some of the ionised gas
observed. Millimetre continuum emission was also detected in NGC 2768, now one
of only a few low-luminosity active galactic nuclei with observed millimetre
continuum emission.Comment: Accepted for publication in MNRAS, 11 pages, 8 figure
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