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→\to (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 $\rm M_{gal}(H_2)\sim (1-2)\times 10^{10} M_{\odot}$, 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 $\rm SFR\sim 370 M_{\odot} yr^{-1}$, placing it firmly on the upper gas-rich/star-forming end of Ultra Luminous Infrared Galaxies (ULIRGs, $\rm L_{IR}>10^{12} L_{\odot}$). 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$\to$(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