Pixel and Micro-lensing with NGST

Within 8 years, the current microlensing surveys of M31 will provide several hundred events affecting unresolved stars. They will thus allow a statistical study of the dark matter in M31's halo. The NGST will resolve these stars and constrain the mass of the corresponding lenses. In case of on-line alerts from ground-based observations, real-time NGST follow-up with high signal-to-noise ratio will provide further constraints on the lenses. In addition, high resolution observations with NGST will complement XMM and the previous optical data and thus enable a closer insight of X-ray binaries within M31 to be obtained. The optimal instrumentation to achieve these scientific goals will be discussed. Last, the study of the dark matter encompassed in the galaxy clusters would be possible with high angular resolution observations on a large field camera and would open a new field of research.Comment: 5 pages -- presented at the NGST Science and Technology Exposition (Hyannis, USA) 13-16 september 1999, published by the PAS

Molecular content of a type-Ia SN host galaxy at z=0.6

We study the properties and the molecular content of the host of a type-Ia supernova (SN1997ey). This z=0.575 host is the brightest submillimetre source of the sample of type-Ia supernova hosts observed at 450um and 850um by Farrah et al.. Observations were performed at IRAM-30m to search for CO(2-1) and CO(3-2) lines in good weather conditions but no signal was detected. The star formation rate cannot exceed 50 M_sol/yr. These negative results are confronted with an optical analysis of a Keck spectrum and other data archives. We reach the conclusion that this galaxy is a late-type system (0.7 L^B_*), with a small residual star-formation activity (0.2 M_sol/yr) detected in the optical. No source of heating (AGN or starburst) is found to explain the submillimetre-continuum flux and the non-CO detection excludes the presence of a large amount of cold gas. We thus suggest that either the star formation activity is hidden in the nucleus (with A_V ~ 4) or this galaxy is passive or anemic and this flux might be associated with a background galaxy.Comment: 8 pages, 7 figures, accepted for publication in MNRA

CO investigation of z=0.4-1.5 galaxies

We report on the results of an IRAM-30m search for CO emission lines in three galaxies at intermediate redshifts. The idea was to investigate the molecular content of galaxies bright in the infrared at z=0.4-1.5, a redshift desert for molecular line studies, poorly investigated as of yet. We integrated 8-10h per source and did not succeed in detecting any of the sources. From our upper limits, we are able to constrain the molecular gas content in these systems to less than 4 to 8 x 10^9 Mo, assuming a CO-to-H_2 conversion factor (\alpha=0.8 Mo/(K km s^-1 pc^2)). We stress the current difficulty of selecting sources with a detectable molecular content, a problem that will be faced by the ALMA First Science projects.Comment: 6 pages, 5 figures. Accepted for publication in Astronomy and Astrophysic

AGAPEROS: Searching for variable stars in the LMC Bar with the Pixel Method. I. Detection, astrometry and cross-identification

We extend the work developed in previous papers on microlensing with a selection of variable stars. We use the Pixel Method to select variable stars on a set of 2.5 x 10**6 pixel light curves in the LMC Bar presented elsewhere. The previous treatment was done in order to optimise the detection of long timescale variations (larger than a few days) and we further optimise our analysis for the selection of Long Timescale and Long Period Variables (LT&LPV). We choose to perform a selection of variable objects as comprehensive as possible, independent of periodicity and of their position on the colour magnitude diagram. We detail the different thresholds successively applied to the light curves, which allow to produce a catalogue of 632 variable objects. We present a table with the coordinate of each variable, its EROS magnitudes at one epoch and an indicator of blending in both colours, together with a finding chart. A cross-correlation with various catalogues shows that 90% of those variable objects were undetected before, thus enlarging the sample of LT&LPV previously known in this area by a factor of 10. Due to the limitations of both the Pixel Method and the data set, additional data -- namely a longer baseline and near infrared photometry -- are required to further characterise these variable stars, as will be addressed in subsequent papers.Comment: 11 pages with 10 figure

Detection of CO in the inner part of M31's bulge

We report the first detection of CO in M31's bulge. The 12CO (1-0) and (2-1) lines are both detected in the dust complex D395A/393/384, at 1.3" (~0.35 kpc) from the centre. From these data and from visual extinction data, we derive a CO-luminosity to reddening ratio (and a CO-luminosity to H_2 column density ratio) quite similar to that observed in the local Galactic clouds. The (2-1) to (1-0) line intensity ratio points to a CO rotational temperature and a gas kinetic temperature > 10 K. The molecular mass of the complex, inside a 25' (100 pc) region, is 1.5 10^4 Mo.Comment: 5 pages including 4 figures (2 in colour

Star formation efficiency in galaxy interactions and mergers: a statistical study

We investigate the enhancement of star formation efficiency in galaxy interactions and mergers, by numerical simulations of several hundred galaxy collisions. All morphological types along the Hubble sequence are considered in the initial conditions of the two colliding galaxies, with varying bulge-to-disk ratios and gas mass fractions. Different types of orbits are simulated, direct and retrograde, according to the initial relative energy and impact parameter, and the resulting star formation history is compared to that occuring in the two galaxies when they are isolated. Our principal results are: (1) retrograde encounters have a larger star formation efficiency (SFE) than direct encounters; (2) the amount of gas available in the galaxy is not the main parameter governing the SFE in the burst phase; (3) there is an anticorrelation between the amplitude of the star forming burst and the tidal forces exerted per unit of time, which is due to the large amount of gas dragged outside the galaxy by tidal tails in strong interactions; (4) globally, the Kennicutt-Schmidt law is retrieved statistically for isolated galaxies, interacting pairs and mergers; (5) the enhanced star formation is essentially occurring in nuclear starbursts, triggered by inward gas flows driven by non-axisymmetries in the galaxy disks. Direct encounters develop more pronounced asymmetries than retrograde ones. Based on these statistical results, we derive general laws for the enhancement of star formation in galaxy interactions and mergers, as a function of the main parameters of the encounter.Comment: 22 pages, 37 figures, 4 tables. Accepted on Astronomy & Astrophysic