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 greater 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 a negative correlation 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 seen to apply statistically for isolated galaxies, interacting pairs and mergers, (5) enhanced star formation occurs essentially 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.

We investigate the intensity enhancement and the duration of starburst episodes triggered by major galaxy interactions and mergers. We analyze two large statistical datasets of numerical simulations. These have been obtained using two independent and different numerical techniques to model baryonic and dark matter evolution that are extensively compared for the first time. One is a Tree-SPH code, the other one is a grid-based N-body sticky-particles code. We show that, at low redshift, galaxy interactions and mergers in general trigger only moderate star formation enhancements. Strong starbursts where the star formation rate is increased by a factor greater than 5 are rare and found only in about 15% of major galaxy interactions and mergers. Merger-driven starbursts are also rather short-lived, with a typical duration of activity of a few 10 8 yr. These conclusions are found to be robust, independent of the numerical techniques and star formation models. At higher redshifts where galaxies contain more gas, gas inflow-induced starbursts are neither stronger nor longer than their local counterparts. In turn, the formation of massive gas clumps, results of local Jeans instability that can occur spontaneously in gas-rich disks or be indirectly favored by galaxy interactions, could play a more important role in determining the duration and intensity of star formation episodes.

To compare photometric properties of galaxies at different redshifts, the fluxes need to be corrected for the changes of effective rest-frame wavelengths of filter bandpasses, called K-corrections. Usual approaches to compute them are based on the template fitting of observed spectral energy distributions (SED) and, thus, require multicolour photometry. Here, we demonstrate that, in cases of widely used optical and near-infrared (NIR) filters, K-corrections can be precisely approximated as two-dimensional low-order polynomials of only two parameters: redshift and one observed colour. With this minimalist approach, we present the polynomial fitting functions for K-corrections in Sloan Digital Sky Survey (SDSS) ugriz, United Kingdom Infrared Telescope (UKIRT) Wide Field Camera Y J H K, JohnsonCousins UBV R c I c and Two Micron All Sky Survey J H K s bands for galaxies at redshifts Z < 0.5 based on empirically computed values obtained by fitting combined optical-NIR SEDs of a set of 10 5 galaxies constructed from SDSS Data Release 7 (DR7) and UKIRT Infrared Deep Sky Survey DR5 photometry using the Virtual Observatory. For luminous red galaxies we provide K-corrections as functions of their redshifts only. In two filters, g and r, we validate our solutions by computing K-corrections directly from SDSS DR7 spectra. We also present a K-corrections calculator, a web-based service for computing K-corrections online.

We present the GalMer database, a library of galaxy merger simulations, that has been produced and made available to users by means of tools compatible with the Virtual Observatory (VO) standards adapted specially for this theoretical database. To investigate the physics of galaxy formation through hierarchical merging, it is necessary to simulate galaxy interactions varying a large number of parameters, e.g. morphological types, mass ratios, orbital configurations. On the one hand, these simulations have to be performed in a cosmological context, capable of providing a large number of galaxy pairs, with boundary conditions given by the large-scale simulation. On the other hand, the resolution has to be high enough on galaxy scales, to provide realistic physics. The GalMer database is a library of thousands of simulations of galaxy mergers at moderate spatial resolution and represents a compromise between considering a diverse range of initial conditions and optimising the details of underlying physics. We provide all coordinates and data of simulated particles in FITS binary tables. The main advantages of the database are VO access interfaces and valueadded services that allow users to compare the results of the simulations directly to observations: stellar population modelling, dust extinction, spectra, images, visualisation using dedicated VO tools. The GalMer value-added services can be used as a virtual telescope producing broadband images, 1D spectra, 3D spectral datacubes, thus enhancing the utility of our database to observers. We present several examples of the GalMer database scientific usage obtained by analyzing simulations and modelling their stellar population properties, including: (1) studies of the star formation efficiency in interactions; (2) creation of old counter-rotating components; (3) reshaping metallicity profiles in elliptical galaxies; (4) orbital to internal angular momentum transfer; (5) reproducing observed colour bimodality of galaxies.

Abstract.We have carried out a survey of the Andromeda galaxy for unresolved microlensing (pixel lensing). We present a subset of four short timescale, high signal-to-noise microlensing candidates found by imposing severe selection criteria: the source flux variation exceeds the flux of an R = 21 magnitude star and the full width at half maximum timescale is less than 25 days. Remarkably, in three out of four cases, we have been able to measure or strongly constrain the Einstein crossing time of the event. One event, which lies projected on the M 31 bulge, is almost certainly due to a stellar lens in the bulge of M 31. The other three candidates can be explained either by stars in M 31 and M 32 or by MACHOs.

We report the discovery of a short-duration microlensing candidate in the northern field of the POINT-AGAPE pixel lensing survey toward M31. Almost certainly, the source star has been identified on

POINT-AGAPE is an Anglo-French collaboration which is employing the Isaac Newton Telescope (INT) to conduct a pixel-lensing survey towards M31. Pixel lensing is a technique which permits the detection of microlensing against unresolved stellar fields. The survey aims to constrain the stellar population in M31 and the distribution and nature of massive compact halo objects (MACHOs) in both M31 and the Galaxy.In this paper we investigate what we can learn from pixel-lensing observables about the MACHO mass and fractional contribution in M31 and the Galaxy for the case of spherically-symmetric near-isothermal haloes. We employ detailed pixel-lensing simulations which include many of the factors which affect the observables, such as non-uniform sampling and signal-to-noise ratio degradation due to changing observing conditions. For a maximum MACHO halo we predict an event rate in V of up to 100 per season for M31 and 40 per season for the Galaxy. However, the Einstein radius crossing time is generally not measurable and the observed full-width halfmaximum duration provides only a weak tracer of lens mass. Nonetheless, we find that the near-far asymmetry in the spatial distribution of M31 MACHOs provides significant information on their mass and density contribution. We present a likelihood estimator for measuring the fractional contribution and mass of both M31 and Galaxy MACHOs which permits an unbiased determination to be made of MACHO parameters, even from data-sets strongly contaminated by variable stars. If M31 does not have a significant population of MACHOs in the mass range 10 −3 M ⊙ − 1 M ⊙ strong limits will result from the first season of INT observations. Simulations based on currently favoured density and mass values indicate that, after three seasons, the M31 MACHO parameters should be constrained to within a factor four uncertainty in halo fraction and an order of magnitude uncertainty in mass (90% confidence). Interesting constraints on Galaxy MACHOs may also be possible. For a campaign lasting ten years, comparable to the lifetime of current LMC surveys, reliable estimates of MACHO parameters in both galaxies should be possible.

A B S T R A C TWe report the first detection of CO in the bulge of M31. The 12 CO (1±0) and (2±1) lines are both detected in the dust complex D395A/393/384, at 1.3 arcmin (,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 of .10 K. The molecular mass of the complex, inside a 25-arcsec (100 pc) region, is 1X5 Â 10 4 M ( X