A.-L. Melchior

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 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.

On the frequency, intensity and duration of starburst episodes triggered by galaxy interactions and mergers

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.

Analytical approximations of K‐corrections in optical and near‐infrared bands

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 YJHK, Johnson-Cousins U BV 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.

The GalMer database: galaxy mergers in the virtual observatory

We present the GalMer database, a library of galaxy merger simulations, made available to users through tools compatible with the Virtual Observatory (VO) standards adapted specially for this theoretical database. To investigate the physics of gal axy formation through hierarchical merging, it is necessary to simulate galaxy interactions varying a large number of parameters: morphological types, mass ratios, orbital configurations, etc. On one side , these simulations have to be run in a cosmological context, able to provide a large number of galaxy pairs, with boundary conditions given by the large-scale simulations, on the other side the reso lution has to be high enough at galaxy scales, to provide realistic physics. The GalMer database is a library of thousands simulations of galaxy mergers at moderate spatial resolution and it is a compromise between the diversity of initial conditions and the detail s 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 value-added services which allow users to compare the results of the simulations directly to obse rvations: stellar population modelling, dust extinction, spectra, images, v isualisation using dedicated VO tools. The GalMer value-added services can be used as virtual telescope producing broadband images, 1D spectra, 3D spectral datacubes, thus making our database oriented towards the usage by observers. We present several examples of the GalMer database scientific usage obtained from the ana lysis of simulations and modelling their stellar population proper ties, including: (1) studies of the star formation effi ciency in interactions; (2) creation of old counter-rotating components; (3) reshaping metallicity profiles in elliptical galaxies; (4) orbit al to internal angular momentum transfer; (5) reproducing observed colour bimodality of galaxies.

Old stellar counter-rotating components in early-type galaxies from elliptical-spiral mergers

We use numerical simulations to investigate the possibility of forming counter-rotating old stellar components by major mergers between an elliptical and a spiral galaxy. We show that counterrotation can appear both in dissipative and dissipationless retrograde mergers, and it is mostly associated to the presence of a disk component, which preserves part of its initial spin. In turn, the external regions of the two interacting galaxies acquire part of the orbital angular momentum, from the action of tidal forces exerted on each galaxy by the companion.

Molecular gas in the inner 0.7 kpc-radius ring of M 31

The gaseous disc of M31, traced by atomic, ionised and molecular gas together with dust emission, is mainly characterise d by a large ring of 10kpc radius, and an inner ring of 0.7kpc radius, in addition to an underlying spiral structure. This inner ring is conspicuous in dust emission and extinction, and co-exists with an inner disc detected in ionised and atomic gas. Up to now, there was very little neutral gas detected in these inner parts, and the dyn amics of this ring was unknown. We have observed with the IRAM-30m telescope the CO lines in several regions of the inner kpc of M31. While the inner ring is offset by 0.5kpc from the nucleus, the observed positions, selected on the extinction map, are located on the North-Western side of the inner disc, corresponding to the near side. We detect CO in most of these regions, with a complex kinematics. In several points, multiple velocity components are detected with line splittings up to 260 km s −1 . One of the component, the broader one, corresponds to the rotation pattern of the galaxy, while the narrower is in counter-rotation. In addition, the veloc ities measured in the molecular gas do not match the one measured with ionised gas, especially along the minor axis, where one would expect the gas to have the systemic velocity, for a disc in ci rcular rotation. The North-East part of the ring is weak but detected in HI, while a disc speed pattern is tentatively detected af ter removal of the main disc contribution. We have studied several scenarios to account for the abnormal kinematics, in terms of bars, warps or tilted rings. Our best model which reproduces the main observed features assumes that the peculiar component comes from a tilted ring-like material with 40 deg inclination and PA =−35 deg. This ring corresponds to that discovered in the dust emission by Block et al. (2006). It could have formed in the annular wave provoked by the head-on collision, when the companion M32 crossed the plane of M31. The reasons why the ring appears in counter-rotation are discussed.

A cold-gas reservoir to fuel the M 31 nuclear black hole and stellar cluster

With IRAM-30 m/HERA, we have detected CO(2–1) gas complexes within 30 arcsec (∼100 pc) from the center of M 31 that amount to a minimum total mass of 4.2×10 4 M� (one third of the positions are detected). Averaging the whole HERA field, we show that there is no additional undetected diffuse component. Moreover, the gas detection is associated with gas lying on the far side of the M 31 center as no extinction is observed in the optical, but some emission is present on infrared Spitzer maps. The kinematics is complex. (1) The velocity pattern is mainly redshifted: the dynamical center of the gas differs from the black hole position and the maximum of optical emission, and only the redshifted side is seen in our data. (2) Several velocity components are detected in some lines of sight. Our interpretation is supported by the reanalysis of the effect of dust on a complete planetary nebula sample. Two dust components are detected with respective position angles of 37 deg and –66 deg. This is compatible with a scenario where the superposition of the (PA = 37 deg) disk is dominated by the 10 kpc ring and the inner 0.7 kpc ring detected in infrared data, whose position angle (–66 deg) we measured for the first time. The large-scale disk, which dominates the HI data, is steeply inclined (i = 77 deg), warped and superposed on the line of sight on the less inclined inner ring. The detected CO emission might come from both components.

The surface brightness of the Galaxy at the solar neighbourhood

We present a new determination of the surface brightness of our Galaxy at the Solar Neighbourhood as observed from outside the Galaxy. We rely on various existing optical and infra-red surveys to obtain a multiwavelength estimate. On the one hand, scattered light does not contribute significantly to the surface brightness. On the other hand, optical and infrared integrated all-sky surveys (Pioneer 10/11 and COBE/DIRBE) show a systematically larger value than our synthetic local estimate based on Hipparcos data. This local estimate is also compatible with our Galactic simulations normalised at the Solar Neighbourhood and assuming a homogeneous stellar distribution. We interpret this disagreement as a signature of the presence of a local minimum of the stellar density compatible with Goulds belt. According to this result, the global luminosity of the Milky Way should follow the Tully-Fisher relation established for external galaxies.

Molecular content of a Type Ia supernova 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 450 and 850 μm by Farrah et al. Observations were performed at the Institut de Radioastronomie Millimetrique 30 m telescope (IRAM 30 m) 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 ⊙ yr -1 . 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 (0.7L B * ), with a small residual star formation activity (0.2 M ⊙ yr -1 ) detected in the optical. No source of heating (active galactic nucleus 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.

Dust content of core-collapse supernova hosts

We study 13 core-collapse supernova (CCSN) host galaxies at z = 0.1−0.6. Continuum observations at 250 GHz were performed using MAMBO at the IRAM-30 m telescope. No source is detected and the error-weighted mean flux is 0.25 ± 0.32 mJy. Upper limits on their dust masses are derived and the corresponding sample mean corresponds to 1.4 ± 2.2 × 10 8 M� . These results are comparable to previous submillimetre observations of SN-Ia hosts completed by Farrah et al. and Clements et al. We conclude that CCSN hosts are not extreme sources at millimetre wavelengths, and as confirmed using the optical luminosities of a subset of our sample, they are typical of the local galaxy population.