Ph. Prugniel

Evolutionary synthesis of galaxies at high spectral resolution with the code PEGASE-HR. Metallicity and age tracers

We present PEGASE-HR, a new stellar population synthesis program generating high resolution spectra (R = 10 000) over the optical range λλ = 400-680 nm. It links the spectro-photometric model of galaxy evolution PEGASE.2 (Fioc & Rocca-Volmerange 1997) to an updated version of the ELODIE library of stellar spectra observed with the 193 cm telescope at the Observatoire de Haute-Provence (Prugniel & Soubiran 2001a). The ELODIE star set gives a fairly complete coverage of the Hertzprung-Russell (HR) diagram and makes it possible to synthesize populations in the range (Fe/H) = − 2t o+0.4. This code is an exceptional tool for exploring signatures of metallicity, age, and kinematics. We focus on a detailed study of the sensitivity to age and metallicity of the high-resolution stellar absorption lines and of the classical metallic indices proposed until now to solve the age-metallicity degeneracy. Validity tests on several stellar lines are performed by comparing our predictions for Lick indices to the models of other groups. The comparison with the lower resolution library BaSeL (Lejeune et al. 1997) confirms the quality of the ELODIE library when used for single stellar populations (SSPs) from 10 7 to 2 × 10 10 yr. Predictions for the evolved populations of globular clusters and elliptical galaxies are given and compared to observational data. Two new high- resolution indices are proposed around the Hγ line. They should prove useful in the analysis of spectra from the new generation of telescopes and spectrographs.

Spectroscopic ages and metallicities of stellar populations: validation of full spectrum fitting

Fitting whole spectra at intermediate spectral resolution (R= 1000–3000), to derive physical properties of stellar populations, appears as an optimized alternative to methods based on spectrophotometric indices: it uses all the redundant information contained in the signal. This paper addresses the validation of the method and it investigates the quality of the population models together with the reliability of the fitting procedures. Our method compares observed optical spectra with models to derive the age, metallicity and line broadening due to the internal kinematics. It is insensitive to the shape of the continuum and the results are consistent with Lick indices but three times more precise. We are using two algorithms: steckmap, a non-parametric regularized program and nbursts a parametric non-linear minimization. We compare three spectral synthesis models for single stellar populations (SSPs): Pegase-HR, Galaxev and Vazdekis/Miles, and we analyse spectra of Galactic clusters whose populations are known from studies of colour–magnitude diagrams (CMD) and spectroscopy of individual stars. We find the following. (1) The quality of the models critically depends on the stellar library they use, and in particular on its coverage in age, metallicity and surface gravity of the stars. Pegase-HR and Vazdekis/Miles are consistent, while the comparison between Pegase-HR and Bruzual & Charlot shows some systematics reflecting the limitations of the stellar library (STELIB) used to generate the latter models. (2) The two fitting programs are consistent. (3) For globular clusters and M67 spectra, the method restitutes metallicities in agreement with spectroscopy of stars with a precision of 0.14 dex. (4) The spectroscopic ages are very sensitive to the presence of a blue horizontal branch (BHB) or of blue stragglers. A BHB morphology results in a young SSP-equivalent age. Fitting a free amount of blue stars in addition to the SSP model to mimic the BHB improves and stabilizes the fit and restores ages in agreement with CMDs studies. This method is potentially able to disentangle age or BHB effects in extragalactic clusters. Full spectrum fitting is reliable method to derive the parameters of a stellar population.

Kinematical data on early-type galaxies. VI

We present the result of spectroscopic observations of a sample of 73 galaxies, completing the database published in this series of articles. The sample contains mostly low-luminosity early-type objects, including four dwarfs of the Local Group (in particular, deep spectra of NGC 205), 15 dEs or dS0s in the Virgo cluster, and UGC 05442, a spheroidal dwarf of the M 81 group. We have measured the central velocity dispersion for all but one object, and determined the major-axis rotation and velocity-dispersion profiles for 59 objects. For the current sample of diffuse (or dwarf) elliptical galaxies, we have compared stellar rotation to velocity dispersion; the analysis suggests that these objects may be nearly rotationally flattened, and therefore that anisotropy may be less important than previously thought.

The X-shooter Spectral Library (XSL) - I. DR1: Near-ultraviolet through optical spectra from the first year of the survey

We present the first release of the X-shooter Spectral Library (XSL). This release contains 237 stars. The spectra in this release span a wavelength range of 3000-10 200 A and have been observed at a resolving power of R ≡ λ/ Δλ ~ 10 000. The spectra were obtained at ESOs 8-m Very Large Telescope (VLT). The sample contains O-M, long-period variable, C and S stars. The spectra are flux-calibrated and telluric-corrected. We describe a new technique for the telluric correction. The wavelength coverage, spectral resolution, and spectral type of this library make it well suited to stellar population synthesis of galaxies and clusters, kinematical investigation of stellar systems, and the study of the physics of cool stars. Full Table 3 and Table A.1 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/565/A117

Structure and stellar content of dwarf galaxies - III.

We have carried out CCD photometry in the Cousins B and R bands of 21 dwarf galaxies in and around the M101 group. Eleven are members and suspected members of the group and ten are field galaxies in the projected vicinity of the group. We have derived total magnitudes, effective radii, effective surface brightnesses, as well as galaxy diameters at various isophotal levels in both colours. Best-fitting exponential parameters and colour gradients are also given for these galaxies. Some of the galaxies show a pronounced luminosity excess above the best-fitting exponential at large radii, or surface brightnesses fainter than in R . This feature, while non-significant for a single case and technically difficult to interpret, might be an indication of the existence of an extended old stellar halo in dwarf irregulars. The photometric parameters of the galaxies presented here will be combined with previously published data for nearby dwarf galaxies and statistically analysed in a forthcoming paper.

B

We have carried out CCD photometry in the Cousins B and R bands of 25 dwarf galaxy members and suspected members of the M 81 group of galaxies. Based on azimuthally averaged brightness proles we have de- rived total magnitudes, eective radii, eective surface brightnesses, as well as galaxy diameters at various isopho- tal levels in both photometric bands. Best-tting exponen- tial parameters and B R colour gradients are also given for these galaxies. The data will be interpreted, along with those of another supplementary paper, in Paper III of this series (to appear in the main journal). In two appendices we discuss the nature of a clustering of unusual low-surface brightness objects in the south-east corner of the M 81 group and show the luminosity function of its presently known members.

and

Aims. We compare six popularly used evolutionary population synthesis (EPS) models by fitting the full optical spectra of six representative types of galaxies (star-forming and composite galaxies, Seyfert 2s, LINERs, E+A, and early-type galaxies) taken from the Sloan Digital Sky Survey (SDSS). We explore the dependence of stellar population synthesis results on the main ingredients of the EPS models and study whether there is an age sequence among these types of galaxies. Methods. We use the simple stellar populations (SSPs) of each EPS model and the software STARLIGHT to perform our fits. Firstly, we explore the dependence of stellar population synthesis on EPS models by fixing the age, metallicity, and initial mass function (IMF) to construct a standard SSP group. We then use the standard SSP group of each EPS model (BC03, CB07, Ma05, GALEV, GRASIL, and Vazdekis/Miles) to fit the spectra of star-forming and E+A galaxies. Secondly, we fix the IMF and alter the age and metallicity to construct eight additional SSP groups. We then use these SSP groups to fit the spectra of star-forming and E+A galaxies to verify the effects of age and metallicity on stellar populations. Finally, we study the effect of stellar evolution tracks and stellar spectral libraries on our results, and present a possible age sequence among these types of galaxies. Results. Using different EPS models, the numerical values of contributing light fractions obviously change, even though the dominant populations are unaltered. The stellar population synthesis does depend on the selection of age and metallicity, but does not depend significantly on the stellar evolution track. The importance of young populations decreases from star-forming, composite, Seyfert 2, LINER, to early-type galaxies, and the properties of E+A galaxies are between composite galaxies and Seyfert 2s in most cases. Conclusions. Different EPS models infer different stellar population parameters, so that it is not reasonable to directly compare stellar populations estimated from different EPS models. To obtain reliable results, we should use the same EPS model to derive the parameter values that we compare.

R

Context. NGC404 is a nearly face-on nearby low-luminosity lenticular galaxy. Probing its characteristics provides a wealth of information on the details of possible evolution processes of dS0 galaxies which may not be possible in other, more distant objects. Aims. We study the internal kinematics and the spatial distribution of the star formation history in NGC404. Methods. We obtained long slit spectroscopy at the OHP 1m93 telescope along the major and minor axes of NGC404. The spectra have a resolution R = 3600 covering a wavelength range from 4600 to 5500 A. The data are fitted against the Pegase.HR stellar population models to derive simultaneously the internal stellar kinematics, ages and metallicities. All this is done while taking into account any instrumental contamination to the line of sight velocity distribution. Firstly, the global properties of the galaxy are analyzed by fitting a single model and to the data and looking at the kinematic variations and SSP equivalent age and metallicities as a function of radius. Afterwards, the stellar populations are decomposed into 4 components that are individually analyzed. Results. NGC404 clearly shows two radial velocity inversions along its major axis. The kinematically decoupled core rotates in the same direction as the neutral hydrogen shell that surrounds the galaxy. We resolved the star formation history in the core of the galaxy ino 4 events: A very young (< 150 Myr, and [Fe/H] = 0.4) component with constant on-going star formation, a second young (430 Myr) component with [Fe/H] = 0.1, an intermediate population (1.7 Gyr) which has [Fe/H] = -0.05 and, finally, an old (12 Gyr) component with [Fe/H] = -1.26. The two young components fade very quickly with radius, leaving only the intermediate and old population at a radius of 25 00 (370 pc) from the centre. Conclusions. We conclude that NGC404 had a spiral morphology about 1 Gyr ago and that one or many merger events has triggered a morphological transition. The interstellar medium in the galaxy has two components, the cold molecular gas is most probably a remnant from its past spiral incarnation and the outer neutral hydrogen layer which has probably been acquired in one of the latest merger.

photometry of dwarf galaxies in the M 101 group and the nearby field

The formation and evolution of galaxies is imprinted on their stellar population radial gradients. Two recent articles present conflicting results concerning the mass dependence of the metallicity gradients for early-type dwarf galaxies. On one side, Spolaor et al. (2009) show a tight positive correlation between the total metallicity [Z/H] and the mass. On the other side, in a distinct sample, we do not find any trend involving [Fe/H] (Koleva et al. 2009). In order to investigate the origin of the discrepancy, we examine various factors that may affect the determination of the gradients: namely the sky subtraction and the signal-to-noise ratio. We conclude that our detection of gradients are well above the possible analysis biases. Then, we measured the [Mg/Fe] relative abundance profile and found moderate gradients. The derived [Z/H] gradients scatter around -0.4 dex/r_e. The two samples contain the same types of objects and the reason of the disagreement is still not understood. Based on observations made with ESO telescopes at La Silla Paranal observatory under program ID076.B-0196

Structure and stellar content of dwarf galaxies - IV.

We present R-band and Hα+[N ii] narrow-band imaging of FCC032, FCC206 and FCCB729, three dwarf elliptical galaxies (dEs) in the Fornax Cluster. These dEs contain significant amounts of ionised gas. FCC032 harbours a large ionised gas complex, consisting of several individual clouds, a superbubble and a filament that extends away from the galaxy centre. The ionised gas structures observed in FCC032 bear a strong resemblance to those observed in more gas-rich and more fiercely star-forming dwarf galaxies. FCC206, a very low surface brightness dE, contains one faint extended emission region, and two compact clouds. In FCCB729, the only nucleated galaxy in this sample, one of the ionised gas clouds coincides with the stellar nucleus. We derive ionised gas masses of a few 10 to 10 M⊙ for these galaxies. This brings our sample of dEs with ionised gas with Hα+[N ii] narrow-band imaging to five objects. The ionised gas morphologies in these galaxies range from pure nuclear emission peaks to extended emission complexes. This morphological diversity could also indicate a diversity in ionising processes in dEs with a warm interstellar medium (active galactic nuclei, starbursts, post-AGB stars,. . . ). Inside FCC206, four star clusters appear to be on the verge of merging to form a nucleus in this presently non-nucleated dE. Understanding the formation of nuclei in dEs could give us important clues to the formation of super-massive black holes (SMBHs).