Bruno Jungwiert

Mass-metallicity relation explored with CALIFA - I. Is there a dependence on the star-formation rate?

We studied the global and local ℳ-Z relation based on the first data available from the CALIFA survey (150 galaxies). This survey provides integral field spectroscopy of the complete optical extent of each galaxy (up to 2−3 effective radii), with a resolution high enough to separate individual H II regions and/or aggregations. About 3000 individual H II regions have been detected. The spectra cover the wavelength range between [OII]3727 and [SII]6731, with a sufficient signal-to-noise ratio to derive the oxygen abundance and star-formation rate associated with each region. In addition, we computed the integrated and spatially resolved stellar masses (and surface densities) based on SDSS photometric data. We explore the relations between the stellar mass, oxygen abundance and star-formation rate using this dataset. We derive a tight relation between the integrated stellar mass and the gas-phase abundance, with a dispersion lower than the one already reported in the literature (σ_Δlog (O/H) = 0.07 dex). Indeed, this dispersion is only slightly higher than the typical error derived for our oxygen abundances. However, we found no secondary relation with the star-formation rate other than the one induced by the primary relation of this quantity with the stellar mass. The analysis for our sample of ~3000 individual H II regions confirms (i) a local mass-metallicity relation and (ii) the lack of a secondary relation with the star-formation rate. The same analysis was performed with similar results for the specific star-formation rate. Our results agree with the scenario in which gas recycling in galaxies, both locally and globally, is much faster than other typical timescales, such like that of gas accretion by inflow and/or metal loss due to outflows. In essence, late-type/disk-dominated galaxies seem to be in a quasi-steady situation, with a behavior similar to the one expected from an instantaneous recycling/closed-box model.

Stellar population gradients in galaxy discs from the CALIFA survey: the influence of bars

While studies of gasphase metallicity gradients in disc galaxies are common, very little has been done towards the acquisition of stellar abundance gradients in the same regions. We present here a comparative study of the stellar metallicity and age distributions in a sample of 62 nearly face-on, spiral galaxies with and without bars, using data from the CALIFA survey. We measure the slopes of the gradients and study their relation with other properties of the galaxies. We find that the mean stellar age and metallicity gradients in the disc are shallow and negative. Furthermore, when normalized to the effective radius of the disc, the slope of the stellar population gradients does not correlate with the mass or with the morphological type of the galaxies. In contrast to this, the values of both age and metallicity at similar to 2.5 scale lengths correlate with the central velocity dispersion in a similar manner to the central values of the bulges, although bulges show, on average, older ages and higher metallicities than the discs. One of the goals of the present paper is to test the theoretical prediction that non-linear coupling between the bar and the spiral arms is an efficient mechanism for producing radial migrations across significant distances within discs. The process of radial migration should flatten the stellar metallicity gradient with time and, therefore, we would expect flatter stellar metallicity gradients in barred galaxies. However, we do not find any difference in the metallicity or age gradients between galaxies with and without bars. We discuss possible scenarios that can lead to this lack of difference.

Near-IR photometry of disk galaxies: Search for nuclear isophotal twist and double bars

We present a near-IR, mainly H band, photometry of 72 nearby ( Mpc) disk galaxies. The main goal of the survey was to search for isophotal twist inside their nuclear regions. As the twist can be due in some cases to projection effects, rather than resulting from a dynamical phenomenon, we deproject – under the simplifying assumption of a 2D geometry – all galaxies whose disk position angle and inclination are known, the latter not exceeding . We show the ellipticity, position angle and surface brightness radial profiles, and discuss how a projection of 2D and 3D bars can distort the isophotes, give an illusion of a non-existing double bar or mask a real one. We report 15 new double-barred galaxies and confirm 2 detected previously. We identify 14 additional twists not known before and we also find nuclear triaxial structures in three SA galaxies. The frequency of Seyferts among galaxies with nuclear bars or twists is high. Since these observations are part of a larger survey, the interpretation of the results will be given in a future paper, as soon as the number of objects grows enough to permit meaningful statistics. As a secondary product, we publish structural parameters (length and axis ratio) of large-scale bars in order to extend still scarce data on bars in the near-IR.

Continuous stellar mass-loss in N -body models of galaxies

We present an N-body computer code { aimed at studies of galactic dynamics { with a CPU-ecient algorithm for a continuous (i.e. time-dependent) stellar mass-loss. First, we summarize available data on stellar mass-loss and derive the long-term (20 Gyr) dependence of mass-loss rate of a coeval stellar population. We then implement it through a simple parametric form into a particle-mesh code with stellar and gaseous particles. We perform several tests of the algorithm reliability and show an illustrative application: a 2D simulation of a disk galaxy, starting as purely stellar but evolving as two-component due to gradual mass-loss from initial stars and to star formation. In a subsequent paper we will use the code to study changes that are induced in galactic disks by the continuous gas recycling compared to the instantaneous recycling approximation, especially the changes in star formation rate and radial inflow of matter.

INSIGHTS ON THE STELLAR MASS-METALLICITY RELATION FROM THE CALIFA SURVEY

We use spatially and temporally resolved maps of stellar population properties of 300 galaxies from the CALIFA integral field survey to investigate how the stellar metallicity (Z) relates to the total stellar mass (M) and the local mass surface density (μ) in both spheroidal- and disk-dominated galaxies. The galaxies are shown to follow a clear stellar mass–metallicity relation (MZR) over the hole 109–1012 M range. This relation is steeper than the one derived from nebular abundances, which is similar to the flatter stellar MZR derived when we consider only young stars. We also find a strong relation between the local values of μ and Z (the μZR), betraying the influence of local factors in determining Z. This shows that both local (μ-driven) and global (M-driven) processes are important in determining metallicity in galaxies. We find that the overall balance between local and global effects varies with the location within a galaxy. In disks, μ regulates Z, producing a strong μZR whose amplitude is modulated by M. In spheroids it is M that dominates the physics of star formation and chemical enrichment, with μ playing a minor, secondary role. These findings agree with our previous analysis of the star formation histories of CALIFA galaxies, which showed that mean stellar ages are mainly governed by surface density in galaxy disks and by total mass in spheroids.

OASIS integral-field spectroscopy of the central kpc in 11 Seyfert 2 galaxies

We map narrow-line regions (NLRs) of 11 nearby Seyfert 2 galaxies with the optical integral-field spectrograph OASIS mounted at CFHT. We model emission-line profiles of 5 forbidden-line doublets and 2 Balmer lines, correcting for the underlying stellar absorption by reconstructing stellar spectra with synthetic evolutionary stellar population models. For each of the 11 targets, we present 2D maps of surface brightness in the observed emission lines, diagnostic line intensity ratios, gas kinematics (mean line-of-sight velocity and velocity dispersion), electron density, and interstellar reddening, and we plot spatially resolved spectral-diagnostic diagrams. The stellar data are represented by maps of mean line-of-sight (LOS) velocities and of the relative mass fractions of the young stellar populations. The gas velocity fields in 80% of the sample exhibit twisted S-shaped isovelocity contours, which are signatures of non-circular orbits and indicate non-axisymmetric gravitational potentials, gas motions out of the galactic plane, or possible outflows and inflows. Based on the kinematic measurements, we identified a possible nuclear ring or radial gas flow in NGC 262 (Mrk 348), not reported before. Eight of the eleven observed objects exhibit strongly asymmetric or multi-component emission-line profiles, in most cases confined to an elongated region passing through the galactic centre, perpendicular to the major axis of emission.

Size and properties of the NLR in the Seyfert-2 galaxy NGC 1386

We study the narrow-line region (NLR) of the Seyfert-2 galaxy NGC 1386 by means of long-slit spectroscopy obtained with FORS 1 at the VLT. We use the galaxy itself for subtracting the stellar template, applying reddening corrections to fit the stellar template to the spectra of the NLR. The continuum gets steadily redder towards the nucleus. The spatial distribution of the reddening derived from the Balmer decrement differs from the continuum reddening, indicating dust within the NLR with a varying column density along the line of sight. Using spatially resolved spectral diagnostics, we find a transition between central line ratios falling into the AGN regime and outer ones in the H II-region regime, occuring at a radius of ∼6 (310 pc) in all three diagnostic diagrams. Applying CLOUDY photoionisation models, we show that the observed distinction between H II-like and AGN-like ratios in NGC 1386 represents a true difference in ionisation source and cannot be explained by variations of physical parameters such as ionisation parameter, electron density or metallicity. We interpret it as a real border between the NLR, i.e. the central AGN-photoionised region and surrounding H II regions. We derive surface brightness, electron density, and ionisation parameter as a function of distance from the nucleus. Both the electron density and the ionisation parameter decrease with radius. We discuss the consequences of these observations for the interpretation of the empirical NLR size-luminosity relation. In the outer part of the NLR, we find evidence for shocks, resulting in a secondary peak of the electron-density and ionisation-parameter distribution north of the nucleus. We compare the NLR velocity curve with the stellar one and discuss the differences.

The origin and orbit of the old, metal-rich, open cluster NGC 6791 - Insights from kinematics

Context. NGC 6791 is a unique stellar system among Galactic open clusters, which is at the same time one of the oldest open clusters and the most metal rich. It is located inside the solar circle , harbors a large population of binary stars, and possibly ex perienced prolonged star formation. The combination of all these properties is puzzling and poses the intriguing question of its o rigin. Aims. One possible scenario is that the cluster formed close to the Galactic Center and later migrated outward to its current location. In this work we study the cluster’s orbit and investigate the possible migration processes that may have displaced NGC 6791 to its present-day position, under the assumption that it actuall y formed in the inner disk. Methods. To this aim we performed integrations of NGC 6791’s orbit in a potential consistent with the main Milky Way parameters. In addition to analytical expressions for halo, bulge and di sk, we also consider the effect of bar and spiral arm perturbations, which are expected to be very important for the disk dynamical evolution, especially inside the solar circle. Starting from state -of-the art initial conditions for NGC 6791, we calculated 1000 orbits back in time for about 1 Gyr turning different non-axisymmetric components of the global potential on and off. We then compared statistical estimates of the cluster’s re cent orbital parameters with the orbital parameters of 10 4 test-particles originating close to the Galactic Center (w ith initial galocentric radii in the range of 3‐5 kpc) and undergoing radial migration during 8 Gyr of forward integration. Results. We find that a model that incorporates a strong bar and spiral a rm perturbations can indeed be responsible for the migration of NGC 6791 from the inner disk (galocentric radii of 3‐5 kpc) to its present-day location. Such a model can provide orbital parameters that are close enough to the observed ones. However, the probability of this scenario as it results from our investigatio ns is very low.

The second-generation VLT instrument MUSE: science drivers and instrument design

The Multi Unit spectroscopic Explorer (MUSE) is a second generation VLT panoramic integral-field spectrograph operating in the visible wavelength range. MUSE has a field of 1 x 1 arcmin2 sampled at 0.2x0.2 arcsec2 and is assisted by a ground layer adaptive optics system using four laser guide stars. The simultaneous spectral range is 0.465-0.93 μm, at a resolution of R~3000. MUSE couples the discovery potential of a large imaging device to the measuring capabilities of a high-quality spectrograph, while taking advantage of the increased spatial resolution provided by adaptive optics. This makes MUSE a unique and tremendously powerful instrument for discovering and characterizing objects that lie beyond the reach of even the deepest imaging surveys. MUSE has also a high spatial resolution mode with 7.5 x 7.5 arcse2 field of view sampled at 25 milli-arcsec. In this mode MUSE should be able to get diffraction limited data-cube in the 0.6-1 μm wavelength range. Although MUSE design has been optimized for the study of galaxy formation and evolution, it has a wide range of possible applications; e.g. monitoring of outer planets atmosphere, young stellar objects environment, supermassive black holes and active nuclei in nearby galaxies or massive spectroscopic survey of stellar fields.

Quadruple-peaked spectral line profiles as a tool to constrain gravitational potential of shell galaxies

Stellar shells observed in many giant elliptical and lenticular as well as a few spiral and dwarf galaxies presumably result from galaxy mergers. Line-of-sight velocity distributions of the shells could, in principle, if measured with a sufficiently high signal-to-noise ratio, constitute a method to constrain the gravitational potential of the host galaxy. Merrifield & Kuijken (1998, MNRAS, 297, 1292) predicted a double-peaked line profile for stationary shells resulting from a nearly radial minor merger. In this paper, we aim at extending their analysis to a more realistic case of expanding shells, inherent to the merging process, whereas we assume the same type of merger and the same orbital geometry. We used an analytical approach as well as test particle simulations to predict the line-of-sight velocity profile across the shell structure. Simulated line profiles were convolved with spectral PSFs to estimate peak detectability. The resulting line-of-sight velocity distributions are more complex than previously predicted due to nonzero phase velocity of the shells. In principle, each of the Merrifield & Kuijken (1998) peaks splits into two, giving a quadruple-peaked line profile, which allows more precise determination of the potential of the host galaxy and contains additional information. We find simple analytical expressions that connect the positions of the four peaks of the line profile and the mass distribution of the galaxy, namely, the circular velocity at the given shell radius and the propagation velocity of the shell. The analytical expressions were applied to a test-particle simulation of a radial minor merger, and the potential of the simulated host galaxy was successfully recovered. Shell kinematics can thus become an independent tool to determine the content and distribution of the dark matter in shell galaxies up to ~100 kpc from the center of the host galaxy.