Showing papers by "Ortwin Gerhard published in 2010"

An extremely top-heavy initial mass function in the galactic center stellar disks

Abstract: We present new observations of the nuclear star cluster in the central parsec of the Galaxy with the adaptive optics assisted, integral field spectrograph SINFONI on the ESO/VLT. Our work allows the spectroscopic detection of early- and late-type stars to m{sub K} >= 16, more than 2 mag deeper than our previous data sets. Our observations result in a total sample of 177 bona fide early-type stars. We find that most of these Wolf Rayet (WR), O-, and B-stars reside in two strongly warped disks between 0.''8 and 12'' from Sgr A*, as well as a central compact concentration (the S-star cluster) centered on Sgr A*. The later type B-stars (m{sub K} >15) in the radial interval between 0.''8 and 12'' seem to be in a more isotropic distribution outside the disks. The observed dearth of late-type stars in the central few arcseconds is puzzling, even when allowing for stellar collisions. The stellar mass function of the disk stars is extremely top heavy with a best-fit power law of dN/dm propor to m {sup -0.45+}-{sup 0.3}. WR/O-stars were formed in situ in a single star formation event approx6 Myr ago, this mass function probably reflects the initial mass function (IMF).more » The mass functions of the S-stars inside 0.''8 and of the early-type stars at distances beyond 12'' are compatible with a standard Salpeter/Kroupa IMF (best-fit power law of dN/dm propor to m {sup -2.15+}-{sup 0.3}).« less

The old and heavy bulge of M 31 - I. Kinematics and stellar populations

Abstract: We present new optical long-slit data along six position angles of the bulge region of M 31. We derive accurate stellar and gas kinematics reaching 5 arcmin from the center, where the disk light contribution is always less than 30%, and out to 8 arcmin along the major axis, where the disk provides 55% of the total light. We show that the velocity dispersions of McElroy (1983) are severely underestimated (by up to 50 km s-1 ). As a consequence, previous dynamical models have underestimated the stellar mass of M 31's bulge by a factor of 2. As a further consequence, the light-weighted velocity dispersion of the galaxy grows to 166 km s-1 and to 170 km s-1 if rotation is also taken into account, thus reducing the discrepancy between the predicted and measured mass of the black hole at the center of M 31 from a factor of 3 to a factor of 2. The kinematic position angle varies with distance, pointing to triaxiality, but a quantitative conclusion can be reached only after simultaneous proper dynamical modeling of the bulge and disk components is performed. We detect gas counterrotation near the bulge minor axis. We measure eight emission-corrected Lick indices. They are approximately constant on circles. Using simple stellar population models we derive the age, metallicity and α -element overabundance profiles. Except for the region in the inner arcsecs of the galaxy, the bulge of M 31 is uniformly old (12 Gyr, with many best-fit ages at the model grid limit of 15 Gyr), slightly α -elements overabundant ([ α /Fe] ≈ 0.2) and of solar metallicity, in agreement with studies of the resolved stellar components. The predicted , and Sloan color profiles match the dust-corrected observations reasonably well, within the known limitations of current simple stellar population models. The stellar populations have approximately radially constant mass-to-light ratios (M /LR ≈ 4-4.5 for a Kroupa IMF), which is in agreement with the stellar dynamical estimates based on our new velocity dispersions. In the inner arcsecs the luminosity-weighted age drops to 4–8 Gyr, while the metallicity increases to above three times the solar value. Starting from 6 arcmin from the center along the major axis, the mean age drops to ≤ 8 Gyr with slight supersolar metallicity (+0.1 dex) and α -element overabundance (+0.2 dex) for a mass-to-light ratio ≤ 3 . Diagnostic diagrams based on the [OIII]/Hβ and [NI]/Hβ emission line equivalent widths (EWs) ratios indicate that the gas is ionized by shocks outside 10 arcsec, but an AGN-like ionizing source could be present near the center. We speculate that a gas-rich minor merger happened some 100 Myr ago, causing the observed minor axis gas counterrotation, the recent star formation event and possibly some nuclear activity.

Distinct core and halo stellar populations and the formation history of the bright Coma cluster early-type galaxy NGC 4889

Abstract: We study the stellar population far into the halo of one of the two brightest galaxies in the Coma cluster, NGC 4889, based on deep medium-resolution spectroscopy with FOCAS at the Subaru 8.2-m telescope. We fit single stellar population models to the measured line-strength (Lick) indices (Hβ, Mgb, [MgFe]′ and 〈Fe〉). Combining with literature data, we construct radial profiles of metallicity, [α/Fe] element abundance ratio and age for NGC 4889, from the centre out to ∼60 kpc (∼ 4 Re). We find evidence for different chemical and star formation histories for stars inside and outside 1.2 Re= 18 kpc radius. The inner regions are characterized by a steep [Z/H] gradient and high [α/Fe] at ∼2.5 solar value. In the halo, between 18 and 60 kpc, the [Z/H] is near-solar with a shallow gradient, while [α/Fe] shows a strong negative gradient, reaching solar values at 60 kpc. We interpret these data in terms of different formation histories for both components. The data for the inner galaxy are consistent with a rapid, quasi-monolithic, dissipative merger origin at early redshifts, followed by one or at most a few dry mergers. Those for the halo argue for later accretion of stars from old systems with more extended star formation histories. The half-light radius of the inner component alone is estimated as ∼6 kpc, suggesting a significantly smaller size of this galaxy in the past. This may be the local stellar population signature of the size evolution found for early-type galaxies from high-redshift observations.

The PN.S Elliptical Galaxy Survey: a standard LCDM halo around NGC 4374?

Abstract: As part of our current programme to test LCDM predictions for dark matter (DM) haloes using extended kinematical observations of early-type galaxies, we present a dynamical analysis of the bright elliptical galaxy NGC 4374 (M84) based on ~450 Planetary Nebulae (PNe) velocities from the PN.Spectrograph, along with extended long-slit stellar kinematics. This is the first such analysis of a galaxy from our survey with a radially constant velocity dispersion profile. We find that the spatial and kinematical distributions of the PNe agree with the field stars in the region of overlap. The velocity kurtosis is consistent with zero at almost all radii. We construct a series of Jeans models, fitting both velocity dispersion and kurtosis to help break the mass-anisotropy degeneracy. Our mass models include DM halos either with shallow cores or with central cusps as predicted by cosmological simulations - along with the novel introduction in this context of adiabatic halo contraction from baryon infall. Both classes of models confirm a very massive dark halo around NGC 4374, demonstrating that PN kinematics data are well able to detect such haloes when present. Considering the default cosmological mass model, we confirm earlier suggestions that bright galaxies tend to have halo concentrations higher than LCDM predictions, but this is found to be solved if either a Salpeter IMF or adiabatic contraction with a Kroupa IMF is assumed. Thus for the first time a case is found where the PN dynamics may well be consistent with a standard dark matter halo. A cored halo can also fit the data, and prefers a stellar mass consistent with a Salpeter IMF. The less dramatic dark matter content found in lower-luminosity "ordinary" ellipticals suggests a bimodality in the halo properties which may be produced by divergent baryonic effects during their assembly histories.

Comparison of approximately isothermal gravitational potentials of elliptical galaxies based on X-ray and optical data

Abstract: We analyze six X-ray bright elliptical galaxies, observed with Chandra and XMM-Newton, and approximate their gravitational potentials by isothermal spheres ϕ =v 2c logr over a range of radii from ∼0.5 to ∼ 25 kpc. We then compare the circularspeed v c derived from X-ray data with the estimators available from optical data. Inparticular we discuss two simple and robust procedures for evaluating the circularspeed of the galaxy using the observed optical surface brightness and the line-of-sight velocity dispersion profiles. The best fitting relation between the circular speedsderived from optical observations of stars and X-ray observations of hot gas is v c,opt ≃η v c,X , where η = 1.10 − 1.15 (depending on the method), suggesting, albeit withlarge statistical and systematic uncertainties, that non-thermal pressure on averagecontributes ∼20-30% of the gas thermal pressure.Key words: Galaxies: Kinematics and Dynamics, X-Rays: Galaxies: Clusters 1 INTRODUCTIONIn spiral galaxies, disk rotation curves offer an accurateand robust way of measuring total gravitational potentialsto distances as large as 10–30 kpc. To a first approxi-mation the rotation curves are flat over a broad rangeof radii, suggesting an isothermal (logarithmic) potentialcharacterized by ϕ(r) = v

Steepening mass profiles, dark matter and environment of X-ray bright elliptical galaxies

Abstract: We use a new non-parametric Bayesian approach to obtain the most probable mass distributions and circular velocity curves along with their confidence ranges, given deprojected density and temperature profiles of the hot gas surrounding X-ray bright elliptical galaxies. For a sample of six X-ray bright ellipticals, we find that all circular velocity curves are rising in the outer parts due to a combination of a rising temperature profile and a logarithmic pressure gradient that increases in magnitude. Therefore at large radii, mass density profiles rise more steeply than isothermal profiles, implying that we are probing the more massive group-sized haloes in which these galaxies are embedded. Comparing the circular velocity curves we obtain from X-rays to those obtained from dynamical models, we find that the former are often lower in the central � 10 kpc. This is probably due to a combination of: i) Non-thermal contributions of up to � 35% in the pressure (with stronger effects in NGC 4486), ii) multiple-temperature components in the hot gas, iii) incomplete kinematic spatial coverage in the dynamical models, and iv) mass profiles that are insufficiently general in the dynamical modelling. Complementing the total mass information from the Xrays with photometry and stellar population models to infer the dark matter content, we find evidence for massive dark matter haloes with dark matter mass fractions of � 35–80% at 2Re, rising to a maximum of 80–90% at the outermost radii. We also find that the six galaxies follow a Tully-Fisher relation with slope � 4 and that their circular velocities at 1Re correlate strongly with the velocity dispersion of the local environment. As a result, the galaxy luminosity at 1Re also correlates with the velocity dispersion of the environment. These relations suggest a close link between the properties of central X-ray bright elliptical galaxies and their environments.

Counter-dispersed slitless-spectroscopy technique: planetary nebula velocities in the halo of NGC 1399

Abstract: Aims. Using a counter-dispersed slitless spectroscopy technique, we detect and measure the line-of-sight velocities of 187 planetary nebulae (PNe) around one of the nearest cD galaxies, NGC 1399, with FORS1 on the VLT. Methods. We describe the method for identifying and classifying the emission-line sources and the procedure for computing their J2000 coordinates and velocities. The number of PN detections and the errors in the velocity measurements (37 km s −1 ) indicate that this technique is comparable to other methods. Results. We present the spatial distribution of the PNe and a basic analysis of their velocities. The PN two-dimensional velocity field shows marginal rotation consistent with other studies. We also find a low-velocity substructure in the halo and a flatter velocity-dispersion profile compared to previous observations that extends to ∼400 �� . The detection of a low-velocity subcomponent underscores the importance of discrete velocity tracers for the detection of un-mixed components. Conclusions. The new velocity-dispersion profile is in good agreement with revised velocity dispersions for the red globular clusters in NGC 1399. The outer parts of this profile are consistent with one of the dynamical models, which corresponds to a circular velocity of � 340 kms −1 and a rescaled B-band mass-to-light ratio of � 20 at 7 � radius. These measurements trace the kinematics of the outer halo and disentangle the heterogenous populations in the Fornax Cluster core. The new data set the stage for a revised dynamical model of the outer halo of NGC 1399.

Chemical abundances in the polar disk of NGC4650A: implications for cold accretion scenario

Abstract: The aim of the present study is to test whether the cold accretion of gas through a "cosmic filament" Macci\`o et al. 2006 is a possible formation scenario for the polar disk galaxy NGC 4650A. If polar disks form from cold accretion of gas, the abundances of the HII regions may be similar to those of very late-type spiral galaxies, regardless of the presence of a bright central stellar spheroid, with total luminosity of few 10^9 Lsun. We use deep long slit spectra obtained with the FORS2 spectrograph at the VLT in the optical and near-infrared wavelength ranges for the brightest HII regions in the disk polar disk of NGC 4650A. The strongest emission lines ([OII] Hbeta, [OIII], Halpha) were used to derived oxygen abundances, metallicities and the global star formation rates for the disk. The deep spectra available allowed us to measure the Oxygen abundances (12 + log (O/H)) using the "Empirical method" based on intensities of the strongest emission lines, and the "Direct method", based on the determination of the electron temperature from the detection of weak auroral lines, as the [OIII] at 4363 Angstrom. The Oxygen abundance measured for the polar disk is then compared with those measured for different galaxy types of similar total luminosities, and then compared against the predictions of different polar ring formation scenarios. The average metallicity values for the polar disk in NGC 4650A is Z=0.2 Zsun, and it is lower that the values measured for ordinary spirals of similar luminosity. Moreover the gradient of the metallicity is flat along the polar disk major axis, which implies none or negligible metal enrichment from the stars in the older central spheroid. The low metallicity value in the polar disk NGC 4650A and the flat metallicity gradient are both consistent with a later infall of metal-poor gas, as expected in the cold accretion processes.

Chemical abundances in the polar disk of ngc 4650a: implications for cold accretion scenario

Abstract: The aim of the present study is to test whether the cold accretion of gas through a 'cosmic filament' is a possible formation scenario for the polar disk galaxy NGC 4650A. If polar disks form from cold accretion of gas, the abundances of the H II regions may be similar to those of very late-type spiral galaxies, regardless of the presence of a bright central stellar spheroid, with total luminosity of few 10{sup 9} L{sub sun}. We use deep long-slit spectra obtained with the FORS2 spectrograph at the Very Large Telescope in the optical and near-infrared wavelength ranges for the brightest H II regions in the polar disk of NGC 4650A. The strongest emission lines ([O II] H{sub {beta}}, [O III], H{sub {alpha}}) were used to derive oxygen abundances, metallicities, and the global star formation rates for the disk. The available deep spectra allowed us to measure the oxygen abundances (12 + log(O/H)) using the empirical method based on intensities of the strongest emission lines and the direct method based on the determination of electron temperature from the detection of weak auroral lines, as the [O III] at 4363 A. The oxygen abundance measured for the polar disk is thenmore » compared with those measured for different galaxy types of similar total luminosities and then compared against the predictions of different polar ring formation scenarios. The average metallicity values for the polar disk in NGC 4650A is Z = 0.2 Z{sub sun}, and it is lower than the values measured for ordinary spirals of similar luminosity. Moreover, the gradient of the metallicity is flat along the polar disk major axis, which implies none or negligible metal enrichment from the stars in the older central spheroid. The low-metallicity value in the polar disk NGC 4650A and the flat metallicity gradient are both consistent with a latter infall of metal-poor gas, as expected in the cold accretion processes.« less

Comparison of an approximately isothermal gravitational potentials of elliptical galaxies based on X-ray and optical data

Abstract: We analyze six X-ray bright elliptical galaxies, observed with Chandra and XMM-Newton, and approximate their gravitational potentials by isothermal spheres phi(r)=v_c^2 ln(r) over a range of radii from ~0.5 to ~25 kpc. We then compare the circular speed v_c derived from X-ray data with the estimators available from optical data. In particular we discuss two simple and robust procedures for evaluating the circular speed of the galaxy using the observed optical surface brightness and the line-of-sight velocity dispersion profiles. The best fitting relation between the circular speeds derived from optical observations of stars and X-ray observations of hot gas is v_{c,opt}~ \eta * v_{c,X}, where \eta=1.10-1.15 (depending on the method), suggesting, albeit with large statistical and systematic uncertainties, that non-thermal pressure on average contributes ~20-30% of the gas thermal pressure.

The dynamically hot stellar halo around NGC 3311: a small cluster-dominated central galaxy ,

Abstract: Context. An important open question is the relation between intracluster light and the halos of central galaxies in galaxy clusters. Aims. Here we report results from an on going project with the aim to characterize the dynamical state in the core of the Hydra I (Abell 1060) cluster around NGC 3311. Methods. We analyze deep long-slit absorption line spectra reaching out to ~25 kpc in the halo of NGC 3311. Results. We find a very steep increase in the velocity dispersion profile from a central σ 0 = 150 km s -1 to σ out ≃ 450 km s -1 at R ≃ 12 kpc. Farther out, to ~25 kpc, σ appears to be constant at this value, which is ~60% of the velocity dispersion of the Hydra I galaxies. With its dynamically hot halo kinematics, NGC 3311 is unlike other normal early-type galaxies. Conclusions. These results and the large amount of dark matter inferred from X-rays around NGC 3311 suggest that the stellar halo of this galaxy is dominated by the central intracluster stars of the cluster, and that the transition from predominantly galaxy-bound stars to cluster stars occurs in the radial range 4 to 12 kpc from the center of NGC 3311. We comment on the wide range of halo kinematics observed in cluster central galaxies, depending on the evolutionary state of their host clusters.

Kinematics and line strength indices in the halos of the Coma brightest cluster galaxies NGC 4874 and NGC 4889

Abstract: Aims. We investigate the stellar kinematics and line strength indices in the outer halos of brightest cluster galaxies (BCGs) in the Coma cluster to obtain the outer halo V rot and σ profiles and to derive constraints on the formation history of these objects.Methods. We analyzed absorption lines in deep, medium-resolution, long-slit spectra in the wavelength range ~ 4500–5900 A, out to ~ 50 kpc for NGC 4874 and ~ 65 kpc for NGC 4889, probing regions with a surface brightness down to μ R ~ 24 mag arcsec-2 .Results. These data provide stellar velocity and velocity dispersion profiles along the major axes of both BCGs, and also along the minor axis of NGC 4889. The kinematic properties of NGC 4874 and NGC 4889 halos extend the previous relations of early-type galaxy halos to bright luminosities and indicate that the stars in the outer regions are still bound to these galaxies. For NGC 4889 we also determine Hβ Mg and Fe line strength indices, finding strong radial gradients for Mg and Fe. The current dataset for NGC 4889 is one of the most extended in radius, including both stellar kinematics and line strength index measurements.

Diffuse Light in Galaxy Clusters

Abstract: Diffuse intracluster light (ICL) has now been observed in nearby and in intermediate redshift clusters. Individual intracluster stars have been detected in the Virgo and Coma clusters and the first color-magnitude diagram and velocity measurements have been obtained. Recent studies show that the ICL contains of the order of 10% and perhaps up to 30% of the stellar mass in the cluster, but in the cores of some dense and rich clusters like Coma, the local ICL fraction can be high as 40%-50%. What can we learn from the ICL about the formation of galaxy clusters and the evolution of cluster galaxies? How and when did the ICL form? What is the connection to the central brightest cluster galaxy? Cosmological N-body and hydrodynamical simulations are beginning to make predictions for the kinematics and origin of the ICL. The ICL traces the evolution of baryonic substructures in dense environments and can thus be used to constrain some aspects of cosmological simulations that are most uncertain, such as the modeling of star formation and the mass distribution of the baryonic component in galaxies.

The Outer Halos of Elliptical Galaxies

Abstract: Recent progress is summarized on the determination of the density distributions of stars and dark matter, stellar kinematics,and stellar population properties in the extended, low-surface brightness halo regions of elliptical galaxies. With integral field absorption spectroscopy and with planetary nebulae as tracers,velocity dispersion and rotation profiles have been followed to ~ 4 and ~ 5–8 effective radii, respectively, and in M87 to the outer edge at ~ 150 kpc. The results are generally consistent with the known dichotomy of elliptical galaxy types, but some galaxies show more complex rotation profiles in their halos and there is a higher incidence of misalignments, indicating triaxiality. Dynamical models have shown a range of slopes for the total mass profiles and that the inner dark matter densities in ellipticals are higher than in spiral galaxies, indicating earlier assembly redshifts. Analysis of the hot X-ray emitting gas in X-ray bright ellipticals and comparison with dynamical mass determinations indicate that non-thermal components to the pressure may be important in the inner ~ 10 kpc and that the properties of these systems are closely related to their group environments. First results on the outer halo stellar population properties do not yet give a clear picture. In the halo of one bright galaxy, lower [α/Fe] abundances indicate longer star formation histories pointing toward late accretion of the halo. This is consistent with independent evidence for ongoing accretion and suggests a connection to the observed size evolution of elliptical galaxies with redshift.

Pattern speeds in the Milky Way

Abstract: A brief review is given of different methods used to determine the pattern speeds of the Galactic bar and spiral arms. The Galactic bar rotates rapidly, with corotation about halfway between the Galactic center and the Sun, and outer Lindblad resonance not far from the solar orbit, R0. The Galactic spiral arms currently rotate with a distinctly slower pattern speed, such that corotation is just outside R0. Both structures therefore seem dynamically decoupled.

Kinematics and line strength indices in the halos of the Coma Brightest Cluster Galaxies NGC 4874 and NGC 4889

Abstract: We investigate the stellar kinematics and line strength indices in the outer halos of brightest cluster galaxies (BCGs) in the Coma cluster to obtain the outer halo V_rot and sigma profiles and to derive constraints on the formation history of these objects. Methods: We analyzed absorption lines in deep, medium-resolution, long-slit spectra in the wavelength range ~ 4500 - 5900 Angstrom, out to ~50 kpc for NGC 4874 and ~65 kpc for NGC 4889, probing regions with a surface brightness down to mu_R ~24 mag/arcsec^2. Results: These data provide stellar velocity and velocity dispersion profiles along the major axes of both BCGs, and also along the minor axis of NGC 4889. The kinematic properties of NGC 4874 and NGC 4889 halos extend the previous relations of early-type galaxy halos to bright luminosities and indicate that the stars in the outer regions are still bound to these galaxies. For NGC 4889 we also determine Hbeta, Mg and Fe line strength indices, finding strong radial gradients for Mg and Fe. The current dataset for NGC 4889 is one of the most extended in radius, including both stellar kinematics AND line strength index measurements.

Counter-dispersed slitless-spectroscopy technique: planetary nebula velocities in the halo of NGC 1399

Abstract: Using a counter-dispersed slitless spectroscopy technique, we detect and measure the line-of-sight velocities of 187 planetary nebulae (PNe) around one of the nearest cD galaxies, NGC 1399, with FORS1 on the VLT. We describe the method for identifying and classifying the emission-line sources and the procedure for computing their J2000 coordinates and velocities. The number of PN detections and the errors in the velocity measurements (37 km/s indicate that this technique is comparable to other methods, such as that described by Teodorescu et al. (2005). We present the spatial distribution of the PNe and a basic analysis of their velocities. The PN two-dimensional velocity field shows marginal rotation consistent with other studies. We also find a low-velocity substructure in the halo and a flatter velocity-dispersion profile compared to previous observations that extends to ~400 arcsec. The detection of a low-velocity subcomponent underscores the importance of discrete velocity tracers for the detection of un-mixed components. The new velocity-dispersion profile is in good agreement with revised velocity dispersions for the red globular clusters in NGC 1399, using the data of Schuberth et al. (2009). The outer parts of this profile are consistent with one of the dynamical models of Kronawitter et al. (2000), which corresponds to a circular velocity of ~340 km/s and a rescaled B-band mass-to-light ratio of ~20 at 7' radius. These measurements trace the kinematics of the outer halo and disentangle the heterogenous populations in the Fornax Cluster core. The new data set the stage for a revised dynamical model of the outer halo of NGC 1399.

VizieR Online Data Catalog: Counter-dispersed slitless-spectroscopy technique: planetary nebula velocities in the halo of NGC 1399.

Abstract: Aims. Using a counter-dispersed slitless spectroscopy technique, we detect and measure the line-of-sight velocities of 187 planetary nebulae (PNe) around one of the nearest cD galaxies, NGC 1399, with FORS1 on the VLT. Methods. We describe the method for identifying and classifying the emission-line sources and the procedure for computing their J2000 coordinates and velocities. The number of PN detections and the errors in the velocity measurements (37 km s −1 ) indicate that this technique is comparable to other methods. Results. We present the spatial distribution of the PNe and a basic analysis of their velocities. The PN two-dimensional velocity field shows marginal rotation consistent with other studies. We also find a low-velocity substructure in the halo and a flatter velocity-dispersion profile compared to previous observations that extends to ∼400 �� . The detection of a low-velocity subcomponent underscores the importance of discrete velocity tracers for the detection of un-mixed components. Conclusions. The new velocity-dispersion profile is in good agreement with revised velocity dispersions for the red globular clusters in NGC 1399. The outer parts of this profile are consistent with one of the dynamical models, which corresponds to a circular velocity of � 340 kms −1 and a rescaled B-band mass-to-light ratio of � 20 at 7 � radius. These measurements trace the kinematics of the outer halo and disentangle the heterogenous populations in the Fornax Cluster core. The new data set the stage for a revised dynamical model of the outer halo of NGC 1399.

Steepening mass profiles, dark matter and environment of X-ray bright elliptical galaxies

Abstract: We use a new non-parametric Bayesian approach to obtain the most probable mass distributions and circular velocity curves along with their confidence ranges, given deprojected density and temperature profiles of the hot gas surrounding X-ray bright elliptical galaxies. For a sample of six X-ray bright ellipticals, we find that all circular velocity curves are rising in the outer parts due to a combination of a rising temperature profile and a logarithmic pressure gradient that increases in magnitude. Comparing the circular velocity curves we obtain from X-rays to those obtained from dynamical models, we find that the former are often lower in the central ~10 kpc. This is probably due to a combination of: i) Non-thermal contributions of up to ~35% in the pressure (with stronger effects in NGC 4486), ii) multiple-temperature components in the hot gas, iii) incomplete kinematic spatial coverage in the dynamical models, and iv) mass profiles that are insufficiently general in the dynamical modelling. Complementing the total mass information from the X-rays with photometry and stellar population models to infer the dark matter content, we find evidence for massive dark matter haloes with dark matter mass fractions of ~35-80% at 2Re, rising to a maximum of 80-90% at the outermost radii. We also find that the six galaxies follow a Tully-Fisher relation with slope ~4 and that their circular velocities at 1Re correlate strongly with the velocity dispersion of the local environment. As a result, the galaxy luminosity at 1Re also correlates with the velocity dispersion of the environment. These relations suggest a close link between the properties of central X-ray bright elliptical galaxies and their environments (abridged).

A survey for Ly alpha galaxies at z~3.1

Abstract: We describe the results of a deep survey for Ly alpha emission line galaxies at z~3.1, carried out with the multislit imaging spectroscopy (MSIS) technique, with the FORS2 spectrograph on VLT-UT1. We discuss the criteria used to select the emission line galaxies and present the main physical characteristics of the sample: redshift, observed flux and equivalent width distributions.

Distinct core and halo stellar populations and the formation history of the bright Coma cluster early-type galaxy NGC 4889

Abstract: We study the stellar population far into the halo of one of the two brightest galaxies in the Coma cluster, NGC 4889, based on deep medium resolution spectroscopy with FOCAS at the Subaru 8.2m telescope. We fit single stellar population models to the measured line-strength (Lick) indices (Hbeta, Mgb, [MgFe]' and ). Combining with literature data, we construct radial profiles of metallicity, [alpha/Fe] element abundance ratio and age for NGC 4889, from the center out to ~60 kpc (~4Re). We find evidence for different chemical and star formation histories for stars inside and outside 1.2Re = 18 kpc radius. The inner regions are characterized by a steep [Z/H] gradient and high [alpha/Fe] at ~2.5 times solar value. In the halo, between 18 and 60 kpc, the [Z/H] is near-solar with a shallow gradient, while [alpha/Fe] shows a strong negative gradient, reaching solar values at ~60 kpc. We interpret these data in terms of different formation histories for both components. The data for the inner galaxy are consistent with a rapid, quasi-monolithic, dissipative merger origin at early redshifts, followed by one or at most a few dry mergers. Those for the halo argue for later accretion of stars from old systems with more extended star formation histories. The half-light radius of the inner component alone is estimated as ~6 kpc, suggesting a significantly smaller size of this galaxy in the past. This may be the local stellar population signature of the size evolution found for early-type galaxies from high-redshift observations.

The outer halos of elliptical galaxies

Abstract: Recent progress is summarized on the determination of the density distributions of stars and dark matter, stellar kinematics, and stellar population properties, in the extended, low surface brightness halo regions of elliptical galaxies. With integral field absorption spectroscopy and with planetary nebulae as tracers, velocity dispersion and rotation profiles have been followed to ~4 and ~5-8 effective radii, respectively, and in M87 to the outer edge at ~150 kpc. The results are generally consistent with the known dichotomy of elliptical galaxy types, but some galaxies show more complex rotation profiles in their halos and there is a higher incidence of misalignments, indicating triaxiality. Dynamical models have shown a range of slopes for the total mass profiles, and that the inner dark matter densities in ellipticals are higher than in spiral galaxies, indicating earlier assembly redshifts. Analysis of the hot X-ray emitting gas in X-ray bright ellipticals and comparison with dynamical mass determinations indicates that non-thermal components to the pressure may be important in the inner ~10 kpc, and that the properties of these systems are closely related to their group environments. First results on the outer halo stellar population properties do not yet give a clear picture. In the halo of one bright galaxy, lower [alpha/Fe] abundances indicate longer star formation histories pointing towards late accretion of the halo. This is consistent with independent evidence for on-going accretion, and suggests a connection to the observed size evolution of elliptical galaxies with redshift.