arXiv subjects

Harald Kuntschner

Publications and source records attributed to Harald Kuntschner.

At least 109 records · Page 6Linked to original sources

arXiv2007

Fast and Slow Rotators: The build-up of the Red Sequence

Using the unique dataset obtained within the course of the SAURON project, a radically new view of the structure, dynamics and stellar populations of early-type galaxies has emerged. We show that galaxies come in two broad flavours (slow and fast rotators), depending on whether or not they exhibit clear large-scale rotation, as indicated via a robust measure of the specific angular momentum of baryons. This property is also linked with other physical characteristics of early-type galaxies, such as: the presence of dynamically decoupled cores, orbital structure and anisotropy, stellar populations and dark matter content. I here report on the observed link between this baryonic angular momentum and a mass sequence, and how this uniquely relates to the building of the red sequence via dissipative/dissipationless mergers and secular evolution.

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arXiv2007

The SAURON project -- IX. A kinematic classification for early-type galaxies

2D stellar kinematics of 48 representative E and S0 galaxies obtained with the SAURON IFS reveal that early-type galaxies appear in two broad flavours, depending on whether they exhibit clear large-scale rotation or not. We define a new parameter LambdaR which involves luminosity weighted averages over the full 2D kinematic field, as a proxy to quantify the observed projected stellar angular momentum per unit mass. We use it as a basis for a new kinematic classification: early-type galaxies are separated into slow and fast rotators (SRs, FRs), depending on whether they have LambdaR values within their effective radius Re below or above 0.1, respectively. SRs and FRs are shown to be physically distinct classes of galaxies, a result which cannot simply be the consequence of a biased viewing angle. FRs tend to be relatively low luminosity galaxies. SRs tend to be brighter and more massive galaxies, but are still spread over a wide range of absolute magnitude. 3 slow rotators of our sample, among the most massive ones, are consistent with zero rotation. Remarkably, all other SRs contain a large kpc-scale KDC. All FRs show well aligned photometric and kinemetric axes, and small velocity twists, in contrast with most SRs which exhibit significant misalignments and velocity twists. In a companion paper (Paper X), we also show that FRs and SRs are distinct classes in terms of their orbital distribution. We suggest that gas is a key ingredient in the formation and evolution of FRs, and that the slowest rotators are the extreme evolutionary end point reached deep in gravitational potential wells where dissipationless mergers had a major role in the evolution, and for which most of the baryonic angular momentum was expelled outwards. (abridged)

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arXiv2007

The SAURON project - XI. Stellar Populations from Absorption Line Strength Maps of 24 Early-Type Spirals

We present absorption line strength maps of a sample of 24 representative early-type spiral galaxies, mostly of type Sa, obtained as part of the SAURON survey of nearby galaxies using our custom-built integral-field spectrograph. Using high-quality spectra, spatially binned to a constant signal-to-noise, we measure several key age, metallicity and abundance ratio sensitive indices from the Lick/IDS system over a contiguous two-dimensional field including bulge and inner disc. We present maps of H beta, Fe 5015 and Mg b, for each galaxy The absorption line maps show that many galaxies contain some younger populations (<= 1 Gyr), distributed in small or large inner discs, or in circumnuclear star forming rings. In many cases these young stars are formed in circumnuclear mini-starbursts, which are dominating the light in the centres of some of the early-type spirals. These mini-starburst cause a considerable scatter in index-index diagrams such as Mg b- H beta and Mg b -Fe 5015, more than is measured for early-type galaxies. We find that the central regions of Sa galaxies display a wide range in ages, even within the galaxies. 50% of the sample show velocity dispersion drops in their centres. All of the galaxies of our sample lie on or below the Mg b- $σ$ relation for elliptical galaxies in the Coma cluster, and above the H beta absorption line - $σ$ relation for elliptical galaxies. If those relations are considered to be relations for the oldest local galaxies we see that our sample of spirals has a considerable scatter in age, with the largest scatter at the lowest $σ$. This is in disagreement with highly inclined samples, in which generally only old stellar populations are found in the central regions. All this can be understood if ... (see paper for rest of abstract)

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arXiv2007

Absorption line-strengths of 18 late-type spiral galaxies observed with SAURON

We present absorption line-strength maps for a sample of 18 Sb-Sd galaxies observed using the integral-field spectrograph SAURON. The SAURON spectral range allows the measurement of the Lick/IDS indices Hbeta, Fe5015 and Mgb, which can be used to estimate the stellar population parameters. We present here the two-dimensional line-strength maps for each galaxy. From the maps, we learn that late-type spiral galaxies tend to have high Hbeta and low Fe5015 and Mgb values, and that the Hbeta index has often a positive gradient over the field, while the metal indices peak in the central region. We investigate the relations between the central line-strength indices and their correlations with morphological type and central velocity dispersion, and compare the observed behaviour with that for ellipticals, lenticulars and early-type spirals from the SAURON survey. We find that our galaxies lie below the Mg - sigma relation determined for elliptical galaxies and that the indices show a clear trend with morphological type. From the line-strength maps we calculate age, metallicity and abundance ratio maps and discuss the results from a one-SSP approach and from a two-SSP approach. Late-type galaxies are generally younger and more metal poor than ellipticals and have abundance ratios closer to solar values. We also explore a continuous star formation scenario, and try to recover the star formation history using the evolutionary models of Bruzual & Charlot (2003), assuming constant or exponentially declining star formation rate (SFR). We find a correlation between the e-folding time-scale tau of the starburst and the central velocity dispersion: more massive galaxies tend to have shorter tau, suggesting that the star formation happened long ago and has now basically ended, while for smaller objects with larger values of tau it is still active.

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arXiv2007

The Nature of Galactic Bulges from SAURON Absorption Line Strength Maps

We discuss SAURON absorption line strength maps of a sample of 24 early-type spirals, mostly Sa. From the Lick indices H$β$, Mg b and Fe 5015 we derive SSP-ages and metallicities. By comparing the scaling relations of Mg b and H beta and central velocity dispersion with the same relation for the edge-on sample of Falcon-Barroso et al. (2002) we derive a picture in which the central regions of Sa galaxies contain at least 2 components: one (or more) thin, disc-like component, often containing recent star formation, and another, elliptical-like component, consisting of old stars and rotating more slowly, dominating the light above the plane. If one defines a bulge to be the component responsible for the light in excess of the outer exponential disc, then many Sa-bulges are dominated by a thin, disc-like component containing recent star formation.

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arXiv2006

Young Kinematically Decoupled Components in Early-Type Galaxies

We present results from a series of follow-up observations of a sub-sample of the representative SAURON survey elliptical (E) and lenticular (S0) galaxies using the OASIS integral-field spectrograph. These observations focus on the central 10" x 10", with roughly double the spatial resolution of the SAURON observations. This increased spatial resolution reveals a number of interesting and previously unresolved features in the measured stellar kinematics and absorption-line strengths. We find that galaxies exhibiting the youngest global stellar populations (as measured with SAURON) often contain a distinctly young central region (on scales of a few hundred parsec or less) compared to the rest of the galaxy. Moreover, these compact, young components are found to be mostly counter-rotating with respect to the rest of the galaxy. Given that there is no well-established reason for such young components to `prefer' counter- over co-rotation, this finding raises the following questions: How common are these small KDCs as a function of age? Why are there more young than old compact KDCs? Where are the equivalent co-rotating components? We explore these questions using simple simulated velocity fields and stellar population models, and find that the fading of the young component as it evolves, coupled with the fact that counter-rotating components are more easily detected in the velocity field, may help explain the observed trends.

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arXiv2006

GMOS Spectroscopy of the S0 galaxy NGC 3115

We present Gemini GMOS longslit spectroscopy of the isolated S0 galaxy NGC 3115. We have determined kinematical data and Lick/IDS absorption line-strength indices for the major axis out to around 9 kpc and for the minor axis out to around 5 kpc (around 2Re). Using stellar population models which include the effects of variable [alpha/Fe] ratios we derive metallicities, abundance ratios and ages for the stellar population of NGC 3115. We find that [alpha/Fe] remains fairly constant with increasing radius at around [alpha/Fe] = 0.17 for the major axis but increases rapidly for the minor axis to around [alpha/Fe] = 0.3. We also find that to first order this behaviour can be explained by a simple spheroid + disc model, where the spheroid has [alpha/Fe] = 0.3 and the disc shows close to solar abundance ratios. The disc also appears considerably younger than the spheroid, having an age of around 6 Gyr compared to 12 Gyr for the spheroid. We compare these results to those previously presented for the globular cluster system of NGC 3115.

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arXiv2005

Internal kinematics and stellar populations of early-type galaxies in the Fornax cluster

We present a study of the internal kinematics and stellar populations of early-type galaxies in the Fornax cluster based on integral-field spectroscopic observations with Gemini South GMOS and VLT-VIMOS. Seven galaxies in a luminosity range of -21.3 < M_B < -17.7 have been observed with these integral field units (IFU). As first results, velocity and line strength maps are presented for NGC 1427 and NGC 1419.

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arXiv2005

Old Globular Clusters Masquerading as Young in NGC 4365?

High signal-to-noise, low-resolution spectra have been obtained for 22 globular clusters (GCs) in NGC 4365. Some of these were selected as probable representatives of an intermediate-age (2-5 Gyr), extremely metal-rich GC subpopulation. The presence of such a subpopulation had been inferred from the unusual optical and near-IR color distributions of GCs in this otherwise typical Virgo elliptical galaxy. However, ages derived from Lick indices are consistent with uniformly old mean ages for all GCs in our sample. The metallicities of the clusters show some evidence for a trimodal distribution. The most metal-poor and metal-rich peaks are consistent with the values expected for an elliptical galaxy of this luminosity, but there appears to be an additional, intermediate-metallicity peak lying between them. New Hubble Space Telescope photometry is consistent with this result. A plausible scenario is that in earlier data these three peaks merged into a single broad distribution. Our results suggest that it is difficult to identify intermediate-age GC subpopulations solely with photometry, even when both optical and near-infrared colors are used.

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arXiv2004

Line-of-sight velocity distribution corrections for Lick/IDS indices of early-type galaxies

We investigate line-of-sight velocity distribution (LOSVD) corrections for absorption line-strength indices of early-type galaxies in the Lick/IDS system. This system is often used to estimate basic stellar population parameters such as luminosity weighted ages and metallicities. Using single stellar population model spectral energy distributions by Vazdekis (1999) we find that the LOSVD corrections are largely insensitive to changes in the stellar populations for old galaxies (age >3 Gyr). Only the Lick/IDS Balmer series indices show an appreciable effect, which is on the order of the correction itself. Furthermore, we investigate the sensitivity of the LOSVD corrections to non-Gaussian LOSVDs. In this case the LOSVD can be described by a Gauss-Hermite series and it is shown that typical values of h_3 and h_4 observed in early-type galaxies can lead to significant modifications of the LOSVD corrections and thus to changes in the derived luminosity weighted ages and metallicities. A new, simple parameterisation for the LOSVD corrections, taking into account the h_3 and h_4 terms, is proposed and calibrations given for a subset of the Lick/IDS indices and two additional indices applicable to old (>3 Gyr) stellar populations.

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arXiv2002

VLT spectroscopy of globular cluster in NGC 3115

We present results derived from VLT-FORS2 spectra of 17 globular clusters associated with the nearby lenticular galaxy NGC3115. Comparing line-strength indices to new stellar population models by Thomas et al. we determine ages, metallicities and element abundance ratios. Our data are also compared with the Lick/IDS observations of Milky Way and M31 globular clusters. Our best age estimates show that the observed clusters which sample the bimodal colour distribution of NGC3115 globular clusters are coeval within our observational errors (2-3 Gyr). Our best calibrated age/metallicity diagnostic diagram (Hbeta vs [MgFe]) indicates an absolute age of 11-12 Gyr consistent with the luminosity weighted age for the central part of NGC3115. We confirm with our accurate line-strength measurements that the (V-I) colour is a good metallicity indicator within the probed metallicity range (-1.5 < [Fe/H] < 0.0). The abundance ratios for globular clusters in NGC3115 give an inhomogeneous picture. We find a range from solar to super-solar ratios for both blue and red clusters. This is similar to the data for M31 while the Milky Way seems to harbour clusters which are mainly consistent with [alpha/Fe] =~ 0.3.

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arXiv2002

VLT spectroscopy of NGC3115 globular clusters

We present results derived from VLT-FORS2 spectra of 24 different globular clusters associated with the lenticular galaxy NGC3115. A subsample of 17 globular clusters have sufficiently high signal-to-noise to allow precision measurements of absorption line-strengths. Comparing these indices to new stellar population models by Thomas et al. we determine ages, metallicities and element abundance ratios. Our data are also compared with the Lick/IDS observations of Milky Way and M31 globular clusters. Unpublished higher order Balmer lines (HgammaA,F and HdeltaA,F) from the Lick/IDS observations are given in the Appendix. Our best age estimates show that the observed clusters which sample the bimodal colour distribution of NGC3115 are coeval within our observational errors (2-3 Gyr). Our best calibrated age/metallicity diagnostic diagram (Hbeta vs [MgFe]) indicates an absolute age of 11-12 Gyr. We confirm with our accurate line-strength measurements that the (V-I) colour is a good metallicity indicator within the probed metallicity range. The abundance ratios for globular clusters in NGC3115 give an inhomogeneous picture. We find a range from solar to super-solar ratios for both blue and red clusters. From our accurate recession velocities we detect, independent of metallicity, clear rotation in the sample of globular clusters. In order to explain the metallicity and abundance ratio pattern, particularly the range in abundance ratios for the metal rich globular clusters in NGC3115, we favour a formation picture with more than two distinct formation episodes.(Abridged)

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arXiv2002

Early-type galaxies in low-density environments

We describe the construction and study of an objectively-defined sample of early-type galaxies in low-density environments. The sample galaxies are selected from a recently-completed redshift survey using uniform and readily-quantified isolation criteria, and are drawn from a sky area of ~700 deg^2, to a depth of 7000 k/ms and an apparent magnitude limit of b_J < 16.1. Their early-type (E/S0) morphologies are confirmed by subsequent CCD imaging. Five out of the nine sample galaxies show signs of morphological peculiarity such as tidal debris or blue circumnuclear rings. We confirm that E/S0 galaxies are rare in low-density regions, accounting for only ~8% of the total galaxy population in such environments. We present spectroscopic observations of nine galaxies in the sample, which are used, in conjunction with updated stellar population models, to investigate star-formation histories. Environmental effects on early-type galaxy evolution are investigated by comparison with a sample of Fornax cluster E/S0s. Results from both samples are compared with predictions from semi-analytic galaxy formation models. The Mg-sig relation of E/S0s in low-density regions is shown to be indistinguishable from that of the Fornax sample. Luminosity-weighted stellar ages and metallicities are determined by considering various combinations of line-indices. At a given luminosity, the E/S0 galaxies in low-density regions are younger than the E/S0s in clusters (by ~2-3 Gyr), and also more metal-rich (by ~0.2 dex). We infer that an anti-correlation of age and metallicity effects is responsible for maintaining the zero-point of the Mg-sig relation. The youngest galaxies in our sample show clear morphological signs of interaction. (Abridged)

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arXiv2002

Two-dimensional kinematics and stellar populations of early-type galaxies: First results from the SAURON survey

We present the SAURON project, which is aimed at studying the morphology, two-dimensional kinematics and stellar populations of a representative sample of elliptical galaxies and spiral bulges. SAURON, a dedicated integral-field spectrograph that is optimized for wide-field observations and has high throughput, was built in Lyon and is now operated at the WHT 4.2m telescope. At present, we have observed approximately two thirds of the seventy-two sample galaxies with SAURON. A comparison with published long-slit measurements demonstrates that the SAURON-data is of equal or better quality, and provides full two-dimensional coverage. The velocity and velocity dispersion fields exhibit a large variety of morphologies: from simple rotating systems to cylindrical, disky and triaxial velocity fields, bars and decoupled cores. Most of these kinematical signatures do not have counterparts in the light distribution. While some galaxies are consistent with axisymmetry, most are more complex systems than assumed previously. This suggests that the kinematical properties of nearby E/S0 galaxies do not agree with the often assumed simplistic two-family model, in which the giant non-rotating triaxial ellipticals are opposed to the fast-rotating axisymmetric faint ellipticals and S0s.

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arXiv2002

Stellar populations in early-type Coma cluster galaxies - I. The data

We present a homogeneous and high signal-to-noise data set (mean S/N of ~60 per Å) of Lick/IDS stellar population line indices and central velocity dispersions for a sample of 132 bright (b_j < 18.0) galaxies within the central 1 deg (= 1.26 h^-1 Mpc) of the nearby rich Coma cluster (A1656). Our observations include 73 per cent (100 out of 137) of the total early-type galaxy population (b_j < 18.0). Observations were made with the WHT 4.2 metre and the AUTOFIB2/WYFFOS multi-object spectroscopy instrument (resolution of 2.2 ÅFWHM) using 2.7'' diameter fibres (= 0.94 h^-1 kpc). The data in this paper have well characterised errors, calculated in a rigorous and statistical way. Data are compared to previous studies and are demonstrated to be of high quality and well calibrated on to the Lick/IDS system. Our data have median errors of ~0.1 Åfor atomic line indices, ~0.008 mag for molecular line indices, and 0.015 dex for velocity dispersions. This work provides a well-defined, high-quality baseline at z~0 for studies of medium to high redshift clusters. Subsequent papers will use this data set to probe the stellar populations (which act as fossil records of galaxy formation and evolution) and the spectro-photometric relations of the bright early-type galaxies within the core of the Coma cluster.

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arXiv2002

Probing the stellar populations of early-type galaxies: the SAURON survey

The SAURON project will deliver two-dimensional spectroscopic data of a sample of nearby early-type galaxies with unprecedented quality. In this paper, we focus on the mapping of their stellar populations using the SAURON data, and present some preliminary results on a few prototypical cases.

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arXiv2001

On the Origin of the Colour-Magnitude Relation in the Virgo Cluster

We explore the origin of the colour-magnitude relation (CMR) of early type galaxies in the Virgo cluster using spectra of very high S/N ratio for six elliptical galaxies selected along the CMR. The data are analysed using a new evolutionary stellar population synthesis model to generate galaxy spectra at the resolution given by their velocity dispersions. In particular we use a new age indicator that is virtually free of the effects of metallicity. We find that the luminosity weighted mean ages of Virgo ellipticals are greater than ~8 Gyr, and show no clear trend with galaxy luminosity. We also find a positive correlation of metallicity with luminosity, colour and velocity dispersion. We conclude that the CMR is driven primarily by a luminosity-metallicity correlation. However, not all elements increase equally with the total metallicity and we speculate that the CMR may be driven by both a total metallicity increase and by a systematic departure from solar abundance ratios of some elements along the CMR. A full understanding of the role played by the total metallicity, abundance ratios and age in generating the CMR requires the analysis of spectra of very high quality, such as those reported here, for a larger number of galaxies in Virgo and other clusters.

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arXiv2001

The SAURON project. I. The panoramic integral-field spectrograph

A new integral-field spectrograph, SAURON, is described. It is based on the Tiger principle, and uses a lenslet array. SAURON has a large field of view and high throughput, and allows simultaneous sky subtraction. Its design is optimized for studies of the stellar kinematics, gas kinematics, and line-strength distributions of nearby early-type galaxies. The instrument design and specifications are described, as well as the extensive analysis software which was developed to obtain fully calibrated spectra, and the associated kinematic and line-strength measurements. A companion paper reports on the first results obtained with SAURON on the William Herschel Telescope.

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