arXiv subjects

Tom Richtler

Publications and source records attributed to Tom Richtler.

At least 37 records · Page 2Linked to original sources

arXiv2014

A 3D view of the Hydra I cluster core - II. Stellar populations

Several observations of the central region of the Hydra I galaxy cluster point to a multi-epoch assembly history. Using our novel FORS2/VLT spectroscopic data set, we were able to map the luminosity-weighted age, [Fe/H] and [$α$/Fe] distributions for the stellar populations around the cD galaxy NGC 3311. Our results indicate that the stellar populations follow the trends of the photometric substructures, with distinct properties that may aid to constrain the evolutionary scenarios for the formation of the cluster core.

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arXiv2014

News from the isolated ellipticals NGC 5812, NGC 7507, and NGC 7796

We report on ongoing photometric and spectroscopic work on a sample of isolated elliptical galaxies. We investigate their globular cluster systems, and use the kinematics of globular clusters and the integrated galaxy light to constrain their dark halos, which are not found in the cases of NGC 5812 and NGC 7507

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arXiv2014

The globular cluster system of NGC1316. III. Kinematic complexity

The merger remnant NGC 1316 (Fornax A) is one of the most important objects regarding the investigation of merger-related processes. We use kinematical data of globular clusters (GCs) and the diffuse stellar light to investigate the global structure of NGC 1316 and to constrain the dark matter content. We perform multi-object-spectroscopy with VLT/FORS2 and MXU. Out of 562 slits, we extract radial velocities for 177 GCs. Moreover, we measure radial velocities of the integrated galaxy light, using slits with a sufficiently bright "sky". To these data, we add 20 cluster velocities from Goudfrooij et al. (2001). In an appendix, we identify new morphological features of NGC 1316 and its companion galaxy NGC 1317. The GC sample based on radial velocities confirms the colour peaks already found in our photometric study. The bright clusters, which probably have their origin in a 2 Gyr-old starburst and younger star formation events, avoid the systemic velocity. A Gaussian velocity distribution is found only for clusters fainter than about m_R=22 mag. The velocity distribution of clusters shows a pronounced peak at 1600 km/s. These clusters populate a wide area in the south-western region which we suspect to be a disk population. Globular clusters or subsamples of them do not show a clear rotation signal. This is different from the galaxy light, where rotation along the major axis is discernable out to 3 arcmin radius. A simple spherical model like that suggested by dynamical analyses of planetary nebulae reproduces also the velocity dispersions of the faint GCs. The central dark matter density of the present model resembles a giant elliptical galaxy. This contradicts population properties which indicate spiral galaxies as pre-merger components. MOND would provide a solution, but the kinematical complexity of NGC 1316 does not allow a really firm conclusion. (abridged)

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arXiv2014

Ultra-Compact Dwarfs around NGC 3268

We present radial velocities (from Gemini/GMOS) of the second sample of ultra-compact dwarfs (UCDs) and bright globular clusters (GCs) in the Antlia cluster. Twenty-three objects are located around the giant elliptical NGC 3268, and one is close to the fainter lenticular NGC 3273. Together with previously found UCDs around NGC 3258 a total of 35 UCDs and bright GCs has been now identified in the Antlia cluster. Their colours and magnitudes are compared with those of the nuclei of dE,N galaxies already confirmed as Antlia members. For a subsample that lie on ACS images and are brighter than M_V = -9 mag, the effective radii (R_eff) have been measured, the maximum radius being approximately 10 pc. In addition to the radial velocity sample, we find 10 objects in the magnitude range corresponding to GCs but with 10 < R_eff < 17 pc, resembling the so-called `extended clusters'. By number and magnitude, the new UCDs fit to the GC luminosity function, supporting their interpretation as bright GCs. Additionally, we use a tracer mass estimator to calculate the mass enclosed up to ~47 kpc from NGC 3268, which results in 2.7 x 10^12 M_o.

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arXiv2012

Isolated ellipticals and their globular cluster systems I: Washington photometry of NGC 3585 and NGC 5812

The globular cluster (GC) systems of isolated elliptical galaxies have only recently begun to be studied in detail, and may exhibit morphological connections to the evolutionary histories of their hosts. Here we present the first in a series of wide-field analyses of the GC systems of the isolated ellipticals - Washington C and R photometry of NGC 3585 and NGC 5812 down to ~24 mag. The GC systems are characterised, with each system displaying both the "Universal" blue peak at (C-R)~1.3, and a red peak, but each with differing strengths. The total number of GCs in each system, and their specific frequencies, are estimated. The GC colours and specific frequencies are highly indicative that the host galaxy environment plays a role in shaping its GC system. We produce, and subtract, accurate models of each galaxy, revealing interesting underlying features, including the first definitive evidence that NGC 5812 is interacting with a dwarf companion galaxy. From the galaxy models we also determine surface brightness and colour profiles. Both colour profiles appear quite flat and with (C-R)~1.7 and we discuss the apparent youth of NGC 3585 in the context of this work.

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arXiv2012

Globular cluster systems of early-type galaxies - do we understand them?

I review recent and less recent work on globular cluster systems in early-type galaxies. Explaining their properties and possible assembly scenarios, touches on a variety of astrophysical topics from cluster formation itself to galaxy formation and evolution and even details of observational techniques. The spectacular cluster systems of central galaxies in galaxy clusters may owe their richness to a plethora of less spectacular galaxies and their star formation processes. It seems that dwarf galaxies occupy a particularly important role.

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arXiv2012

Dynamics of the NGC 4636 globular cluster system II. Improved constraints from a large sample of globular cluster velocities

We present new radial velocities for 289 globular clusters around NGC 4636, the southernmost giant elliptical galaxy of the Virgo cluster. The data were obtained with FORS2/MXU at the Very Large Telescope. Together with data analysed in an earlier study (Schuberth et al. 2006), we now have a sample of 460 globular cluster velocities out to a radius of 12 arcmin (60 kpc) available - one of the largest of its kind. This new data set also provides a much more complete angular coverage. Moreover, we present new kinematical data of the inner stellar population of NGC 4636. We perform an updated Jeans analysis, using both stellar and GC data, to better constrain the dark halo properties. We find a stellar M/L-ratio of 5.8 in the R-band, higher than expected from single stellar population synthesis. We model the dark halo by cored and cuspy analytical halo profiles and consider different anisotropies for the tracer populations. Properties of NFW halos lie well within the expected range of cosmological simulations. Cored halos give central dark matter densities, which are typical for elliptical galaxies of NGC 4636's luminosity. The surface densities of the dark matter halos are higher than those of spiral galaxies. We compare the predictions of Modified Newtonian Dynamics with the derived halo properties and find satisfactory agreement. Therefore NGC 4636 therefore falls onto the baryonic Tully-Fisher relation for spiral galaxies. The comparison with the X-ray mass profile of Johnson et al. (2009) reveals satisfactory agreement only, if the abundance gradient of hot plasma has been taken into account. This might indicate a general bias towards higher masses for X-ray based mass profiles in all systems, including galaxy clusters, with strong abundance gradients.

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arXiv2011

Galaxy populations in the Antlia cluster - III. Properties of faint early-type galaxies

(Abridge) We present a new analysis of the early-type galaxy population in the central region of the Antlia cluster, focusing on the faint systems like dwarf ellipticals (dE) and dwarf spheroidals (dSph). We confirm 22 early-type galaxies as Antlia members, using GEMINI-GMOS and MAGELLAN-MIKE spectra. Among them, 2 belong to the rare type of compact ellipticals (cE), and 5 are new faint dwarfs that had never been catalogued before. In addition, we present 16 newly identified low surface brightness galaxy candidates, almost half of them displaying morphologies consistent with being Antlia's counterparts of Local Group dSphs, that extend the faint luminosity limit of our study down to MB = -10.1 (BT = 22.6) mag. We built an improved CMR in the Washington photometric system, i.e. integrated T1 magnitudes versus (C - T1) colours, which extends \sim 4 mag faintwards the limit of spectroscopically confirmed Antlia members. When only confirmed early-type members are considered, this relation extends over 10 mag in luminosity with no apparent change in slope or increase in colour dispersion towards its faint end. The intrinsic colour scatter of the relation is compared with those reported for other clusters of galaxies; we argue that it is likely that the large scatter of the CMR, usually reported at faint magnitudes, is mostly due to photometric errors along with an improper membership/morphological classification. The distinct behaviour of the luminosity versus mean effective surface brightness relation at the bright and faint ends is analyzed, while it is confirmed that dE galaxies on the same relation present a very similar effective radius, regardless of their colour. The projected spatial distribution of the member sample confirms the existence of two groups in Antlia, each one dominated by a giant elliptical galaxy and with one cE located close to each giant.

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arXiv2011

Remarks on the properties of elliptical galaxies in modified Newtonian dynamics

Two incorrect arguments against MOND in elliptical galaxies could be that the equivalent circular velocity curves tend to become flat at much larger accelerations than in spiral galaxies, and that the Newtonian dark matter halos are more concentrated than in spirals. Here, we compare published scaling relations for the dark halos of elliptical galaxies to the scaling relations expected for MONDian phantom halos. We represent the baryonic content of galaxies by spherical profiles, and their corresponding MONDian phantom halos by logarithmic halos. We then derive the surface densities, central densities, and phase space densities and compare them with published scaling relations. We conclude that it is possible to get flat circular velocity curves at high acceleration in MOND, and that this happens for baryonic distributions described by Jaffe profiles in the region where the circular velocity curve is flat. Moreover, the scaling relations of dark halos of ellipticals are remarkably similar to the scaling relations of phantom halos of MOND.

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arXiv2011

Crazy heart: kinematics of the "star pile" in Abell 545

We study the structure and internal kinematics of the "star pile" in Abell 545 - a low surface brightness structure lying in the center of the cluster.We have obtained deep long-slit spectroscopy of the star pile using VLT/FORS2 and Gemini/GMOS, which is analyzed in conjunction with deep multiband CFHT/MEGACAM imaging. As presented in a previous study the star pile has a flat luminosity profile and its color is consistent with the outer parts of elliptical galaxies. Its velocity map is irregular, with parts being seemingly associated with an embedded nucleus, and others which have significant velocity offsets to the cluster systemic velocity with no clear kinematical connection to any of the surrounding galaxies. This would make the star pile a dynamically defined stellar intra-cluster component. The complicated pattern in velocity and velocity dispersions casts doubts on the adequacy of using the whole star pile as a dynamical test for the innermost dark matter profile of the cluster. This status is fulfilled only by the nucleus and its nearest surroundings which lie at the center of the cluster velocity distribution.

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arXiv2010

Two confirmed compact elliptical galaxies in the Antlia cluster

We confirm the existence of two compact elliptical (cE) galaxies in the central region of the Antlia cluster through MAGELLAN-MIKE and GEMINI-GMOS spectra. Only about a dozen galaxies of this rare type are known today up to a distance of 100 Mpc. With this finding, Antlia becomes the nearest galaxy cluster harbouring more than one cE galaxy among its galaxy population. One of these galaxies shows evidence of interaction with one of the giant ellipticals that dominate the central region of the cluster.

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arXiv2008

Galaxy populations in the Antlia cluster. II. Compact elliptical galaxy candidates

We present a photometric analysis of four galaxies classified as compact elliptical (cE) galaxies in the FS90 Antlia Group catalogue. Only 6 members of this rare type of galaxies are known until now. Using data in various photometric systems, we measured brightness and colour profiles, as well as structural parameters. By comparing them with those of other galaxies in the Antlia cluster, as well as with confirmed cE galaxies from the literature, we found that two of the FS90 cE candidates, albeit being spectroscopically confirmed Antlia members, are not cE galaxies. However, one of these objects presents strong ellipticity and position angle variations that resemble those already reported for M32, leading us to speculate about this kind of objects being progenitors of cE galaxies. The other two FS90 cE candidates, for which radial velocities are not available, match some features typical of cE galaxies like being close in projection to a larger galaxy, displaying flat colour profiles, and having a high degree of compactness. Only one of the remaining cE candidates shows a high central surface brightness, two components in its brightness profile, distinct changes in ellipticity and position angle where the outer component begins to dominate, and seems to follow the same trend as other confirmed cE galaxies in a luminosity versus mean effective surface brightness diagram. Moreover, it shows a distorted inner structure with similar characteristics to those found by simulations of interacting galaxies, and an extremely faint structure that seems to link this object with one of the Antlia dominant galaxies, has been detected in MOSAIC-CTIO, FORS1-VLT, and ACS-HST images. The cE nature of this galaxy as well as the possible interaction with its bright companion, still have to be confirmed through spectroscopy.

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arXiv2008

The Nature of UCDs: Internal Dynamics from an Expanded Sample and Homogeneous Database

We have obtained high-resolution spectra of 23 ultra-compact dwarf galaxies (UCDs) in the Fornax cluster with -10.4>M_V>-13.5 mag (10^6 2*10^6 M_*) have M/L ratios twice as large as GCs (M < 2*10^6 M_*). We show that stellar population models tend to under-predict dynamical M/L ratios of UCDs and over-predict those of GCs. Considering the scaling relations of stellar spheroids, UCDs align well along the 'Fundamental Manifold', constituting the small-scale end of the galaxy sequence. The alignment for UCDs is especially clear for r_e >~ 7 pc, which corresponds to dynamical relaxation times that exceed a Hubble time. In contrast, GCs exhibit a broader scatter and do not appear to align along the manifold. We argue that UCDs are the smallest dynamically un-relaxed stellar systems, with M > 2*10^6 M_* and 7<r_e<100 pc. Future studies should aim at explaining the elevated M/L ratios of UCDs and the environmental dependence of their properties.

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arXiv2008

VLT photometry in the Antlia Cluster: the giant ellipticals NGC 3258 and NGC 3268 and their globular cluster systems

We present a deep VLT photometry in the regions surrounding the two dominant galaxies of the Antlia cluster, the giant ellipticals NGC 3258 and NGC 3268. We construct the luminosity functions of their globular cluster systems (GCSs) and determine their distances through the turn-over magnitudes. These distances are in good agreement with those obtained by the SBF method. There is some, but not conclusive, evidence that the distance to NGC 3268 is larger by several Mpc. The GCSs colour distributions are bimodal but the brightest globular clusters (GCs) show a unimodal distribution with an intermediate colour peak. The radial distributions of both GCSs are well fitted by de Vaucouleurs laws up to 5 arcmin. Red GCs present a steeper radial density profile than the blue GCs, and follow closely the galaxies' brightness profiles. Total GC populations are estimated to be about 6000+/-150 GCs in NGC 3258 and 4750+/-150 GCs in NGC 3268. We discuss the possible existence of GCs in a field located between the two giant galaxies (intracluster GCs). Their luminosity functions and number densities are consistent with the two GCSs overlapping in projection.

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arXiv2006

Medium resolution spectroscopy in Omega Centauri: abundances of 400 subgiant and turn-off region

Medium resolution spectra of more than 400 subgiant and turn-off region stars in Omega Centauri were analysed. The observations were performed at the VLT/Paranal with FORS2/MXU. In order to determine the metallicities of the sample stars, we defined a set of line indices (mostly iron) adjusted to the resolution of our spectra. The indices as determined for Omega Centauri were then compared to line indices from stars in the chemically homogeneous globular cluster M55, in addition to standard stars and synthetic spectra. The uncertainties in the derived metallicities are of the order of 0.2 dex. Our study confirms the large variations in iron abundances found on the giant branch in earlier studies (-2.2 <[Fe/H]< -0.7 dex). In addition, we studied the alpha-element and CN/CH abundances. Stars of different metallicity groups not only show distinct ages, but also different behaviours in their relative abundances. The alpha abundances increase smoothly with increasing metallicity resulting in a flat [alpha/Fe] ratio over the whole observed metallicity range. The combined CN+CH abundance increases smoothly with increasing iron abundance. The most metal-rich stars are CN-enriched. In a CN vs. CH plot, though, the individual abundances divide into CN- and CH-rich branches. The large abundance variations observed in our sample of (unevolved) subgiant branch stars most probably have their origin in the pre-enriched material rather than in internal mixing effects. Together with the age spread of the different sub-populations, our findings favour the formation of Omega Centauri within a more massive progenitor.

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arXiv2005

Some Remarks on Extragalactic Globular Clusters

I comment (in a review fashion) on a few selected topics in the field of extragalactic globular clusters with strong emphasis on recent work. The topics are: bimodality in the colour distribution of cluster systems, young massive clusters, and the brightest old clusters. Globular cluster research, perhaps more than ever, has lead to important (at least to astronomers) progress and problems in galaxy structure and formation.

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arXiv2005

Globular cluster systems in low-luminosity early-type galaxies near the Fornax Cluster centre

We present a photometric study of the globular cluster systems of the Fornax cluster galaxies NGC 1374, NGC 1379, and NGC 1387. The data consists of images from the wide-field MOSAIC Imager of the CTIO 4-m telescope, obtained with Washington C and Kron-Cousins R filters. The images cover a field of 36 x 36 arcmin, corresponding to 200 x 200 kpc at the Fornax distance. Two of the galaxies, NGC 1374 and NGC 1379, are low-luminosity ellipticals while NGC 1387 is a low-luminosity lenticular. Their cluster systems are still embedded in the cluster system of NGC 1399. Therefore the use of a large field is crucial and some differences to previous work can be explained by this. The colour distributions of all globular cluster systems are bimodal. NGC 1387 presents a particularly distinct separation between red and blue clusters and an overproportionally large population of red clusters. The radial distribution is different for blue and red clusters, red clusters being more concentrated towards the respective galaxies. The different colour and radial distributions point to the existence of two globular cluster subpopulations in these galaxies. Specific frequencies are in the range S_N= 1.4-2.4, smaller than the typical values for elliptical galaxies. These galaxies might have suffered tidal stripping of blue globular clusters by NGC 1399.

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arXiv2005

VLT Spectroscopy of Globular Cluster Systems, I. The Photometric and Spectroscopic Dataset

We present Lick line-index measurements of extragalactic globular clusters in seven early-type galaxies (NGC 1380, 2434, 3115, 3379, 3585, 5846, and 7192) with different morphological types (E-S0) located in field and group/cluster environments. High-quality spectra were taken with the FORS2 instrument at ESO's Very Large Telescope. About 50% of our data allow an age resolution dt/t ~ 0.3 and a metallicity resolution ~0.25-0.4 dex, depending on the absolute metallicity. Globular cluster candidates are selected from deep B, V, R, I, K FORS2/ISAAC photometry with 80-100% success rate inside one effective radius. Using combined optical/near-infrared colour-colour diagrams we present a method to efficiently reduce fore-/background contamination down to <10%. We find clear signs for bi-modality in the globular cluster colour distributions of NGC 1380, 3115, and 3585. The colour distributions of globular clusters in NGC 2434, 3379, 5846, and 7192 are consistent with a broad single-peak distribution. For the analysed globular cluster systems the slopes of projected radial surface density profiles, of the form Sigma(R) ~ R^-Gamma, vary between ~0.8 and 2.6. Using globular clusters as a tracer population we determine total dynamical masses of host galaxies out to large radii (~1.6 - 4.8 Reff). For the sample we find masses in the range ~8.8*10^10 Msolar up to ~1.2*10^12 Msolar. The line index data presented here will be used in accompanying papers of this series to derive ages, metallicities and abundance ratios. A compilation of currently available high-quality Lick index measurements for globular clusters in elliptical, lenticular, and late-type galaxies is provided and will serve to augment the current data set. [abridged]

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