T. Richtler

University of Concepción, Ciudad de Concepcion, Biobío, Chile

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Publications (237)402.99 Total impact

  • Source
    Michael Hilker · Tom Richtler
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    ABSTRACT: The properties of globular cluster systems (GCSs) in the core of the nearby galaxy clusters Fornax and Hydra I are presented. In the Fornax cluster we have gathered the largest radial velocity sample of a GCS system so far, which enables us to identify photometric and kinematic sub-populations around the central galaxy NGC 1399. Moreover, ages, metallicities and [alpha/Fe] abundances of a sub-sample of 60 bright globular clusters (GCs) with high S/N spectroscopy show a multi-modal distribution in the correlation space of these three parameters, confirming heterogeneous stellar populations in the halo of NGC 1399. In the Hydra I cluster very blue GCs were identified. They are not uniformly distributed around the central galaxies. 3-color photometry including the U-band reveals that some of them are of intermediate age. Their location coincides with a group of dwarf galaxies under disruption. This is evidence of a structurally young stellar halo 'still in formation', which is also supported by kinematic measurements of the halo light that point to a kinematically disturbed system. The most massive GCs divide into generally more extended ultra-compact dwarf galaxies (UCDs) and genuine compact GCs. In both clusters, the spatial distribution and kinematics of UCDs are different from those of genuine GCs. Assuming that some UCDs represent nuclei of stripped galaxies, the properties of those UCDs can be used to trace the assembly of nucleated dwarf galaxies into the halos of central cluster galaxies. We show via semi-analytical approaches within a cosmological simulation that only the most massive UCDs in Fornax-like clusters can be explained by stripped nuclei, whereas the majority of lower mass UCDs belong to the star cluster family.
    Preview · Article · Dec 2015
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    Karina Voggel · Michael Hilker · Tom Richtler
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    ABSTRACT: We present a novel approach to constrain the formation channels of Ultra-Compact Dwarf Galaxies (UCDs). This inhomogeneous class of objects of remnants of tidally stripped dwarf elliptical galaxies and high mass globular clusters. We use three methods to unravel their nature: 1) we analysed their surface brightness profiles, 2) we carried out a direct search for tidal features around UCDs and 3) we compared the spatial distribution of GCs and UCDs in the halo of their host galaxy. Based on FORS2 observations, we have studied the detailed structural composition of a large sample of 97 UCDs in the halo of NGC1399, the central galaxy of the Fornax cluster, by analysing theirsurface brightness profiles. We derived the structural parameters of 13 extended UCDs modelling them with a single Sersic function and decomposing them into composite King and Sersic profiles. We find evidence for faint stellar envelopes at mu=~26 mag\arcsec^-2 surrounding the UCDs up to an extension of 90pc in radius. We also show new evidence for faint asymmetric structures and tidal tail-like features surrounding several of these UCDs, a possible tracer of their origin and assembly history within their host galaxy halos. In particular, we present evidence for the first discovery of a significant tidal tail with an extension of ~350pc around UCD-FORS2. We searched for local overdensities in the spatial distribution of globular clusters within the halo of NGC1399, to see if they are related to the positions of the UCDs. We found a local overabundance of globular clusters on a scale of <1kpc around UCDs, when we compare it to the distribution of globulars from the host galaxy. This effect is strongest for the metal-poor blue GCs. We discuss how likely it is that these clustered globulars were originally associated with the UCD.
    Preview · Article · Oct 2015 · Astronomy and Astrophysics
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    J. P. Caso · T. Richtler
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    ABSTRACT: The present literature does not give a satisfactory answer to the question about the nature of the "Antlia galaxy cluster". The radial velocities of galaxies found in the region around the giant ellipticals NGC 3258/3268 range from about 1000 km/s to 4000 km/s. We characterise this region and its possible kinematical and population substructure. We have obtained VLT--VIMOS multi-object spectra of the galaxy population in the inner part of the Antlia cluster and measure radial velocities for 45 potential members. We supplement our galaxy sample with literature data, ending up with 105 galaxy velocities. We find a large radial velocity dispersion for the entire sample as reported in previous papers. However, we find three groups at about 1900 km/s, 2800 km/s, and 3700 km/s, which we interpret as differences in the recession velocities rather than peculiar velocities. The high radial velocity dispersion of galaxies in the Antlia region reflects a considerable extension along the line of sight.
    Preview · Article · Oct 2015 · Astronomy and Astrophysics
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    ABSTRACT: We report the discovery of 158 previously undetected dwarf galaxies in the Fornax cluster central regions using a deep coadded $u, g$ and $i$-band image obtained with the DECam wide-field camera mounted on the 4-meter Blanco telescope at the Cerro Tololo Interamerican Observatory as part of the {\it Next Generation Fornax Survey} (NGFS). The new dwarf galaxies have quasi-exponential light profiles, effective radii $0.1\!<\!r_e\!<\!2.8$ kpc and average effective surface brightness values $22.0\!<\!\mu_i\!<\!28.0$ mag arcsec$^{-2}$. We confirm the existence of ultra-diffuse galaxies (UDGs) in the Fornax core regions that resemble counterparts recently discovered in the Virgo and Coma galaxy clusters.~We also find extremely low surface brightness NGFS dwarfs, which are several magnitudes fainter than the classical UDGs. The faintest dwarf candidate in our NGFS sample has an absolute magnitude of $M_i\!=\!-8.0$\,mag. The nucleation fraction of the NGFS dwarf galaxy sample appears to decrease as a function of their total luminosity, reaching from a nucleation fraction of $>\!75\%$ at luminosities brighter than $M_i\!\simeq\!-15.0$ mag to $0\%$ at luminosities fainter than $M_i\!\simeq\!-10.0$ mag. The two-point correlation function analysis of the NGFS dwarf sample shows an excess on length scales below $\sim\!100$ kpc, pointing to the clustering of dwarf galaxies in the Fornax cluster core.
    Full-text · Article · Oct 2015
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    ABSTRACT: We present the first large-scale study of the photometric and structural relations followed by early-type galaxies (ETGs) in the Antlia cluster. Antlia is the third nearest populous galaxy cluster after Fornax and Virgo (d $\sim 35$ Mpc). A photographic catalog of its galaxy content was built by Ferguson & Sandage in 1990 (FS90). Afterwards, we performed further analysis of the ETG population located at the cluster centre. Now, we extend our study covering an area four times larger, calculating new total magnitudes and colours, instead of isophotal photometry, as well as structural parameters obtained through S\'ersic model fits extrapolated to infinity. Our present work involves a total of 177 ETGs, out of them 56 per cent have been cataloged by FS90 while the rest (77 galaxies) are newly discovered ones. Medium-resolution GEMINI and VLT spectra are used to confirm membership when available. Including radial velocities from the literature, 59 ETGs are confirmed as Antlia members. Antlia scaling relations mainly support the existence of unique functions (linear and curved) that join bright and dwarf ETGs, excluding compact ellipticals (cEs). Lenticular galaxies are outliers only with respect to the curved relation derived for effective surface brightness versus absolute magnitude. The small number of bright ellipticals and cEs present in Antlia, prevents us from testing if the same data can be fitted with two different linear sequences, for bright and dwarf ETGs. However, adding data from other clusters and groups, the existence of such sequences is also noticeable in the same scaling relations.
    Full-text · Article · May 2015 · Monthly Notices of the Royal Astronomical Society
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    R. Salinas · A. Alabi · T. Richtler · R. R. Lane
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    ABSTRACT: As tracers of star formation, galaxy assembly and mass distribution, globular clusters have provided important clues to our understanding of early-type galaxies. But their study has been mostly constrained to galaxy groups and clusters where early-type galaxies dominate, leaving the properties of the globular cluster systems (GCSs) of isolated ellipticals as a mostly uncharted territory. We present Gemini-South/GMOS $g'i'$ observations of five isolated elliptical galaxies: NGC 3962, NGC 2865, IC 4889, NGC 2271 and NGC 4240. Photometry of their GCSs reveals clear color bimodality in three of them, remaining inconclusive for the other two. All the studied GCSs are rather poor with a mean specific frequency $S_N\sim 1.5$, independently of the parent galaxy luminosity. Considering also previous work, it is clear that bimodality and especially the presence of a significant, even dominant, population of blue clusters occurs at even the most isolated systems, casting doubts on a possible accreted origin of metal-poor clusters as suggested by some models. Additionally, we discuss the possible existence of ultra-compact dwarfs around the isolated elliptical NGC 3962.
    Full-text · Article · Feb 2015 · Astronomy and Astrophysics
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    Richard R. Lane · Ricardo Salinas · Tom Richtler
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    ABSTRACT: Previous studies have shown that the kinematics of the field elliptical galaxy NGC 7507 do not necessarily require dark matter. This is troubling because, in the context of LCDM cosmologies, all galaxies should have a large dark matter component. We use penalised pixel fitting software to extract velocities and velocity dispersions from GMOS slit mask spectra. Using Jeans and MONDian modelling we produce best fit models to the velocity dispersion. We find that NGC 7507 has a two component stellar halo, with the outer halo and inner haloes counter rotating. The velocity dispersion profile exhibits an increase at ~70" (~7.9 kpc), reminiscent of several other elliptical galaxies. Our best fit models are those under mild anisotropy which include ~100 times less dark matter than predicted by LCDM, although mildly anisotropic models that are completely dark matter free fit almost equally well. Our MONDian models, both isotropic and anisotropic, systematically fail to reproduce the measured velocity dispersions at almost all radii. The counter rotating outer halo implies a merger remnant, as does the increase in velocity dispersion at ~70". From simulations it seems plausible that the merger that caused the increase in velocity dispersion was a spiral-spiral merger. Our Jeans models are completely consistent with a no dark matter scenario, however, some dark matter can be accommodated, although at much lower concentrations that predicted by LCDM simulations. This indicates that NGC 7507 may be a dark matter free elliptical galaxy. Whether NGC 7507 is completely dark matter free or very dark matter poor, this is at odds with predictions from current LCDM cosmological simulations. It may be possible that the observed velocity dispersions could be reproduced if the galaxy is significantly flattened along the line of sight (e.g. due to rotation), however, invoking this flattening is problematic.
    Preview · Article · Dec 2014 · Astronomy and Astrophysics
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    T. Richtler · R. Salinas · R. R. Lane · M. Hilker · M. Schirmer
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    ABSTRACT: We investigate the globular cluster system of the isolated elliptical NGC 7796, present new photometry of the galaxy, and use published kinematical data to constrain the dark matter content. Deep images in B and R, obtained with the VIsible MultiObject Spectrograph (VIMOS) at the VLT, form the data base. We present isotropic and anisotropic Jeans-models and give a morphological description of the companion dwarf galaxy. The globular cluster system has about 2000 members, so it is not as rich as those of giant ellipticals in galaxy clusters with a comparable stellar mass, but richer than many cluster systems of other isolated ellipticals. The colour distribution of GCs is bimodal, which does not necessarily mean a metallicity bimodality. The kinematic literature data are somewhat inconclusive. The velocity dispersion in the inner parts can be reproduced without dark matter under isotropy. Radially anisotropic models need a low stellar mass-to-light ratio, which would contrast with the old age of the galaxy. A MONDian model is supported by X-ray analysis and previous dynamical modelling, but better data are necessary for a confirmation. The dwarf companion galaxy NGC 7796-1 exhibits tidal tails, multiple nuclei, and very boxy isophotes. NGC 7796 is an old, massive isolated elliptical galaxy with no indications of later major star formation events as seen frequently in other isolated ellipticals. Its relatively rich globular cluster system shows that isolation does not always mean a poor cluster system. The properties of the dwarf companion might indicate a dwarf-dwarf merger. (abridged)
    Preview · Article · Nov 2014 · Astronomy and Astrophysics
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    ABSTRACT: 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 [$\alpha$/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.
    Full-text · Article · Oct 2014 · Proceedings of the International Astronomical Union
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    ABSTRACT: We used FORS2 in MXU mode to mimic a coarse 'IFU' in order to measure the 3D large-scale kinematics around the central Hydra I cluster galaxy NGC 3311. Our data show that the velocity dispersion field varies as a function of radius and azimuthal angle and violates point symmetry. Also, the velocity field shows similar dependence, hence the stellar halo of NGC 3311 is a dynamically young structure. The kinematic irregularities coincide in position with a displaced diffuse halo North-East of NGC 3311 and with tidal features of a group of disrupting dwarf galaxies. This suggests that the superposition of different velocity components is responsible for the kinematic substructure in the Hydra I cluster core.
    Full-text · Article · Oct 2014 · Proceedings of the International Astronomical Union
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    ABSTRACT: 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
    Preview · Article · Sep 2014 · Proceedings of the International Astronomical Union
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    ABSTRACT: 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)
    Preview · Article · Jun 2014 · Astronomy and Astrophysics
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    ABSTRACT: We present radial velocities (from Gemini/GMOS) of the second sample of ultracompact dwarfs (UCDs) and bright globular clusters (GCs) in the Antlia cluster. 23 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 (Advanced Camera for Surveys) images and are brighter than MV = −9 mag, the effective radii (Reff) 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 < Reff < 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 × 1012 M⊙.
    Full-text · Article · Jun 2014 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present a catalog of point-like sources around NGC 7507, which forms the photometric database for our paper. The sources were obtained from the PSF photometry of MOSAIC images in filters R and C. The catalog contains coordinates, T1 magnitudes with uncertainties, and C-T1 colors and their uncertainties. Magnitudes and colour are corrected by absorption and reddening. (1 data file).
    No preview · Article · Jul 2013
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    ABSTRACT: Context: There is strong evidence that globular cluster systems (GCSs) of massive galaxies are largely assembled by infall/accretion processes. Therefore, we expect the GCSs of isolated elliptical galaxies to be poor. Alhough not completely isolated, NGC 7507 is a massive field elliptical galaxy with an apparently very low dark matter content. Aims: We determine the richness, the colour distribution, and the structural properties of the GCS of NGC 7507. Methods: We perform wide-field Washington photometry with data obtained with the MOSAIC II camera at the 4m-Blanco telescope, CTIO. Results: The GCS is very poor with S_N ~ 0.6. We identify three subpopulations with peaks at (C-T1) colours of 1.21, 1.42, and 1.72. The bluest population may represent the old, metal-poor component. This interpretation is supported by its shallow density profile. The red population is more concentrated, resembling the galaxy light. The intermediate-colour population is strongly peaked in colour and we interpret this population as the signature of a starburst, whose age depends on the metallicity, but should be quite old, since no signatures of a merger are identifiable. In addition, we find a main sequence in the stellar foreground population, which we attribute to the Sagittarius dwarf tidal stream. Conclusions: The extraordinarily poor GCS of NGC 7507, a massive elliptical galaxy, is an illustration of how important the environmental conditions are for producing rich GCSs.
    Preview · Article · May 2013 · Astronomy and Astrophysics
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    ABSTRACT: We present the first compact stellar systems with luminosities in the range of ultracompact dwarfs (UCDs), discovered in the Antlia galaxy cluster (−10.5 < MV < −11.6). The magnitude limit between UCDs and globular clusters (CGs) is discussed. By means of imaging from VLT (FORS1), CTIO (MOSAIC) and the Hubble Space Telescope (ACS) archive, 11 UCDs/bright GCs are selected on the basis of photometry and confirmed as Antlia members through radial velocities measured on new Gemini (GMOS) spectra. In addition, nine UCD candidates are identified taking into account properties derived from their surface brightness profiles. All of them, members and candidates, are located in the proximity of NGC 3258, one of the two brightest elliptical galaxies in the cluster core. Antlia UCDs in this sample present absolute magnitudes fainter than MV ∼ −11.6 mag and most of them have colours within the blue GC range, falling only two within the red GC range. Effective radii measured for the ones lying on the ACS field are in the range Reff = 3–11 pc and are similar to equivalent objects in other clusters, obtained from the literature. The UCD sample shares the same behaviour on the size–luminosity plane: a linear relation between Reff and MV is present for UCDs brighter than MV ∼ −10.5 to −11 mag while no trend is detected for fainter ones, that have an approximately constant Reff. The projected spatial distribution of UCDs, GCs and X-ray emission points to an ongoing merger between two Antlia groups, dominated by NGC 3258 and NGC 3268. Nuclei of dwarf elliptical galaxies and blue UCDs share the same locus on the colour–magnitude diagram, supporting the hypothesis that some blue UCDs may be remnants of stripped nucleated dwarfs.
    No preview · Article · Apr 2013 · Monthly Notices of the Royal Astronomical Society
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    Richard R. Lane · Ricardo Salinas · Tom Richtler
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    ABSTRACT: 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.
    Preview · Article · Dec 2012 · Astronomy and Astrophysics
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    Tom Richtler
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    ABSTRACT: 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.
    Preview · Article · Sep 2012
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    ABSTRACT: 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.
    Full-text · Article · Aug 2012 · Astronomy and Astrophysics
  • T. Richtler · L. P. Bassino · B. Dirsch · B. Kumar
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    ABSTRACT: We present a catalog of point-like sources around NGC 1316, which forms the photometric database for our paper. This catalogue basically is a selection from a larger photometric database. The selection criteria are specified in the paper. The catalog contains coordinates, R magnitudes with uncertainties, C-R colors and their uncertainties as well as "sharp" and "chi" values from DAOPHOT II. The coordinates are for identification purposes only. Depending on the position, deviations of up to 1 arcsec may occur. See the paper for validity limits of the photometric calibration, particularly for very red colors. (1 data file).
    No preview · Article · Jul 2012

Publication Stats

2k Citations
402.99 Total Impact Points

Institutions

  • 2000-2015
    • University of Concepción
      • • Departamento de Astronomía
      • • Departamento de Física
      Ciudad de Concepcion, Biobío, Chile
  • 2006
    • National University of La Plata
      Eva Perón, Buenos Aires, Argentina
  • 2002
    • Moscow State Forest University
      Mytishi, MO, Russia
  • 1989-1999
    • University of Bonn
      • Argelander-Institute of Astronomy
      Bonn, North Rhine-Westphalia, Germany
  • 1992
    • University of Santiago, Chile
      CiudadSantiago, Santiago Metropolitan, Chile
  • 1991
    • Indian Institute of Astrophysics
      Bengalūru, Karnātaka, India
    • Space Telescope Science Institute
      Baltimore, Maryland, United States
  • 1986-1991
    • Observatoire de Paris
      Lutetia Parisorum, Île-de-France, France
  • 1988
    • Christian-Albrechts-Universität zu Kiel
      • Institute for Theoretical Physics and Astrophysics (ITAP)
      Kiel, Schleswig-Holstein, Germany