M. Hilker

European Southern Observatory, Arching, Bavaria, Germany

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Publications (206)464.7 Total impact

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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.
    10/2014;
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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.
    10/2014;
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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
    09/2014;
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    ABSTRACT: Ultra-compact dwarf galaxies are among the densest stellar systems in the Universe. These systems have masses of up to 2 × 10(8) solar masses, but half-light radii of just 3-50 parsecs. Dynamical mass estimates show that many such dwarfs are more massive than expected from their luminosity. It remains unclear whether these high dynamical mass estimates arise because of the presence of supermassive black holes or result from a non-standard stellar initial mass function that causes the average stellar mass to be higher than expected. Here we report adaptive optics kinematic data of the ultra-compact dwarf galaxy M60-UCD1 that show a central velocity dispersion peak exceeding 100 kilometres per second and modest rotation. Dynamical modelling of these data reveals the presence of a supermassive black hole with a mass of 2.1 × 10(7) solar masses. This is 15 per cent of the object's total mass. The high black hole mass and mass fraction suggest that M60-UCD1 is the stripped nucleus of a galaxy. Our analysis also shows that M60-UCD1's stellar mass is consistent with its luminosity, implying a large population of previously unrecognized supermassive black holes in other ultra-compact dwarf galaxies.
    Nature 09/2014; 513(7518):398-400. · 38.60 Impact Factor
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    ABSTRACT: We use the Millennium II cosmological simulation combined with the semi-analytic galaxy formation model of Guo et al. (2011) to predict the contribution of galactic nuclei formed by the tidal stripping of nucleated dwarf galaxies to globular cluster (GC) and ultra-compact dwarf galaxy (UCD) populations of galaxies. We follow the merger trees of galaxies in clusters back in time and determine the absolute number and stellar masses of disrupted galaxies. We assume that at all times nuclei have a distribution in nucleus-to-galaxy mass and nucleation fraction of galaxies similar to that observed in the present day universe. Our results show stripped nuclei follow a mass function $N(M) \sim M^{-1.5}$ in the mass range $10^6 < M/M_\odot < 10^8$, significantly flatter than found for globular clusters. The contribution of stripped nuclei will therefore be most important among high-mass GCs and UCDs. For the Milky Way we predict between 1 and 3 star clusters more massive than $10^5 M_\odot$ come from tidally disrupted dwarf galaxies, with the most massive cluster formed having a typical mass of a few times $10^6 M_\odot$, like omega Centauri. For a galaxy cluster with a mass $7 \times 10^{13} M_\odot$, similar to Fornax, we predict $\sim$19 UCDs more massive than $2\times10^6 M_\odot$ and $\sim$9 UCDs more massive than $10^7 M_\odot$ within a projected distance of 300 kpc come from tidally stripped dwarf galaxies. The observed number of UCDs are $\sim$200 and 23, respectively. We conclude that most UCDs in galaxy clusters are probably simply the high mass end of the GC mass function.
    08/2014;
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    ABSTRACT: CRIRES, the ESO high resolution infrared spectrometer, is a unique instrument which allows astronomers to access a parameter space which up to now was largely uncharted. In its current setup, it consists of a single-order spectrograph providing long-slit, single-order spectroscopy with resolving power up to R=100,000 over a quite narrow spectral range. This has resulted in sub-optimal efficiency and use of telescope time for all the scientific programs requiring broad spectral coverage of compact objects (e.g. chemical abundances of stars and intergalactic medium, search and characterization of extra-solar planets). To overcome these limitations, a consortium was set-up for upgrading CRIRES to a cross-dispersed spectrometer, called CRIRES+. This paper presents the updated optical design of the crossdispersion module for CRIRES+. This new module can be mounted in place of the current pre-disperser unit. The new system yields a factor of >10 increase in simultaneous spectral coverage and maintains a quite long slit (10"), ideal for observations of extended sources and for precise sky-background subtraction.
    07/2014;
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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)
    06/2014;
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    ABSTRACT: We investigate the colour-magnitude relation of metal-poor globular clusters, the 'blue tilt', in the Hydra and Centaurus galaxy clusters and constrain the primordial conditions for star cluster self-enrichment. We analyse U,I photometry for about 2500 globular clusters in the central regions of Hydra and Centaurus, based on FORS1@VLT data. We convert the measured colour-magnitude relations into mass-metallicity space and obtain a scaling of Z \propto M^{0.27 \pm 0.05} for Centaurus GCs and Z \propto M^{0.40 \pm 0.06} for Hydra GCs, consistent with results in other environments. We find that the GC mass-metallicity relation already sets in at present-day masses of a few 10^5 solar masses and is well established in the luminosity range of massive MW clusters like omega Centauri. We compare the mass-metallicity relation with predictions from the star cluster self-enrichment model by Bailin & Harris (2009). For this we include effects of dynamical and stellar evolution and a physically well motivated primordial mass-radius scaling. The self-enrichment model reproduces the observed relations well for average primordial half-light radii r_h ~ 1-1.5 pc, star formation efficiencies f_* ~ 0.3-0.4, and pre-enrichment levels of [Fe/H] ~ -1.7 dex. Within the self-enrichment scenario, the observed blue tilt implies a correlation between GC mass and width of the stellar metallicity distribution. We find that this implied correlation matches the trend of width with GC mass measured in Galactic GCs, including extreme cases like omega Cen and M54. We conclude that 1. A primordial star cluster mass-radius relation provides a significant improvement to the self-enrichment model fits. 2. Broadenend metallicity distributions as found in some massive MW globular clusters may have arisen naturally from self-enrichment processes, without the need of a dwarf galaxy progenitor.
    06/2014;
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    ABSTRACT: (Abridged) Atmospheric dispersion and field differential refraction impose severe constraints on widefield MOS observations. Flux reduction and spectral distortions must be minimised by a careful planning of the observations -- which is especially true for instruments that use slits instead of fibres. This is the case of VIMOS at the VLT, where MOS observations have been restricted, since the start of operations, to a narrow two-hour range from the meridian to minimise slit losses. We revisit in detail the impact of atmospheric effects on the quality of VIMOS-MOS spectra. We model slit losses across the entire VIMOS FOV as a function of target declination. We explore two different slit orientations at the meridian: along the parallactic angle (North-South), and perpendicular to it (East-West). We show that, for fields culminating at zenith distances larger than 20 deg, slit losses are minimised with slits oriented along the parallactic angle at the meridian. The two-hour angle rule holds for these observations using N-S orientations. Conversely, for fields with zenith angles smaller than 20 deg at culmination, losses are minimised with slits oriented perpendicular to the parallactic angle at the meridian. MOS observations can be effectively extended to plus/minus three hours from the meridian in these cases. In general, night-long observations of a single field will benefit from using the E-W orientation. All-sky or service mode observations, however, require a more elaborate planning that depends on the target declination, and the hour angle of the observations. We establish general rules for the alignment of slits in MOS observations that will increase target observability, enhance the efficiency of operations, and speed up the completion of programmes -- a particularly relevant aspect for the forthcoming spectroscopic public surveys with VIMOS.
    02/2014;
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    ABSTRACT: We present results of the metallicities, velocity dispersions, mass-to-light ratios, and dark matter content of a sample of Virgo and Fornax Cluster dwarf elliptical galaxies from their globular cluster systems. Between 5 and 9 globular cluster candidates have been identified in each galaxy from HST imaging and Gemini GMOS optical spectroscopy. Lick line-index measurements indicate that the globular clusters are old and metal-poor with low [α/Fe]. The velocity dispersions of the globular cluster systems are between 25 and 40 km/sec. These are consistent with the predictions based on the total optical luminosity and the mass-to-light ratios from the optical colors of the galaxies. Therefore, the globular clusters are likely to be bound. These relatively massive, ~109 Msun, dwarf elliptical galaxies do not seem to be dominated by dark matter within a projected radius of 5 kpc.
    01/2014;
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    ABSTRACT: In 2013 the eleven currently ongoing ESO public surveys successfully completed the submission and publication of their science data products via the ESO Science Archive Facility. An overview of the public survey projects in terms of telescope time allocation, observation progress and expected date of completion is presented. The science data products available in the ESO archive and their usage by the astronomical community are discussed with regard to the legacy value and scientific impact of these projects. This overview represents a natural introduction to the special section of the Messenger dedicated to the ESO public survey projects, in which the survey teams present their scientific aims and selected results in a series of dedicated articles.
    11/2013;
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    ABSTRACT: Multi-object spectroscopic (MOS) observations with VIMOS have traditionally been limited to a narrow two-hour range from the meridian to minimise slit losses caused by atmospheric dispersion and differential refraction. We revisit the impact of these effects on the quality of VIMOS-MOS spectra through extensive simulations of slit losses. We show that MOS observations can be effectively extended to plus/minus three hours from the meridian for fields with zenith angles smaller than 20 degrees at culmination — provided a nonstandard rotator offset angle of 0 degrees is used. The increase in target observability will enhance the efficiency of operations, and hasten the completion of programmes — a particularly relevant aspect for the forthcoming spectroscopic public surveys with VIMOS.
    11/2013;
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    ABSTRACT: The science operations process of the VLT Survey Telescope (VST) camera, OmegaCAM, is described. OmegaCAM is a 267-megapixel CCD camera imaging a 1 × 1 degree field of view with a pixel scale of 0.21 arcseconds. It began operations in October 2011. The telescope and camera provide a survey speed that is five times greater than the now-decommissioned Wide Field Imager on the MPG/ESO 2.2-metre telescope at La Silla. OmegaCAM is currently used for three public surveys, guaranteed time observations for the OmegaCAM and VST consortia, and Chilean programmes. The execution of OmegaCAM observations, real-time quality control and the calibration plan are outlined.
    11/2013;
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    ABSTRACT: Fossil groups are considered the end product in a galaxy group's evolution -- a massive central galaxy that dominates the luminosity budget of the group, as the outcome of efficient merging between intermediate-luminosity members. Little is however known about the faint satellite systems of fossil groups. Here we present a SUBARU/Suprime-Cam wide-field, deep imaging study in the B- and R-band of the nearest fossil group NGC 6482 (M_{tot}\sim4\times10^{12}M_{\sun}), covering the virial radius out to 310 kpc. We perform detailed completeness estimations and select group member candidates by a combination of automated object detection and visual inspection. A fiducial sample of 48 member candidates down to M_R -10.5 mag is detected, making this study the deepest of a fossil group up to now. We investigate the photometric scaling relations, the colour-magnitude relation, and the luminosity function of our galaxy sample. We find evidence of recent and ongoing merger events among bright group galaxies. The colour-magnitude relation is comparable to that of nearby galaxy clusters, and exhibits significant scatter at the faintest luminosities. The completeness-corrected luminosity function is dominated by early-type dwarfs and is characterized by a faint end slope \alpha=-1.32\pm0.05. We conclude that the NGC 6482 fossil group shows photometric properties consistent with those of regular galaxy clusters and groups, including a normal abundance of faint satellites.
    Astronomy and Astrophysics 09/2013; · 5.08 Impact Factor
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    ABSTRACT: CONTEXT: The dynamical mass-to-light (M/L) ratios of massive ultra-compact dwarf galaxies (UCDs) are about 50% higher than predicted by stellar population models. AIMS: Here we investigate the possibility that these elevated M/L ratios are caused by a central black hole (BH), heating up the internal motion of stars. We focus on a sample of ~50 extragalactic UCDs for which velocity dispersions and structural parameters have been measured. METHODS: Using up-to-date distance moduli and a consistent treatment of aperture and seeing effects, we calculate the ratio Psi=(M/L)_{dyn}/(M/L)_{pop} between the dynamical and the stellar population M/L of UCDs. For all UCDs with Psi>1 we estimate the mass of a hypothetical central BH needed to reproduce the observed integrated velocity dispersion. RESULTS: Massive UCDs (M>10^7 M_*) have an average Psi = 1.7 +-0.2, implying notable amounts of dark mass in them. We find that, on average, central BH masses of 10-15% of the UCD mass can explain these elevated dynamical M/L ratios. The implied BH masses in UCDs range from several 10^5 M_* to several 10^7 M_*. In the M_BH-Luminosity plane, UCDs are offset by about two orders of magnitude in luminosity from the relation derived for galaxies. Our findings can be interpreted such that massive UCDs originate from progenitor galaxies with masses around 10^9 M_*, and that those progenitors have SMBH occupation fractions of 60-100%. The suggested UCD progenitor masses agree with predictions from the tidal stripping scenario. Lower-mass UCDs (M<10^7 M_*) exhibit a bimodal distribution in Psi, suggestive of a coexistence of massive globular clusters and tidally stripped galaxies in this mass regime. CONCLUSIONS: Central BHs as relict tracers of tidally stripped progenitor galaxies are a plausible explanation for the elevated dynamical M/L ratios of UCDs.
    Astronomy and Astrophysics 08/2013; · 5.08 Impact Factor
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    ABSTRACT: We present preliminary results of the velocity dispersions, mass-to-light ratios, and dark matter content of a sample of Virgo and Fornax Cluster dwarf elliptical galaxies from their globular cluster systems. Between 5 and 8 globular cluster candidates have been identified in each galaxy from HST imaging and Gemini GMOS optical spectroscopy. The velocity dispersions of the globular cluster systems are between 30 and 45 km/sec. These are consistent with the predictions based on the total optical luminosity and the mass-to-light ratios from the optical colors of the galaxies. Therefore, these relatively massive, ~109 M⊙ dwarf elliptical galaxies do not seem to be dominated by dark matter within a projected radius of 5 kpc.
    07/2013;
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    ABSTRACT: B and V images centred on Pal 4 were obtained with LRIS on the night of 1999 January 14. On three different nights in 1999 February and March, spectra for 24 candidate red giants in the direction of Pal 4 were obtained using HIRES mounted on the Keck I telescope. (1 data file).
    VizieR Online Data Catalog. 04/2013;
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    ABSTRACT: VIMOS is the powerful visible (360-1000 nm) imager and multi-object/integral field spectrometer mounted on the VLT Unit Telescope 3, Melipal. Its high multiplex advantage makes it ideal for undertaking large-scale spectroscopic surveys of faint sources. In order to extend the life of the instrument, improve its performance and prepare for possible large-scale surveys, in 2009 it was decided to upgrade VIMOS. The first phase of the upgrade, which included replacing the detectors and the fitting of an active flexure compensation system, has been previously reported; this article describes the second stage of the upgrade, which has improved the delivered image quality and stability.
    The Messenger. 03/2013;
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    ABSTRACT: The high multiplex advantage of VIMOS, the VLT visible imager and multi-object/integral-field spectrometer, makes it a powerful instrument for large-scale spectroscopic surveys of faint sources. Following community input and recommendations by ESO's Science and Technology Committee, in 2009 it was decided to upgrade the instrument. This included installing an active flexure compensation system and replacing the detectors with CCDs that have a far better red sensitivity and less fringing. Significant changes have also been made to the hardware, maintenance and operational procedures of the instrument with the aim of improving availability and productivity. Improvements have also been made to the data reduction pipeline. The upgrade will end in 2012 and the results of the program will be presented here.
    Proc SPIE 09/2012;
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    ABSTRACT: The ESO policies for public surveys include regular monitoring and reviews of the progress of the surveys to ensure their legacy value and scientific competitiveness. The review process is carried out on the basis of reports by the ESO survey team and the survey PIs submitted to the relevant Public Survey Panel (PSP) - the VISTA PSP in this case. A summary of the time allocation to VISTA surveys, the service mode observations and the current progress after two years of telescope operation is provided. Furthermore the content of the data products delivery by the survey teams to the ESO Science Archive Facility is described.
    The Messenger. 09/2012;

Publication Stats

1k Citations
464.70 Total Impact Points

Institutions

  • 2007–2014
    • European Southern Observatory
      Arching, Bavaria, Germany
    • University of Queensland
      • School of Mathematics and Physics
      Brisbane, Queensland, Australia
    • University of Central Lancashire
      Preston, England, United Kingdom
  • 2009
    • Universität Heidelberg
      • Centre for Astronomy (ZAH)
      Heidelburg, Baden-Württemberg, Germany
  • 1996–2009
    • University of Bonn
      • • Argelander-Institut für Astronomie (AIfA)
      • • Institute of Anatomy
      Bonn, North Rhine-Westphalia, Germany
  • 1994–2001
    • Pontifical Catholic University of Chile
      • Instituto de Astrofísica
      Santiago, Region Metropolitana de Santiago, Chile