arXiv subjects

Ortwin Gerhard

Publications and source records attributed to Ortwin Gerhard.

At least 127 records · Page 7Linked to original sources

arXiv2007

NMAGIC: Fast Parallel Implementation of a Chi-Squared-Made-to-Measure Algorithm for Modelling Observational Data

We describe a made-to-measure algorithm for constructing N-particle models of stellar systems from observational data (Chi-Squared-M2M), extending earlier ideas by Syer and Tremaine. The algorithm properly accounts for observational errors, is flexible, and can be applied to various systems and geometries. We implement this algorithm in a parallel code NMAGIC and carry out a sequence of tests to illustrate its power and performance: (i) We reconstruct an isotropic Hernquist model from density moments and projected kinematics and recover the correct differential energy distribution and intrinsic kinematics. (ii) We build a self-consistent oblate three-integral maximum rotator model and compare how the distribution function is recovered from integral field and slit kinematic data. (iii) We create a non-rotating and a figure rotating triaxial stellar particle model, reproduce the projected kinematics of the figure rotating system by a non-rotating system of the same intrinsic shape, and illustrate the signature of pattern rotation in this model. From these tests we comment on the dependence of the results from Chi-Squared-M2M on the initial model, the geometry, and the amount of available data.

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arXiv2007

Planetary Nebulae Surveys Beyond the Local Group

Distant planetary nebulae (PNe) are used to measure distances through the PN luminosity function, as kinematic tracers in determining the mass distribution in elliptical galaxies, and most recently, for measuring the kinematics of the diffuse stellar population in galaxy clusters. This article reviews the photometric and spectroscopic survey techniques that have been used to detect PNe beyond the Local Group, out to the Coma cluster at 100 Mpc distance. Contaminations by other emission sources and ways to overcome them will be discussed as well as some science highlights and future perspectives.

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arXiv2006

The Resolved Stellar Populations of a Dwarf Spheroidal Galaxy in the Virgo Cluster

We report on the discovery of a faint (M_V ~ -10.6 +/- 0.2) dwarf spheroidal galaxy on deep F606W and F814W Hubble Space Telescope images of a Virgo intracluster field. The galaxy is easily resolved in our images, as our color magnitude diagram (CMD) extends > 1 magnitude beyond the tip of the red giant branch (RGB). Thus, it is the deepest CMD for a small dwarf galaxy inside a cluster environment. Using the colors of the RGB stars, we derive a metal abundance for the dwarf of [M/H]= -2.3 +/- 0.3, and show that the metallicity dispersion is less than 0.6 dex at 95% confidence. We also use the galaxy's lack of AGB stars and the absence of objects brighter than M_bol ~ -4.1 +/- 0.2 to show that the system is old (t >~10 Gyr). Finally, we derive the object's structural parameters, and show that the galaxy displays no obvious evidence of tidal threshing. Since the tip of the red giant branch distance ((m-M)_0 = 31.23 +/- 0.17 or D = 17.6 +/- 1.4 Mpc) puts the galaxy near the core of the Virgo cluster, one might expect the object to have undergone some tidal processing. Yet the chemical and morphological similarity between the dwarf and the dSph galaxies of the Local and M81 Group demonstrates that the object is indeed pristine, and not the shredded remains of a much larger galaxy. We discuss the possible origins of this galaxy, and suggest that it is just now falling into Virgo for the first time.

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arXiv2006

The Metallicity Distribution of Intracluster Stars in Virgo

We have used the Hubble Space Telescope's Advanced Camera for Surveys (ACS) to detect and measure ~5300 stars in a single intracluster field in the Virgo Cluster. By performing F606W and F814W photometry on these stars, we have determined their metallicity distribution function, and constrained the types of stars present in this portion of Virgo's intracluster space. Based on the small number of stars detected brighter than the red giant branch (RGB) tip, we suggest that in this region, Virgo's intracluster stars are mostly old (>~10 Gyr). Through analysis of the RGB stars themselves, we determine that the population contains the full range of metallicities probed (-2.3<[M/H]<0.0). We also present evidence that the younger (<10 Gyr) component of the population is more metal-rich, with [M/H]>-0.5. The spatial distribution of the most metal-poor stars in the field shows significantly more structure than that of the metal-rich stars, indicating that the intracluster population is not well-mixed. We discuss the implications these observations have for the production of intracluster stars and the dynamical evolution of the Virgo Cluster.

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arXiv2006

Virgo's Intracluster Globular Clusters as Seen by the Advanced Camera for Surveys

We report the discovery of 4 candidate intracluster globular clusters (IGCs) in a single deep HST ACS field of the Virgo Cluster. We show that each cluster is roughly spherical, has a magnitude near the peak of the Virgo globular cluster luminosity function, has a radial profile that is best-fit by a King model, and is surrounded by an excess of point sources which have the colors and magnitudes of cluster red giant stars. Despite the fact that two of our IGC candidates have integrated colors redder than the mean of the M87 globular cluster system, we propose that all of the objects are metal-poor with [M/H] < -1. We show that the tidal radii of our intracluster globulars are all larger than the mean for Milky Way clusters, and suggest that the clusters have undergone less tidal stress than their Galactic counterparts. Finally, we normalize our globular cluster observations to the luminosity of intracluster stars, and derive a value of S_N ~ 6 for the specific frequency of Virgo intracluster globular clusters. We use these data to constrain the origins of Virgo's intracluster population, and suggest that globular clusters in our intracluster field have a different origin than globular clusters in the vicinity of M87. In particular, we argue that dwarf elliptical galaxies may be an important source of intracluster stars.

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arXiv2006

Mass Distribution and Bulge Formation in the Milky Way Galaxy

In its first part, this paper summarizes recent work on the mass and shape of the Galactic dark halo. The second part presents a review of the large-scale structure of the Milky Way, and of the evidence that the inner Galaxy is dominated by baryonic matter. This is briefly compared with the predictions of LCDM and MOND. Finally, a summary is given of bulge formation from clumpy, gas-rich disks, a process which may give rise to old, disk-like, alpha-rich bulges similar to the Galactic bulge.

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arXiv2005

Kinematic evidence for different Planetary Nebulae Populations in the elliptical galaxy NGC 4697

We have analysed the magnitudes, kinematics and positions of a complete sample of 320 PNs in the elliptical galaxy NGC 4697. We show (i) that the PNs in NGC 4697 do not constitute a single population that is a fair tracer of the distribution of all stars. The radial velocity distributions, mean velocities, and dispersions of bright and faint subsamples differ with high statistical confidence. (ii) Using the combined data for PNs brighter than 26.2, we have identified a subpopulation of PNs which is azimuthally unmixed and kinematically peculiar, and which thus neither traces the distribution of all stars nor can it be in dynamical equilibrium with the galaxy potential. (iii) The planetary nebula luminosity functions (PNLF) of two kinematic subsamples in NGC 4697 differ with 99.7% confidence, ruling out a universal PNLF. We estimate that the inferred secondary PN population introduces an uncertainty in the bright cutoff magnitude of ~ 0.15 mag for this galaxy. -- We argue that this secondary PN distribution may be associated with a younger, >~ 1 Gyr old stellar population, perhaps formed in tidal structures that have now fallen back onto the galaxy, as has previously been suggested for the X-ray point sources in this galaxy, or coming from a more recent merger/accretion with a red galaxy. The use of PNs for extragalactic distance determinations is not necessarily compromised, but their use as dynamical tracers of dark halos will require deep observations and careful analysis of large PN samples.

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arXiv2005

The line-of-sight velocity distributions of intracluster planetary nebulae in the Virgo cluster core

Radial velocities of 40 intracluster planetary nebulae (ICPNe) in the Virgo cluster were obtained with the new multi-fiber FLAMES spectrograph on UT2 at VLT. For the first time, the lambda 4959 AA line of the [OIII] doublet is seen in a large fraction (50%) of ICPNe spectra, and a large fraction of the photometric candidates with m(5007) <\= 27.2 is spectroscopically confirmed. ICPNe with the velocity dispersion of the Virgo cluster are found in our CORE field 1 deg from M87. These may have originated from tidal mass loss of smaller galaxies in the M87 subcluster halo. In a field 0.25 deg from M87, we see an extended stellar halo of M87 in approximate dynamical equilibrium, but with few ICPNe. Finally, in a field near M84/M86, the ICPNe velocities are highly correlated with the galaxy velocities, showing that any well-mixed intracluster population is yet to form. Overall, the measured velocity distributions confirm the non-uniform dynamical structure and on-going assembly of the Virgo cluster.(Based on data collected with the FLAMES spectrograph at the UT2 of the VLT at Cerro Paranal, Chile, operated by ESO, during observing run 71.B-0147(A))

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arXiv2005

Modelling Kinematics and Dark Matter: The Halos of Elliptical Galaxies

This review is focussed on the outer halos of elliptical galaxies. Its emphasis is on (i) planetary nebulae as test particles to trace the stellar kinematics at large radii, (ii) the observed angular momentum in elliptical galaxy halos and its theoretical relevance, (iii) dynamical modelling of stellar-kinematic data, and (iv) a discussion of the evidence for dark matter halos in ellipticals from a wide range of measurements.

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arXiv2005

Detection of Intracluster Planetary Nebulae in the Coma Cluster

[OIII] lambda 5007AA emission lines of 16 intracluster planetary nebulae candidates in the Coma cluster were detected with a Multi-Slit Imaging Spectroscopy (MSIS) technique using FOCAS on the Subaru telescope. The identification of these faint emission sources as PNe is supported by (i) their point-like flux distribution in both space and wavelength, with tight limits on the continuum flux; (ii) the identification of the second [OIII] lambda 4959 line in the only object at high enough velocity that this line too falls into the filter bandpass; (iii) emission line fluxes consistent with PNe at 100 Mpc distance, in the range 2.8 x 10^{-19} - 1.7 x 10^{-18} erg/s/cm^2; and (iv) a narrow velocity distribution approximately centered on the systemic velocity of the Coma cluster. Comparing with the velocities of galaxies in our field, we conclude that the great majority of these candidates would be intracluster PNe, free floating in the Coma cluster core. Their velocity dispersion is ~760 km/s, and their mean velocity is lower than that of the galaxies. The velocity distribution suggests that the intracluster stellar population has different dynamics from the galaxies in the Coma cluster core.

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arXiv2004

Sub-Galactic Clumps at High Redshift: A Fragmentation Origin?

We investigate the origin of the clumpy structures observed at high redshift, like the chain galaxies. We use a three dimensional chemodynamical simulation describing the dynamics of stars and a two-phase interstellar medium, as well as feedback processes from the stars. For high efficiency of energy dissipation in the cold cloud medium, the initially gaseous disk fragments and develops several massive clumps of gas and stars. We follow the evolution of the individual clumps and determine their masses, metallicities and velocities. A few dynamical times after fragmentation of the disk, the clumps merge to build a massive bulge. Calculating HST- and UBVRIJHKLM-colors, including absorption by interstellar dust, we determine the morphologies and colors of this model in HST images. Several peculiar morphological structures seen in the HDF can be well-explained by a fragmented galactic disk model, including chain galaxies and objects consisting of several nearby knots.

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arXiv2003

Large-Scale Model of the Milky Way: Stellar Kinematics and Microlensing Event Timescale Distribution in the Galactic Bulge

We build a stellar-dynamical model of the Milky Way barred bulge and disk, using a newly implemented adaptive particle method. The underlying mass model has been previously shown to match the Galactic near-infrared surface brightness as well as gas-kinematic observations. Here we show that the new stellar-dynamical model also matches the observed stellar kinematics in several bulge fields, and that its distribution of microlensing event timescales reproduces the observed timescale distribution of the {\it MACHO} experiment with a reasonable stellar mass function. The model is therefore an excellent basis for further studies of the Milky Way. We also predict the observational consequences of this mass function for parallax shifted events.

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arXiv2003

Gas Physics, Disk Fragmentation, and Bulge Formation in Young Galaxies

We investigate the evolution of star-forming gas-rich disks, using a 3D chemodynamical model including a dark halo, stars, and a two-phase interstellar medium with feedback processes from the stars. We show that galaxy evolution proceeds along very different routes depending on whether it is the gas disk or the stellar disk which first becomes unstable, as measured by the respective Q-parameters. This in turn depends on the uncertain efficiency of energy dissipation of the cold cloud component from which stars form. When the cold gas cools efficiently and drives the instability, the galactic disk fragments and forms a number of massive clumps of stars and gas. The clumps spiral to the center of the galaxy in a few dynamical times and merge there to form a central bulge component in a strong starburst. When the kinetic energy of the cold clouds is dissipated at a lower rate, stars form from the gas in a more quiescent mode, and an instability only sets in at later times, when the surface density of the stellar disk has grown sufficiently high. The system then forms a stellar bar, which channels gas into the center, evolves, and forms a bulge whose stars are the result of a more extended star formation history. We investigate the stability of the gas-stellar disks in both regimes, as well as the star formation rates and element enrichment. We study the morphology of the evolving disks, calculating spatially resolved colours from the distribution of stars in age and metallicity, including dust absorption. We then discuss morphological observations such as clumpy structures and chain galaxies at high redshift as possible signatures of fragmenting, gas-rich disks. Finally, we investigate abundance ratio distributions as a means to distinguish the different scenarios for bulge formation.

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arXiv2003

Formation of the Galactic Halo

Recent observational and theoretical work suggests that the formation of the Galactic stellar halo involved both dissipative processes and the accretion of subfragments. With present data, the fraction of the halo for which an accretion origin can be substantiated is small, of order 10 percent. The kinematics of the best halo field star samples show evidence for both dissipative and dissipationless formation processes. Models of star-forming dissipative collapse, in a cosmological context and including feedback from star formation, do not confirm the simple relations between metallicity, rotation velocity, and orbital eccentricity for halo stars as originally predicted. The new model predictions are much closer to the observed distributions, which have generally been interpreted as evidence for an accretion origin. These results are broadly consistent with a hierarchical galaxy formation model, but the details remain to be worked out.

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arXiv2003

Narrow band survey for intragroup light in the Leo HI cloud. Constraints on the galaxy background contamination in imaging surveys for intracluster planetary nebulae

We have observed emission line objects located in a 0.26 deg^2 field in the M96 (Leo) group, coincident with the intergalactic HI cloud. The emission line objects were selected using the same procedure as used for the search for intracluster planetary nebulae in the Virgo cluster, and their m_5007 luminosity function has a bright cut-off ~1.2 magnitude fainter than for the PNLF associated with the elliptical galaxies in the M96 group.Therefore the vast majority of these emission line objects are compatible with not being intragroup planetary nebulae at the Leo group distance of 10 Mpc. Spectroscopic follow-up of two emission line objects in this Leo field showed that indeed these do not have the [OIII] doublet expected for a real PN. The brighter source is identified as a starburst object at redshift z = 3.128, because of a second emission in the near infrared, identified as FeII (λ2220 AA). From these data we derive three main results: (i) from the absence of PN we can determine a more stringent upper limit to the surface brightness in any old stellar population associated with the Leo HI cloud. (ii) This translates to an upper limit of 1.6% for the fraction of luminosity in a diffuse intragroup component in the densest 3 x 2 deg area of the Leo group,relative to the light in galaxies. (iii) Using this Leo field as a blank field, we derive an average fraction of 13.6% background emission line objects that enter in surveys of Virgo intracluster PN. The small fraction confirms the validity of the selection criteria for intracluster PN candidates in Virgo.

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arXiv2003

The origin of IRS 16: dynamically driven inspiral of a dense star cluster to the Galactic center?

We use direct N-body simulations to study the inspiral and internal evolution of dense star clusters near the Galactic center. These clusters sink toward the center due to dynamical friction with the stellar background, and may go into core collapse before being disrupted by the Galactic tidal field. If a cluster reaches core collapse before disruption, its dense core, which has become rich in massive stars, survives to reach close to the Galactic center. When it eventually dissolves, the cluster deposits a disproportionate number of massive stars in the innermost parsec of the Galactic nucleus. Comparing the spatial distribution and kinematics of the massive stars with observations of IRS 16, a group of young He I stars near the Galactic center, we argue that this association may have formed in this way.

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arXiv2003

The Formation of a Disk Galaxy within a Growing Dark Halo

We present a dynamical model for the formation and evolution of a massive disk galaxy, within a growing dark halo whose mass evolves according to cosmological simulations of structure formation. The galactic evolution is simulated with a new 3D chemo-dynamical code, including dark matter, stars and a multi-phase ISM. The simulations start at redshift z=4.85 with a small dark halo in a LCDM universe and we follow the evolution until the present epoch. The energy release by massive stars and SNe prevents a rapid collapse of the baryonic matter and delays the maximum star formation until z=1. The galaxy forms radially from inside-out and vertically from halo to disk. The first galactic component that forms is the halo, followed by the bulge, the disk-halo transition region, and the disk. At z=1, a bar begins to form which later turns into a triaxial bulge. There is a pronounced deficiency of low-metallicity disk stars due to pre-enrichment of the disk ISM with metal-rich gas from the bulge and inner disk (G-dwarf problem). The mean rotation and the distribution of orbital eccentricities for all stars as a function of metallicity are not very different from those observed in the solar neighbourhood, showing that homogeneous collapse models are oversimplified. The approach presented here provides a detailed description of the formation and evolution of an isolated disk galaxy in a LCDM universe, yielding new information about the kinematical and chemical history of the stars and the ISM, but also about the evolution of the luminosity, the colours and the morphology of disk galaxies.

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arXiv2002

Gas Dynamics in the Milky Way: Second Pattern Speed and Large-Scale Morphology

We present new gas flow models for the Milky Way inside the solar circle. To this end we use SPH simulations in gravitational potentials determined from the NIR luminosity distribution (including spiral arms) which are based on the COBE/DIRBE maps. Gas flows in models which include massive spiral arms clearly match the observed 12CO lvplot better than if the potential does not include spiral structure. Besides single pattern speed models we investigate models with separate pattern speeds for the bar and spiral arms. The most important difference is that in the latter case the gas spiral arms go through the bar corotation region, keeping the gas aligned with the arms there. In the (l,v) plot this results in characteristic regions which appear to be nearly void of gas.In single pattern speed models these regions are filled with gas because the spiral arms dissolve in the bar corotation region. Comparing with the 12CO data we find evidence for separate pattern speeds in the Milky Way.From a series of models the preferred range for the bar pattern speed is Om_p=60\pm5 /Gyr, corresponding to corotation at 3.4\pm0.3kpc. The spiral pattern speed is less well constrained, but our preferred value is Om_sp\approx 20 /Gyr. A further series of gas models is computed for different bar angles, using separately determined luminosity models and gravitational potentials in each case. We find acceptable gas models for 20<=\phibar<=25. The model with (\phibar=20, Om_p=60 /Gyr, Om_sp=20 /Gyr) gives an excellent fit to the spiral arm ridges in the observed (l,v) plot.

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