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Ortwin Gerhard

Publications and source records attributed to Ortwin Gerhard.

At least 145 records · Page 8Linked to original sources
arXiv2002

Isolated Star Formation: A Compact HII Region in the Virgo Cluster

We report on the discovery of an isolated, compact HII region in the Virgo cluster. The object is located in the diffuse outer halo of NGC 4388, or could possibly be in intracluster space. Star formation can thus take place far outside the main star forming regions of galaxies. This object is powered by a small starburst with an estimated mass of $\sim 400\msun$ and age of $\sim 3\myr$. From a total sample of 17 HII region candidates, the present rate of isolated star formation estimated in our Virgo field is small, $\sim 10^{-6} Msun arcmin}^{-2} yr^{-1}$. However, this mode of star formation might have been more important at higher redshifts and be responsible for a fraction of the observed intracluster stars and total cluster metal production. This object is relevant also for distance determinations with the planetary nebula luminosity function from emission line surveys, for high-velocity clouds and the in situ origin of B stars in the Galactic halo, and for local enrichment of the intracluster gas by Type II supernovae.

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arXiv2002

The Pattern Speed of the OH/IR Stars in the Milky Way

We show how the continuity equation can be used to determine pattern speeds in the Milky Way Galaxy (MWG). This method, first discussed by Tremaine & Weinberg in the context of external galaxies, requires projected positions, $(l,b)$, and line-of-sight velocities for a spatially complete sample of relaxed tracers. If the local standard of rest (LSR) has a zero velocity in the radial direction ($u_{\rm LSR}$), then the quantity that is measured is $ΔV \equiv Ω_p R_0 - V_{\rm LSR}$, where $Ω_p$ is the pattern speed of the non-axisymmetric feature, $R_0$ is the distance of the Sun from the Galactic centre and $V_{\rm LSR}$ is the tangential motion of the LSR, including the circular velocity. We use simple models to assess the reliability of the method for measuring a single, constant pattern speed of either a bar or spiral in the inner MWG. We then apply the method to the OH/IR stars in the ATCA/VLA OH 1612 MHz survey of Sevenster et al, finding $ΔV = 252 \pm 41$ km/s, if $u_{\rm LSR} = 0$. Assuming further that $R_0 = 8$ kpc and $V_{\rm LSR} = 220$ \kms, this gives $Ω_p = 59\pm 5$ km/s/kpc with a possible systematic error of perhaps 10 km/s/kpc. The non-axisymmetric feature for which we measure this pattern speed must be in the disc of the MWG.

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arXiv2002

Colour Evolution of Disk Galaxy Models from z=4 to z=0

We calculate synthetic UBVRIJHKLM images, integrated spectra and colours for the disk galaxy formation models of Samland & Gerhard (2002), from redshift z=4 to z=0. Two models are considered, an accretion model based on LambdaCDM structure formation simulations, and a classical collapse model in a dark matter halo. Both models provide the star formation history and dynamics of the baryonic component within a three-dimensional chemo-dynamical description. To convert to spectra and colours, we use the latest, metallicity-calibrated spectral library of Westera et al. (2002), including internal absorption. As a first application, we compare the derived colours with Hubble Deep Field North bulge colours and find good agreement. With our model, we disentangle metallicity effects and absorption effects on the integrated colours, and find that absorption effects are dominant for redshift z < 1.5. Furthermore, we confirm the quality of m_K as a mass tracer, and find indications for a correlation between (J-K) and metallicity gradients.

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arXiv2002

The Galactic Bar

The Milky Way has a barred bulge. This article summarizes the current understanding of the main structural parameters and pattern speed of the bar, and compares predicted values for the microlensing optical depth with the bulge microlensing observations.

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arXiv2002

Dynamical Masses of Elliptical Galaxies

Recent progress in the dynamical analysis of elliptical galaxy kinematics is reviewed. Results reported briefly include (i) the surprisingly uniform anisotropy structure of luminous ellipticals, (ii) their nearly flat (to $\sim 2R_e$) circular velocity curves, (iii) the Tully-Fisher and $M/L - L$ relations and the connection to the Fundamental Plane, and (iv) the large halo mass densities implied by the dynamical models.

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arXiv2002

Mass distribution in our Galaxy

This article summarizes recent work on the luminosity and mass distribution of the Galactic bulge and disk, and on the mass of the Milky Way's dark halo. A new luminosity model consistent with the COBE NIR data and the apparent magnitude distributions of bulge clump giant stars has bulge/bar length of $\simeq 3.5\kpc$, axis ratios of 1:(0.3-0.4):0.3, and short disk scale-length ($\simeq 2.1\kpc$). Gas-dynamical flows in the potential of this model with constant M/L fit the terminal velocities in $10°\le |l| \le 50°$ very well. The luminous mass distribution with this M/L is consistent with the surface density of known matter near the Sun, but still underpredicts the microlensing optical depth towards the bulge. Together, these facts argue strongly for a massive, near-maximal disk in our $\sim L^\ast$, Sbc spiral Galaxy. While the outer rotation curve and global mass distribution are not as readily measured as in similar spiral galaxies, the dark halo mass estimated from satellite velocities is consistent with a flat rotation curve continuing on from the luminous mass distribution.

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arXiv2001

Spiral arms, bar shape and bulge microlensing in the Milky Way

A new model for the luminosity distribution in the inner Milky Way is found, using a non-parametric penalized maximum-likelihood algorithm to deproject a dereddened COBE/DIRBE L-band map of the inner Galaxy. The model is also constrained by the apparent magnitude (line-of-sight) distributions of clump giant stars in certain bulge fields. An important new feature is the inclusion of a spiral arm model in the disk. Spiral arms make the model appear broader on the sky, thus our bar is more elongated than in previous eight-fold symmetric models. They also lead to a smoother disk model interior to the Sun. The bar length is approx. 3.5kpc and its axis ratios are 1:(0.3-0.4):0.3, independent of whether the spiral arm model is 4-armed or 2-armed. The larger elongation in the plane makes it possible to reproduce the observed clump giant distributions as well. For the bar angle we estimate 15^o <= phi <= 30^o, with the best models obtained for 20^o <= phi <= 25^o. We use our reference model to predict a microlensing optical depth map towards the bulge, normalising its mass by the observed terminal velocity curve. For clump giant sources at (l,b)=(3.9,-3.8) we find tau_{-6}:=tau/10^{-6}=1.27, within 1.8*sigma of the new MACHO measurement given by Popowski et al. The value for all sources at (l,b)=(2.68,-3.35) is tau_{-6}=1.1, still >3*sigma away from the published MACHO DIA value. The dispersion of these tau_{-6} values within our models is 10%. Because the distribution of sources is well-fit by the NIR model, increasing the predicted optical depths by >20% will be difficult. Thus the high value of the measured clump giant optical depth argues for a near-maximal disk in the Milky Way. (abridged)

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arXiv2000

Dynamical family properties and dark halo scaling relations of giant elliptical galaxies

Based on a uniform dynamical analysis of line-profile shapes for 21 luminous round elliptical galaxies, we have investigated the dynamical family relations of ellipticals: (i) The circular velocity curves (CVCs) of elliptical galaxies are flat to within ~10% for R>~0.2R_e. (ii) Most ellipticals are moderately radially anisotropic; their dynamical structure is surprisingly uniform. (iii) Elliptical galaxies follow a Tully-Fisher (TF) relation, with v_c^max=300 km/s for an L_B^* galaxy. At given v_c^max, they are ~1 mag fainter in B and appear to have slightly lower baryonic mass than spirals even for maximum M/L_B. (iv) The luminosity dependence of M/L_B is confirmed. The tilt of the Fundamental Plane is not caused by dynamical non-homology, nor only by an increasing dark matter fraction with L. It is, however, consistent with stellar population models based on published metallicities and ages. The main driver is therefore probably metallicity, and a secondary population effect is needed to explain the K-band tilt. (v) These results make it likely that elliptical galaxies have nearly maximal M/L_B (minimal halos). (vi) Despite the uniformly flat CVCs, there is a spread in cumulative M/L_B(r). Some galaxies have no indication for dark matter within 2R_e, whereas others have local M/L_Bs of 20-30 at 2R_e. (vii) In models with maximum stellar mass, the dark matter contributes ~10-40% of the mass within R_e. (viii) The corresponding halo core densities and phase-space densities are at least ~25 times larger and the halo core radii ~4 times smaller than in spiral galaxies of the same v_c^max. The increase in M/L sets in at ~10 times larger acceleration than in spirals. This could imply that elliptical galaxy halos collapsed at high redshift or that some of the dark matter in ellipticals might be baryonic. (abridged)

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arXiv2000

Structure and Mass Distribution of the Milky Way Bulge and Disk

This article summarizes the structural parameters of the Galactic bulge and disk, and discusses the interpretation of the bulge microlensing observations and the determination of the Milky Way's luminous mass from the terminal velocity curve and the Oort limit. The bulge is a rotating bar with corotation radius around 4 kpc. The NIR disk has a short scale-length. The measured surface density of the local disk is in good agreement with the prediction of a maximum NIR disk model for the Milky Way. The preliminary new value for the microlensing optical depth of clump giant sources is within $1.7σ$ of the prediction by the maximum NIR disk model, while the optical depth for all sources is still significantly higher. These results imply that cold dark matter cannot dominate inside the solar radius.

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arXiv2000

The Galactic Center He I Stars: Remains of a Dissolved Young Cluster?

A massive young star cluster, initially embedded in its parent molecular cloud, will spiral into the Galactic Center from $\lta 30m_6^{1/2}\pc$ during the life-time of its most massive stars, if the combined total mass is $\sim 10^6m_6\msun$. On its way inwards the system loses most of its mass to the strong tidal field, until the dense cluster core of high-mass stars is finally disrupted near the central black hole. A simple model is presented to argue that this scenario may under plausible conditions explain the observed location and rotation of the Galactic Center HeI stars. Accretion of star clusters into the Galactic Center could be recurrent, and play an important role in regulating the activity of Sgr A$^\ast$.

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arXiv2000

Dynamical masses, time-scales, and evolution of star clusters

This review discusses (i) dynamical methods for determining the masses of Galactic and extragalactic star clusters, (ii) dynamical processes and their time-scales for the evolution of clusters, including evaporation, mass segregation, core collapse, tidal shocks, dynamical friction and merging. These processes lead to significant evolution of globular cluster systems after their formation.

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arXiv2000

Is Galactic Structure Compatible with Microlensing Data?

We generalize to elliptical models the argument of Kuijken (1997), which connects the microlensing optical depth towards the Galactic bulge to the Galactic rotation curve. When applied to the latest value from the MACHO collaboration for the optical depth for microlensing of bulge sources, the argument implies that the Galactic bar cannot plausibly reconcile the measured values of the optical depth, the rotation curve and the local mass density. Either there is a problem with the interpretation of the microlensing data, or our line of sight to the Galactic centre is highly atypical in that it passes through a massive structure that wraps only a small distance around the Galactic centre.

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arXiv1999

The orbital structure and potential of NGC 1399

Accurate and radially extended stellar kinematic data reaching R=97'' from the center are presented for the cD galaxy of Fornax, NGC 1399. The stellar rotation is small (<= 30 km/s); the stellar velocity dispersion remains constant at 250-270 km/s. The deviations from Gaussian line of sight velocity distributions are small, at the percent level. We construct dynamical models of the galaxy, deprojecting its nearly round (E0-E1) surface brightness distribution, and determining the spherical distribution function that best fits (at the 4 percent level) the kinematic data on a grid of parametrized potentials. We find that the stellar orbital structure is moderately radial, with beta=0.3 +- 0.1 for R<=60'', similar to results found for some normal giant ellipticals. The gravitational potential is dominated by the luminous component out to the last data point, with a mass-to-light ratio M/L_B=10 solar units, although the presence of a central black hole of M approx 5 x 10^8 solar masses is compatible with the data in the inner 5 arcsec. The influence of the dark component is marginally detected starting from R approx 60''. Using the radial velocities of the globular clusters and planetary nebulae of the galaxy we constrain the potential more strongly, ruling out the self-consistent case and finding that the best fit solution agrees with X-ray determinations. The resulting total mass and mass-to-light ratio are M=1.2-2.5 x 10^12 solar masses and M/L_B=22-48 M solar units inside R=417'' or 35 kpc for D=17.6 Mpc.

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arXiv1999

Dynamics of the Boxy Elliptical Galaxy NGC 1600

We use three--integral models to infer the distribution function (DF) of the boxy E3-E4 galaxy NGC 1600 from surface brightness and line profile data on the minor and major axes. We assume axisymmetry and that the mass-to-light ratio is constant in the central ~1 R_e. Stars in the resulting gravitational potential move mainly on regular orbits. We use an approximate third integral K from perturbation theory, and write the DF as a sum of basis functions in the three integrals E, L_z and K. We then fit the projected moments of these basis functions to the kinematic observables and deprojected density, using a non-parametric algorithm. The deduced dynamical structure is radially anisotropic, with sigma_theta/sigma_r ~ sigma_phi/sigma_r ~ 0.7 on the major axis. Both on the minor axis and near the centre the velocity distribution is more isotropic; thus the model is flattened by equatorial radial orbits. The kinematic data is fit without need for a central black hole; the central mass determined previously from ground-based data therefore overestimates the actual black hole mass. The mass-to-light ratio of the stars is M/L_V = 6 h_50. The anisotropy structure of NGC 1600 with a radially anisotropic main body and more nearly isotropic centre is similar to that found recently in NGC 1399, NGC 2434, NGC 3379 and NGC 6703, suggesting that this pattern may be common amongst massive elliptical galaxies. We discuss a possible merger origin of NGC 1600 in the light of these results.

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arXiv1998

Gas Dynamics and Large-Scale Morphology of the Milky Way Galaxy

We present a new model for the gas dynamics in the galactic disk inside the Sun's orbit. Quasi-equilibrium flow solutions are determined in the gravitational potential of the deprojected COBE NIR bar and disk, complemented by a central cusp and, in some models, an outer halo. These models generically lead to four-armed spiral structure between corotation of the bar and the solar circle; their large-scale morphology is not sensitive to the precise value of the bar's pattern speed, to the orientation of the bar with respect to the observer, and to whether or not the spiral arms carry mass. Our best model provides a coherent interpretation of many observed gas dynamical features. Its four-armed spiral structure outside corotation reproduces quantitatively the directions to the five main spiral arm tangents at |l|<=60deg observed in a variety of tracers. The 3-kpc-arm is identified with one of the model arms emanating from the ends of the bar, extending into the corotation region. The model features an inner gas disk with a cusped orbit shock transition to an x_2 orbit disk of radius R~150pc. The bar's corotation radius is fairly well--constrained at R_c=3.5 +/- 0.5 kpc. The best value for the orientation angle of the bar is probably 20-25deg, but the uncertainty is large since no detailed quantitative fit to all features in the observed lv-diagrams is yet possible. The Galactic terminal velocity curve from HI and CO observations out to l=+/-45deg (=5 kpc) is approximately described by a maximal disk model with constant mass-to-light ratio for the NIR bulge and disk.

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arXiv1998

Dynamical Mass Determination for Elliptical Galaxies

The mass and anisotropy of an elliptical galaxy can be simultaneously determined from velocity dispersion and line profile shape measurements. We describe the principles, techniques, and limitations of this approach, and the results obtained sofar. We briefly discuss how best to combine these stellar-dynamical results with X-ray measurements and gravitational lensing analyses.

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arXiv1997

The Galactic Bar

We summarize recent work on the structure and dynamics of the Galactic bar and inner disk. Current work focusses on constructing a quantitative model which integrates NIR photometry, source count observations, gas kinematics, stellar dynamical observations, and microlensing. Some avenues for future research are discussed.

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