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Erwin P.,Max Planck Insitut fur Extraterrestrische Physik | Erwin P.,Universitats Sternwarte Munich
Astrophysical Journal | Year: 2015

I describe a new, open-source astronomical image-fitting program called imfit, specialized for galaxies but potentially useful for other sources, which is fast, flexible, and highly extensible. A key characteristic of the program is an object-oriented design that allows new types of image components (two-dimensional surfacebrightness functions) to be easily written and added to the program. Image functions provided with imfit include the usual suspects for galaxy decompositions (Sérsic, exponential, Gaussian), along with Core-Sérsic and brokenexponential profiles, elliptical rings, and three components that perform line-of-sight integration through threedimensional luminosity-density models of disks and rings seen at arbitrary inclinations. Available minimization algorithms include Levenberg-Marquardt, Nelder-Mead simplex, and Differential Evolution, allowing trade-offs between speed and decreased sensitivity to local minima in the fit landscape. Minimization can be done using the standard χ2 statistic (using either data or model values to estimate per-pixel Gaussian errors, or else user-supplied error images) or Poisson-based maximum-likelihood statistics; the latter approach is particularly appropriate for cases of Poisson data in the low-count regime. I show that fitting low-signal-to-noise ratio galaxy images using χ2 minimization and individual-pixel Gaussian uncertainties can lead to significant biases in fitted parameter values, which are avoided if a Poisson-based statistic is used; this is true even when Gaussian read noise is present. © 2015. The American Astronomical Society. All rights reserved.

Wilman D.J.,Max Planck Insitut fur Extraterrestrische Physik | Erwin P.,Max Planck Insitut fur Extraterrestrische Physik | Erwin P.,Universitats Sternwarte Munich
Astrophysical Journal | Year: 2012

We present results of an analysis of the local (z ∼ 0) morphology-environment relation for 911 bright (MB < -19) galaxies, based on matching classical RC3 morphologies with the Sloan Digital Sky Survey based group catalog of Yang et al., which includes halo mass estimates. This allows us to study how the relative fractions of spirals, lenticulars, and ellipticals depend on halo mass over a range of 10 11.7-1014.8 h -1 M·, from isolated single-galaxy halos to massive groups and low-mass clusters. We pay particular attention to how morphology relates to central versus satellite status (where "central" galaxies are the most massive within their halo). The fraction of galaxies which are elliptical is a strong function of stellar mass; it is also a strong function of halo mass, but only for central galaxies. We interpret this as evidence for a scenario where elliptical galaxies are always formed, probably via mergers, as central galaxies within their halos, with satellite ellipticals being previously central galaxies accreted onto a larger halo. The overall fraction of galaxies which are S0 increases strongly with halo mass, from ∼10% to ∼70%. Here, too, we find striking differences between the central and satellite populations. 20% 2% of central galaxies with stellar masses M* > 1010.5 M · are S0 regardless of halo mass, but satellite S0 galaxies are only found in massive (>1013 h -1 M ·) halos, where they are 69% ± 4% of the M * > 1010.5 M· satellite population. This suggests two channels for forming S0 galaxies: one which operates for central galaxies and another which transforms lower-mass (M * ≲ 1011 M·) accreted spirals into satellite S0 galaxies in massive halos. Analysis of finer morphological structure (bars and rings in disk galaxies) shows some trends with stellar mass, but none with halo mass; this is consistent with other recent studies which indicate that bars are not strongly influenced by galaxy environment. Radio sources in high-mass central galaxies are common, similarly so for elliptical and S0 galaxies, with a frequency that increases with the halo mass. Emission-line active galactic nuclei (mostly LINERs) are more common in S0s, but show no strong trends with environment. © 2012 The American Astronomical Society. All rights reserved.

Tristram K.R.W.,Max Planck Institute for Radio Astronomy | Schartmann M.,Max Planck Institute for Extraterrestrial Physics | Schartmann M.,Universitats Sternwarte Munich
Astronomy and Astrophysics | Year: 2011

Context. Interferometric measurements in the mid-infrared have shown that the sizes of the warm dust distributions in active galactic nuclei (AGN) are consistent with their scaling with the square root of their luminosity. Aims. We carry out a more detailed analysis of this size-luminosity relation to investigate which of the general properties of the dusty tori in AGN can be derived from this relation. We are especially interested in the cases, where only a very small number of interferometric measurements are available and the sizes are directly calculated from the measured visibilities assuming a Gaussian brightness distribution. Methods. We improve the accuracy of the size-luminosity relation by adding a few additional size measurements from more recent interferometric observations and compare the measured sizes to those derived from hydrodynamical and radiative transfer models of AGN tori. Results. We find that a Gaussian approximation yields a reasonable estimate of the size of the brightness distribution, as long as the visibilities are within 0.2 ≤ V ≤ 0.9. The uncertainty in the size estimate depends on the true brightness distribution and is up to a factor of four for the models used in our investigation. The size estimates derived from the models are consistent with those determined from the measurements. However, the models predict a significant offset between the sizes derived for face-on (Seyfert 1 case) and edge-on (Seyfert 2 case) tori: the face-on tori should appear significantly more compact for the same luminosity. This offset is not observed in the current data, probably because of the large uncertainties and low statistics of the present interferometric measurements. Furthermore, we find a ratio of the mid- to near-infrared sizes of approximately 30, whereas the first probes the body of the torus and the second is an estimate of the inner rim. Conclusions. The size-luminosity relation of AGN tori using Gaussian size estimates is a very simple and effective tool to investigate the internal structure and geometry of AGN tori and obtain constraints on the differences between type 1 and type 2 AGN. However, to fully exploit the possibilities of investigating the nuclear distributions of gas and dust in AGN using this size-luminosity relation, more accurate interferometric measurements of a larger sample of AGN are needed. © 2011 ESO.

Grillo C.,Max Planck Institute for Extraterrestrial Physics | Grillo C.,Universitats Sternwarte Munich | Gobat R.,CEA Saclay Nuclear Research Center
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2010

We investigate the most plausible stellar initial mass function (IMF) and the main origin of the tilt of the fundamental plane (FP) for old, massive early-type galaxies. We consider a sample of 13 bright galaxies of the Coma cluster and combine our results with those obtained from a sample of 57 lens galaxies in the same luminous mass range. We estimate the luminous mass and stellar mass-to-light ratio values of the sample galaxies by fitting their Sloan Digital Sky Survey multiband photometry with composite stellar population models computed with different dust-free, solar-metallicity templates and IMFs. We compare these measurements and those derived from two-component orbit-based dynamical modelling. The photometric and dynamical luminous mass estimates of the galaxies in our sample are consistent, within the errors, if a Salpeter IMF is adopted. On the contrary, with a Kroupa or Chabrier IMF the two luminous mass diagnostics differ at a more than 4σ level. For the massive Coma galaxies, their stellar mass-to-light ratio scales with luminous mass as the corresponding effective quantities are observed to scale on the FP. This indicates that the tilt of the FP is primarily caused by stellar population properties. We conclude that old, massive lens and non-lens early-type galaxies obey the same luminous and dynamical scaling relations, favour a Salpeter IMF, and suggest a stellar population origin for the tilt of the FP. The validity of these results for samples of early-type galaxies with different age and mass properties still remains to be tested. © 2010 The Authors. Journal compilation © 2009 RAS.

Naab T.,Universitats Sternwarte Munich | Johansson P.H.,Universitats Sternwarte Munich | Ostriker J.P.,Princeton University
Astrophysical Journal Letters | Year: 2010

Using a high-resolution hydrodynamical cosmological simulation of the formation of a massive spheroidal galaxy we show that elliptical galaxies can be very compact and massive at high redshift in agreement with recent observations. Accretion of stripped infalling stellar material increases the size of the system with time and the central concentration is reduced by dynamical friction of the surviving stellar cores. In a specific case of a spheroidal galaxy with a final stellar mass of 1.5 × 1011 M ⊙ we find that the effective radius re increases from 0.7 ± 0.2 kpc at z = 3 to re = 2.4 ± 0.4 kpc at z = 0 with a concomitant decrease in the effective density of an order of magnitude and a decrease of the central velocity dispersion by approximately 20% over this time interval. A simple argument based on the virial theorem shows that during the accretion of weakly bound material (minor mergers) the radius can increase as the square of the mass in contrast to the usual linear rate of increase for major mergers. By undergoing minor mergers compact high-redshift spheroids can evolve into present-day systems with sizes and concentrations similar to observed local ellipticals. This indicates that minor mergers may be the main driver for the late evolution of sizes and densities of early-type galaxies. © 2009 The American Astronomical Society.

Grillo C.,TU Munich | Grillo C.,Max Planck Institute for Extraterrestrial Physics | Grillo C.,Universitats Sternwarte Munich
Astrophysical Journal | Year: 2010

We investigate in massive early-type galaxies the variation of their two-dimensional central fraction of dark over total mass and dark matter density as a function of stellar mass, central stellar velocity dispersion, effective radius, and central surface stellar mass density. We use a sample of approximately 1.7 × 105 galaxies from the Sloan Digital Sky Survey Data Release Seven (SDSS DR7) at redshift smaller than 0.33. We apply conservative photometric and spectroscopic cuts on the SDSS DR7 and the MPA/JHU value-added galaxy catalogs, to select galaxies with physical properties similar to those of the lenses studied in the Sloan Lens ACS Survey. The values of the galaxy stellar and total mass projected inside a cylinder of radius equal to the effective radius are obtained, respectively, by fitting the SDSS multicolor photometry with stellar population synthesis models, under the assumption of a Chabrier stellar initial mass function (IMF), and adopting a one-component isothermal total mass model with effective velocity dispersion approximated by the central stellar velocity dispersion. The plausibility of an isothermal model to represent the galaxy total mass distribution is supported by independent gravitational lensing and stellar-dynamical analyses performed in the lens subsample, which is found here to represent nicely the entire galaxy sample. We find that within the effective radius the stellar mass estimates differ from the total ones by only a relatively constant proportionality factor. In detail, we observe that the values of the projected fraction of dark over total mass and the logarithmic values of the central surface dark matter density (measured in M⊙ kpc-2) have almost Gaussian probability distribution functions, with median values of 0.64+0.08 0.11 and 9.1+0.2-0.2, respectively. We discuss the observed correlations between these quantities and other galaxy global parameters and show that our results disfavor an interpretation of the tilt of the fundamental plane in terms of differences in the galaxy dark matter content and give useful information on the possible variations of the galaxy stellar IMF and dark matter density profile. Finally, we provide some observational evidence on the likely significant contribution of dry minor mergers, feedback from active galactic nuclei, and/or coalescence of binary black holes on the formation and evolution of massive early-type galaxies. © 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A.

Bastian N.,Liverpool John Moores University | Niederhofer F.,European Southern Observatory | Niederhofer F.,Universitats Sternwarte Munich
Monthly Notices of the Royal Astronomical Society | Year: 2015

A recent surprise in stellar cluster research, made possible through the precision of Hubble Space Telescope photometry, was that some intermediate-age (1-2 Gyr) clusters in the Large and Small Magellanic Clouds have main-sequence turn-off (MSTO) widths that are significantly broader than would be expected for a simple stellar population (SSP). One interpretation of these extended MSTOs (eMSTOs) is that age spreads of the order of 500 Myr exist within the clusters, radically redefining our view of stellar clusters, which are traditionally thought of as single-age, single-metallicity stellar populations.Herewe test this interpretation by studying other regions of the CMD that should also be affected by such large age spreads, namely the width of the sub-giant branch (SGB) and the red clump (RC). We study two massive clusters in the LMC that display the eMSTO phenomenon (NGC 1806 and NGC 1846) and show that both have SGB and RC morphologies that are in conflict with expectations if large age spreads exist within the clusters. We conclude that the SGB and RC widths are inconsistent with extended star formation histories within these clusters, hence age spreads are not likely to be the cause of the eMSTO phenomenon. Our results are in agreement with recent studies that also have cast doubt on whether large age spreads can exist in massive clusters; namely the failure to find age spreads in young massive clusters, a lack of gas/dust detected within massive clusters, and homogeneous abundances within clusters that exhibit the eMSTO phenomenon. © 2015 The Authors.

Mashonkina L.,Universitats Sternwarte Munich | Mashonkina L.,Russian Academy of Sciences
Astronomy and Astrophysics | Year: 2013

Context. Magnesium abundances of cool stars with different metallicities are important for understanding the galactic chemical evolution. Aims. This study tests atomic data used in stellar magnesium abundance analyses. Methods. We evaluate nonlocal thermodynamical equilibrium (NLTE) line formation for Mg i, using the most up-to-date theoretical and experimental atomic data available so far, and check the Mg abundances from individual lines in the Sun, four well studied A-type stars, and three reference metal-poor stars. Results. With the adopted gf-values, NLTE abundances derived from the Mg i 4703 Å, 5528 Å, and Mg ib lines are consistent within 0.05 dex for each A-type star. The same four Mg i lines in the solar spectrum give consistent NLTE abundances at log NMg/NH =-4.45, when correcting van der Waals damping constants inferred from the perturbation theory. Inelastic Mg+H collisions as treated by Barklem, Belyaev, Spielfiedel, Guitou, and Feautrier serve as an efficient thermalizing process for the statistical equilibrium of Mg i in the atmospheres of metal-poor stars. The use of Mg+H collision data improves Mg abundance determinations for HD 84937 and HD 122563, though it does not completely remove the differences between different lines. © 2013 ESO.

Dale J.E.,Excellence Cluster Universe | Ngoumou J.,Universitats Sternwarte Munich | Ercolano B.,Excellence Cluster Universe | Bonnell I.A.,University of St. Andrews
Monthly Notices of the Royal Astronomical Society | Year: 2013

We examine the effect of momentum-driven OB-star stellar winds on a parameter space of simulated turbulent giant molecular clouds using smoothed particle hydrodynamic simulations. By comparison with identical simulations in which ionizing radiation was included instead of winds, we show that momentum-driven winds are considerably less effective in disrupting their host clouds than are HII regions. The wind bubbles produced are smaller and generally smoother than the corresponding ionization-driven bubbles. Winds are roughly as effective in destroying the very dense gas in which the O stars are embedded, and thus shutting down the main regions of star-forming activity in the model clouds. However, their influence falls off rapidly with distance from the sources, so they are not as good at sweeping up dense gas and triggering star formation further out in the clouds. As a result, their effect on the star formation rate and efficiency is generally more negative than that of ionization, if they exert any effect at all © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Erwin P.,Max Planck Insitut fur extraterrestrische Physik | Erwin P.,Universitats Sternwarte Munich | Debattista V.P.,University of Central Lancashire
Monthly Notices of the Royal Astronomical Society | Year: 2013

We show that direct detection and measurement of the vertically thickened parts of bars (so-called 'boxy' or 'peanut-shaped' bulges) are possible not only for edge-on galaxies but also for galaxies with moderate inclinations (i< 70°), and that examples are relatively common in the nearby Universe. The analysis of a sample of 78 nearby, moderately inclined (i ≲ 65°) early-type (S0-Sb) barred galaxies shows that the isophotal signature of the box/peanut can usually be detected for inclinations as low as i ∼ 40° - and in exceptional cases down to i ∼ 30°. In agreement with the predictions from N-body simulations, the signature is most easily detectable when the bar's position angle is within ∼50° of the galaxy major axis; in particular, galaxies where the bar lies very close to the minor axis do not show the signature clearly or at all. For galaxies with i= 40°-65° and relative angles <45°, we find evidence for the signature ≈2/3 of the time; the true frequency of box/peanut structures in bars may be higher. Comparison with N-body models also allows us to link observed photometric morphology with 3D physical structures, and thus estimate the relative sizes of box/peanut structures and bars. For our local sample, we find that box/peanut structures range in radial size (measured along the bar major axis) from 0.4 to 3.8 kpc (mean 1.5 ± 0.9 kpc) and span 0.26-0.58 of the bar length (mean of 0.38 ± 0.08). This is a clear observational confirmation that when bars thicken, it is not the entire bar which does so, but only the inner part. This technique can also be used to identify galaxies with bars which have not vertically thickened. We suggest that NGC 3049 and IC 676 may be particularly good examples, and that the fraction of S0-Sb bars which lack box/peanut structures is at least ∼13 per cent. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

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