arXiv subjects

Stephane Courteau

Publications and source records attributed to Stephane Courteau.

At least 55 records · Page 3Linked to original sources

arXiv2007

A Revised Model for the Formation of Disk Galaxies: Low Spin and Dark-Halo Expansion

We use observed rotation velocity-luminosity (VL) and size-luminosity (RL) relations to single out a specific scenario for disk galaxy formation in the LCDM cosmology. Our model involves four independent log-normal random variables: dark-halo concentration c, disk spin lam_gal, disk mass fraction m_gal, and stellar mass-to-light ratio M/L_I. A simultaneous match of the VL and RL zero points with adiabatic contraction requires low-c halos, but this model has V_2.2~1.8 V_vir (where V_2.2 and V_vir are the circular velocity at 2.2 disk scale lengths and the virial radius, respectively) which will be unable to match the luminosity function (LF). Similarly models without adiabatic contraction but standard c also predict high values of V_2.2/V_vir. Models in which disk formation induces an expansion rather than the commonly assumed contraction of the dark-matter halos have V_2.2~1.2 V_vir which allows a simultaneous fit of the LF. This may result from non-spherical, clumpy gas accretion, where dynamical friction transfers energy from the gas to the dark matter. This model requires low lam_gal and m_gal values, contrary to naive expectations. However, the low lam_gal is consistent with the notion that disk galaxies predominantly survive in halos with a quiet merger history, while a low m_gal is also indicated by galaxy-galaxy lensing. The smaller than expected scatter in the RL relation, and the lack of correlation between the residuals of the VL and RL relations, respectively, imply that the scatter in lam_gal and in c need to be smaller than predicted for LCDM halos, again consistent with the idea that disk galaxies preferentially reside in halos with a quiet merger history.

astro-ph↗

arXiv2006

The Bulge-Halo Connection in Galaxies: A Physical Interpretation of the Vcirc-sigma_0 Relation

We explore the dependence of the ratio of a galaxy's circular velocity, Vcirc, to its central velocity dispersion, sigma_0, on morphology, or equivalently total light concentration. Such a dependence is expected if light traces the mass. Over the full range of galaxy types, masses and brightnesses, and assuming that the gas velocity traces the circular velocity, we find that galaxies obey the relation log(Vcirc/sigma_0)= 0.63-0.11*C28 where C28=5log(r80/r20) and the radii are measured at 80 percent and 20 percent of the total light. Massive galaxies scatter about the Vcirc = sqrt(2)*sigma_0 line for isothermal stellar systems. Disk galaxies follow the simple relation Vcirc/sigma_0=2(1-B/T), where B/T is the bulge-to-total light ratio. For pure disks, C28~2.8, B/T -> 0, and Vcirc~=2*sigma_0. Self-consistent equilibrium galaxy models from Widrow & Dubinski (2005) constrained to match the size-luminosity and velocity-luminosity relations of disk galaxies fail to match the observed Vcirc/sigma_0 distribution. Furthermore, the matching of dynamical models for Vcirc(r)/sigma(r) with observations of dwarf and elliptical galaxies suffers from limited radial coverage and relatively large error bars; for dwarf systems, however, kinematical measurements at the galaxy center and optical edge suggest Vcirc(Rmax) > 2*sigma_0 (in contrast with past assumptions that Vcirc = sqrt(2)*sigma_0 for dwarfs.) The Vcirc-sigma_0-C28 relation has direct implications for galaxy formation and dynamical models, galaxy scaling relations, the mass function of galaxies, and the links between respective formation and evolution processes for a galaxy's central massive object, bulge, and dark matter halo.

astro-ph↗

arXiv2005

M31's Heavy Element Distribution and Outer Disk

Hubble Space Telescope imaging of 11 fields in M31 were reduced to color-magnitude diagrams. The fields were chosen to sample all galactocentric radii to 50 kpc. Assuming that the bulk of the sampled stellar populations are older than a few Gyr, the colors of the red giants map to an abundance distribution with errors of order 0.1 dex in abundance. The radially sampled abundance distributions are all about the same width, but show a mild abundance gradient that flattens outside ~20 kpc. The various distributions were weighted and summed with the aid of new surface brightness profile fits to obtain an abundance distribution representative of the entirety of M31. M31 is a system near chemical maturity. This ``observed closed box'' is compared to analytical closed box models. M31 suffers from a lack of metal-poor stars and metal-rich stars relative to the simplest closed-box model in the same way as the solar neighborhood.Comparing to several simple chemical evolution models, neither complete mixing of gas at all times nor zero mixing, inhomogeneous models give the most convincing match to the data. As noted elsewhere, the outer disk of M31 is a factor of ten more metal-rich than the Milky Way halo, ten times more metal-rich than the dwarf spheroidals cospatial with it, and more metal-rich than most of the globular clusters at the same galactocentric radius. Difficulties of interpretation are greatly eased if we posit that the M31 disk dominates over the halo at all radii out to 50 kpc. In fact, scaling from current density models of the Milky Way, one should not expect to see halo stars dominating over disk stars until beyond our 50 kpc limit. A corollary conclusion is that most published studies of the M31 "halo" are actually studies of its disk.

astro-ph↗

arXiv2005

A Library of Integrated Spectra of Galactic Globular Clusters

We present a new library of integrated spectra of 40 Galactic globular clusters, obtained with the Blanco 4-m telescope and the R-C spectrograph at the Cerro Tololo Interamerican Observatory. The spectra cover the range ~ 3350 -- 6430 A with ~ 3.1 A (FWHM) resolution. The spectroscopic observations and data reduction were designed to integrate the full projected area within the cluster core radii in order to properly sample the light from stars in all relevant evolutionary stages. The S/N values of the flux-calibrated spectra range from 50 to 240/A at 4000 A and from 125 to 500/A at 5000 A. The selected targets span a wide range of cluster parameters, including metallicity, horizontal-branch morphology, Galactic coordinates, Galactocentric distance, and concentration. The total sample is thus fairly representative of the entire Galactic globular cluster population and should be valuable for comparison with similar integrated spectra of unresolved stellar populations in remote systems. For most of the library clusters, our spectra can be coupled with deep color-magnitude diagrams and reliable metal abundances from the literature to enable the calibration of stellar population synthesis models. In this paper we present a detailed account of the observations and data reduction. The spectral library is publicly available in electronic format from the National Optical Astronomical Observatory website.

astro-ph↗

arXiv2005

Mass Modeling of Disk Galaxies: Constraints, Degeneracies and Adiabatic Contraction

This paper addresses available constraints on mass models fitted to rotation curves. Mass models of disk galaxies have well-known degeneracies, that prevent a unique mass decomposition. The most notable is due to the unknown value of the stellar mass-to-light ratio (the disk-halo degeneracy); even with this known, degeneracies between the halo parameters themselves may prevent an unambiguous determination of the shape of the dark halo profile, which includes the inner density slope of the dark matter halo. The latter is often referred to as the ``cusp-core degeneracy''. We explore constraints on the disk and halo parameters and apply these to four mock and six observed disk galaxies with high resolution and extended rotation curves. Our full set of constraints consists of mass-to-light (M/L) ratios from stellar population synthesis models based on B-R colors, constraints on halo parameters from N-body simulations, and constraining the halo virial velocity to be less than the maximum observed velocity. These constraints are only partially successful in lifting the cusp-core degeneracy. The effect of adiabatic contraction of the halo by the disk is to steepen cores into cusps and reduce the best-fit halo concentration and M/L values (often significantly). We also discuss the effect of disk thickness, halo flattening, distance errors, and rotation curve error values on mass modeling. Increasing the imposed minimum rotation curve error from typically low, underestimated values to more realistic estimates decreases the chi-square substantially and makes distinguishing between a cuspy or cored halo profile even more difficult. In spite of the degeneracies and uncertainties present, our constrained mass modeling favors sub-maximal disks (i.e., a dominant halo) at 2.2 disk scale lengths, with Vdisk/Vtot < 0.6.

astro-ph↗

arXiv2005

Origin of the Joint Distribution of Structural Parameters in Disc Galaxies

We use a simple model of disc-galaxy formation in the LCDM cosmology to simultaneously reproduce the slopes, zero-points, scatter and uncorrelated residuals in the observed velocity-luminosity (VL) and radius-luminosity (RL) relations. Observed I-band luminosities are converted to stellar masses using the IMF-dependent relation between stellar mass-to-light ratio, M/L, and color. The model treats halo concentration, spin parameter, and disc mass fraction as independent log-normal random variables. Our main conclusion is that the VL and RL zero-points and the uncorrelated residuals can only be reproduced simultaneously if adiabatic contraction is avoided. One or the other could be fixed by appealing to unrealistic values for M/L, halo concentration c, and spin parameter lambda, but not all together. The small VL scatter is naturally determined by the predicted scatter in c and in M/L, quite independent of the large scatter in lambda. However, the RL scatter, driven by the scatter in lambda, can be as low as observed only if sigma_lambda simeq 0.25, about half the value predicted for CDM haloes. This may indicate that discs form in a special subset of haloes. The VL slope is reproduced once star formation occurs only above a threshold surface density, and the RL slope implies that the disc mass fraction is increasing with halo mass, consistent with feedback effects. A model that incorporates the above ingredients provides a simultaneous fit to all the observed features. In particular, the elimination of adiabatic contraction allows 80% disc contribution to the observed rotation velocity of bright discs at 2.2 disc scale lengths. The lack of halo contraction may indicate that disc formation is not as smooth as typically envisioned, but instead may involve clumpy, cold streams.

astro-ph↗

arXiv2004

The Identification of Blue Horizontal Branch Stars in the Integrated Spectra of Globular Clusters

A major uncertainty in the spectroscopic dating of extragalactic globular clusters concerns the degenerate effect that age and horizontal branch morphology have on the strength of Balmer lines. In this Letter we show that the ratio between the equivalent widths of Hdelta and Hbeta is far more sensitive to horizontal branch morphology than to age, thus making it possible to break the degeneracy. We show that it is possible to distinguish intermediate-age globular clusters from those whose Balmer lines are strengthened by the presence of blue horizontal branch stars, purely on the basis of the clusters' integrated spectra. The degeneracy between age and horizontal branch morphology can be lifted with Hbeta and Hdelta line strengths from spectra with S/N >= 30 per Angstrom, which is typical of current studies of integrated spectroscopy of extragalactic globular clusters.

astro-ph↗

arXiv2004

Structure of Disk Dominated Galaxies: II. Color Gradients and Stellar Population Models

(Abridged) We investigate optical and near-IR color gradients in a sample of 172 low-inclination galaxies spanning Hubble types S0--Irr. The colors are compared to stellar population synthesis models from which luminosity-weighted average ages and metallicities are determined. We explore the effects of different underlying star formation histories and additional bursts of star formation. Our results are robust in a relative sense under the assumption that our galaxies shared a similar underlying star formation history and that no bursts involving more than ~10% of the galaxy mass have occurred in the past 1-2 Gyr. We find strong correlations in age and metallicity with Hubble-type, rotational velocity, total magnitude, and central surface brightness in the sense that earlier-type, faster rotating, more luminous, and higher surface brightness galaxies are older and more metal-rich, suggesting an early and more rapid star formation history for these galaxies. The increasing trends with rotational velocity and total magnitude level off for V_rot > 120 km/s and M_K < -23 mag respectively. This effect is stronger for metallicity (than age) which could reflect a threshold potential above which all metals are retained and thus metallicity saturates at the yield. Trends in gradients with galaxy parameters are compared with model predictions: these trends do not agree with predictions of semi-analytic models of hierarchical galaxy formation, possibly due to the effect of bar-induced radial flows. The observed trends are in agreement with chemo-spectrophotometric models of spiral galaxy evolution based on CDM-motivated scaling laws but none of the hierarchical merging characteristics, implying a strong dependence of the star formation history of spiral galaxies on the galaxy potential and halo spin parameter.

astro-ph↗

arXiv2003

Properties of Dark Matter Halos in Disk Galaxies

We present a simple technique to estimate mass-to-light (M/L) ratios of stellar populations based on two broadband photometry measurements, i.e. a color-M/L relation. We apply the color-M/L relation to galaxy rotation curves, using a large set of galaxies that span a great range in Hubble type, luminosity and scale size and that have accurately measured HI and/or Halpha rotation curves. Using the color-M/L relation, we construct stellar mass models of the galaxies and derive the dark matter contribution to the rotation curves. We compare our dark matter rotation curves with adiabatically contracted Navarro, Frenk, & White (1997, NFW hereafter) dark matter halos. We find that before adiabatic contraction most high surface brightness galaxies and some low surface brightness galaxies are well fit by a NFW dark matter profile. However, after adiabatic contraction, most galaxies are poorly fit in the central few kpc. The observed angular momentum distribution in the baryonic component is poorly matched by LambdaCDM model predictions, indicating that the angular momentum distribution is not conserved during the galaxy assembly process. We find that in most galaxies the dark matter distribution can be derived by scaling up the HI gas contribution. However, we find no consistent value for the scaling factor among all the galaxies.

astro-ph↗

arXiv2003

Scaling Relations of Local Spiral Galaxies.I. Observational Foundations

(Abridged) This paper presents an exploration of two fundamental scaling relations of spiral galaxies, the luminosity-rotation speed (or Tully-Fisher; TF) relation, and the size-luminosity (SL) relation, and the dependences of their scatter, at red and IR bands. We verify that the observed virial relations of disk galaxies are given by Vobs \prop L^0.31 and Rd \prop L^0.33 using distance-redshift surveys of high surface brightness (HSB) and low surface brightness (LSB) non-interacting galaxies. Thus the galaxy surface brightness (SB) is Sigma \prop Rd \prop Vobs \prop Mvir^0.33. We derive the first extensive J-band TF and SL relations using 2MASS data products. The TF residuals are fully independent of SB and other tested observables. TF relations (TFRs) for various morphological types have different zero points, but a common slope, such that early-type (redder) disks rotate faster than later-types (bluer) of the same luminosity. This dependence of the TFR is a direct consequence of the more fundamental dependence on color, which itself is related to the galaxy star formation history. The SL relation scatter is mostly dominated by SB, but color also plays a small role. The TF and SL residuals for HSB and LSB galaxies are weakly correlated, in agreement with the earlier claim by Courteau & Rix (1999). This suggests that spiral disks of all color, brightness, and barredness are, on average, dominated by dark matter even in their inner parts. The observed scaling relations of disk galaxies are likely the result of the simplest scenario of galaxy formation, but their color dependence remains a challenge to hierarchical structure formation models. The scaling relations and their derivatives are contrasted against galaxy formation models in our companion paper.

astro-ph↗

arXiv2003

The Tully-Fisher Relation of Barred Galaxies

We present new data exploring the scaling relations, such as the Tully-Fisher relation (TFR), of bright barred and unbarred galaxies. A primary motivation for this study is to establish whether barredness correlates with, and is a consequence of, virial properties of galaxies. Various lines of evidence suggest that dark matter is dominant in disks of bright unbarred galaxies at 2.2 disk scale lengths, the point of peak rotation for a pure exponential disk. We test the hypothesis that the TF plane of barred high surface brightness galaxies is offset from the mean TFR of unbarred galaxies, as might be expected if barred galaxies are ``maximal'' in their inner parts. We use existing and new TF data to search for basic structural differences between barred and unbarred galaxies. Our new data consist of 2-dimensional Halpha velocity fields derived from SparsePak integral field spectroscopy (IFS) and V,I-band CCD images collected at the WIYN Observatory for 14 strongly barred galaxies. We use WIYN/SparsePak (2-D) velocity fields to show that long-slit (1-D) spectra yield reliable circular speed measurements at or beyond 2.2 disk scale lengths, far from any influence of the bar. This enables us to consider line width measurements from extensive TF surveys which include barred and nonbarred disks and derive detailed scaling relation comparisons. We find that for a given luminosity, barred and unbarred galaxies have comparable structural and dynamical parameters, such as peak velocities, scale lengths, or colors. In particular, the location of a galaxy in the TF plane is independent of barredness. In a global dynamical sense, barred and unbarred galaxies behave similarly and are likely to have, on average, comparable fractions of luminous and dark matter at a given radius. (abridged)

astro-ph↗

arXiv2002

Structure of Disk Dominated Galaxies I. Bulge/Disk Parameters, Simulations, and Secular Evolution

(Abridged) A robust analysis of galaxy structural parameters, based on the modeling of bulge and disk brightnesses in the BVRH bandpasses, is presented for 121 face-on and moderately inclined late-type spirals. Each surface brightness (SB) profile is decomposed into a sum of a generalized Sersic bulge and an exponential disk. The reliability and limitations of our bulge-to-disk (B/D) decompositions are tested with extensive simulations of galaxy brightness profiles (1D) and images (2D). Galaxy types are divided into 3 classes according to their SB profile shapes; Freeman Type-I and Type-II, and a third ``Transition'' class for galaxies whose profiles change from Type-II in the optical to Type-I in the infrared. We discuss possible interpretations of Freeman Type-II profiles. The Sersic bulge shape parameter for nearby Type-I late-type spirals shows a range between n=0.1-2 but, on average, the underlying surface density profile for the bulge and disk of these galaxies is adequately described by a double-exponential distribution. We confirm a coupling between the bulge and disk with a scale length ratio r_e/h=0.22+/-0.09, or h_bulge/h_disk=0.13+/-0.06 for late-type spirals, in agreement with recent N-body simulations of disk formation and models of secular evolution. This ratio increases from ~0.20 for late-type spirals to ~0.24 for earlier types. The similar scaling relations for early and late-type spirals suggest comparable formation and/or evolution scenarios for disk galaxies of all Hubble types.

astro-ph↗

arXiv2002

Galaxy Evolution from Emission Linewidths

The major thrust of the Tully-Fisher (TF) surveys of distant galaxies is the measurement of linewidths rather than mere redshifts or colors. Linewidths are a measure of galaxy mass and should therefore be a more stable indicator of size than galaxy brightness, which can be badly affected by luminosity evolution. Masses may provide the best way to relate galaxies at different epochs, but for such a program to work, we must control systematic effects that could bias linewidth measurements at high redshift and skew comparisons with local Tully-Fisher calibrations. Potential sources of confusion in TF studies of galaxy structure and evolution include central or extended star bursts, infalling gas, turbulence and outflows, dust extinction, calibration of emission linewidths, and improper application of local TF calibrations to high redshift galaxies.

astro-ph↗

arXiv2001

Cosmic Flows: A Status Report

We give a brief review of recent developments in the study of the large-scale velocity field of galaxies since the international workshop on Cosmic Flows held in July 1999 in Victoria, B.C. Peculiar velocities (PVs) yield a tight and unique constraint on cosmological characteristics, independent of Lambda and biasing, such as the cosmological matter density parameter Omega_m and the convergence of bulk flows on large scales. Significant progress towards incorporating non-linear dynamics and improvements of velocity field reconstruction techniques have led to a rigorous control of errors and much refined cosmic flow analyses. Current investigations favor low-amplitude (< 250 km/s) bulk flows on the largest scales (< ~100 Mpc/h) probed reliably by existing redshift-distance surveys, consistent with favored LambdaCDM cosmogonies. Tidal field analyses also suggest that the Shapley Concentration, located behind the Great Attractor (GA), might play an important dynamical role, even at the Local Group. Low-amplitude density fluctuations on very large scales generate the overall large-scale streaming motions while massive attractors like the GA, and Perseus-Pisces account for smaller scale motions which are superposed on the large-scale flow. Likelihood analyses of galaxy PVs, in the framework of flat CDM cosmology, now provide tight constraints of Omega_m = 0.35 +/- 0.05. A four-fold size increase of our data base is expected in the next 4-5 years with the completion of next generation FP/TF surveys and automated supernovae searches within 20,000 km/s.

astro-ph↗

arXiv1999

An Atlas for Structural Studies of Spiral Galaxies

This is an announcement of a new database of structural properties for 304 late-type (Sb-Sc) spiral galaxies drawn from the UGC catalogue. These data were compiled from the kinematic and photometric studies of Courteau (1996, 1997), and are made available to the community via the Canadian Astronomy Data Centre. The data base contains redshift information and Tully-Fisher distances, various measures of optical (Halpha) line width and rotational velocity, isophotal diameters and magnitudes, disk scale lengths, B-r colour, rotation curve model parameters, and more. The main table includes 66 entries (columns); it can be down-loaded as one single file, or searched for any range of parameters using our search engine. The data files for each rotation curve and luminosity profile (including multiple observations) are also available and can be retrieved as two separate tar files. These data were originally obtained for cosmic flow studies (e.g. Courteau etal 1993, Courteau 1993) and have been included in the Mark III Catalog of Galaxy Peculiar Velocities (Willick etal 1997). The high spatial and spectral resolution of these data make them ideal for structural and dynamical investigations of spiral galaxies (e.g. Broeils & Courteau 1997; Mo, Mao, & White 1998; Somerville & Primack 1999 (astro-ph/9802268); Courteau & Rix 1999 (astro-ph/9707290).) The data base can be accessed at http://cadcwww.hia.nrc.ca/astrocat/courteau.html

astro-ph↗

arXiv1999

The Solar Motion Relative to the Local Group

New data on the membership of the Local Group, in conjunction with new and improved radial velocity data, are used to refine the derivation of the motion of the Sun relative to the Local Group (hereafter LG). The Sun is found to be moving with a velocity of Vbulk = 306 \pm 18 km/s, towards an apex at l = 99 \pm 5 and b = -4 \pm 4 degrees. This agrees very well with previous analyses, but we discuss the possibility of a bias if the phase-space distribution of LG galaxies is bimodal. The LG radial velocity dispersion is 61 \pm 8 km/s. We use various mass estimators to compute the mass of the Local Group and the Andromeda subgroup. We find M_LG = (2.3 \pm 0.6)*10^12 \Msol, from which M/L_V = 44 \pm 12 (in solar units). For an assumed LG age of 14 \pm 2 Gyr, the radius of an idealized LG zero-velocity surface is r_0 = 1.18 \pm 0.15 Mpc. The Local Group is found to have 35 likely members. Only three of those have (uncertain) distances \grtapr 1.0 Mpc from the LG barycenter. Barring new discoveries of low surface brightness dwarfs, this suggests that the Local Group is more compact, and isolated from its surroundings, than previously believed.

astro-ph↗

arXiv1998

Maximal Disks and the Tully-Fisher Relation

We show that for luminous, non-barred, high-surface brightness (HSB) spirals the Tully-Fisher (TF) relation residuals can be used to estimate the relative mass contributions of the stellar disk and the dark halo at the peak of the disk rotation, near 2.2 exponential scale lengths (Rh). For "maximal disks", a large fraction (0.85 +/- 0.10) of the total rotational support, V2.2, at such radii should arise from their stellar mass. We find that the disk size, or surface brightness, is not a significant additional parameter in the TF relation and the case of a maximal disk is ruled out for most HSB galaxies. We model adiabatic infall of varying amounts of luminous matter into dark matter halos and find that the TF relation residuals require a mean value of V2.2/Vtotal ~ 0.6 +/- 0.1, fairly insensitive to the details of the initial dark matter halo and to the presence of a bulge. This translates to Mhalo/Mtotal ~ 0.6 within 2.2*Rh, or roughly twice more dark matter in the inner parts of late-type spirals than previously accounted for by maximum disk fits. We show that any stellar population differences between disks of different scalelengths lead to lower values of V2.2/Vtotal. Our result is independent of the shape of the luminosity profile and relies only on the assumption of adiabatic contraction and that the dark matter halo rotation rises in the central parts of the galaxy. Sub-maximal disks establish a natural continuity between HSB and low-surface brightness (LSB) galaxies which appear to be completely dark matter dominated even in their inner regions. (Abridged)

astro-ph↗

arXiv1997

Optical Rotation Curves and Linewidths for Tully-Fisher Applications

We present optical long-slit rotation curves (RCs) for 304 northern Sb-Sc UGC galaxies originally selected for Tully-Fisher (TF) applications. 20% of the galaxies were observed twice or more, allowing for a proper determination of systematic errors. Various measures of maximum rotational velocity to be used as input in the TF relation are tested on the basis of their repeatability, minimization of TF scatter, and match with 21cm linewidths. The best measure of TF velocity, v2.2, is given at the location of peak rotational velocity of a pure exponential disk. Optical TF calibrations yield internal scatter comparable to, if not smaller than, the best calibrations based on single-dish 21cm radio linewidths. Even though resolved HI RCs are more extended than their optical counterpart, a tight match between optical and radio linewidths exists since the bulk of the HI surface density is enclosed within the optical radius. We model the 304 RCs presented here and a sample of 958 curves from Mathewson etal. (1992) with various fitting functions. An arctan function provides an adequate simple fit (not accounting for non-circular motions and spiral arms). More elaborate, empirical models may yield a better match at the expense of strong covariances. We caution against physical or "universal" parameterizations for TF applications. (abridged)

astro-ph↗