Stellar Density Profiles Research Articles

Fitting isochrones to open cluster photometric data

We present a new technique to fit color-magnitude diagrams of open clusters based on the Cross-Entropy global optimization algorithm. The method uses theoretical isochrones available in the literature and maximizes a weighted likelihood function based on distances measured in the color-magnitude space. The weights are obtained through a non parametric technique that takes into account the star distance to the observed center of the cluster, observed magnitude uncertainties, the stellar density profile of the cluster among others. The parameters determined simultaneously are distance, reddening, age and metallicity. The method takes binary fraction into account and uses a Monte-Carlo approach to obtain uncertainties on the determined parameters for the cluster by running the fitting algorithm many times with a re-sampled data set through a bootstrapping procedure. We present results for 9 well studied open clusters, based on 15 distinct data sets, and show that the results are consistent with previous studies. The method is shown to be reliable and free of the subjectivity of most previous visual isochrone fitting techniques.

Numerical studies of the efficiency of resonant relaxation around a massive black hole

Resonant relaxation (RR) is a rapid relaxation process that operates in the nearly-Keplerian potential near a massive black hole (MBH). RR dominates the dynamics of compact remnants that inspiral into a MBH and emit gravitational waves (extreme mass ratio inspiral events, EMRIs). RR can either increase the EMRI rate, or strongly suppress it, depending on its still poorly-determined efficiency. We use small-scale Newtonian N-body simulations to measure the RR efficiency and to explore its possible dependence on the stellar number density profile around the MBH, and the mass-ratio between the MBH and a star (a single-mass stellar population is assumed). We present a suite of simulations with a range of stellar density profiles and mass-ratios, and measure the mean RR efficiency in the near-Keplerian limit. We do not find a strong dependence on the density profile or the mass-ratio. Our numerical determination of the RR efficiency in the Newtonian, single-mass population approximations, suggests that RR will likely enhance the EMRI rate by a factor of a few over the rates predicted assuming only slow stochastic two-body relaxation.

Probing the stellar populations in the outskirts of spiral galaxies

We present our results on the stellar population properties of the outskirts of disk galaxies. In particular, we focus on spiral galaxies with stellar disk truncations in their radial surface brightness profiles. Using SDSS data we show color gradients. We find that the color radial profile exhibits a "U'-shape", showing a minimum at the position of the break of the surface brightness profile. We obtain stellar surface mass density profiles of truncated galaxies as well, these show a peculiar behaviour: they follow very closely an exponential decrement. This suggests the idea that the observed properties of truncated galaxies are not caused by a drop in the mass distribution but by a different stellar population in the outer regions of the disks. Confronting this with current theoretical scenarios we find that this is likely to be a result of secular evolution, in which scenario stars are being formed inside the break and then being motioned outwards which would result in an inverted age gradient corresponding to the observed color profiles. Having maintained this idea, using multiwave-length data from GALEX, SDSS, UKIDSS and SPITZER we showed a dependence of the inner-outer scale-length ratio of truncated disks. Our results suggest that there is an existing general trend of the scale-length ratio: from bluer to redder bands the scale-length ratio decreases, which is in accordance with the stellar disk being dominated by an older stellar population in the outer disk.

NGC 2309: A Relatively Young Open Cluster Projected onto a Random Stellar Concentration

ABSTRACTWe have obtained CCD UBVIKC photometry down to V ∼ 22.0 for the open cluster NGC 2309 and its surrounding field. Twenty three probable cluster members have been identified for the first time on the basis of sound photometric criteria. Because NGC 2309 is a relatively poor cluster projected onto a rich star field showing density fluctuations, the frequently used membership criteria based on stellar density profiles or proper motion measurements would not be enough to assess the star membership status if employed independently. We estimated a cluster radius of 7.9′ and a radius at half the maximum of the cluster density profile of 1.7′. Based on the best fits of isochrones computed by the Geneva group to the cluster color–magnitude and color–color diagrams, previously shifted by the cluster reddening (E(B - V) = 0.32) and distance (d = 2.5 kpc), we derived the cluster age (250 Myr) and metallicity ([Fe/H] ≃ 0.0). If an 8.5 kpc distance from the Sun to the center of the Galaxy is assumed, then NGC 2309 is found to be located just in front of the Perseus spiral arm.

LIGHT CURVE TEMPLATES AND GALACTIC DISTRIBUTION OF RR LYRAE STARS FROM SLOAN DIGITAL SKY SURVEY STRIPE 82

We present an improved analysis of halo substructure traced by RR Lyrae stars in the SDSS stripe 82 region. With the addition of SDSS-II data, a revised selection method based on new ugriz light curve templates results in a sample of 483 RR Lyrae stars that is essentially free of contamination. The main result from our first study persists: the spatial distribution of halo stars at galactocentric distances 5--100 kpc is highly inhomogeneous. At least 20% of halo stars within 30 kpc from the Galactic center can be statistically associated with substructure. We present strong direct evidence, based on both RR Lyrae stars and main sequence stars, that the halo stellar number density profile significantly steepens beyond a Galactocentric distance of ~30 kpc, and a larger fraction of the stars are associated with substructure. By using a novel method that simultaneously combines data for RR Lyrae and main sequence stars, and using photometric metallicity estimates for main sequence stars derived from deep co-added u-band data, we measure the metallicity of the Sagittarius dSph tidal stream (trailing arm) towards R.A.2h-3h and Dec~0 deg to be 0.3 dex higher ([Fe/H]=-1.2) than that of surrounding halo field stars. Together with a similar result for another major halo substructure, the Monoceros stream, these results support theoretical predictions that an early forming, smooth inner halo, is metal poor compared to high surface brightness material that have been accreted onto a later-forming outer halo. The mean metallicity of stars in the outer halo that are not associated with detectable clumps may still be more metal-poor than the bulk of inner-halo stars, as has been argued from other data sets.

New fundamental parameters of the Galactic open clusters Berkeley 26, Czernik 27, Melotte 72, NGC 2479 and BH 37

We have obtained CCD UBVI_{KC} photometry down to V ~ 21.0 for the open clusters Berkeley 26, Czernik 27, Melotte 72, NGC 2479 and BH 37. The latter has never been studied before. Cluster stellar density profiles were obtained from star counts in appropriate-sized boxes distributed throughout the entire observed fields. Based on different measured indices, we estimate the ages of Berkeley 26, Melotte 72 and NGC 2479. On the other hand, we indicate possible solutions for the cluster fundamental parameters by matching theoretical isochrones which reasonably reproduce the main cluster features in their CMDs. In the case of NGC 2479, the cluster E(B-V) and E(V-I) colour excesses and apparent distance modulus were estimated from the fit of the Zero-Age Main Sequence (ZAMS) to the colour-colour and colour-magnitude diagrams, respectively.

The structure of the outer Galactic disc as revealed by IPHAS early A stars

This study is an investigation of the stellar density profile of the Galactic disc in the anticentre direction. We select over 40 000 early A stars from INT/WFC Photometric Hα Survey of the Northern Galactic Plane (IPHAS) photometry in the Galactic longitude range 160° < l < 200° close to the equatorial plane (−1° < b < +1°). We then compare their observed reddening-corrected apparent magnitude distribution with simulated photometry obtained from parametrized models in order to set constraints on the anticentre stellar density profile. By selecting A stars, we are appraising the properties of a population only ∼100 Myr old. We find that the stellar density profile of young stars is well fit to an exponential with length-scale of (3020 ± 120statistical± 180systematic) pc, which is comparable to that obtained in earlier studies, out to a Galactocentric radius of RT= (13.0 ± 0.5statistical± 0.6systematic) kpc. At larger radii, the rate of decline appears to increase with the scalelength dropping to (1200 ± 300statistical± 70systematic) pc. This result amounts to a refinement of the conclusions reached in previous studies that the stellar density profile is abruptly truncated. The IPHAS A star data are not compatible with models that propose a sudden change in metallicity at RG= 10 kpc.

Open cluster survival within the solar circle: Teutsch 145 and Teutsch 146

Teutsch145 and Teutsch146 are shown to be open clusters (OCs) orbiting well inside the Solar circle, a region where several dynamical processes combine to disrupt most OCs on a time-scale of a few 10^8yrs. BVI photometry from the GALILEO telescope is used to investigate the nature and derive the fundamental and structural parameters of the optically faint and poorly-known OCs Teutsch145 and 146. These parameters are computed by means of field-star decontaminated colour-magnitude diagrams (CMDs) and stellar radial density profiles (RDPs). Cluster mass estimates are made based on the intrinsic mass functions (MFs). We derive the ages 200+100-50Myr and 400+/-100Myr, and the distances from the Sun 2.7+/-0.3kpc and 3.8+/-0.2kpc, respectively for Teutsch145 and 146. Their integrated apparent and absolute magnitudes are m_V ~ 12.4, m_V ~ 13.3, M_V ~- 5.6 and M_V ~- 5.3. The MFs (detected for stars with m>1Msun) have slopes similar to Salpeter's IMF. Extrapolated to the H-burning limit, the MFs would produce total stellar masses of ~1400Msun, typical of relatively massive OCs. Both OCs are located deep into the inner Galaxy and close to the Crux-Scutum arm. Since cluster-disruption processes are important, their primordial masses must have been higher than the present-day values. The conspicuous stellar density excess observed in the innermost bin of both RDPs might reflect the dynamical effects induced by a few 10^8yrs of external tidal stress.

HIGH ANGULAR RESOLUTION INTEGRAL-FIELD SPECTROSCOPY OF THE GALAXY'S NUCLEAR CLUSTER: A MISSING STELLAR CUSP?

We report on the structure of the nuclear star cluster in the innermost 0.16 pc of the Galaxy as measured by the number density profile of late-type giants. Using laser guide star adaptive optics in conjunction with the integral field spectrograph, OSIRIS, at the Keck II telescope, we are able to differentiate between the older, late-type ($\sim$ 1 Gyr) stars, which are presumed to be dynamically relaxed, and the unrelaxed young ($\sim$ 6 Myr) population. This distinction is crucial for testing models of stellar cusp formation in the vicinity of a black hole, as the models assume that the cusp stars are in dynamical equilibrium in the black hole potential. Based on the late-type stars alone, the surface stellar number density profile, $\Sigma(R) \propto R^{-\Gamma}$, is flat, with $\Gamma = -0.27\pm0.19$. Monte Carlo simulations of the possible de-projected volume density profile, n(r) $\propto r^{-\gamma}$, show that $\gamma$ is less than 1.0 at the 99.73 % confidence level. These results are consistent with the nuclear star cluster having no cusp, with a core profile that is significantly flatter than predicted by most cusp formation theories, and even allows for the presence of a central hole in the stellar distribution. Of the possible dynamical interactions that can lead to the depletion of the red giants observable in this survey -- stellar collisions, mass segregation from stellar remnants, or a recent merger event -- mass segregation is the only one that can be ruled out as the dominant depletion mechanism. The lack of a stellar cusp around a supermassive black hole would have important implications for black hole growth models and inferences on the presence of a black hole based upon stellar distributions.

NGC 2587: a sparse open cluster projected on to a populous star field

We present CCD photometry in the Johnson U, B and V and Kron―Cousins I passbands for the open cluster NGC 2587. The sample consists of 4406 stars reaching down to V ∼ 21.0. We developed a new method to clean statistically the colour-magnitude diagrams. NGC 2587 appears to be a sparse, relatively bright open cluster, with a few tens of members projected on to a populous star field. The comparatively bright F7/8 II type star HD 70927, located close to the cluster centre, seems not to be a member. Our analysis suggests that NGC 2587 is slightly younger than the Hyades and probably of solar metallicity. A cluster radius of roughly 8 arcmin was estimated from the radial stellar density profile. From 18 probable cluster members with measured proper motions, we derive the following mean values for NGC 2587: μ α = ―4.3 ± 3.6 mas yr ―1 and μ δ = ―2.5 ± 3.4 mas yr ―1 . Adopting the theoretical metal content Z = 0.02, which provides the best global fit, we derive a cluster age of 500 +60 ―50 . Simultaneously, colour excesses E(B - V) = 0.10 and E(V - I) = 0.15 and an apparent distance modulus of V -M v = 12.50 are obtained. The interstellar extinction in the cluster direction is found to follow the normal law. NGC 2587 is located at a distance of (2.70 ± 0.70) kpc from the Sun and ∼9.8 kpc from the Galactic centre.

The nature of the young and low-mass open clusters Pismis 5, vdB 80, NGC 1931 and BDSB 96

We investigate the nature of 4 young and low-mass open clusters (OCs) located in the $2^{nd}$ and $3^{rd}$ quadrants with near-IR 2MASS photometry (errors $<0.1$ mag). After field decontamination, the colour-magnitude diagrams (CMDs) display similar morphologies: a poorly-populated main sequence (MS) and a dominant fraction of pre-MS (PMS) stars somewhat affected by differential reddening. Pismis 5, vdB 80 and BDSB 96 have MS ages within $5\pm4$ Myr, while the MS of NGC 1931 is $10\pm3$ Myr old. However, non-instantaneous star formation is implied by the wider ($\sim20$ Myr) PMS age spread. The cluster masses derived from MS + PMS stars are low, within $\sim60-180 \ms$, with mass functions (MFs) significantly flatter than Salpeter's initial mass function (IMF). Distances from the Sun are within $1.0-2.4$ kpc, and the visual absorptions are in the range $\aV=1.0-2.0$. From the stellar radial density profiles (RDPs), we find that they are small ($\rc\la0.48$ pc, $\rl\la5.8$ pc), especially Pismis 5 with $\rc\approx0.2$ pc and $\rl\approx1.8$ pc. Except for the irregular and cuspy inner regions of NGC 1931 and Pismis 5, the stellar RDPs follow a King-like profile. At $\sim10$ Myr, central cusps - which in old clusters appear to be related to advanced dynamical evolution - are probably associated with a star-formation and/or molecular cloud fragmentation effect. Despite the flat MFs, vdB 80 and BDSB 96 appear to be typical young, low-mass OCs. NGC 1931 and especially Pismis 5, with irregular RDPs, low cluster mass and flat MFs, do not appear to be in dynamical equilibrium. Both may be evolving into OB associations and/or doomed to dissolution in a few $10^7$ yr.

Multi-wavelength observations of the GRB 080319B afterglow and the modeling constraints

We present observations of the afterglow of GRB 080319B at optical, mm and radio frequencies from a few hours to 67 days after the burst. Present observations along with other published multi-wavelength data have been used to study the light-curves and spectral energy distributions of the burst afterglow. The nature of this brightest cosmic explosion has been explored based on the observed properties and it's comparison with the afterglow models. Our results show that the observed features of the afterglow fits equally good with the Inter Stellar Matter and the Stellar Wind density profiles of the circum-burst medium. In case of both density profiles, location of the maximum synchrotron frequency $\nu_m$ is below optical and the value of cooling break frequency $\nu_c$ is below $X-$rays, $\sim 10^{4}$s after the burst. Also, the derived value of the Lorentz factor at the time of naked eye brightness is $\sim 300$ with the corresponding blast wave size of $\sim 10^{18}$ cm. The numerical fit to the multi-wavelength afterglow data constraints the values of physical parameters and the emission mechanism of the burst.

Galaxy density profiles and shapes - I. Simulation pipeline for lensing by realistic galaxy models

Studies of strong gravitational lensing in current and upcoming wide and deep photometric surveys, and of stellar kinematics from (integral-field) spectroscopy at increasing redshifts, promise to provide valuable constraints on galaxy density profiles and shapes. However, both methods are affected by various selection and modelling biases, whch we aim to investigate in a consistent way. In this first paper in a series we develop a flexible but efficient pipeline to simulate lensing by realistic galaxy models. These galaxy models have separate stellar and dark matter components, each with a range of density profiles and shapes representative of early-type, central galaxies without significant contributions from other nearby galaxies. We use Fourier methods to calculate the lensing properties of galaxies with arbitrary surface density distributions, and Monte Carlo methods to compute lensing statistics such as point-source lensing cross-sections. Incorporating a variety of magnification bias modes lets us examine different survey limitations in image resolution and flux. We rigorously test the numerical methods for systematic errors and sensitivity to basic assumptions. We also determine the minimum number of viewing angles that must be sampled in order to recover accurate orientation-averaged lensing quantities. We find that for a range of non-isothermal stellar and dark matter density profiles typical of elliptical galaxies, the combined density profile and corresponding lensing properties are surprisingly close to isothermal around the Einstein radius. The converse implication is that constraints from strong lensing and/or stellar kinematics, which are indeed consistent with isothermal models near the Einstein radius, cannot trivially be extrapolated to smaller and larger radii.

ANALYTIC STUDY OF MASS SEGREGATION AROUND A MASSIVE BLACK HOLE

We analyze the distribution of stars of arbitrary mass function xi(m) around a massive black hole (MBH). Unless xi is strongly dominated by light stars, the steady-state distribution function approaches a power-law in specific energy x=-E/(m*sigma^2)<x_max with index p=m/4M_0, where E is the energy, sigma is the typical velocity dispersion of unbound stars, and M_0 is the mass averaged over m*xi*x_{max}^p. For light-dominated xi, p can grow as large as 3/2 - much steeper than previously thought. A simple prescription for the stellar density profile around MBHs is provided. We illustrate our results by applying them to stars around the MBH in the Milky Way.

THE EFFICIENCY OF RESONANT RELAXATION AROUND A MASSIVE BLACK HOLE

Resonant relaxation (RR) is a rapid relaxation process that operates in the nearly-Keplerian potential near a massive black hole (MBH). RR dominates the dynamics of compact remnants that inspiral into a MBH and emit gravitational waves (extreme mass ratio inspiral events, EMRIs). RR can either increase the EMRI rate, or strongly suppress it, depending on its still poorly-determined efficiency. We use small-scale Newtonian N-body simulations to measure the RR efficiency and to explore its possible dependence on the stellar number density profile around the MBH, and the mass-ratio between the MBH and a star (a single-mass stellar population is assumed). We develop an efficient and robust procedure for detecting and measuring RR in N-body simulations. We present a suite of simulations with a range of stellar density profiles and mass-ratios, and measure the mean RR efficiency in the near-Keplerian limit. We do not find a statistically significant dependence on the density profile or the mass-ratio. Our numerical determination of the RR efficiency in the Newtonian, single-mass population approximations, suggests that RR will likely enhance the EMRI rate by a factor of a few over the rates predicted assuming only slow stochastic two-body relaxation.

THE RELATION BETWEEN COMPACT, QUIESCENT HIGH-REDSHIFT GALAXIES AND MASSIVE NEARBY ELLIPTICAL GALAXIES: EVIDENCE FOR HIERARCHICAL, INSIDE-OUT GROWTH

Recent studies have shown that massive quiescent galaxies at high redshift are much more compact than present-day galaxies of the same mass. Here we compare the radial stellar density profiles and the number density of a sample of massive galaxies at z ~ 2.3 to nearby massive elliptical galaxies. We confirm that the average stellar densities of the z ~ 2.3 galaxies within the effective radius, rho(<r_e), are two orders of magnitude higher than those of local elliptical galaxies of the same stellar mass. However, we also find that the densities measured within a constant physical radius of 1 kpc, rho(<1 kpc), are higher by a factor of 2-3 only. This suggests that inside-out growth scenarios are plausible, in which the compact high redshift galaxies make up the centers of normal nearby ellipticals. The compact galaxies are common at high redshift, which enables us to further constrain their evolution by requiring that the number density of their descendants does not exceed constraints imposed by the z=0 galaxy mass function. We infer that size growth must be efficient, with (r_{1+2}/r_1) ~ (M_{1+2}/M_1)^2. A simple model where compact galaxies with masses ~ 10^11 Msun primarily grow through minor mergers produces descendants with the approximate sizes, stellar densities, and number density of elliptical galaxies with masses 2-3 x10^11 Msun in the local Universe. We note that this model also predicts evolution in the M_BH - sigma relation, such that the progenitors of elliptical galaxies have lower black hole masses at fixed velocity dispersion. The main observational uncertainty is the conversion from light to mass; measurements of kinematics are needed to calibrate the masses and stellar densities of the high redshift galaxies.

THE DETECTION OF INSIDE-OUT DISK GROWTH IN M33

We present resolved stellar photometry of 4 fields along the major axis of the M33 disk from images taken with the Advanced Camera for Surveys aboard the Hubble Space Telescope. The photometry provides a detailed census of the red clump in all fields and reaches the ancient main sequence in the outermost field. Through detailed modeling of the color-magnitude diagrams, we find that the percentage of the stellar mass formed prior to z=1 changes from 71 +/- 9% in the innermost field to 16 +/- 6% in the outermost field. The disk shows a clear trend of increasing scale-length with time, evolving from 1.0 +/- 0.1 kpc 10 Gyr ago to 1.8 +/- 0.1 kpc at times more recent than 5 Gyr ago, in agreement with analytical predictions for disk growth. Beyond the disk truncation radius, however, the stellar density profile steepens with time and the age gradient reverses, in agreement with recent simulations. The late and slow growth of the stellar disk may be due in part to the low mass of M33.

Probing the age and structure of the nearby very young open clusters NGC 2244 and 2239

The very young open cluster (OC) NGC 2244 in the Rosette Nebula was studied with field-star-decontaminated Two-Micron All-Sky Survey (2MASS) photometry, which shows the main-sequence (MS) stars and an abundant pre-MS (PMS) population. Fundamental and structural parameters were derived with colour–magnitude diagrams (CMDs), stellar radial density profiles (RDPs) and mass functions (MFs). Most previous studies centred NGC 2244 close to the bright K0V star 12 Monocerotis, which is not a cluster member. Instead, the near-infrared RDP indicates a pronounced core near the O5 star HD 46150. We derive an age within 1–6 Myr, an absorption AV= 1.7 ± 0.2, a distance from the Sun d⊙= 1.6 ± 0.2 kpc (≈1.5 kpc outside the solar circle), an MF slope χ= 0.91 ± 0.13 and a total (MS+PMS) stellar mass of ∼625 M⊙. Its RDP is characterized by the core and cluster radii Rc≈ 5.6 arcmin (≈2.6 pc) and RRDP≈ 10 arcmin (≈4.7 pc), respectively. Departure from dynamical equilibrium is suggested by the abnormally large core radius and the marked central stellar excess. We also investigate the elusive neighbouring OC NGC 2239, which is low mass (mMS+PMS≈ 301 M⊙), young (5 ± 4 Myr) rather absorbed (AV= 3.4 ± 0.2), and located in the background of NGC 2244 at d⊙= 3.9 ± 0.4 kpc. Its RDP follows a King-like function of Rc≈ 0.5 arcmin ≈ 0.5 pc and RRDP≈ 5.0 arcmin ≈ 5.6 pc. The MF slope, χ= 1.24 ± 0.06, is essentially Salpeter's initial mass function. NGC 2244 is probably doomed to dissolution in a few 107 yr. Wide-field extractions and field-star decontamination increase the stellar statistics and enhance both CMDs and RDPs, which is essential for faint and bright star clusters.

Investigating the age and structure of the infrared old open clusters LK 1, LK 10, FSR 1521 and FSR 1555

The combination of several mass-decreasing processes may critically affect the structure of open clusters (OCs), to the point that most dissolve into the field in a time-scale shorter than ≈1 Gyr. Therefore, as is observed, old OCs should be sparse within the Galaxy. These mass-decreasing processes can only be quantifiably examined given the derivation of the fundamental parameters, age, distance and mass, for a sample of old OCs. In this paper we focus on four candidate old star clusters, namely, LK 1, LK 10, FSR 1521 and FSR 1555. The first two clusters, discovered by Le Duigou & Knodlseder, are projected towards the Cygnus association, whilst the remaining two have been detected in the fourth quadrant as cluster candidates from stellar overdensities by Froebrich, Scholz & Raftery. To analyse the target clusters we construct near-infrared colour-magnitude diagrams (CMDs) and derive stellar radial density profiles (RDPs). The CMDs are constructed using Two Micron All-Sky Survey (2MASS) J, H and K s bands, and the intrinsic morphologies of the target OCs within these diagrams are revealed by applying a field-star decontamination algorithm. Fundamental parameters are estimated with Padova isochrones built for the 2MASS filters. We derive extinctions to the objects within the range 3.4 < A V ≤ 8.9, which makes them suitable for the near-infrared analysis, ages within 1.0-2.0 Gyr, and distances from the Sun within 1.4-4.5 kpc. These distances, in conjunction with the positions in the sky, place the present four OCs close to the solar circle (≤0.6 kpc). For LK 10 our photometry reaches a depth ≈3mag below the main-sequence turn off, from which we derive a relatively steep mass function slope (x = 2.4 ± 0.4) when compared to the Salpeter value (x = 1.35). LK 10 is a rather massive old OC, with a mass within 1360 < m (M ⊙ ) ≤ 4400, for stars in the observed magnitude range and the extrapolation to 0.08 M ⊙ , respectively. The mass estimated in the restricted magnitude range for the remaining, more distant OCs is within 260 ≤ m (M ⊙ ) < 380. However, similarity with the CMD morphology and red clump of LK 10 suggests that they may be as massive as LK 10. The RDPs are well represented by a King-like function, except LK 10, which has a central cusp suggesting post-core collapse. Structurally, LK 1, FSR 1521 and FSR 1555 are similar to a sample of nearby OCs of comparable age.

THE STAR FORMATION EFFICIENCY IN NEARBY GALAXIES: MEASURING WHERE GAS FORMS STARS EFFECTIVELY

We measure the star formation efficiency (SFE), the star formation rate per unit gas, in 23 nearby galaxies and compare it to expectations from proposed star formation laws and thresholds. We use HI maps from THINGS and derive H2 maps from HERACLES and BIMA SONG CO. We estimate the star formation rate by combining GALEX FUV maps and SINGS 24 micron maps, infer stellar surface density profiles from SINGS 3.6 micron data, and use kinematics from THINGS. We measure the SFE as a function of: the free-fall and orbital timescales; midplane gas pressure; stability of the gas disk to collapse (including the effects of stars); the ability of perturbations to grow despite shear; and the ability of a cold phase to form. In spirals, the SFE of H2 alone is nearly constant at 5.25 +/- 2.5 x 10^(-10) yr^(-1) (equivalent to an H2 depletion time of 1.9x10^9 yr) as a function of all of these variables at our 800 pc resolution. Where the ISM is mostly HI, on the other hand, the SFE decreases with increasing radius in both spiral and dwarf galaxies, a decline reasonably described by an exponential with scale length 0.2-0.25 r_25. We interpret this decline as a strong dependence of GMC formation on environment. The ratio of H2 to HI appears to be a smooth function of radius, stellar surface density, and pressure spanning from the H2-dominated to HI-dominated ISM. The radial decline in SFE is too steep to be reproduced only by increases in the free-fall time or orbital time. Thresholds for large-scale instability suggest that our disks are stable or marginally stable and do not show a clear link to the declining SFE. We suggest that ISM physics below the scales that we observe - phase balance in the HI, H2 formation and destruction, and stellar feedback - governs the formation of GMCs from HI.