Synthetic Color-magnitude Diagrams Research Articles

The Complex Star Formation History of NGC 1569

We present new results on the star formation history of the dwarf irregular galaxy NGC 1569. The data were obtained with Hubble Space Telescope's NICMOS/NIC2 in the F110W (J) and F160W (H) near-infrared (NIR) filters and interpreted with the synthetic color-magnitude diagram method. The galaxy experienced a complex star formation (SF) activity. The best fit to the data is found by assuming three episodes of activity in the last 1-2 Gyr. The most recent and strong episode constrained by these NIR data started ~37 Myr ago and ended ~13 Myr ago, although we cannot exclude the possibility that up to three SF episodes occurred in this time interval. The average star-formation rate (SFR) of the episode is 3.2 Msun yr-1 kpc-2, in agreement with literature data. A previous episode produced stars between 150 Myr and 40 Myr ago, with a mean SFR about 2/3 lower than the mean SFR of the youngest episode. An older SF episode occurred about 1 Gyr ago. All these SFRs are 2-3 orders of magnitude higher than those derived for late-type dwarfs of the Local Group. In all cases an initial mass function similar to Salpeter's allows for a good reproduction of the data, but we cannot exclude flatter mass functions. These results have been obtained adopting a distance of 2.2 Mpc and a reddening E(B-V)=0.56. A larger distance would require younger episodes and higher SFRs. We have explored some possible scenarios using the astrated mass in the best fit model, in order to constrain the past star formation history. We cannot rule out a low rate in the past SF but we can safely conclude that the last 1-2 Gyr have been peculiar.

The stellar populations of local dwarfs

Recent progress in our knowledge of stellar populations in local dwarf spheroidal galaxies is briefly discussed. A few results are summarized including wide field observations of stellar populations and their spatial variations, studies of AGB and variable stars, extension to near-infrared wavelengths, and the interpretation effort based on synthetic color-magnitude diagrams and chemical evolution models.

The star formation history of the Fornax dSph galaxy using the new synthetic color–magnitude diagram code IAC-STAR

We present VLT-FORS1 color-magnitude diagrams (CMDs) reaching the oldest main-sequence turnoffs for three fields at two galactocentric distances in the Fornax dSph galaxy. We derive the star formation history (SFH) for each field through comparison with synthetic CMDs and show that the main epoch of star formation has extended to more recent epochs toward the center of the galaxy. A small burst of star formation about 2 Gyr ago is a robust feature in the SFH of all fields. It might be related to the shell structure found in Fornax by Coleman et al. (2004), which in turn has been interpreted as the relic of a merger event that occurred in Fornax about 2 Gyr ago. © 2005 International Astronomical Union.

BINARIES IN OPEN STAR CLUSTERS: PHOTOMETRIC APPROACH WITH APPLICATION TO THE HYADES

A new method has been developed to solve the star cluster membership problem. It is based on synthetic photometry employing the Black Body concept as stellar radiation simulator. Synthetic color-magnitude diagram is constructed showing the main sequence band and the positions of binary star systems of combinations of various components through different photometric tracks. The method has been applied to the Hyades. The cluster membership problem has been re-appraised for the cluster (both single and binary) stars. For the binary members, the components' spectral types have been derived by the method. The results obtained agree very well with those found in literature, The method is simpler than the others and can be developed to undertake other cases as multiple star systems.

IAC-STAR: A Code for Synthetic Color-Magnitude Diagram Computation

The code IAC-star is presented. It generates synthetic HR and color-magnitude diagrams (CMDs) and is mainly aimed to star formation history studies in nearby galaxies. Composite stellar populations are calculated on a star by star basis, by computing the luminosity, effective temperature and gravity of each star by direct bi-logarithmic interpolation in the metallicity and age grid of a library of stellar evolution tracks. Visual (broad band and HST) and infrared magnitudes are also provided for each star after applying bolometric corrections. The Padua (Bertelli et al. 1994, Girardi et al. 2000) and Teramo (Pietrinferni et al. 2004) stellar evolution libraries and various bolometric corrections libraries are used in the current version. A variety of star formation rate functions, initial mass functions and chemical enrichment laws are allowed and binary stars can be computed. Although the main motivation of the code is the computation of synthetic CMDs, it also provides integrated masses, luminosities and magnitudes as well as surface brightness fluctuation luminosities and magnitudes for the total synthetic stellar population, and therefore it can also be used for population synthesis research. The code is offered for free use and can be executed at the site {\tt http://iac-star.iac.es}, with the only requirement of referencing this paper and crediting as indicated in the site.

Berkeley 29, the most distant old open cluster

We present CCD BVI photometry of the old open cluster (OC) Berkeley 29, located in the anticentre direction. Using the synthetic colour-magnitude diagrams (CMDs) technique we estimate at the same time its age, reddening, distance and approximate metallicity using three types of stellar evolutionary tracks. The best solutions give: age = 3.4 or 3.7 Gyr, (m - M) 0 = 15.6 or 15.8 with E(B - V) = 0.13 or 0.10, and metallicity lower than solar (Z = 0.006 or 0.004), depending on the adopted stellar models. Using these derived values, Be 29 turns out to be the most distant OC known, with Galactocentric distance R GC = 21.4 to 22.6 kpc. Hence, Be 29 qualitatively follows both the age-metallicity relation and the metal abundance gradient typical of Galactic disc objects. The cluster position and radial velocity, however, appear to link Be 29 to the family of the Canis Maior debris.

Probing the Canis Major stellar over-density as due to the Galactic warp

Proper-motion, star counts and photometric catalog simulations are used to explain the detected stellar over-density in the region of Canis Major, claimed to be the core of a disrupted dwarf galaxy (CMa, Martin et al. [CITE], MNRAS, 348, 12; Bellazzini et al. [CITE], [arXiv:astro-ph/0311119]), as due to the Galactic warp and flare in the external disk. We compare the kinematics of CMa M-giant selected sample with surrounding Galactic disk stars in the UCAC2 catalog and find no peculiar proper motion signature: CMa stars mimic thick disk kinematics. Moreover, when taking into account the Galactic warp and flare of the disk, 2MASS star count profiles reproduce the CMa stellar over-density. This star count analysis is confirmed by direct comparison with synthetic color-magnitude diagrams simulated with the Besancon models (Robin et al. [CITE], AA Yanny et al. 2003, ApJ, 588, 824).

Interpreting the colour–magnitude diagrams of open star clusters through numerical simulations

We present detailed comparisons between high quality observational colour-magnitude diagrams (CMDs) of open star clusters and synthetic CMDs based on Monte Carlo numerical simulations. The comparisons account for all of the main parameters which determine the shape of the CMD for a stellar population. For the four clusters studied, NGC 6819, 2099 (M37), 2168 (M35) and 2323 (M50), we derive reddening, distance, age, binary fraction, star formation rate and indicative metallicity by comparing the locations and density of points in the observed CMDs to the simulated CMDs. We estimate the uncertainties related to stellar evolution theories by adopting various sets of stellar models for all of the synthetic CMDs and discuss which stellar models provide the theoretical CMDs that best reproduce the observations.

Modeling the Star Population of Resolved Galaxies

A program is developed for determining the history of star-forming galaxies based on the use of two- or multicolor photometry of the resolved stars in a given galaxy. We create a library of synthetic color-magnitude diagrams from theoretical stellar isochrones, taking the initial mass function, distance to the galaxy, internal and external absorption, and photometric errors into account. The resulting synthetic diagrams are combined linearly and compared quantitatively with photometric data for stars in a galaxy in order to determine star formation rates as a function of age and metallicity. This program is tested in detail under different conditions using artificial color-magnitude diagrams. Special attention is given to the limiting case when only the brightest stars of a galaxy can be seen in the color-magnitude diagram and the number of resolved stars is at most a few hundred. This limiting case corresponds to a large fraction of the nearby galaxies at distances of 3-5 Mpc observed by large ground based telescopes and the Hubble Space Telescope.

The Stellar Content and Star Formation History of the Late-Type Spiral Galaxy NGC 300 fromHubble Space TelescopeObservations

We present the first Hubble Space Telescope (HST) WFPC2 V and I photometry for the Sculptor Group galaxy NGC 300 in four fields ranging from the center to the outer edge. We have made the first measurement of the star formation histories in two disk fields: the oldest stars were born at similar epochs and formation activity increased but at different mean rates. The main disk stellar population is predominantly old, consisting of red giant branch (RGB) and asymptotic giant branch (AGB) stars, based on a synthetic color-magnitude diagram analysis. The metallicity Z is found to have been less than 0.006 (or 0.33 Z⊙), with no evidence for significant change in mean Z over time in both disk fields. In the central region, we find a dearth of bright stars with respect to the two disk fields that cannot be explained by observational effects. Taken at face value, this finding would agree with the Davidge report of suppressed star formation there during the past 109 yr with respect to his disk fields at larger radii; but the possibility remains that significant central extinction affects our finding. We have also determined the first distance modulus estimate based on the tip of the red giant branch method. On the Cepheid distance scale of Ferrarese et al., we find (m-M)0 = 26.56 ± 0.07(±0.13) mag and a similar value from the Cepheid-independent empirical method of Lee, Freedman, & Madore, both in good agreement with the Cepheid distance determined by Freedman et al. A discrepancy between this value and the theoretical calibration of the red giant branch tip magnitude method remains. Finally, we report a newly detected young (up to about 10 Myr) stellar association of about average size (∼140 pc) in one of the disk fields.

The Star Formation History of NGC 1705: A Poststarburst Galaxy on the Verge of Activity

We infer the star formation history in different regions of the blue compact dwarf NGC 1705 by comparing synthetic color-magnitude diagrams with the Hubble Space Telescope optical and near-infrared photometry. We find that NGC 1705 is not a young galaxy, because its star formation commenced at least 5 Gyr ago. On the other hand, we confirm the existence of a recent burst of star formation between 15 and 10 Myr ago. We also find evidence for new strong activity, which started 3 Myr ago and is still continuing. The old population is spread across the entire galaxy, while the young and intermediate stars are more concentrated in the central regions. We derive an almost continuous star formation with variable rate and exclude the presence of long quiescent phases between the episodes during the last ≈1 Gyr. The central regions experienced an episode of star formation of ~0.07 M⊙ yr-1 (for a Salpeter initial mass function [IMF]) 15 to 10 Myr ago. This coincides with the strong activity in the central super–star cluster. We find a rate of ~0.3 M⊙ yr-1 for the youngest ongoing burst, which started ~3 Myr ago. This is higher than in other dwarfs and comparable to the rate of NGC 1569. The star formation rate of earlier episodes is not especially high and falls in the range 10-3 to 10-1 M⊙ yr-1. The IMF is close to the Salpeter value or slightly steeper.

Photometric binary content in intermediate/old open clusters

The stellar evolutionary models by Bertelli et al. ([CITE]) have been used to generate synthetic colour–magnitude diagrams (CMDs) of open clusters. A comparison of the synthetic integrated luminosity function (ILF) and synthetic colour distribution with the corresponding observed distributions is used to estimate the photometric binary content in three intermediate age open clusters, which is found to be about 30% in these clusters.

Clues to the Evolution of the Carina Dwarf Spheroidal Galaxy from the Color Distribution of its Red Giant Stars

The thin red giant branch (RGB) of the Carina dwarf spheroidal galaxy appears at first sight quite puzzling and seemingly in contrast to the presence of several distinct bursts of star formation. In this Letter, we provide a measurement of the color spread of red giant stars in Carina, based on new BVI wide-field observations, and model the width of the RGB by means of synthetic color-magnitude diagrams. The measured color spread, σV-I = 0.021 ± 0.005, is quite naturally accounted for by the star formation history of the galaxy. The thin RGB appears to be essentially related to the limited age range of its dominant stellar populations, with no need for a metallicity dispersion at a given age. This result is relatively robust with respect to changes in the assumed age-metallicity relation, as long as the mean metallicity over the galaxy lifetime matches the observed value ([Fe/H] = -1.91 ± 0.12 after correction for the age effects). This analysis of photometric data also sets some constraints on the chemical evolution of Carina by indicating that the chemical abundance of the interstellar medium in Carina remained low throughout each episode of star formation even though these episodes occurred over many gigayears.

Testing Intermediate-Age Stellar Evolution Models with VLT Photometry of Large Magellanic Cloud Clusters. III. Padova Results

The color-magnitude diagrams (CMDs) of three intermediate-age Large Megallanic Cloud clusters, NGC 2173, SL 556, and NGC 2155, are analyzed to determine their age and metallicity based on Padova stellar models. Synthetic CMDs are compared with cluster data. The best match is obtained using two fitting functions based on star counts in the different bins of the cluster CMD. Two different criteria are used. One of them takes into account the uncertainties in the color of the red clump stars. Given the uncertainties on the experimental values of the clusters metallicity, we provide a set of acceptable solutions. They define the corresponding values of metallicity, age, reddening and distance modulus (for the assumed initial mass function). The comparison with Padova models suggests for NGC 2173 a prolonged star formation (spanning a period of about 0.3 Gyr), beginning 1.7 Gyr and ending 1.4 Gyr ago. The metallicity Z is in the range 0.0016–0.003. For SL 556 an age of 2.0 Gyr is obtained. The metallicity value is in the range 0.002–0.004, depending on the adopted comparison criterion. The derived age for NGC 2155 is 2.8 Gyr, and its metallicity Z is in the range 0.002–0.003. The CMD features of this cluster suggest that a more efficient overshoot should be adopted in the evolutionary models. A period of extended star formation is not required to fit the SL 556 and NGC 2155 observations.

Testing Intermediate-Age Stellar Evolution Models with VLT Photometry of Large Magellanic Cloud Clusters. II. Analysis with the Yale Models

Using Yale stellar evolution models, we present an analysis of the color-magnitude diagrams (CMDs) of three intermediate-age LMC clusters, namely, NGC 2173, SL 556, and NGC 2155, obtained with the Very Large Telescope. The main goal of our project is to investigate the amount of convective core overshoot necessary to reproduce the CMDs of relatively metal-poor, intermediate-age stellar populations, to check whether the extrapolation that is usually made from solar metallicity is valid. In the process, we obtained values for the binary fraction of each cluster, together with refined age estimates. Our method involved the comparison of the observed CMDs with synthetic CMDs computed using various values of the overshoot parameter and binary fraction. We conclude that a moderate amount of overshoot and some fraction of binary stars are essential for reproducing the observed shapes around the turnoff in the CMDs of all three clusters: unresolved binary stars fill in the expected core contraction gap and make a unique sequence near the gap, which cannot be reproduced by single stars alone, even with a larger amount of overshoot. We utilize ratios of the number of stars in different areas around the core contraction gap to constrain the binary fraction, which is around 10%–20% (for primary-to-secondary mass ratio ≥0.7) in all three clusters. Even if binary stars contaminate the core contraction gap, it is shown that the overshoot parameter can be inferred from the color dispersion of the stars around the contraction gap, regardless of the assumed binary fraction. From our overall analysis such as shape of isochrones, star counts, color distribution, and synthetic CMD comparisons, we conclude that overshoot ∼20% of the local pressure scale height best reproduces the CMD properties of all three clusters. The best age estimates are 1.5, 2.1, and 2.9 Gyr for NGC 2173, SL 556, and NGC 2155, respectively.

Tidal dwarfs in the M81 group: The second generation?

We derive quantitative star formation histories of the four suspected tidal dwarf galaxies in the M 81 group, Holmberg IX, BK3N, Arp-loop (A0952+69), and Garland, using Hubble Space Telescope/Wide Field Planetary Camera 2 images in F606W and F814W obtained as part of a Snapshot survey of dwarf galaxies in the Local Universe. We consider the spatial distribution and ages of resolved stellar populations in these dwarf irregular galaxies. We use synthetic color-magnitude diagrams to derive the ages of the major star formation episodes, star formation rates, and approximate metallicity ranges. All the galaxies show evidence of continuous star formation between about 20 and 200 Myr ago with star formation rates in the range – yr-1. The metallicity of the detected stars spans a wide range, and have lower than solar abundance. A possible scenario is that all four dwarf galaxies were formed from material in the metal-poor outer part of the giant spiral galaxy M 81 after the tidal interaction between M 81, M 82, and NGC 3077 Myr ago. While we do not directly detect pronounced old stellar populations, the photometric limits of our data are such that the presence of such a population is not entirely ruled out.

The intermediate-age open cluster NGC 2112

ABSTRA C T We report on UBVRI CCD photometry of two overlapping fields in the region of the intermediate-age open cluster NGC 2158 down to Va 21. By analysing colour ‐colour (CC) and colour ‐magnitude diagrams (CMDs) we infer a reddening EOB 2 VUa0:55 ^ 0:10, a distance of 3600 ^ 400 pc, and an age of about 2 Gyr. Synthetic CMDs constructed with these parameters (but fixing EOB 2 VUa0:60 andâFe=Haa20:60U, and including binaries, field contamination and photometric errors, yield a good description of the observed CMD. The elongated shape of the clump of red giants in the CMD is interpreted as resulting from a differential reddening of about DEOB 2 VUa0:06 across the cluster, in the direction perpendicular to the Galactic plane. NGC 2158 turns out to be an intermediate-age open cluster with an anomalously low metal content. The combination of these parameters, together with the analysis of the cluster orbit, suggests that the cluster belongs to the old thin disc population.

The Oldest Stars of the Extremely Metal-Poor Local Group Dwarf Irregular Galaxy Leo A

We present deep Hubble Space Telescope (HST) single-star photometry of Leo A in B, V, and I. Our new field of view is offset from the centrally located field observed by Tolstoy et al. in order to expose the halo population of this galaxy. We report the detection of metal-poor red horizontal branch stars, which demonstrate that Leo A is not a young galaxy. In fact, Leo A is as least as old as metal-poor Galactic Globular Clusters that exhibit red horizontal branches and are considered to have a minimum age of about 9 Gyr. We discuss the distance to Leo A and perform an extensive comparison of the data with stellar isochrones. For a distance modulus of 24.5, the data are better than 50% complete down to absolute magnitudes of 2 or more. We can easily identify stars with metallicities between 0.0001 and 0.0004, and ages between about 5 and 10 Gyr, in their post–main-sequence phases, but we lack the detection of main-sequence turnoffs that would provide unambiguous proof of ancient (>10 Gyr) stellar generations. Blue horizontal branch stars are above the detection limits but difficult to distinguish from young stars with similar colors and magnitudes. Synthetic color-magnitude diagrams show it is possible to populate the blue horizontal branch in the halo of Leo A. The models also suggest ≈50% of the total astrated mass in our pointing to be attributed to an ancient (>10 Gyr) stellar population. We conclude that Leo A started to form stars at least about 9 Gyr ago. Leo A exhibits an extremely low oxygen abundance, only 3% of solar, in its ionized interstellar medium. The existence of old stars in this very oxygen-deficient galaxy illustrates that a low oxygen abundance does not preclude a history of early star formation.

Numerical methods of star formation history measurement and applications to seven dwarf spheroidals

A comprehensive study of the measurement of star formation histories from colour-magnitude diagrams (CMDs) is presented, with an emphasis on a variety of subtle issues involved in the generation of model CMDs and maximum likelihood solution. Among these are the need for a complete sampling of the synthetic CMD, the use of proper statistics for dealing with Poisson-distributed data (and a demonstration of why χ2 must not be used), measuring full uncertainties in all reported parameters, quantifying the goodness-of-fit, and questions of binning the CMD and incorporating outside information. Several example star formation history measurements are given. Two examples involve synthetic data, in which the input and recovered parameters can be compared to locate possible flaws in the methodology (none were apparent) and measure the accuracy with which ages, metallicities and star formation rates can be recovered. Solutions of the histories of seven Galactic dwarf spheroidal companions (Carina, Draco, Leo I, Leo II, Sagittarius, Sculptor and Ursa Minor) illustrate the ability to measure star formation histories given a variety of conditions — numbers of stars, complexity of star formation history and amount of foreground contamination. Significant measurements of ancient >8 Gyr star formation are made in all seven galaxies. Sculptor, Draco and Ursa Minor appear entirely ancient, while the other systems show varying amounts of younger stars.

The Star Formation History of the Blue Compact Dwarf Galaxy UGCA 290

We present the star formation history of UGCA 290, a galaxy with properties intermediate between blue compact dwarfs (BCDs) and dwarf irregulars. This galaxy is particularly interesting because its young stellar population is extremely similar to that of the well-studied type iE blue compact dwarf VII Zw 403, despite its different spatial morphology and old stellar content. Our Hubble Space Telescope/Wide Field Planetary Camera 2 single-star photometry for UGCA 290 extends over 9 magnitudes in I and allows a detailed study of its star formation history. Using synthetic color-magnitude diagrams, we show that the recent burst which gives this galaxy its BCD status is a moderate enhancement in star formation which lasted for approximately 20 Myr, at a rate about 10 times above its previous rate. The star formation history for most of the previous billion years is consistent with a constant rate, although enhancements as large as the current one are possible at times earlier than 400 Myr ago. We estimate that the total mass converted into stars in UGCA 290 more than one billion years ago is about 3 times the astrated mass since that time. The initial mass function is consistent with a Salpeter slope, and the stellar metallicity is bracketed by Z☉/50 and Z☉/5, with evidence for metallicity evolution. Similar results for the star formation history over the past 600 Myr apply to VII Zw 403. Our main result is that despite the traditional picture of BCDs, the current bursts in these two galaxies are neither remarkably intense nor short-lived, and that most of their star formation occurred more than a billion years ago.