The Star Formation History of NGC 1705: a Post-Starburst 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 HST 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 Msun/yr (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 Msun/yr 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}-10^{-1} Msun/yr. The IMF is close to the Salpeter value or slightly steeper.Comment: 34 pages, including 6 tables and 14 .ps figures (9 in colour), AJ in pres

Spectro-photometric Evolution of Elliptical Galaxies. II. Models with infall

In this paper we present new chemo-spectro-photometric models of elliptical galaxies in which infall of primordial gas is allowed to occur. They aim to simulate the collapse of a galaxy made of two components, i.e. luminous material and dark matter. The mass of the dark component is assumed to be constant in time, whereas that of the luminous material is supposed to accrete at a suitable rate. They also include the effect of galactic winds powered by supernova explosions and stellar winds from massive, early-type stars. The models are constrained to match a number of properties of elliptical galaxies, i.e. the slope and mean colours of the colour-magnitude relation (CMR), V versus (V--K), the UV excess as measured by the colour (1550--V) together with the overall shape of the integrated spectral energy distribution (ISED) in the ultraviolet, the relation between the Mg2 index and (1550--V), the mass to blue luminosity ratio M/Lb as a function of the B luminosity, and finally the broad-band colours (U--B), (B--V), (V--I), (V--K), etc.Comment: pages 22, 20 postscript figures, 2 external table (tab2_infall using supertabular.sty and tab5_infall using supertabular1.sty

The Resolved Stellar Populations in NGC 1705

We present HST photometry of the resolved stellar population in the dwarf irregular galaxy NGC 1705. The galaxy has been observed with both WFPC2 and NICMOS, and successful images have been obtained in the F555W, F814W, F110W and F160W bands. The optical fields cover most of the galaxy, while the infrared field (NIC2) maps only its central regions. The optical photometry provides \~20000 objects down to m_F555W ~ 29 in the PC field of view and ~ 20000 in the three WFCs. In the infrared we have been able to resolve ~ 2400 stars down to m_{F110W}, m_{F160W} ~ 26. A subsample of 1834 stars have been unambiguously measured in all the four bands. The corresponding color-magnitude diagrams (CMDs) confirm the existence of an age gradient, showing that NGC 1705 hosts both young (a few Myr old) and very old (up to 15 Gyr old) stars, with the former strongly concentrated toward the galactic center and the latter present everywhere, but much more easily visible in the external regions. The tip of the red giant branch (TRGB) is clearly visible both in the optical and in the infrared CMDs and allows us to derive the galaxy distance. Taking into account the uncertainties related to both the photometry and the TRGB magnitude -- distance relation, we find that the distance modulus of NGC 1705 is (m-M)_0=28.54 +- 0.26, corresponding to a distance D=5.1 +- 0.6 Mpc.Comment: aastex, 37 pages, 13 (reduced resolution) .ps figures. Accepted by AJ. Full resolution figures are available upon request to the first autho

The evolution of H{\sc ii} galaxies: Testing the bursting scenario through the use of self-consistent models

We have computed a series of realistic and self-consistent models of the emitted spectra of H{\sc ii} galaxies. Our models combine different codes of chemical evolution, evolutionary population synthesis and photoionization. The emitted spectrum of H{\sc ii} galaxies is reproduced by means of the photoionization code CLOUDY, using as ionizing spectrum the spectral energy distribution of the modelled H{\sc ii} galaxy, which in turn is calculated according to a Star Formation History (SFH) and a metallicity evolution given by a chemical evolution model that follows the abundances of 15 different elements. The contribution of emission lines to the broad-band colours is explicitly taken into account. The results of our code are compared with photometric and spectroscopic data of H{\sc ii} galaxies. Our technique reproduces observed diagnostic diagrams, abundances, equivalent width-colour and equivalent width-metallicity relations for local H{\sc ii} galaxies.Comment: 13 figures and 2 tables, accepted for publication in MNRAS Main Journa

Stellar age versus mass of early-type galaxies in the Virgo Cluster

The flux excess of elliptical galaxies in the far-ultraviolet can be reproduced by population synthesis models when accounting for the population of old hot helium-burning subdwarf stars. This has been achieved by Han and coworkers through a quantitative model of binary stellar evolution. Here, we compare the resulting evolutionary population synthesis model to the GALEX far-near ultraviolet colors (FUV-NUV) of Virgo cluster early-type galaxies that were published by Boselli and coworkers. FUV-NUV is reddest at about the dividing luminosity of dwarf and giant galaxies, and becomes increasingly blue for both brighter and fainter luminosities. This behavior can be easily explained by the binary model with a continuous sequence of longer duration and later truncation of star formation at lower galaxy masses. Thus, in contrast to previous conclusions, the GALEX data do not require a dichotomy between the stellar population properties of dwarfs and giants. Their apparently opposite behavior in FUV-NUV occurs naturally when the formation of hot subdwarfs through binary evolution is taken into account.Comment: 8 pages, 4 figures. Accepted for publication in Ap

The red stellar population in NGC 1569

We present HST NICMOS photometry of the resolved stellar population in NGC 1569. The CMD in the F110W and F160W photometric bands contains ~2400 stars with a formal photometric error < 0.1 mag down to J~23.5 and H~22.5. We describe the data processing which is required to calibrate the instrumental peculiarities of NICMOS. Two different packages for PSF-fitting photometry are used to strengthen the photometric results in the crowded stellar field of NGC 1569. The resulting CMD is discussed in terms of the major evolutionary properties of the resolved stellar populations. For a distance modulus of 26.71 and a reddening E(B-V)=0.56, our CMD samples stars down to ~0.8 Mo, corresponding to look-back times > 15 Gyr. This is clear indication of SF activity spanning an entire Hubble time. The metallicity of the reddest RGB stars is in better agreement with Z=0.004 as measured in HII regions, than with Z=0.0004 as expected from the stellar ages. The presence of - yet undetected - very metal-poor stars embedded in the stellar distribution around J=22.75 and J-H=1.15 is, however, not ruled out. The youngest stars (< 50 Myr) are preferentially found around the two central super star clusters, whereas the oldest population has a more uniform spatial distribution. A SFR per unit area of 1 Mo yr*(-1) kpc*(-2) and a mass formed in stars of ~ 1.4x10*6 Mo in the last 50 Myr are derived from the CMD. The NIR CMD places strong constraints on the lower limit of the onset of SF in NGC 1569. The exceptionally high crowding in the NICMOS images of NGC 1569 is a challenge for the photometric analysis. As a result, optical and NIR images of NGC 1569 sample different populations and cannot be cross-correlated. Nevertheless, we demonstrate the consistency of the SF histories derived from the optical and NIR CMDs.Comment: 41 pages including 1 table (Latex) and 14 figures (postscript). Accepted for publication in the Astronomical Journal, March 2001 issu

HST Studies of the WLM Galaxy. I. The Age and Metallicity of the Globular Cluster

We have obtained V and I images of the lone globular cluster that belongs to the dwarf Local Group irregular galaxy known as WLM. The color-magnitude diagram of the cluster shows that it is a normal old globular cluster with a well-defined giant branch reaching to M_V=-2.5, a horizontal branch at M_V=+0.5, and a sub-giant branch extending to our photometry limit of M_V=+2.0. A best fit to theoretical isochrones indicates that this cluster has a metallicity of [Fe/H]=-1.52\pm0.08 and an age of 14.8\pm0.6 Gyr, thus indicating that it is similar to normal old halo globulars in our Galaxy. From the fit we also find that the distance modulus of the cluster is 24.73\pm0.07 and the extinction is A_V=0.07\pm0.06, both values that agree within the errors with data obtained for the galaxy itself by others. We conclude that this normal massive cluster was able to form during the formation of WLM, despite the parent galaxy's very small intrinsic mass and size.Comment: 14 pages, 5 figures, 1 tabl

IR Surface Brightness Fluctuations of Magellanic Star Clusters

We present surface brightness fluctuations (SBFs) in the near--IR for 191 Magellanic star clusters available in the Second Incremental and All Sky Data releases of the Two Micron All Sky Survey (2MASS), and compare them with SBFs of Fornax Cluster galaxies and with predictions from stellar population models. Our goals are twofold. First, to provide an empirical calibration of near--IR SBFs, given that existing stellar population synthesis models are particularly discrepant in the near--IR. Second, whereas most previous SBF studies have focused on old, metal rich populations, this is the first application to a system with such a wide range of ages (~ 10^6 to more than 10^10 yr, i.e., 4 orders of magnitude), at the same time that the clusters have a very narrow range of metallicities (Z ~ 0.0006 -- 0.01, ie., 1 order of magnitude only). Since stellar population synthesis models predict a more complex sensitivity of SBFs to metallicity and age in the near--IR than in the optical, this analysis offers a unique way of disentangling the effects of age and metallicity. We find a satisfactory agreement between models and data. We also confirm that near--IR fluctuations and fluctuation colors are mostly driven by age in the Magellanic cluster populations, and that in this respect they constitute a sequence in which the Fornax Cluster galaxies fit adequately. Moreover, fluctuation colors display a tendency to redden with age that can be fit by a straight line. Finally, we use for the first time a Poissonian approach to establish the error bars of the fluctuation measurements, instead of the customary Monte Carlo simulations.Comment: 54 pages, 12 figures, accepted by Ap

Spectral Energy Distributions of M81 Globular Clusters in BATC Multicolor Survey

In this paper, we give the spectral energy distributions of 42 M81 globular clusters in 13 intermediate-band filters from 4000 to 10000 A, using the CCD images of M81 observed as part of the BATC multicolor survey of the Sky. The BATC multicolor filter system is specifically designed to exclude most of the bright and variable night-sky emission lines including the OH forest. Hence, it can present accurate SEDs of the observed objects. These spectral energy distributions are low-resolution spectra, and can reflect the stellar populations of the globular clusters. This paper confirms the conclusions of Schroder et al. that, M81 contains clusters as young as a few Gyrs, which also were observed in both M31 and M33Comment: Accepted for Publication in PASP, 10 pages, 3 figure