The Star Formation Histories of Two Northern LMC Fields

Ground-based UBV photometry of two fields in the northern disk of the LMC are presented. A distance modulus of (m-M)_0 = 18.41 +/- 0.04 and an extinction of A_V = 0.30 +/- 0.05 has been calculated for these fields. The measurable star formation history of the LMC began no more than 12 Gyr ago with a strong star forming episode with [Fe/H] = -1.63 +/- 0.10 that accounted for approximately half (by mass) of the LMC's total star formation in the first 3 Gyr. The data does not give accurate star formation rates during intermediate ages, but there appears to have been a recent increase in the star formation rate in these fields, beginning approximately 2.5 Gyr ago, with the current metallicity in the region being [Fe/H] = -0.38 +/- 0.10. The two fields have had very similar star formation rates until 200 Myr ago, at which point one shows a large increase.Comment: 8 pages, 11 figures Accepted for publication in MNRA

The Star-Forming Region NGC 346 in the Small Magellanic Cloud with Hubble Space Telescope ACS Observations I. Photometry

We present a photometric study of the star-forming region NGC 346 and its surrounding field in the Small Magellanic Cloud, using data taken with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST). The data set contains both short and long exposures for increased dynamic range, and photometry was performed using the ACS module of the stellar photometry package DOLPHOT. We detected almost 100,000 stars over a magnitude range of V ~ 11 to V ~ 28 mag, including all stellar types from the most massive young stars to faint lower main sequence and pre-main sequence stars. We find that this region, which is characterized by a plethora of stellar systems and interesting objects, is an outstanding example of mixed stellar populations. We take into account different features of the color-magnitude diagram of all the detected stars to distinguish the two dominant stellar systems: The stellar association NGC 346 and the old spherical star cluster BS 90. These observations provide a complete stellar sample of a field about 5 arcmin x 5 arcmin around the most active star-forming region in this galaxy. Considering the importance of these data for various investigations in the area, we provide the full stellar catalog from our photometry. This paper is the first part of an ongoing study to investigate in detail the two dominant stellar systems in the area and their surrounding field.Comment: 8 pages, 5 figures, Accepted for publication in The Astrophysical Journal Supplement Series, A high-resolution version can be found at: http://www.astro.uni-bonn.de/~dgoulier/Science/Gouliermis+_NGC346_I.pd

Calibration of BVRI Photometry for the Wide Field Channel of the HST Advanced Camera for Surveys

We present new observations of two Galactic globular clusters, PAL4 and PAL14, using the Wide-Field Channel of the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope (HST), and reanalyze archival data from a third, NGC2419. We matched our photometry of hundreds of stars in these fields from the ACS images to existing, ground-based photometry of faint sequences which were calibrated on the standard BVRI system of Landolt. These stars are significantly fainter than those generally used for HST calibration purposes, and therefore are much better matched to supporting precision photometry of ACS science targets. We were able to derive more accurate photometric transformation coefficients for the commonly used ACS broad-band filters compared to those published by Sirianni, et al. (2005), owing to the use of a factor of several more calibration stars which span a greater range of color. We find that the inferred transformations from each cluster individually do not vary significantly from the average, except for a small offset of the photometric zeropoint in the F850LP filter. Our results suggest that the published prescriptions for the time-dependent correction of CCD charge-transfer efficiency appear to work very well over the ~3.5 yr interval that spans our observations of PAL4 and PAL14 and the archived images of NGC2419.Comment: 22 pages, 12 figures. Accepted for publication in PAS

HST Survey of Clusters in Nearby Galaxies. II. Statistical Analysis of Cluster Populations

We present a statistical system that can be used in the study of cluster populations. The basis of our approach is the construction of synthetic cluster color-magnitude-radius diagrams (CMRDs), which we compare with the observed data using a maximum likelihood calculation. This approach permits a relatively easy incorporation of incompleteness (a function of not only magnitude and color, but also radius), photometry errors and biases, and a variety of other complex effects into the calculation, instead of the more common procedure of attempting to correct for those effects. We then apply this procedure to our NGC 3627 data from Paper I. We find that we are able to successfully model the observed CMRD and constrain a number of parameters of the cluster population. We measure a power law mass function slope of alpha = -1.50 +/- 0.07, and a distribution of core radii centered at r_c = 1.53 +/- 0.15 pc. Although the extinction distribution is less constrained, we measured a value for the mean extinction consistent with that determined in Paper I from the Cepheids.Comment: 21 pages, 3 figures accepted for publication by A

Faint BVRI Photometric Sequences in Selected Fields

The results from work done to extend the Johnson-Cousins BVRI photometric standard sequence to faint levels of V ~ 21 mag in compact fields is presented. Such calibration and extension of sequences is necessary to fill a calibration gap, if reliable photometry from modest aperture telescopes in space (e.g. HST), or terrestrial telescopes with apertures exceeding 4-m is to done. Sequences like the ones presented here, which cover a large range in brightness as well as color, will allow photometric calibration to be done efficiently, as well as for such work to be less prone to systematic sources of error. Photometry of stars in approximately 10 x 10 arc-minute fields around 3 globular clusters, NGC 2419, Pal 4 and Pal 14 are presented. In each field, several stars are measured in B, V, R and I passbands, with standard errors less than 0.015 mag from random errors, to levels fainter than V=21 mag. The primary context for the work presented here is that parts of these fields were observed repeatedly by the Wide Field Planetary Camera-2 WFPC2) of the HST, and thus these newly calibrated sequences can be used to retroactively calibrate WFPC2 at over various times of its operating life. The sequences presented here agree at the 0.02 mag level with other extant calibrations of these targets, except in the I band, where there are color dependent deviations of up to 0.05 mag versus one other photometric sequence. Possible causes for this discrepancy are examined.Comment: 56 pages, 14 figures, 14 table

A new galaxy near the Local Group in Draco

We present HST WFPC2 and ground-based images of the low surface brightness dwarf Irr/Sph galaxy KKR~25 in Draco. Its colour-magnitude diagram shows red giant branch stars with the tip at I = 22.32 mag, and the presence of some blue stars. The derived true distance modulus, 26.35 \+- 0.14 mag, corresponds to linear distances of KKR25 from the Milky Way and from the Local Group centroid of 1.86 and 1.79 Mpc, respectively. The absolute magnitude of the galaxy, M_V = -10.48, its linear diameter (0.54 Kpc) and central surface brightness (24.0 +- 0.2 mag\arcsec^2) are typical of other dIrr/dSphs in the Local Group. Being situated just beyond the radius of the zero-velocity surface of the Local Group, KKR25 moves away from the LG centroid at a velocity of V_{LG} = + 72 km/s.Comment: 9 pages, 5 figures, accepted for publication in Astronomy & Astrophysic