Science with the Hubble Space Telescope -- II
Book Editors: P. Benvenuti, F. D. Macchetto, and E. J. Schreier
Electronic Editor: H. Payne

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HST Photometry of the Post-Starburst Galaxy NGC 1569

A. Vallenari
Osservatorio di Padova, Padova, Italy

D.J. Bomans
University of Illinois, Dept. of Astronomy, Urbana, IL 61801 USA
Sternwarte, Bonn University, Germany

S. Ortolani
Dipartimento di Astronomia, Universitá di Padova, Italy

Abstract:

We present color-magnitude diagrams (CMDs) derived from HST WFPC images in the F555W and F785LP filters of the post-starburst galaxy NGC 1569 and we discuss its star formation history. This galaxy has experienced a global burst of star formation from 1yr to about 4Myr ago. Older episodes of star formation going from 1.5Gyr to 1.5 yr ago are evident from the data.

Introduction

The process of star formation in irregular and starburst galaxies is still far from being understood, in spite of the efforts made in the recent past. While many small irregular galaxies show continuous star formation (Gallagher et al. 1984), some larger amorphous irregular galaxies are clearly bursting systems (e.g., Meurer et al. 1992, Marlowe et al. 1995).

The study of the color-magnitude diagrams is the most direct way to get information on the star formation for those galaxies that are well resolved into stars. The HST with its superior spatial resolution is a perfect tool to study nearby galaxies using this method. NGC 1569 is a blue dwarf irregular located at a distance of 2.2 Mpc or (m-M)=26.7 mag (Israel 1988). NGC 1569 is thought to be a prime example of a post-starburst galaxy (Israel 1988). This galaxy has recently been studied by Heckman et al. (1995).

The CMD

We obtain the CMD of NGC 1569 from HST WFPC observations (pre-refurbishment) in the F555W and F785LP filters (hereafter called V and I, because we fitted the magnitudes into the standard V,I system) retrieved from the HST archive. The two super starclusters present in this galaxy are discussed by O'Connell et al. (1994) on the basis of two PC images of the same set we have retrieved. No deep photometry of stars in the other PC chips has been published by these authors. Aperture photometry is done using Inventory (in Midas environment) on the images after the removal of cosmic rays and subtraction of the sky. Two apertures (three and four pixels of diameter) are tested and the smallest one is chosen. Due to the particular shape of the point spread function of HST before the refurbishment, only 15% of the light is included in a diameter of about three pixels: even the use of such a small aperture in the photometry allow us to detect stars unresolved in previous ground-based studies. The sky is measured in a sub-array of 1717 pixels around each object. Particular attention is paid to avoid spurious detections in the spikes and tendrils around bright stars: we find that a large value of the Inventory parameter Brgtctrl () can minimize the problem. The reality of each star automatically found by Inventory is checked visually on each frame. We find 1585 stars in the whole galaxy down to I=22.5 mag. All the stars inside the two superclusters are discarded, since the presence of a high concentration of bright objects and, as a consequence, of PSF spikes and tendrils makes the finding procedure unreliable.

 
Figure: CMD of NGC 1569, corrected for internal and foreground reddening, adopting E0.5 (Israel 1988). Isochrones having metallicity Z=0.004 are superposed. The ages, in Gyr, are 2.5, 1.5, 1.0, 0.6, 0.4, 0.1, 0.025, 0.015, 0.01, 0.006, 0.004

The Star Formation History

To discuss the star formation in NGC 1569, we superpose on the CMD the whole galaxy (see Fig.1) the isochrones calculated by Fagotto et al. (1994) for Z=0.004 as appropriate, since Dufour et al. (1982) and Hunter et al. (1982) derive for NGC 1569 a metal content in between those found for the LMC and the SMC. The data presented here suggest a well-defined scenario for the recent history of this galaxy. There is evidence of a recent burst of star formation from about 1.5 yr to 4Myr. The distribution of the red stars shows that some star formation was going on at older ages, from 1.5Gyr to 1.5yr. The presence of stars at intermediate colors is related to an episode of star formation from 1 to 1yr. The maximum quiescent time allowed by our data is about 5yr. However, longer intervals of inactivity cannot be excluded for ages older than 1.5yr.

Conclusions

The star formation of NGC 1569 was probably continuous in the past 1.5Gyr, with very brief episodes of quiescence. Waller (1991) suggests that this galaxy has experienced about six bursts of comparable intensity having a duration of about 2yr. In this scenario, the proposed cycling time is quite uncertain, going from 2Gyr to 3yr. On the basis of the integrated colors of NGC 1569, Israel (1988) predicts quiescent phases of about 6yr. As result of our analysis, these times seem to be too long to account for the distribution of stars in the CMD. This behavior is in agreement with the results found by Marconi et al. (1995) and Tosi et al. (1991) for other nearby Irregular galaxies where the star formation rate in the past 1Gyr was proceeding in a gasping way.

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