GK Workshop - R. Elson et al.

Proceedings of the Workshop
"The Magellanic Clouds and Other Dwarf Galaxies"
of the Bonn/Bochum-Graduiertenkolleg

The Local Group at High Redshift

R. Elson, R. Abraham, T. Kodama, B. Poggianti, and M.S. Oey

Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK

Received 02nd February 1998

Deep observations of faint galaxies, for example the Hubble Space Telescope's Hubble Deep Field, are contributing a great deal to our knowledge of galaxies out to high redshifts, and hence to our understanding of galaxy formation and evolution. In particular, many recent investigations have focussed on the `excess' of faint blue galaxies, and their possible identification as dwarf galaxies undergoing intense star bursts. The aim of this project is to clarify the relationship between the objects referred to as dwarf galaxies by cosmologists, and the objects we call dwarf galaxies in the Local Group, and to determine what can be learned about the former from the latter. We use observed star formation histories, metallicities, and structural parameters compiled from the literature for 30 Local Group galaxies, together with population synthesis models, to simulate images of Local Group galaxies at a range of redshifts, and compare their properties with those of galaxies at redshifts ≥0.1.

Surface brightness profiles were modelled as either the superposition of an exponential disk and bulge, or as an empirical King model. Each galaxy was scaled (both spatially and in terms of surface brightness) according to the appropriate cosmology, and artificially ``re-imaged'' with the photometric zero-point and noise characteristics of the Hubble Space Telescope's Wide Field Camera (WFC) image of the Hubble Deep Field in the I-band (F814W). K-corrections and evolutionary corrections were applied. The population synthesis models we used are described in Kodama & Arimoto (1997) and Barbaro & Poggianti (1997). The models compute the integrated spectra of galaxies from the far UV region up to the near-IR, and make use of evolutionary tracks from the Padova group, as well as the latest Kurucz stellar atmosphere models. They include stellar evolutionary phases up to the AGB and post-AGB, and were computed for a Salpeter IMF between 0.1 and 60 (100) M_sun.

Simulated images of eight of the most luminous galaxies in the Local Group are shown in Fig. 1 for redshifts z=0.1, 0.5 and 1.0. Even at moderate redshifts (z>0.5) most members of the Local Group would be undetectable in the Hubble Deep Field. At z≥1.5 none of the galaxies would be visible. Some of the dwarfs may become visible briefly during bursts of star formation. For z∼0.1 the projected extent of the Local Group is much greater than the WFC of view (2.7 arcmin on a side), and at z≥1 its projected size is comparable to the field of view. Thus, at any redshift it would be difficult to identify the Local Group as a physical association in a WFC image.

We also compare the total star formation rate as a function of time in the Local Group, derived by summing the star formation rates of all 30 galaxies in our sample, with the results of Madau et al. (1996), who estimate the total star formation rate as a function of redshift from U-band observations of galaxies in the Hubble Deep Field and in redshift surveys.

Further simulations, which include substructure within the galaxies, are being carried out, and a detailed comparison of the properties of the Local Group galaxies at various redshifts with those of galaxies in the Hubble Deep Field and in redshift surveys is underway.

[Click here to see Fig. 1!]

References

Barbaro G., Poggianti B., 1997, A&A 324, 490
Kodama T., Arimoto N., 1997, A&A 320, 41
Madau P., Ferguson H., Dickinson M., Giavalisco M., Steidel C., Fruchter A., 1996, MNRAS 283, 1388

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First version:	19th	February,	1998
Last update:	03rd	October,	1998

Jochen M. Braun & Tom Richtler
(E-Mail: jbraun|richtler@astro.uni-bonn.de)