Working Group Members:

• U C Joshi, PRL, Ahmedabad

• Noah Brosch, TA University, Israel

• A Subrahmaniam, IIA, Bangalore

• Prajval Shastri, IIA, Bangalore

• K P Singh, TIFR, Mumbai

Starburst Galaxies science areas:

Studies of star formation regions, star burst phenomenon, the reddening distribution and the UV morphology.
A sample of 20 starburst galaxies was observed from Mount Abu Observatory. These galaxies emit large fraction of energy in UV spectral region. To have a better understanding of their morphology, the observations in UV spectral bands are very useful.

Galaxies observations with TAUVEX:

Galaxy observations will be performed as part of the deep TAUVEX survey and as surveys of specific sky regions. The goal would be to understand aspects of star formation and stellar population evolution. In survey mode TAUVEX uses primarily its three principal filters SF-1, SF-2 and SF-3. These filters span the spectral region from somewhat longer than Lyman alpha to 320 nm with three well-defined bands. Three filters define two color indices in the UV and the combination of these measurements with data from the optical and infrared allows the derivation of even more color indices. As known from the optical domain, color indices are magnitude differences, representing spectral slopes, and these are indicative of the mechanisms producing the radiation.

Since the stellar population in a galaxy evolves, the overall shape of the spectral energy distribution (SED) changes with time. The peak of the SED of a single stellar population becomes redder as time passes since the last major starburst event, moving from the far-UV to the near-UV then to the optical. The use of three UV bands that sample the SED peaks of B, A, and F-type stars will allow, in conjunction with optical information (e.g., from a number of SDSS bands), proper accounting for the stellar populations composing the galaxies covered by the survey. It would be possible to disentangle star formation histories during the last few Gyrs and reconstruct the influence of the neighborhood on the evolution of galaxies in clusters, groups, and in the field.

The importance and relevance of global galaxy surveys with TAUVEX, over a significant fraction of the sky, depends on the depth and extent of these surveys. In order to achieve more significant results than those of the GALEX MIS (our reference comparison) we must cover a solid angle 2.5 times larger, thus survey 3250 square degrees. This is the reason that the first-year proposed survey goal is to cover with deep exposures, of about 5000 sec per source, approximately 1500 square degrees around each Celestial Pole. The important difference in comparison with GALEX is that this survey would be done with three UV spectral bands instead of two (FUV and NUV). The selection of the Celestial Polar Caps (CPC) for this survey is predicated by the necessary observing time per source; achieving this in the first year of the mission for lower declinations would require a prohibitive amount of time.

Since the CPCs do not contain a nearby, rich cluster of galaxies, we propose to add to the survey area the Ursa Major cluster of galaxies (Tully et al. 1996 AJ, 112, 2471). This is a relatively nearby cluster, at only 15 Mpc, in which some 80 galaxies have been identified of which 62 are intrinsically brighter than the Small Magellanic Cloud [M(B)=-16.5 mag]. The cluster is not relaxed and most of it objects are spirals or irregulars, thus expected to show a significant amount of star formation that will show in the UV. HI observations with WSRT showed that 49 galaxies have detectable amounts of hydrogen and this is the raw material for star formation. Surveying the UMa cluster with TAUVEX will provide understanding of the star formation process in clusters.

Other very interesting TAUVEX targets are the two Magellanic Clouds and the stellar and gas bridge linking them (the Magellanic Bridge). The LMC and SMC were observed already with the rocket-borne telescope flown by Smith et al. (1987 ApJ 320, 609), with the FAUST telescope on the Space Shuttle (Courtes et al. 1995 A&A, 297, 338), and parts were observed with UIT (Parker et al. 2001 AJ, 121, 891) and HST (Almoznino et al. 2005 MNRAS 357, 645). While the HST observations have an exquisite angular resolution and resolve individual stars, the two first observations sample arcmin-sized regions and only the UIT images offer an angular resolution somewhat similar, but inferior, to what TAUVEX will produce. It is therefore beneficial to survey both galaxies and the regions connecting them, extending this survey also somewhat away from the galaxies outward, so as to sample the tidal tail and the star formation that is taking place these.

A final topic for UV galaxy surveys with TAUVEX is coving areas for which there are already observations by the ALFALFA survey. This is the HI blind survey conducted at the Arecibo radio observatory where all the extragalactic sky accessible by this telescope is covered from -3,000 km/sec to 20,000 km/sec. The sensitivity is such that objects with a few million solar masses of HI can be detected in the Local Group and nearby, and objects of ten million solar masses of HI can be detected in the Virgo Cluster. Among the first 700+ objects contained in the first survey paper (submitted in December 2006) more than 60% are new in that their redshift was not known previously, and many of the objects are very low surface brightness galaxies. The advantage of observing in the UV is the low sky background that facilitates the detection of such LSB objects. A partial survey of some ALFALFA regions with TAUVEX would make an interesting topic for the first-year plan.

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