Working Group Members:

• Hagai Netzer, TAU, Israel

• Noah Brosch, TAU, Israel

• Elhanan Elmoznino, TAU, Israel

• Swara Ravindranath, IUCAA, Pune

• Prajval Shastri, IIA, Bangalore

• A Subramaniam, IIA, Bangalore

• UC Joshi, PRL, Ahmedabad

• KP Singh, TIFR, Mumbai

Deep surveys with TAUVEX: AGNs, WDs, nebulae

Some aspects of deep surveys with TAUVEX are treated in more detail elsewhere in these pages. For completeness, we mention here galaxies that are included in the survey areas, comets, and asteroids. Additional survey targets that will be briefly discussed here are AGNs, white dwarfs and peculiar binary stars, and diffuse ISM nebulae.

Since the beginning of AGN studies these objects have been identified based on two properties that are relevant to TAUVEX; the tendency of most AGNs to emit a large fraction of their radiation in the UV, and the tendency of the emission from AGNs to vary, sometimes of short time scales.

These two properties may be used to select a UV-based sample of AGNs from among the sources detected by TAUVEX. Preliminary estimates are that the Celestial Pole Cap surveys, that will cover in total some 3000 square degrees of the sky, would reveal tens of thousands of AGNs. These would be separable from the stars by using the three measurements of TAUVEX coupled with ground-based optical measurements, yielding colour indices to be used in “diagnostic diagrams”.

The optical data would be derived from SDSS data or would be the target of special observations. At the Wise Observatory, for instance, we are considering the use of a new, soon to be in operation, CCD mosaic camera that is able to cover one square degree in a single exposure. This, equipped with two SDSS filters (g and r, for example) would allow not only the derivation of an addition number of colour indices required for the diagnostics, but also linking these to the data sets that rely on SDSS survey data proper.

Other AGNs would be identified first from the variability, for the cases where TAUVEX will scan the same sky region a number of times in order to reach fainter brightness levels. The candidates may have sufficient information to decide on their nature only from the UV measurements but it is more likely that follow-up observations, or a combination of optical colors, would be required.

A situation similar to the case of AGNs exists for the survey of white dwarfs. These objects emit a large fraction of their radiation in the UV. Traditionally, white dwarfs were discovered since they showed an excess of blue-UV radiation, or because parallax surveys revealed intrinsically faint and blue objects. Recently the SDSS photometric data set serves to reveal many WDs based on their colors (e.g., Hu et al. astro-ph/ 0701197) and many more are expected from the further data releases. The use of TAUVEX in this field will be complementary to the other surveys and the WDs will be detected by using the UV and optical color indices. The creation of a large data base on WDs is important in studies of the structure and evolution of the Milky Way.

A specifically important field where TAUVEX can make a definite contribution is the detection of peculiar binary stellar systems where an optically-luminous star orbits together with a hot white dwarf. Such systems were discovered with ROSAT and EUVE, but there the white dwarf had to be very hot to emit at very short wavelengths. TAUVEX with a supplement of optical observations would be able to extend this to somewhat cooler WDs.

The topic of diffuse ISM nebulae seems covered by optical surveys, but this is true only for emission-line nebulae. Since space contains many dust clouds, and these mostly scatter preferentially shorter-wavelength light, it should be possible to detect with TAUVEX extended reflection nebulae (a well-known example is the Merope nebula in the Pleiades). This is true also for extremely faint emission nebulae, such as one recently found with GALEX around Z Cam (Shara et al. 2007, Nature, in press). Such discoveries would, by their nature, be serendipitous, although some targeted searches aimed at reaching very faint UV surface brightness levels around a few selected sources could be attempted.

Return to top