Extragalactic Deep Fields

(C. H. Ishwara-Chandra, Nissim Kanekar, Y. Wadadekar, Jayaram N. Chengalur, Apurba Bera, Aditya Chowdhury, Former members: Sandeep K. Sirothia)

An important area of astronomical research in recent years has been the use of deep multi-wavelength studies of specific extragalactic fields to study in detail how galaxies and their stars and gas evolve through the age of the Universe. Radio and infrared imaging is especially important in this area, because most actively star-forming galaxies are obscured by dust and are hence not visible in optical images. Researchers at NCRA-TIFR use deep radio images of such extra-galactic fields to address a number of issues, including quantifying the number of sources of different types as a function of source luminosity and redshift, finding massive radio galaxies at high redshifts via their ultra-steep radio spectra, distinguishing between star-forming and active galaxies, etc.

Recent Results
Probing Star Formation in Galaxies at z~1
Bera et al. carried out deep GMRT 610 MHz imaging of four fields of the DEEP2 Galaxy Redshift Survey, and stacked the radio emission from a sample of nearly 4000 blue star-forming galaxies at 0.7<z< 1.45 to detect the median rest-frame 1.4 GHz radio continuum emission of the galaxies. The authors used the local relation between total star formation rate (SFR) and radio 1.4 GHz luminosity to infer a median total SFR of (24.4 +/- 1.4) solar masses per year for blue star-forming galaxies at these redshifts. They detect the main-sequence relation between SFR and stellar mass, and find that the power-law index of the main sequence shows no change over z~0.7-1.45. They also find that the nebular line emission suffers less extinction than the stellar continuum, contrary to the situation in the local Universe; further, the ratio of nebular extinction to stellar extinction increases with decreasing redshift. They combined their results with earlier GMRT HI 21cm emission studies of the DEEP2 fields to obtain an upper limit of 0.87 Gyr to the atomic gas depletion time of star-forming galaxies at z~1.3. Neutral atomic gas thus appears to be a transient phase in high-z star-forming galaxies. The left panel of the figure shows the stacked rest-frame 1.4 GHz radio emission of the galaxies of the sample, detected at high statistical significance; the right panel shows a similar stack at neighbouring locations ("off-source") which shows no signal, indicating that the detected signal of the left panel is very unlikely to arise from systematic effects.

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