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Gemini Obtains First Spectrum From Furthest Known Dusty Galaxy

September 26, 2002

A US-UK team led by Gemini South astronomer Michael Ledlow, using the Gemini Multi-Object Spectrograph (GMOS) on Gemini North, has obtained a spectrum from the optical counterpart of the most distant "dusty" galaxy known. This object was selected for study based on a detection at submillimeter wavelengths using SCUBA* on the JCMT (Cowie, Barger, & Kneib 2002). The optical counterpart was found in deep multiwavelength imaging from the Kitt Peak National Observatory, but its nature was not understood (only a handful of SCUBA/submm galaxies have measured spectra). Gemini was able, with its great light gathering power, to obtain a spectrum that allows an analysis of this distant galaxy that is thought to represent some of the most massive galaxies of the early universe.

Michael Ledlow and his collaborators studied the optical counterpart of the submillimeter source, SMMJ09431+4700 that is seen through an intervening galaxy cluster (Abell 851) at redshift 0.41. Their results ("Gemini Multi-Object Spectrograph Observations of SCUBA Galaxies behind A851") are published in the October 1, 2002 issue of the Astrophysical Journal Letters.

Ledlow and his colleagues surmise that this galaxy is representative of the most massive galaxies in the early universe and that their intense activity is related to the growth of a massive bulge and an intense starburst. According to Ledlow, "These galaxies possibly represent the bulk of star formation occurring in galaxies at this epoch, and yet they are nearly completely obscured at optical wavelengths."

SCUBA* galaxy, SMMJ09431+4700, observed with GMOS has the highest spectroscopic redshift measured so far, at z = 3.35, which corresponds to a distance of 8 billion light-years. This observation breaks the z = 3 barrier for this population of galaxies for the first time. Its luminosity of LFIR ~ 1.5 × 1013 times the luminosity of the sun makes this galaxy a Hyperlumimous Infrared system. If powered only by massive star formation, its immense luminosity would require a star formation rate of about 10,000 solar masses per year!

However, the spectrum shows the signature of a weak Active Galactic Nucleus (AGN), hence some of the luminosity may come from a young supermassive central black hole. This is the second detection of a Hyperluminous Infrared Galaxy in the submm population. The other (SMMJ02399-0136, z=2.8) being the previously most distant known SCUBA galaxy, which also showed evidence of AGN activity.

If SCUBA galaxies are the progenitors of the most massive galaxies in the local universe, then the close relationships between AGN's, quasar activity, supermassive black holes and massive spheroids could be a natural consequence.

The AGN activity also appears to have a profound impact on the evolution of these galaxies. Others have speculated that the AGN SMMJ02399-0136 is driving a substantial outflow of gas (galactic wind) that may in time sweep the central regions of the galaxy clear of gas and dust (Smail et al. and Ivison et al.). Similarly in the spectrum of SMMJ09431+4700, the contrast between the spatially extended spectral line of Lyman-alpha and the broader, but spatially unresolved high excitation lines like Carbon IV and Nitrogen V, suggests that the former arises from emission in massive outflows. Winds powered by AGN (and central starbursts) are central to our understanding of the growth of the spheroidal components in these massive, young galaxies.

* Note: SCUBA is an extremely sensitive and versatile new device that allows astronomers to make images of astronomical objects at submillimeter wavelengths. SCUBA is mounted on the 15-meter James Clerke Maxwell Telescope of the Joint Astronomy Center on Mauna Kea in Hawaii. For more information about SCUBA, see:

The members of the GMOS team that obtained these observations are Michael Ledlow (Gemini Observatory), Ian Smail (University of Durham, UK), F. N. Owen (NRAO), W. C. Keel (University of Alabama), R. J. Ivison (Astronomy Technology Center, UK) and G. E. Morrison (CalTech).

Images of SMMJ09429 +4659 (left) and images of SMMJ09431+4700 (right) in B, R and K Bands
 Images of SMMJ09429 +4659 (left; H8) and images of SMMJ09431+4700 (right; H6) in B, R and K bands obtained with the 4-meter Mayall Telescope of NOAO.

 GMOS Spectrum of SMMJ09431+4700

This spectrum shows a combination of normal spectral emission lines, indicative of active star formation and high excitation emission lines from an active nucleus.

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