Stellar Populations in Two Massive Galaxies at z~gt1.5

Details Stellar Populations in Two Massive Galaxies at z~gt1.5 Stellar Populations in Two Massive Galaxies at z~gt1.5

Feb 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001noao.prop..118d&link_type=abstract

NOAO Proposal ID #2001A-0118

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Scientific paper

The oldest and most massive galaxies at high redshift provide stringent constraints on the earliest epoch of galaxy formation, and thereby the age of the Universe and the fundamental cosmological parameters. The host galaxies of radio sources are the only z> 1.5 galaxies known to contain old stellar populations as indicated by their red optical-infrared colors. Indeed, deep optical spectra of two z 1.5 weak radio sources (LBDS 53W069 and 53W091) with R- K gt5.5 are remarkably similar to those of low-z ellipticals like M 32, and population synthesis modelling of their UV spectra suggests ages of 3.5 Gyr for these galaxies. The existence of such old objects at early epochs implies a high formation redshift for massive galaxies, and rules out Ω_0=1, (Lambda)_0=0 cosmologies for H_0 ≥ 40 hnotunit. At present, the ages of these objects are constrained only by their UV spectra: by z 1 the more familiar spectral features (such as the 4000Abreak) have moved into the near- infrared. Here, we propose to obtain NIRI low-resolution spectra of two z> 1 radio galaxies (LBDS 53W091 at z=1.55 and 3C 239 at z=1.78) that are believed to contain intermediate age stellar populations. By obtaining rest-frame optical spectra we will be able to detect at least the well-calibrated age-indicator, the 4000Abreak, and possibly the G-band ( 4300Å). The near-IR spectra in combination with existing data will place a much tighter constraint on the ages of both galaxies, and will allow us to compare the stellar contents and ages of two radio galaxies of very different radio power with greater precision than previously possible.

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