The First Hour of Extragalactic Data of the Sloan Digital Sky Survey Spectroscopic Commissioning: The Coma Cluster
2001
On 1999 May 26, one of the Sloan
Digital Sky Survey(SDSS) fiber-fed spectrographs saw astronomical
first light. This was followed by the first spectroscopic commissioning run during the dark period of 1999 June. We present here the first hour of extragalactic spectroscopy taken during these early commissioning stages: an observation of the
Coma clusterof
galaxies. Our data samples the southern part of this cluster, out to a radius of 15 (1.8 h-1 Mpc, approximately to the virial radius) and thus fully covers the NGC 4839 group. We outline in this paper the main characteristics of the SDSS spectroscopic systems and provide redshifts and spectral classifications for 196
Coma
galaxies, of which 45 redshifts are new. For the 151
galaxiesin common with the literature, we find excellent agreement between our redshift determinations and the published values, e.g., for the largest homogeneous sample of
galaxiesin common (63
galaxiesobserved by Colless & Dunn) we find a mean offset of 3 km s -1 and an rms scatter of only 24 km s -1. As part of our analysis, we have investigated four different spectral classification algorithms: measurements of the spectral line strengths, a principal component decomposition, a wavelet analysis and the fitting of spectral synthesis models to the data. We find that these classification schemes are in broad agreement and can provide physical insight into the evolutionary histories of our cluster
galaxies. We find that a significant fraction (25%) of our observed
Coma
galaxiesshow signs of recent star formation activity and that the velocity dispersion of these active
galaxies(emission-line and poststarburst
galaxies) is 30% larger than the absorption-line
galaxies. We also find no active
galaxieswithin the central (projected) 200 h-1 kpc of the cluster. The spatial distribution of our
Comaactive
galaxiesis consistent with that found at higher redshift for the CNOC1 cluster survey. Beyond the core region, the fraction of bright active
galaxiesappears to rise slowly out to the virial radius and are randomly distributed within the cluster with no apparent correlation with the potential merger or postmerger of the NGC 4839 group. We briefly discuss possible origins of this recent
galaxystar formation.
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