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Astron. Astrophys. 336, 445-454 (1998)


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The ESO Slice Project (ESP) galaxy redshift survey *

IV. A discussion of systematic biases in galaxy redshift determinations

A. Cappi 1, G. Zamorani 1, 2, E. Zucca 1, 2, G. Vettolani 2, R. Merighi 1, M. Mignoli 1, G.M. Stirpe 1, C. Collins 3, L. Guzzo 4, G. Chincarini 4, 5, D. Maccagni 6, C. Balkowski 7, V. Cayatte 7, S. Maurogordato 7, 8, D. Proust 7, S. Bardelli 9, M. Ramella 9, R. Scaramella 10, A. Blanchard 11 and H. MacGillivray 12

1 Osservatorio Astronomico di Bologna, Via Zamboni 33, I-40126 Bologna, Italy
2 Istituto di Radioastronomia del CNR, Via Gobetti 101, I-40129 Bologna, Italy
3 Astrophysics Research Institute, Liverpool John-Moores University, Byrom Street, Liverpool L3 3AF, UK
4 Osservatorio Astronomico di Brera, Via Bianchi 46, I-22055 Merate (LC), Italy
5 Università degli Studi di Milano, Via Celoria 16, I-20133 Milano, Italy
6 Istituto di Fisica Cosmica e Tecnologie Relative, Via Bassini 15, I-20133 Milano, Italy
7 Observatoire de Paris, DAEC, Unité associée au CNRS, D0173 et à l'Université Paris 7, 5 Place J.Janssen, F-92195 Meudon, France
8 CERGA, Observatoire de la Côte d'Azur, B.P. 229, F-06304 Nice Cedex 4, France
9 Osservatorio Astronomico di Trieste, Via Tiepolo 11, I-34131 Trieste, Italy
10 Osservatorio Astronomico di Roma, Via Osservatorio 2, I-00040 Monteporzio Catone (RM), Italy
11 Université L. Pasteur, Observatoire Astronomique, 11 rue de l'Université, F-67000 Strasbourg, France
12 Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK

Received 9 February 1998 / Accepted 11 May 1998

Abstract

We present a detailed discussion of the redshift errors associated to the ESO Slice Project measurements. For a subsample of 742 galaxies with redshifts determined both from the absorption lines ([FORMULA]) and from the emission lines ([FORMULA]), we find an average difference [FORMULA] km/s. We find that a similar effect is present in another, deeper redshift survey, the Durham/Anglo-Australian Telescope faint galaxy redshift survey (Broadhurst et al. 1988), while is absent in surveys at brighter magnitude limits. We have investigated in detail many possible sources of such a discrepancy, and we can exclude possible zero-point shifts or calibration problems. We have detected and measured systematic velocity differences produced by the different templates used in the cross-correlation. We conclude that such differences can in principle explain the effect, but in this case the non-trivial implication would be that the best-fitting template does not necessarily give the best velocity estimate. As we do not have any a priori reason to select a template different from the best-fitting one, we did not apply any correction to the ESO Slice Project velocities. However, as for a small number of galaxies the effect is so large that it is likely to have a physical explanation, we have also taken into account the possibility that the discrepancy can be partly real: in this case, it might help to understand the role of gas outflows in the process of galaxy evolution. In view of the future large spectroscopic surveys, we stress the importance of using different templates and making them publicly available, in order to assess the amplitude of systematic effects, and to allow a direct comparison of different catalogues.

Key words: surveys - galaxies: distances and redshift - galaxies: kinematics and dynamics

* based on observations collected at the European Southern Observatory, La Silla, Chile.

Send offprint requests to: Alberto Cappi
Correspondence to: cappi@astbo3.bo.astro.it

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998

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