Direct detection of galaxy stellar halos: NGC 3957 as a test case^*

¹ Laboratoire d'Astrophysique, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire, 1290 Sauverny, Switzerland e-mail: pascale.jablonka@unige.ch
² Université de Genève, Observatoire, 1290 Sauverny, Switzerland
³ Observatoire de Paris, CNRS-UMR8111, Place Jules Janssen, 92190 Meudon, France
⁴ Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh, EH9 3HJ, UK

Received: 18 September 2009
Accepted: 18 January 2010

Abstract

We present a direct detection of the stellar halo of the edge-on S0 galaxy NGC 3957, using ultra-deep VLT/VIMOS V and R images. This is achieved with a sky subtraction strategy based on infrared techniques. These observations allow us to reach unprecedented high signal-to-noise ratios of up to 15 kpc away from the galaxy center, rendering photon-noise negligible. The 1σ detection limits are R = 30.6 mag/arcsec² and V = 31.4 mag/arcsec². We conduct a thorough analysis of the possible sources of systematic errors that could affect the data: flat-fielding, differences in CCD responses, scaling of the sky background, the extended halo itself, and PSF wings. We conclude that the V–R colour of the NGC 3957 halo, calculated between 5 and 8 kpc above the disc plane where the systematic errors are modest, is consistent with an old and preferentially metal-poor normal stellar population, like that revealed in nearby galaxy halos from studies of their resolved stellar content. We do not find support for the extremely red colours found in earlier studies of diffuse halo emission, which we suggest might have been due to residual systematic errors.