The HI content of early-type galaxies from the ALFALFA survey

I. Catalogued HI sources in the Virgo cluster

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50122 Firenze, Italy e-mail: sperello@arcetri.astro.it
² Università di Milano–Bicocca, Piazza delle Scienze 3, 20126 Milano, Italy
³ Center for Radiophysiscs and Space Research, Space Sciences Building, Cornell University, Ithaca, NY 14853, USA
⁴ Department of Physiscs and Astronomy, Union College, Schenectady NY 12308, USA
⁵ National Astronomy and Ionosphere Center (The National Astronomy and Ionosphere Center is operated by Cornell University under a cooperative agreement with the National Science Foundation.) , Space Sciences Building, Cornell University, Ithaca NY 14853, USA
⁶ Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
⁷ INAF - IASF Milano, via Bassini 15, 20133 Milano, Italy
⁸ INAF - Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy

Received: 2 July 2007
Accepted: 8 September 2007

Abstract

Aims.We are using the Arecibo Legacy Fast ALFA survey (ALFALFA), which is covering 17% of the sky at 21 cm, to study the HI content of early-type galaxies (ETG) in an unbiased way. The aim is to get an overall picture of the hot, warm and cold ISM of ETG, as a function of galaxy mass and environment, to understand its origin and fate, and to relate it to the formation and evolution history of these objects.

Methods.This paper deals with the first part of our study, which is devoted to the 8–16 deg. declination strip in the Virgo cluster. In this sky region, using the Virgo Cluster Catalogue (VCC), we have defined an optical sample of 939 ETG, 457 of which are brighter than the VCC completeness limit at B_T = 18.0. We have correlated this optical sample with the catalogue of detected HI sources from ALFALFA.

Results.Out of the 389 ETG from the VCC with B_T ≤ 18.0, outside the 1 deg. region of poor HI detection around M 87, and corrected for background contamination of VCC galaxies without a known radial velocity, only 9 galaxies (2.3%) are detected in HI with a completeness limit of 3.5 and 7.610⁷ of HI for dwarf and giant ETG, respectively. In addition 4 VCC ETG with fainter magnitudes are also detected. Our HI detection rate is lower than previously claimed. The majority of the detected ETG appear to have peculiar morphology and to be located near the edges of the Virgo cluster.

Conclusions.Our preliminary conclusion is that cluster ETG contain very little neutral gas, with the exceptions of a few peculiar dwarf galaxies at the edge of the ETG classification and of very few larger ETG, where the cold gas could have a recent external origin.