Virgo filaments

I. Processing of gas in cosmological filaments around the Virgo cluster^⋆

¹ Dipartimento di Fisica e Astronomia, Alma Mater Studiorum Università di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
e-mail: gianluca.castignani@unibo.it
² INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
³ Institute of Physics, Laboratory of Astrophysics, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
⁴ Observatoire de Paris, PSL University, Sorbonne University, CNRS, LERMA, 75014 Paris, France
⁵ Collège de France, 11 place Marcelin Berthelot, 75231 Paris, France
⁶ GEPI, Observatoire de Paris, Université PSL, CNRS, Place Jules Janssen, 92190 Meudon, France
⁷ Department of Physics & Astronomy, Siena College, 515 Loudon Road, Loudonville, NY 12211, USA
⁸ The University of Kansas, Department of Physics and Astronomy, Malott Room 1082, 1251 Wescoe Hall Drive, Lawrence, KS 66045, USA
⁹ INAF – Osservatorio astronomico di Padova, Vicolo Osservatorio 5, 35122 Padova, Italy
¹⁰ Spitzer Science Center, California Institute of Technology, MS 220-6, Pasadena, CA 91125, USA
¹¹ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA

Received: 16 December 2020
Accepted: 20 September 2021

Abstract

It is now well established that galaxies have different morphologies, gas contents, and star formation rates (SFR) in dense environments like galaxy clusters. The impact of environmental density extends to several virial radii, and galaxies appear to be pre-processed in filaments and groups before falling into the cluster. Our goal is to quantify this pre-processing in terms of gas content and SFR, as a function of density in cosmic filaments. We have observed the two first CO transitions in 163 galaxies with the IRAM-30 m telescope, and added 82 more measurements from the literature, thus forming a sample of 245 galaxies in the filaments around the Virgo cluster. We gathered HI-21cm measurements from the literature and observed 69 galaxies with the Nançay telescope to complete our sample. We compare our filament galaxies with comparable samples from the Virgo cluster and with the isolated galaxies of the AMIGA sample. We find a clear progression from field galaxies to filament and cluster galaxies for decreasing SFR, increasing fraction of galaxies in the quenching phase, an increasing proportion of early-type galaxies, and decreasing gas content. Galaxies in the quenching phase, defined as having a SFR below one-third of that of the main sequence (MS), are only between 0% and 20% in the isolated sample, according to local galaxy density, while they are 20%–60% in the filaments and 30%–80% in the Virgo cluster. Processes that lead to star formation quenching are already at play in filaments; they depend mostly on the local galaxy density, while the distance to the filament spine is a secondary parameter. While the HI-to-stellar-mass ratio decreases with local density by an order of magnitude in the filaments, and two orders of magnitude in the Virgo cluster with respect to the field, the decrease is much less for the H₂-to-stellar-mass ratio. As the environmental density increases, the gas depletion time decreases, because the gas content decreases faster than the SFR. This suggests that gas depletion precedes star formation quenching.