Missing baryons traced by the galaxy luminosity density in large-scale WHIM filaments^⋆

¹ Tartu Observatory, Observatooriumi 1, 61602 Tõravere, Estonia
e-mail: jukka@to.ee
² Dipartimento di Matematica e Fisica, Università degli Studi “Roma Tre”, via della Vasca Navale 84, 00146 Roma, Italy
³ INFN, Sezione di Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
⁴ INAF, Osservatorio Astronomico di Roma, Monte Porzio Catone, 00136 Roma, Italy
⁵ University of Bologna, viale Berti Pichat 6/2, 40127 Bologna, Italy
⁶ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France
⁷ Tuorla Observatory, Väisäläntie 20, 21500 Piikkiö, Finland
⁸ Department of Physics, University of Helsinki, Gustaf Hällströmin katu 2a, 00014 Helsinki, Finland
⁹ University of Alabama in Huntsville, Huntsville, AL 35899, USA

Received: 30 April 2015
Accepted: 6 August 2015

Abstract

We propose a new approach to the problem of the missing baryons. Building on the common assumption that the missing baryons are in the form of the warm hot intergalactic medium (WHIM), we also assume here that the galaxy luminosity density can be used as a tracer of the WHIM. This last assumption is supported by our discovery of a significant correlation between the WHIM density and the galaxy luminosity density in recent hydrodynamical simulations. We also found that the percentage of the gas mass in the WHIM phase is substantially higher (by a factor of ~1.6) within large-scale galactic filaments, i.e. ~70%, compared to the average in the full simulation volume of ~0.1 Gpc³. The relation between the WHIM overdensity and the galaxy luminosity overdensity within the galactic filaments is consistent with a linear one: δ_whim = 0.7 ± 0.1 × δ_LD^0.9±0.2. We then applied our procedure to the line of sight towards the blazar H2356-309 and found evidence of WHIM that corresponds to the Sculptor Wall (SW) (z ~ 0.03 and log N_H = 19.9^{+ 0.1}_-0.3) and Pisces-Cetus (PC) superclusters (z ~ 0.06 and log N_H = 19.7^{+ 0.2}_-0.3), in agreement with the redshifts and column densities of the X-ray absorbers identified recently. This agreement indicates that the galaxy luminosity density and galactic filaments are reliable signposts for the WHIM and that our method is robust for estimating WHIM density. The signal that we detected cannot originate in the halos of nearby galaxies because they cannot account for the high WHIM column densities that our method and X-ray analysis consistently find in the SW and PC superclusters.