EUV excess in the inner Virgo cluster

Dipartimento di Fisica, Università di Lecce and INFN, Sezione di Lecce, Via Arnesano CP 193, 73100 Lecce, Italy

Corresponding author: F. De Paolis, depaolis@le.infn.it

Received: 24 October 2001
Accepted: 21 October 2002

Abstract

Observations with the Extreme UltraViolet Explorer (EUVE) satellite have shown that the inner region of the Virgo cluster (centered in M 87 galaxy) has a strong Extreme UltraViolet (EUV) emission (up to ) in excess to the low-energy tail expected from the hot, diffuse IntraCluster Medium (ICM). Detailed observations of large scale radio emission and upper limits for hard, non-thermal X-ray emission in the  keV energy band have been also reported. Here we show that all available observations can be accounted for by the existence of two electron Populations (indicated as I and II) in the M 87 Galaxy. The mildly relativistic Population I is responsible for the EUV excess emission via IC scattering of CBR and starlight photons. Population II electrons (with higher energy) are instead responsible for the radio emission through synchrotron mechanism. The same electrons also give rise to hard non-thermal X-ray emission (via IC scattering of CBR photons), but the resulting power is always below the upper bounds placed by present observations. The non-negligible energy budget of the two electron populations with respect to that associated with thermal electrons indicates that the M 87 galaxy is not today in a quiescent (relaxed) phase. Nuclear activity and merging processes could have made available this energy budget that today is released in the form of relativistic electrons.