The Cartwheel galaxy with XMM-Newton

INAF - Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy e-mail: anna.wolter@brera.inaf.it

Received: 25 July 2008
Accepted: 2 April 2009

Abstract

Aims. The extreme environment provided by the Cartwheel ring is analyzed to study its X-ray and optical-UV properties. We compare the Cartwheel with the other members of its group and study the system as a whole in the X-ray band.

Methods. We analyze the data of the Cartwheel galaxy obtained with XMM-Newton in two different periods (December 2004 and May 2005). We focus on the X-ray properties of the system and use the OM data to obtain additional information in the optical and UV bands. Each dataset is analyzed separately to study source variability and summed together to study fainter and extended sources.

Results. We detect a total of 8 sources associated with the Cartwheel galaxy and three in its vicinity, including G1 and G2, all at L_X ≥ 10³⁹ erg s^-1, that is the Ultra Luminous X-ray (ULX) source range. The brightest ULX source has been already discussed elsewhere. The spectra of the next three brightest ULX are well fitted by a power-law model with a mean photon index of ~2. We compare the XMM-Newton and Chandra datasets to study the long-term variability of the sources. At least three sources vary in the 5 months between the two XMM-Newton observations and at least four in the 4-year timeframe between Chandra and XMM-Newton observations. One Chandra source disappears and a new one is detected by XMM-Newton in the ring. Optical-UV colors of the Cartwheel ring are consistent with a burst of star formation that is close to reaching its maximum, yielding a mean stellar age of about 40 Myr. The inferred variability and age suggest that high mass X-ray binaries are the counterparts to the ULX sources. The 3 companion galaxies have luminosities in the range 10^39-40 erg/s consistent with expectations. The hot gas of the Cartwheel galaxy is luminous and abundant (a few 10⁸ ) and is found both in the outer ring, and in the inner part of the galaxy, behind the shock wave front. We also detect gas in the group with L_X ~ 10⁴⁰ erg s^-1.