The diverse X-ray properties of four truly isolated elliptical galaxies: NGC 2954, NGC 6172, NGC 7052, and NGC 7785

Abstract

Aims. We investigate the X-ray properties of four isolated elliptical galaxies, selected from the Updated Zwicky Catalog according to strict isolation criteria. Isolated galaxies are not influenced by the group/cluster environment, and their X-ray emission can be studied independently of the often overwhelming contribution of the hot intergalactic medium. They are therefore suited to studying the X-ray characteristics relative to their intrinsic properties. Methods. We analyzed our own XMM-Newton and archival Chandra data in detail for three objects, and derived, when possible, the spatial and spectral characteristics of each source. An upper limit for the fourth one was obtained from archival ASCA data. We compared their characteristics with those of other 23 isolated objects for which X-ray and optical data are available in the literature. We explored possible theoretical explanations to interpret our results. Results. In spite of our attempt to select very homogeneous objects, both in terms of optical properties and environmental characteristics, we find a wide range in X-ray luminosities and L X /L B ratios for the four objects: two of them show a hot gaseous halo, whereas no gas is detected in the other two, to a factor >10 in luminosity. In fact, we find a large spread in the L X /L B for all galaxies considered, suggesting that the presence of hot gas is not easily related to the optical luminosity or to the mass, even in isolated systems. Younger objects tend to be less luminous in X-rays than older systems. However, it appears that older objects could span a wide range in luminosities.

Dark matter halos around isolated ellipticals

“…The net radial surface brightness profiles, centered at the X-ray peak of the EPIC-pn data in the 0.5-2.0 keV energy band, are well reproduced by a β-model whose best-fit values of r c and β are given in Table 1 (see also Memola et al 2009). …”

“…Note that NGC 7052 and NGC 7785, with L X < 2 × 10 41 erg s −1 , and M TOT < 2 × 10 12 M (see Table 3 in Memola et al 2009) do not appear to be fossil groups (e.g., Jones et al 2003;Pompei et al 2007) because they do not show their physical properties. Fossil groups (e.g., NGC 1132, Mulchaey & Zabludoff 1999) have, indeed, so far been observed with larger masses 10 13 −10 14 M , and X-ray luminosity >10 42 erg s −1 (see Fig.…”

“…NGC 7052 was observed by the Chandra ACIS-S instrument in September 2002 for about 10 ks, and NGC 7785 was observed by the XMM-Newton EPIC detectors in June 2004 for about 15 ks (PrimeFullWIndow mode and thin filter). For the spectral analysis of NGC 7052 we refer to Memola et al (2009).…”

The Massive Survey. Iv. The X-Ray Halos of the Most Massive Early-Type Galaxies in the Nearby Universe

“…However, successive generations of X-ray telescopes with improved sensitivity, larger effective areas, and superior spatial resolution are now beginning to allow a more in-depth study of the hotgas component in unprecedented detail, which in turn sheds new light on the formation history of galaxies. Using the excellent angular resolution of the Chandra X-ray telescope, studies are directly detecting and isolating the X-ray emission from stellar processes such as the low-mass X-ray binary population that have plagued the previous generation of X-ray telescopes, particularly in the low stellar mass regime (e.g., Sivakoff et al 2004;Boroson et al 2011;Lehmer et al 2014), as well as exploring the X-ray halos in early-type galaxies as a function of cosmic time (e.g., Lehmer et al 2007;Civano et al 2014;Paggi et al 2015) and probing the thermal gas content (e.g., Diehl & Statler 2007;Jeltema et al 2008;Memola et al 2009;Bogdán et al 2012aBogdán et al , 2012bKim & Fabbiano 2013Su et al 2015).…”

Scaling properties of the hot gas in early-type galaxies

“…The properties of isolated early-type galaxies provide a probe of whether environmental effects can account for the bulk of the scatter in L X /LB . Memola et al (2009) assembled LX -LB data for 27 isolated ellipticals, and found a good deal of scatter. However, excluding probable 'fossil groups' (Ponman et al (1994)) and upper limits, the remaining sample is small.…”