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Paper: Cold Gas in Isolated Elliptical Galaxies
Volume: 421, Galaxies in Isolation: Exploring Nature Versus Nurture
Page: 297
Authors: Wolter, A.; Vergani, D.; Trinchieri, G.
Abstract: Emission from cold gas is rarely found in ellipticals and is not correlated to the optical luminosity. However, HI is detected in on-going mergers as e.g. the NGC5719/13 pair (Vergani et al. 2007). Dynamical modeling require late-type spiral progenitors (Hibbard et al. 1995), which produced HI-rich tidal tails expected to fall back to the main body of the galaxy in about 3 Gyr. Es formed through merging of spirals might possess a reservoir of cold gas which is otherwise missing. Also the large intrinsic scatter in the X-ray properties of early type galaxies could be due to their different merging histories. Galaxies with distinctive signs of disturbances show a strong deficiency of hot ISM, and merger remnants, like NGC3921 and NGC7252 are under-luminous in X-rays compared with the typical mature E in which these remnants are expected to evolve (for details, see discussion in Memola et al 2009).

We are currently studying the hypothesis that the presence of HI is related to a recent merger event and anti-correlated with high X-ray–luminosity gas. If true, this would also help explaining the LX / LK spread in terms of different evolutionary paths. The figure shows that in a sample of isolated ellipticals (Memola et al 2009, Trinchieri, this volume) most of the galaxies for which we have a secure detection of hot X-ray emitting gas have only upper limits in HI, while the objects for which cold gas is present are actually upper limits for the hot gas, hinting at the above hypothesis. However the paucity of data does not allow us to draw conclusions at this time.
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