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The X-ray Evolution of Young Post-Merger

This study explores the X-ray properties and evolution of young post-merger elliptical galaxies, focusing on the regeneration of the X-ray halo. The research is based on observational studies, numerical simulations, and Chandra observations of gas-rich major mergers.

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The X-ray Evolution of Young Post-Merger

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  1. The X-ray Evolution of Young Post-Merger Nicky Brassington The Centre for Astrophysics Research University of Hertfordshire Collaborators: G. Fabbiano, E. O’Sullivan, A. Zezas

  2. SFR AGN Hopkins et al. (2008) Galaxy Evolution • Mergers suggested to be an important process in the formation of some elliptical galaxies • Observational Studies generally support picture of spiral galaxy interactions leading to coalescence • Supported by numerical simulations, e.g. Hopkins et al. (2008) • Are these remnants young elliptical galaxies? The X-ray Universe 2014

  3. Properties of Merger-Remnants • Still unclear if merger-remnants will evolve to appear similar to mature elliptical galaxies • Archetype post-merger NGC 7252 • Central profile relaxed • But prominent tidal tails • Gas-rich major-merger • ~1 Gyr since nuclear Coalescence • X-ray faint compared to mature ellipticals The X-ray Universe 2014

  4. Evolutionary Sequence • Probing evolution in X-ray • Determine X-ray binary population (ULXs) • Observe changes in hot diffuse emission • Chandra survey of nine gas-rich major-mergers • From 1st perigalactic to 3 Gyr post-merger • Post-merger remnants appear X-ray faint compared to mature ellipticals • Increase in LX/LK >1 Gyr after coalescence • Driven by hot gas, regeneration of X-ray halo? Brassington et al. (2007) The X-ray Universe 2014

  5. Determining the Driver of Regeneration • Halo regeneration predicted through hydrodynamical simulations (stellar mass loss/supernovae feedback) • Ciotti et al. (1991) Cox et al. (2006) • Or infall of expelled material shocking to X-ray • Gradual regeneration hinted at through ROSAT observations (O’Sullivan et al. 2001) • Chandra sample only single step between 1 Gyr & 3 Gyr • Observations in this time period required to determine when regeneration begins The X-ray Universe 2014

  6. Sample of ‘Infant’ Ellipticals • To determine timescale of halo regeneration observe sample of post-merger galaxies • Distinguish between infalling gas and different hydrodynamical models • All galaxies: • Within 50 Mpc • Age estimates between 1 Gyr - 5 Gyr • Multi-wavelength indicate single major-merger • Constrained to remove complexities of multiple mergers The X-ray Universe 2014

  7. Evolution of Post-Mergers • Disturbed elliptical galaxy NGC 3656 • Tidal features in R-band • HI observations detect 2 tidal tails (Balcells et al. 2001) • Coalescence 1.5 Gyr (dynamics & colours) • Mass LK= 8x1010 M The X-ray Universe 2014

  8. Chandra Observations of ‘Infant Elliptical’: NGC 3656 • 53 ks observation • D=46 Mpc • 11 point sources within D25 ellipse • Limit LX > 1x1038 erg s-1 The X-ray Universe 2014

  9. Properties of Diffuse Emission • Spectral decomposition required: • Relations of Boroson, Kim & Fabbiano (2012) • Low luminosity LMXBs ------- • CVs & ABs ------ • Diffuse ------- • LxDIFF =4.2x1039 • LxUNRSVL =2.2x1039 • LxCV+AB =0.7x1039 (0.3-8.0 keV) • LxLMXB =8.8x1039 The X-ray Universe 2014

  10. Regeneration of X-ray Halo • Compare NGC 3656 with previous sample • At 1.5 Gyr LX/LK still very low • %Ldiff also low • Halo regeneration may have started • Reject infall scenario • 4 major-mergers >2 Gyr The X-ray Universe 2014

  11. Kim & Fabbiano 2010 Ratio of Lx>5x1038 sources compared to >1x1038 erg s -1 Luminous LMXBs • Suggestion that ‘young’ elliptical galaxies have different XLF compared to older ellipticals (see Lehmer talk next) • Two ellipticals recent minor-merger (Kim & Fabbiano 2010) • Suggests flattening of XLF, similar to HMXBs • No break ~ 5x1038 erg s-1 • NGC 3656 11 LMXBs • 0.22 fraction luminous sources • KF10 average also 0.22 (old ellipticals 0.12) The X-ray Universe 2014

  12. Summary • Moderate depth surveys provide information about the overall behaviour of diffuse gas • At 1.5 Gyr possible evidence of halo regeneration • Systems between 2-5 Gyr required to differentiate • ‘Infant’ elliptical X-ray binary population exhibits shallower XLF than mature galaxies The X-ray Universe 2014

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