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. 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)
Hopkins et al. (2008)
SFR
AGN
Are these remnants young elliptical galaxies?
17th June 2014
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
17th June 2014
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
NGC typical elliptical galaxy
Increase in LX/LK >1 Gyr after coalescence
Driven by hot gas, regeneration of X-ray halo?
Brassington et al. (2007)
17th June 2014
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
17th June 2014
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
17th June 2014
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
17th June 2014
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
17th June 2014
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
( keV)
LxLMXB =8.8x1039
17th June 2014
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
17th June 2014
The X-ray Universe 2014

11. Luminous LMXBs
Kim & Fabbiano 2010
Ratio of Lx>5x1038 sources
compared to >1x1038 erg s -1
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 LMXBs
0.22 fraction luminous sources
KF10 average also 0.22 (old ellipticals 0.12)
17th June 2014
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
17th June 2014
The X-ray Universe 2014