The X-ray Evolution of Young Post-Merger

Nicky BrassingtonThe Centre for Astrophysics Research

University of Hertfordshire

Collaborators: G. Fabbiano, E. O’Sullivan, A. Zezas

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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)

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are needed to see this picture.

Hopkins et al. (2008)

• Are these remnants young elliptical galaxies?

SFR

AGN

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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 tidaltails– Gas-rich major-merger– ~1 Gyr since nuclearCoalescence

• X-ray faint compared to mature ellipticals

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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• Increase in LX/LK >1 Gyr after coalescence

– Driven by hot gas, regeneration of X-ray halo?Brassington et al. (2007)

• Post-merger remnants appear X-ray faint compared to mature ellipticals

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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

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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

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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

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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

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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

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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

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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)

Kim & Fabbiano 2010Ratio of Lx>5x1038 sources

compared to >1x1038 erg s -1

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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