Unravelling the formation and evolutionary histories of the most

massive galaxies

Ilani Loubser(Univ. of the Western Cape)

What is a brightest cluster galaxy?• The central supergiant galaxies in clusters.• Embedded in an extensive luminous halo (cD galaxy).

Typically:• ~1013 MΘ

• ~300 kpc

BCGs have special properties and are found at unique locations in the Universe – Do they require a special process for formation and evolution?

What role does the cluster environment play?

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BCG formation theories:

Galactic cannibalism: Ostriker & Tremaine 1975 etc.. • Mergers and captures of less massive galaxies.• Observational evidence: multiple nuclei; BCG properties correlated with cluster properties.

Tidal Stripping: Gallagher & Ostriker 1972 etc.. • Halo formation.

Abell 3827

Observations:New observations: > 90% BCG mass in place by z ~ 1.5 (Collins et al. 2009; Whiley et al. 2008)

VS.N-body Simulations + Semi-analytic models (De Lucia & Blaizot 2007):

The stars of BCGs formed early, but galaxies assemble late (50% mass in place after z ~ 0.5).

Cooling Flows: Cowie & Binney 1977; Jordan et al 2004 etc.. • Intracluster gas condense and form stars at bottom of potential well. • XMM-Newton results: gas does not cool down enough – AGN heating?

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Sample:

Sample properties:• mb brighter than 16 mag• Mb brighter than -20.7 mag• Distance closer than 340 Mpc• Further than 15° from galactic plain

9 WHT – June 2006

10 Gemini South 2006B

5 Gemini North 2006B

11 Gemini South 2007A

6 Gemini North 2007A 8 Gemini South 2007B

49 BCGs

WHT Observations: (4 nights – PI: Sansom)• ISIS: Longslit spectra, mostly major axis• 5 Flux calibrators & 22 Lick calibrators observed• 3900 – 9600 Å

Gemini Observations: (170 hours queue + 1 week – PI: Loubser)• GMOS: Longslit spectra, mostly major axis • Wavelength range: 3700 to 6500 Å • 22 Lick stars from the Gemini Science Archive

Kinematics: Results

Loubser et al., 2008, MNRAS, 391, 1009 – 1028

• 3 BCGs rotation > 100 km/s but still supported by random motions.

NGC7768

• 15 out of 49 BCGs showed velocity substructure (31%) Ellipticals in high density: 33 – 50%.

IC5358

• 5 out of 49 BCGs were found to have a positive velocity dispersion gradient. Previously for BCGs: IC1101 NGC6166

• Most of the BCG sample lie above the Faber Jackson relation.

ESO552-020

Stellar population results• We compiled the largest, spatially-resolved spectroscopic sample of nearby BCGs: Complete LICK absorption line dataset with high S/N over wide wavelength.

• Ages and abundances:

• In general, the galaxies are very old. However, 11 galaxies were found with log(Age) < 0.8 (in Gyr). They most likely experienced small episodes of recent star formation.

• The BCGs have higher metallicities than the samples of massive elliptical galaxies. This might indicate increased star formation e ciency in BCGs. ffi

• The BCGs have higher alpha enhancement than the comparison elliptical samples. This can be interpreted as a consequence of shorter formation time-scales in BCGs.

Loubser et al., 2009, MNRAS, 398, 133 – 156

Comparison to simulationsMcCarthy et al. (arXiv:0911.2641)

Analysed two high resolution cosmological hydrodynamic simulations from the OverWhelmingly Large Simulations (OWLS) project. Both include galactic winds driven by supernovae, only one of the models includes feedback from accreting black holes.

Current work: stellar population gradients

• Spectra binned for S/N of 40 per Å at Hβ (~ 15% error on ages derived from Hβ).• 30 Galaxies with 4 or more bins (maximum 138 bins).

Future work: combining MeerKAT and SALT observations of nearby BCGs.

Towards an understanding of the role of feedback and accretion in galaxy and cluster evolution.

Previous observations: 25 % of the galaxies show recent star formation.

Cold gas and young stars amount to only a few % of the total mass – but has an important effect.

Current and future multi-wavelength observations: Optical (SALT/Gemini) – hot ionised gas and stellar populations. Radio (MeerKAT) –HI 21 cm. UV (GALEX) – massive young stars. Sub-mm/mm (IRAM/ALMA) – molecular gas. X-ray (Chandra) – host cluster properties.

Why central cluster galaxies: Lie at the bottom of the potential well, often in `cool core’ clusters, where gas from the ISM forms stars or fuel accretion onto the black hole. i.e. they lie at the interface which is crucial to understand the role of feedback in galaxy and cluster evolution.

More likely to host radio-loud AGN than any other massive galaxy.

SALT observations: High S/N long-slit spectroscopy – dynamics, kinematics and stellar populations. – properties of the hot, ionised gas.

MeerKAT observations: Detecting HI on kpc-scale to – trace kinematics and morphology of the neutral gas and relate it to star formation – CO masses in central cluster galaxies (IRAM 30m) of 3 X 108 M to 4 X 1010 M Radio observations of large-scale properties such as tidal structures, combined with stellar population analysis, excellent tool to trace and date galaxy mergers in centers of clusters.