the star formation histories of red sequence galaxies mike hudson u. waterloo / iap steve allanson...

The Star Formation Histories of The Star Formation Histories of

Red Sequence GalaxiesRed Sequence Galaxies

Mike Hudson U. Waterloo / IAP

Steve Allanson (Waterloo) Allanson, MH et al 09, ApJ 702, 1275

Russell Smith (Durham)

Coma/Hectospec team, esp. Ron Marzke

• 70% of the stellar mass density is in present red-sequence galaxies

• Red-sequence galaxies dominated the star formation rate at z > 1

OutlineOutline

“Semi-empirical” galaxy formation models:

• Fit star formation models to low-z spectra

• Compare with high-z observations

What quenches star formation: the role of environment

Cluster Samples

• Red and emission-free but not morphologically-selected.

• Large dynamic range in galaxy mass

• Shapley• 340 galaxies in 3 rich clusters from deep 8hr AAOmega.

Smith et al. 07 MNRAS 381 1035; Smith et al. 09

• Coma Cluster Survey• Deep Hectospec of 90 dwarfs.

Smith et al. 08; Smith et al. 09a

Ages, Metallicities & -Enhancements from Spectra

• Measure Lick indices (typically H, H, H, several Fe lines, Mgb)

• Compare with -enhanced models

e.g. Thomas/Maraston et al., Schiavon.

• Get (“luminosity-weighted”) ages, metallicity, -enhancement

The “Driving Parameter” of Stellar Populations

Velocity dispersion, is the driving scaling parameter, with little or no residual dependence on stellar mass.

Smith, Lucey, MH ‘09

Graves & Faber ‘09

Shapley(Smith et al 07, Allanson et al 09)

70 km/s ~ L*+1.5

The End of Downsizing?The End of Downsizing?

Ages from Balmer LinesAges from Balmer Lines

… are luminosity-weighted ages.

EXP

SSP

QuenchedFR

Very hard to distinguish these toy models with spectra

Ages from Balmer LinesAges from Balmer Lines

only old stars

Linestrengths cannot easily distinguish SF history.

The End of Downsizing?The End of Downsizing?

Stellar M/L vs Dynamical M/LStellar M/L vs Dynamical M/L

Old SSP,Exponential SFR:

M*/L > Mdyn/L

Quenched: OKYoung SSP: OK

Allanson, MH, Smith & Lucey 2009

Dynamical M/L

Stellar M/L(Kroupa)

OutlineOutline

“Semi-empirical” galaxy formation models:

• Fit star formation models to low-z spectra

• Compare with high-z observations

What quenches star formation: the role of environment

Buildup of the red sequenceBuildup of the red sequence

If age is a strong function of mass/velocity dispersion then the red-sequence itself is built “top-down”

Dwarf-to-Giant Ratio as a function of Dwarf-to-Giant Ratio as a function of redshiftredshift

DGR evolution

rules out frosting and exponential models.

The “typical” star formation The “typical” star formation historyhistory

Pure SSP is certainly unrealistic, but high -enhancement suggests most of the stellar mass formed in short (<1 Gyr) burst.

Any residual star formation must have been quenched quite abruptly. It does not continue to the present-day.

OutlineOutline

“Semi-empirical” galaxy formation models:

• Fit star formation models to low-z spectra

• Compare with high-z observations

What quenches star formation: the role of environment

Environmental DependenceEnvironmental Dependence

For giant (~L*) galaxies, dependence of stellar populations on environment is very weak.

Smith et al. ‘06 found a 15% variation in age from the cluster core to the virial radius.

For dwarfs the story is very different …

Recently-quenched Recently-quenched Coma red dwarfsComa red dwarfs

Smith et al. 2008, 2009

SS

P

NGC4839

Smith et al ‘10 in prep

Recently-quenched Recently-quenched Coma dwarfsComa dwarfs

• Coma red dwarfs consistent with having been “quenched” within the last 1-2 Gyr.

• Effect seems stronger in NGC 4839 group

• Morphologically, most of the recently-quenched galaxies appear to be early-type with Sersic n ~ 2 (but ~40% do have disks).

Cluster Red Sequence Cluster Red Sequence GalaxiesGalaxies

• Strong age (“downsizing”), metallicity and -enhancements along red velocity dispersion sequence.

• Downsizing in stellar age stops at ~ 70 km/s.

• Comparisons with both z ~ 0 and high-z RS data favour a star formation history in which the younger ages are due to star formation quenching at intermediate epochs and not due to a small amount of very recent star formation.

• Red sequence dwarf ages show strong environmental dependence.