The Star Formation Histories ofThe 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 inpresent red-sequence galaxies
•Red-sequence galaxies dominated thestar 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 ofenvironment
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 drivingscaling parameter, with little or noresidual 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
… areluminosity-weightedages.
EXP
SSP
Quenched
FR
Very hard to distinguish these toy models with spectra
Ages from Balmer LinesAges from Balmer Lines
only old stars
Linestrengthscannot easilydistinguish SFhistory.
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: OK
Young 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 ofenvironment
Buildup of the red sequenceBuildup of the red sequence
If age is a strong function ofmass/velocity dispersion then the red-sequence itself is built “top-down”
Dwarf-to-Giant Ratio as a function of redshiftDwarf-to-Giant Ratio as a function of redshift
DGRevolution
rules outfrosting andexponentialmodels.
The “typical” star formation historyThe “typical” star formation history
Pure SSP is certainly unrealistic, but high -enhancement suggests most of the stellarmass formed in short (<1 Gyr) burst.
Any residual star formation must have beenquenched quite abruptly. It does not continueto 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 stellarpopulations on environment is very weak.
Smith et al. ‘06 found a 15% variation in agefrom the cluster core to the virial radius.
For dwarfs the story is very different …
Recently-quenchedComa red dwarfsRecently-quenchedComa red dwarfs
Smith et al. 2008, 2009
SSP
NGC4839
Smith et al ‘10 in prep
Recently-quenchedComa dwarfsRecently-quenchedComa 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-quenchedgalaxies appear to be early-type with Sersic n ~ 2(but ~40% do have disks).
Cluster Red Sequence GalaxiesCluster Red Sequence Galaxies
•Strong age (“downsizing”), metallicity and -enhancements along red velocity dispersionsequence.
•Downsizing in stellar age stops at ~ 70 km/s.
•Comparisons with both z ~ 0 and high-z RS datafavour a star formation history in which the youngerages are due to star formation quenching atintermediate epochs and not due to a small amountof very recent star formation.
•Red sequence dwarf ages show strongenvironmental dependence.