Selection and Photometric Properties of K+A Galaxies Alejandro D. Quintero, David W. Hogg, Michael R. Blanton (NYU), et al.

Motivations F SF has been declining since z~1 F Milky Way can’t form stars much longer (next few Gyr) F How universe is shutting off wrt SF. F Look at galaxies that have ‘just’ stopped forming stars. –K+A galaxies (HDS, Post-Starburst Galaxies)

Data F Taken from SDSS –map 1/4 of entire sky (more than 100 million objects) –measure dist to million nearest galaxies. –3d picture of local universe (100x before) –‘most ambitious astronomical survey ever done’ –LSS, galactic evolution, L&D matter, etc…

SDSS F located at Apache Point Observatory, NM

Method In the 80’s Dressler & Gunn found that psb’s are well described sum of A,K spectra –A stars live ~1Gyr –K stars live ~10 Gyr (~ age of universe) F Performed a linear fit of average A,K spectra on every galaxy –gave us ratio A/K (young to old pop) F Also measured H-alpha emission –via Equivalent Width

A and O,B Stars F H-alpha is excited Hydrogen gas (2nd- >1st level emission) –Only stars that can excite in this way are O,B –O,B stars live ~0.03 Gyr –A stars: ~ 1 Gyr F A and O,B stars are naturally correlated

Average A and K spectra F ‘A’, ‘K’ are the number we multiply each spectrum by to make the ‘fit’.

Main Figure F Plot of ‘A/K’ versus H-alpha EW F Clearly shows separate K+A population F Also have simple models –3, 10, 14 Gyr star formation histories –Used to see how objects move in figure over time –Made by convolving PEGASE spectra and performing same measurements

‘A/K’ vs H-alhpa EW

Randomly selected K+A’s

Results F Present day SFR >> cosmic time averaged mean F Verified Kennicutt IMF as opposed to several others F Are found in full range of environments (not a present-day cluster population) F And…

Structure of K+A’s F Are similar to bulge-dominated galaxies but are bluer and…

Surface brighness F Also have higher surface brightness (brightest galaxies in the universe)

Luminosity Function F Lum dist is similar to bright (b-d) end of LF. F LF is lum dist weighted by 1/Vmax

Comoving Rate Density F Crude steady state approximatio n at z~0.1 F Event rate: –~10^4*h^3*M pc^3*-Gyr^-1

What K+A’s look like F Mostly bulge dominated (but not all) F Higher average incidence of ‘close pairs’ –‘close pairs’ ~ mergers

Future Works F Better calculation of even rate (steady state isn’t correct thing to do) –Compare to CDM predictions F K+A populations wrt clustocentric radii F Dark Matter Theory –b-d population is growing by ~1% per Gyr –Does CDM predict this?

The End