Selection and Photometric Properties of K+A GalaxiesSelection and Photometric Properties of K+A Galaxies

Alejandro D. Quintero, David W. Hogg, Alejandro D. Quintero, David W. Hogg, Michael R. Blanton (NYU), et al.Michael R. Blanton (NYU), et al.

Motivations

SF has been declining since z~1SF has been declining since z~1 Milky Way can’t form stars much longer (next Milky Way can’t form stars much longer (next

few Gyr)few Gyr) HowHow universe is shutting off wrt SF. universe is shutting off wrt SF. Look at galaxies that have ‘just’ stopped Look at galaxies that have ‘just’ stopped

forming stars.forming stars.– K+A galaxies (HDS, Post-Starburst Galaxies)K+A galaxies (HDS, Post-Starburst Galaxies)

Data

Taken from SDSSTaken from SDSS– map 1/4 of entire sky (more than 100 map 1/4 of entire sky (more than 100

million objects)million objects)– measure dist to million nearest galaxies.measure dist to million nearest galaxies.– 3d picture of local universe (100x before)3d picture of local universe (100x before)– ‘‘most ambitious astronomical survey ever most ambitious astronomical survey ever

done’done’– LSS, galactic evolution, L&D matter, etc…LSS, galactic evolution, L&D matter, etc…

SDSS located at Apache Point Observatory, NMlocated at Apache Point Observatory, NM

MethodIn the 80’s Dressler & Gunn found that psb’s are In the 80’s Dressler & Gunn found that psb’s are

well described sum of A,K spectrawell described sum of A,K spectra– A stars live ~1GyrA stars live ~1Gyr– K stars live ~10 Gyr (~ age of universe)K stars live ~10 Gyr (~ age of universe)

Performed a linear fit of average A,K spectra Performed a linear fit of average A,K spectra on every galaxyon every galaxy– gave us ratio A/K (young to old pop)gave us ratio A/K (young to old pop)

Also measured H-alpha emissionAlso measured H-alpha emission– via Equivalent Widthvia Equivalent Width

A and O,B Stars H-alpha is excited Hydrogen gas (2nd-H-alpha is excited Hydrogen gas (2nd-

>1st level emission)>1st level emission)– Only stars that can excite in this way are Only stars that can excite in this way are

O,BO,B– O,B stars live ~0.03 GyrO,B stars live ~0.03 Gyr– A stars: ~ 1 GyrA stars: ~ 1 Gyr

A and O,B stars are naturally correlatedA and O,B stars are naturally correlated

Average A and K spectra

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

Main Figure Plot of ‘A/K’ versus H-alpha EWPlot of ‘A/K’ versus H-alpha EW Clearly shows separate K+A populationClearly shows separate K+A population Also have simple modelsAlso have simple models

– 3, 10, 14 Gyr star formation histories3, 10, 14 Gyr star formation histories– Used to see how objects move in figure Used to see how objects move in figure

over timeover time– Made by convolving PEGASE spectra and Made by convolving PEGASE spectra and

performing same measurementsperforming same measurements

‘A/K’ vs H-alhpa EW

Randomly selected K+A’s

Results

Present day SFR >> cosmic time Present day SFR >> cosmic time averaged meanaveraged mean

Verified Kennicutt IMF as opposed to Verified Kennicutt IMF as opposed to several othersseveral others

Are found in full range of environments Are found in full range of environments (not a present-day cluster population)(not a present-day cluster population)

And…And…

Structure of K+A’s Are similar to bulge-dominated galaxies but Are similar to bulge-dominated galaxies but

are bluer and…are bluer and…

Surface brighness Also have higher surface brightness Also have higher surface brightness

(brightest galaxies in the universe)(brightest galaxies in the universe)

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

Comoving Rate Density

Crude steady Crude steady state state approximatioapproximation at z~0.1n at z~0.1

Event rate: Event rate: – ~10^4*h^3*M~10^4*h^3*M

pc^3*-Gyr^-1pc^3*-Gyr^-1

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

– ‘‘close pairs’ ~ mergersclose pairs’ ~ mergers

Future Works

Better calculation of even rate (steady Better calculation of even rate (steady state isn’t correct thing to do)state isn’t correct thing to do)– Compare to CDM predictionsCompare to CDM predictions

K+A populations wrt clustocentric radiiK+A populations wrt clustocentric radii Dark Matter TheoryDark Matter Theory

– b-d population is growing by ~1% per Gyrb-d population is growing by ~1% per Gyr– Does CDM predict this?Does CDM predict this?

The End