Red-Sequence Galaxies with young stars and dust The cluster A901/902 seen with COMBO-17 Christian Wolf (Oxford) Meghan E. Gray (Nottingham) Klaus Meisenheimer (Heidelberg) A&A, in press

A901a A901b A902 Testing ground: the A901/902 supercluster z=0.16: ~5x5 Mpc

COMBO-17 SEDs SED classification ~98% complete at R<23 –Stars (~2000)

COMBO-17 SEDs SED classification ~98% complete at R<23 –Stars (~2000) –White Dwarfs (13)

COMBO-17 SEDs SED classification ~98% complete at R<23 –Stars (~2000) –White Dwarfs (13) –Galaxies (~6000)

COMBO-17 SEDs SED classification ~98% complete at R<23 –Stars (~2000) –White Dwarfs (13) –Galaxies (~6000) –QSOs (~100)

Redshift Accuracy But first photo-z trials in 2002: 1/4 galaxies wrong!!!

Selecting The Cluster Sample z phot = ± galaxies within  cz = 1900 km/s1575 = 3.7 x 3.5 x 120 (Mpc/h) 3

Field Sample COMBO <z<0.25 excluding overdensities 385 galaxies from 9750 (Mpc/h) 3 with =0.172

Colour-Magnitude Diagram COMBO-17 evolving red-sequence 0.2<z< galaxies = 462 red-sequence blue-cloud (field: 62=13+49)

2-D Family: Age x Extinction

What is in the Red Sequence? Examine age x extinction diagram Try splitting red-sequence: –Dust-free old clump Cleaner sample of old red galaxies –Dusty contaminants What are these? Blue cloud as usual Regular star-forming galaxies

Dust-free old clump Dusty red contaminants Blue cloud

Templates: Old vs. Dusty Red Dusty red galaxies are - redder at red end - bluer at blue end than old red galaxies

Old vs. Dusty Near 4000A

Mean Spectra: Old / Dusty Trends in continuum colour and spectral lines!

Mean Spectra

Ratio Spectrum: Dusty/Old

Population Mix: Cluster / Field Enhanced / redder

2-D Clustering Old red = cluster cores Dusty red ~ blue cloud

(x,y):old red = cluster cores dusty red ~ blue cloud (y,z):old red ~ dusty red blue cloud = ‘voids’  v ~ 550 km/s  v ~ 1400 km/s

Luminosity Distribution

Type-Density Relation field cluster

A “Third Party”(??)… Three types of galaxies: 1.Old red, passively evolving 2.Intermediate 3.Young blue, actively star-forming Intermediate properties –Ages: intermediate or “old + young” –Star formation: Little but present –Colour: like red / Dust: like blue galaxies –Luminosities: intermediate –Habitat: medium-density preferred Small contribution from edge-on disks

Origin of Intermediate Type Transformed infalling blue galaxies: Contra: –Luminosity of progenitors –High mean stellar ages Age sequence: cores - intermediates - outskirts - field Pro: –Spirals redder as well (Butcher-Oemler effect) Minor mergers: established large old cluster elliptical +infalling small blue galaxy Pro: –Properties fine without invoking special process –High-luminosity and old-age component required Contra: –Very few undisturbed established old cluster ellipticals in outskirts left

Summary: Data & Galaxy Types COMBO-17 surveys dynamically complex supercluster Abell 901/902 photo-z’s provide ~800 galaxies at M V <-17 Detailed 17-band SED - age x extinction grid More than 1/3 of the red-sequence are not old red galaxies, but dustier intermediate-age ones Added complexity in colour-density relation: dusty red galaxies don’t conform with old red or young blue

Summary: Properties of Dusty Red Galaxies Peak at mid-density (40%) Form interface between blue and old red Currently forming stars Have plenty of old stars Result of –Environmentally triggered transformation of infalling blue galaxies –Age sequence –Minor merger of infalling blue galaxy into old red cluster elliptical

Outlook: Spitzer 24  Analysis Most E B-V >0.2 red-sequence galaxies are luminous 24  sources Bell et al. in prep.

Outlook: Morphology COMBO-17 HST/ACS STAGES Gray et al.

STAGES: Center of A901a