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

A901aA901a

A901b

A901b

A902

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

zphot = 0.170 ± 0.006 795 galaxies within cz = 1900 km/s 1575 = 3.7 x 3.5 x 120 (Mpc/h)3

Field Sample

COMBO-17 field @ 0.05<z<0.25 excluding overdensities385 galaxies from 9750 (Mpc/h)3 with <z>=0.172

Colour-Magnitude Diagram

COMBO-17 evolving red-sequence fit @ 0.2<z<1.2795 galaxies = 462 red-sequence + 333 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-freeold clump

Dusty redcontaminants

Blue cloud

Templates: Old vs. Dusty Red

Dusty red galaxies are- redder at red end- bluer at blue endthan 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 / FieldE

nhanced / red

der

2-D Clustering

Old red = cluster coresDusty red ~ blue cloud

(x,y): old red = cluster coresdusty red ~ blue cloud

(y,z): old red ~ dusty redblue cloud = ‘voids’

v ~ 550 km/sv ~ 1400 km/s

Luminosity Distribution

Type-Density Relation

field cluster

A “Third Party”(??)…

• Three types of galaxies:1. Old red, passively evolving2. Intermediate3. 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 MV<-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 EB-V>0.2 red-sequence galaxies are luminous 24 sources

• Bell et al.in prep.

Outlook: Morphology

WFI@2.2COMBO-17

HST/ACSSTAGESGray et al.

STAGES: Center of A901a

WFI@2.2 HST/ACS