Galaxies in low density environments

Michael BaloghUniversity of Durham

Nature vs. Nurture: galaxy formation and

environmentMichael Balogh

University of Durham

Outline

• Review of cluster galaxy properties• Theoretical expectations• First clues: clusters at intermediate

redshift• New results: tracing star formation

in all environments in the local Universe

• Future work

Cluster Galaxies: background

• Segregation of luminosities, morphologies, and emission line fraction well-known

• Early-types consistent with passive evolution since z>2

• Small fraction of actively star-forming galaxies

Nature or Nurture?

• Nature? Elliptical galaxies only form in protoclusters at high redshift. Rest of population is due to infall.

• or Nurture? Galaxy evolution proceeds along a different path within dense environments.

Butcher-Oemler effect

• Concentrated clusters at high redshift have more blue galaxies than concentrated clusters at low redshift

Butcher & Oemler (1984)

Butcher-Oemler effect

• A lot of scatter– appears to be

mostly due to correlation with cluster richness

• still room to worry about cluster selection?

Margoniner et al. (2001)

Butcher-Oemler effectSDSS: Goto et al. (2003)

• Many of blue galaxies turned out to have post-starburst spectra (Dressler & Gunn 1992; Couch & Sharples 1987)

• Suggested nurture:– ram-pressure stripping

(Gunn & Gott 1972)

– tidal effects (Byrd & Valtonen 1990)

– harassment? (Moore et al. 1999)

But: Field galaxy evolution

• But field population also evolves strongly (Lilly et al. 1996)

• Post-starburst galaxies equally abundant in the field (Zabludoff et al. 1996; Goto et al. 2003)

• So: does BO effect really point to cluster-specific physics, or just the evolving field and infall rate (Ellingson

et al. 2001)?Steidel et al. (1999)

If it is “nurture”…

• Could cosmic SFR evolution be a consequence of environment?

• Only if star formation rates are low in groups and low-density environments as well as clusters

Groups

Clusters

Galaxy formation theory

Cole et al. (2000), Kauffmann et al. (1999); Somerville et al.

(1999) and many others

Galaxy formation cartoon

Radiativecooling

Radiativecooling

Feedback

Feedback

Theory

1. Typical galaxy forms stars at decaying rate

Rocha-Pinto et al. (2000)

Theory

1. Typical galaxy forms stars at exponentially decaying rate

2. Galaxies in dense regions form earlier

Benson et al. (2002)

z=5

z=0

Theory

1. Typical galaxy forms stars at exponentially decaying rate

2. Galaxies in dense regions form earlier

3. “Strangulation”: Galaxies in dense environments lose hot halo

Larson et al. (1980)

Radiativecooling

Radiativecooling

Feedback

Feedback

Theory: predictions

• no ram-pressure stripping, harrassment included, yet achieve reasonable match to observed clusters (Diaferio et al. 2001; Okamoto et al. 2003)

• isolated galaxies should be at centre of cooling flow, hence forming stars

• Galaxies in clusters should be forming stars at a lower rate than those in the field

Observations: z~0.3

CNOC clusters (with Morris, Carlberg, Yee, Ellingson, Schade)AAT spectroscopy (with Couch,

Bower)

Observations: z~0.3

• Average SFR varies gradually with radius

• low average SFR even beyond virial radius

• Gradient much steeper than expected from morphology-density relationBalogh et al. (1998)

Field

CNOC clusters

Morph-density relation

Observations: z~0.3

• Strangulation model:– infall rate +

assumed decay rate of star formation => radial gradient in SFR

• Radial gradients in CNOC clusters suggest ~2 Gyr

Balogh, Navarro & Morris (2000)

Nod & Shuffle: LDSS++

band-limiting filter +microslit = ~800 galaxies per 7’ field

observed 4 clusters at z~0.3

H in Rich Clusters at z~0.3

Balogh et al. (2002)

Couch et al. (2001)

(Field)

• Number of emission lines galaxies is low in all clusters

• However, shape of luminosity function similar to field: – consistent with

shift in normalisation; not in H luminosity

Observations: z~0

2dFGRS (with Bower, Lewis, Eke, Couch et al.)

SDSS (with Nichol, Miller, Gomez et al.)

Observations: z~0

• Analysis of 2dFGRS – H equivalent

widths, within 20 Mpc of known clusters

– dependence of mean SFR on local density

– “critical density”?

Lewis, Balogh et al. (2002)

Observations: z~0

• Analysis of 2dFGRS – H equivalent

widths, within 20 Mpc of known clusters

– dependence of mean SFR on local density

– “critical density”?– independent of

distance to clusterLewis, Balogh et al. (2002)

R > 2 R200

Observations: z~0

• Analysis of SDSS– same trend

observed in SFR from H fluxes

– same value of “critical density”

Gomez et al. (2003)

Sta

r F

orm

atio

n R

ate

(Mo/

yr)

Distance from Cluster Centre (R/Rvirial)

Field 75th percentile

Median

75th percentile

Field median

New results: combining the SDSS and 2dFGRS

• Combined sample of 24,968 galaxies at 0.05<z<0.1 (Balogh, Eke et al. MNRAS submitted)

• Volume limited: Mr<-20.6 (SDSS); Mb<-19.5

• 3 measures of environment:– “traditional” projected distance to 5th nearest

neighbour– 3-dimensional density on 1 and 5 Mpc scales– velocity dispersion of embedding cluster or

group• catalogues of Nichol, Miller et al. and Eke et al.

Bimodality

• SDSS colours show two distinct populations

• Red population may be the result of major mergers at high redshift, followed by passive evolution

Baldry et al. (2003)(u-r)0

Bimodality

• Same is seen in H distribution: SFR is not continuous

• galaxies do not have arbitrarily low SFR

• So mean/median do not necessarily trace a change in SFR

The star-forming population

• Amongst the star-forming population, there is no trend in mean SFR with density!

• Same is seen in z~0.5 cluster H luminosity functions

• Rules out slow-decay models

Correlation with density

• The fraction of star-forming galaxies varies strongly with density

• Correlation at all densities; still a flattening near the critical value

2dFGRS

Isolated Galaxies

• Selection of isolated galaxies:– non-group

members, with low densities on 1 and 5.5 Mpc scales

• ~30% of isolated galaxies show negligible SF– challenge for

models?– environment must

not be only driver of evolution.

All galaxiesBright galaxies

Isolated Galaxies

• Fraction of SF galaxies in lowest density environments is not much larger than the average– So strong evolution

in global average cannot be due only to a change in densities

Average value in full sample

Large scale structure

• Some dependence on cluster velocity dispersion?

• More obvious in 2dF catalogue than in SDSS

200<<400 km/s

>500 km/s

2dFGRS

Large scale structure

• Emission-line fraction appears to depend on 1 Mpc scales and on 5.5 Mpc scales. 5

.5 (

Mp

c-3)

0.050

0.010

0.005 Increasing fraction of Hemitters

Nature vs. Nurture

• Nurture: clusters directly affect SFR?– rule out long-

timescale processes (strangulation)

– trends at low densities and large scales rule out ram-pressure stripping as dominant effect

z~0.3

z~0.1

Nature vs. Nurture

• Nurture: clusters directly affect SFR?– short timescale?

• few (<0.1 %) E+As• normal SFR for colour• however, these don’t

provide strong constraints: it is possible to generate entire non-SF population in this way

Blue galaxies only: (g-r)<0.7

Nature vs. Nurture

• Nurture: clusters directly affect SFR?– short timescale?

• morphology is longer lived

• maybe passive spirals are more common in clusters (Goto et al. 2003; also Poggianti et al. 1999; Balogh et al. 2002; McIntosh et al. 2002)

Goto et al. (2003)

Passive spirals in SDSS

Nature vs. Nurture

• Nature: 1. Dense regions just form a little earlier?

• would expect to see lower SFR among active population in high-z clusters: not observed

2. Early-type population formed at high redshift? • would have to be a substantial fraction of

today’s cluster population: so why does the fraction of SF galaxies evolve? (or does it?)

Most likely scenario (for bright galaxies)?

• Probably several effects: brightest ellipticals likely result of initial conditions

• Galaxy-galaxy interactions:– more common in dense regions– change SFR on short timescale– effective in small groups– evolve strongly with redshift– only environment known to effectively

transform SFR of a galaxy (e.g. Lambas et al. 2002)

Future Work

Groups at z~0.5 (Dave Wilman, R. Bower, J. Mulchaey, A. Oemler, R. Carlberg et al.)

Groups at z~0.5

• Follow-up observations with Magellan to gain higher completeness and depth

• HST data for all groups

• Also infrared data from WHT

• Based on the CNOC2 redshift survey. Group selection and inital look at properties described in Carlberg et al. (2001)

The Future: Groups at z~0.5

• Deep Magellan spectroscopy and HST imaging of ~30 groups at z~0.5

• trace SFR with [OII]

Wilman et al. in prep

0

5

10

1

5

20

2

5

30

Mea

n E

W [

OII

] (Å

)

0 0.3 0.6 0.9 1.2 1.5

Distance from centre (Mpc)

Groups at z~0.5

• Preliminary results suggest SFR distribution in z~0.5 groups is different from that in clusters: enhanced SFR due to interactions?

Conclusions

• Distribution of star formation rates is bimodal, not continuous (unlike morphology?)

• SFR distribution among active population is independent of environment

• Fraction of SF galaxies depends on local and large-scale densities (?)

• Galaxy-galaxy interactions are the most likely cause of observed segregation

Projection Effects?

• Is star-forming population all projected??

• No: at high density, contrast is high, and area is small– at low density,

trend is weak, so signal not diluted by projection

Balogh et al. (2003)

projected population 10 times more dense than field

projected population at field density

Tru

e/o

bse

rved

em

issi

on

lin

e f

ract

ion

Theory: SF in clusters and field

• Age effect only?

Sta

r fo

rmati

on

ra

te

Time

Theory: SF in clusters and field

• Age effect only?– Then SFR in

clusters should be lower at any epoch

Sta

r fo

rmati

on

ra

te

Time

cluster

Theory: SF in clusters and field

• Strangulation?– SF decays more

quickly in clusters, so should still be lower

Sta

r fo

rmati

on

ra

te

Time

cluster

Theory: SF in clusters and field

• Truncation?– Then star-

forming galaxies should all look the same

Sta

r fo

rmati

on

ra

te

Time

cluster

Abell 2390 (z~0.23)3.6 arcmin R image from

CNOC survey(Yee et al. 1996)

H in Abell 23903.6 arcmin

Balogh & Morris 2000

The Future: Environment at z~1.5

• Proposed VLT (FORS2) observations of radio-loud quasar/galaxy environments at z=1.5

• Use narrow-band filters+grism to obtain ~100 [OII] emitters per 7’ field (even in absence of a cluster)

z=1.44