Galaxies in low density environments
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Transcript of Galaxies in low density environments
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