FMOS Workshop, Jan. 2004

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FMOS Workshop, Jan. 2004
The Decline in Cosmic Star Formation is
Environment to blame? or Mapping the
interaction of galaxies with their environment as
a function of cosmic time.
Richard Bower, Univ Durham (special thanks to
Mike Balogh and Dave Wilman)
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The Idea

• Galaxies in high density environments have low
  star formation rates
• can this explain the global decline in star
  formation?
• Compare galaxies in a range of environments at
  different redshifts
• do galaxies in the same environment have similar
  star formation rates at all redshifts?
• do galaxies in all environments show a similar
  trends in star formation rate with redshift?

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The Local UniverseHow do galaxy properties
depend on environment?

• Balogh et al 2003 a look at the star formation
  rates of galaxies in the "field", groups and
  clusters in the 2df and Sloan surveys
• group catalogues by Eke et al. Miller, Nichol et
  al.
• star formation rates from H alpha
• elabortates Lewis et al 2001 - which looks at
  clusters

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MNRAS, submitted
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EWHa distribution

• Ha distribution is distinctly bimodal SFR is not
  continuous
• also seen in colours (Baldry et al. 2003
  Strateva et al. 2001)
• galaxies do not have arbitrarily low SFR
• So mean/median do not necessarily trace a change
  in SFR

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Correlation with density

• The fraction of star-forming galaxies varies
  strongly with density
• Correlation at all densities

2dFGRS
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The star-forming population

• Amongst the star-forming population, there is no
  trend in mean SFR with density!
• Hard to explain with simple, slow-decay models
  (e.g. Balogh et al. 2000)

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Comparison with models
GALFORM model Observations
Cole et al. (2000)
Slow decay models (strangulation) do not work
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How do these trends evolve with Redshift?
First studied using radio-galaxy selected groups
by Allington-Smith et al. 1994
An issue first raised by Butcher-Oelmer 1974... a
long history in clusters...

• Wilman et al. study the CNOC2 groups at z0.5
• CNOC2 spectra supplemented by deeper spectroscopy
  with LDSS2/Magellan

Credits Dave Wilman, Mike Balogh, Richard Bower,
Richard Whitaker, Simon Morris (Durham) John
Mulchaey, Gus Oemler (Carnegie) Ray Carlberg
(Toronto) Ian Lewis (Oxford)
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Redshift evolution?
Comparison of 2dFGRS with CNOC2
groups/field OII distributions for rest
BJ-limited samples
Fraction of OII emitters depends on both
redshift and environment
D. Wilman et al.
Average OII for SF galaxies does not appear to
depend on redshift or environment
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Properties of The Star forming population
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The Analysis Reveals

• The fraction of non-star forming galaxies
  (EWOII lt 5Å), shows
• A significant enhancement in groups with respect
  to the field, where z lt 0.55.
• A strong evolutionary trend, decreasing with
  redshift, both in groups and in the field.
• The distribution of EWOII for star forming
  galaxies shows
• only a weak dependence upon environment.
• A small increase in the number of galaxies with
  EWOII gt 30Å at higher redshift (CNOC2), both in
  groups and the field.
• Brighter galaxies are less likely to be star
  forming in groups and the field than fainter
  galaxies, up to at least z lt 0.55.

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CNOC1 clusters Fraction with EWOII gt 5Å
Field
Clusters
Nakata et al., in prep.
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Implications

• Environment is important in driving the decline
• at z0 and z 0.5, groups have fewer star forming
  galaxies than field
• but galaxies in z0.5 have more star formation
  than their local counter-parts.
• is this just because the galaxies have existed in
  the group environment for longer
• surprisingly little difference in the properties
  of the star forming galaxies

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z 1 -2 The epoch of Galaxy Formation?

• We can only guess at how the trends at z0-0.5
  extrapolate to zgt1
• FMOS will allow us to measure the interaction of
  galaxies with their environment at this crucial
  epoch
• Some example questions
• is the level of star formation supressed in
  cluster? and groups? just like z0-0.5?
• is star formation rate in isolated galaxies
  comparable to local isolated galaxies?
• What are the mechanisms involved - star bursts
  and interactions?

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Its feasible!

• target Halpha at zlt 1.8 or OII zlt3.8
• Need at least K19 (M1) 1 hr exposure gives
  s/n3-5 in continuum.
• emission line galaxies shoul be easy
• passive galaxies are possible, but can use
  "spectro-photometric"redshift to assign z from
  spectral shape
• At this depth 4 gals per sq.arcmin in the field.

Too Optimistic????
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Strategies and Issues

• target regions arround known high-z clusters (or
  QSOs) cf., Nakata et al, 2004 (Lynx field)
• competition from narrowband methods?
• Extract groups/clusters from a blank field
  redshift survey cf., Eke et al 2003 (2df survey)
• redshifts vetoed by skylines?
• surface density of galaxies and sampling ratio.
• close packing of fibres in dense regions?
• Competition for DEEP2/VIMOS/GDDS?