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)

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?

The Local Universe:How 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

MNRAS, submitted

EW[Ha] 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

Correlation with density

The fraction of star-forming galaxies varies strongly with density

Correlation at all densities

2dFGRS

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)

Comparison with models

GALFORM model Observations

Cole et al. (2000)

Slow decay models (strangulation) do not work

How do these trends evolve with Redshift?

Wilman et al. study the CNOC2 groups at z=0.5 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)

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...

Groups at Z=0.5

Durham: follow-up observations with Magellan to gain higher completeness and depth

Aim of comparing star formation rates in groups at z~0.5 and locally

Also infrared data from WHT; HST ACS imaging on the way!

Based on the CNOC2 redshift survey aimed at Z~0.5. Group selection and inital look at properties described in Carlberg et al. (2001)

Redshift evolution?Comparison of 2dFGRS with CNOC2 groups/field[OII] distributions for rest BJ-limited samples

Average [OII] for SF galaxies does not appear to depend on redshift or environment

Fraction of [OII] emitters depends on both redshift and environment

D. Wilman et al.

Properties of The Star forming population

The Analysis Reveals:

The fraction of non-star forming galaxies (EW[OII] < 5Å), shows: A significant enhancement in groups with respect to the

field, where z < 0.55. A strong evolutionary trend, decreasing with redshift, both

in groups and in the field. The distribution of EW[OII] for star forming galaxies shows:

only a weak dependence upon environment.

A small increase in the number of galaxies with EW[OII] > 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 < 0.55.

CNOC1 clusters : Fraction with EW[OII] > 5Å

Nakata et al., in prep.

Clusters

Field

Implications:

Environment is important in driving the decline at z=0 and z= 0.5, groups have fewer star forming

galaxies than field

but galaxies in z=0.5 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

z = 1 -2 The epoch of Galaxy Formation?

We can only guess at how the trends at z=0-0.5 extrapolate to z>1

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 z=0-0.5? is star formation rate in isolated galaxies comparable to local

isolated galaxies? What are the mechanisms involved - star bursts and

interactions?

Its feasible!

target Halpha at z< 1.8 or OII z<3.8 Need at least K=19 (M*+1): 1 hr exposure gives

s/n=3-5 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????

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?