Download W. Couch "Environment of E+A galaxies"

Environments of Galaxies Meeting: Chania, Crete, Aug 2004
The Environments of E+A galaxies
in the local universe
(further clues from the 2dFGRS)
Warrick Couch, Chris Blake,
Mike Pracy, Kenji Bekki
UNSW
(+ the 2dfGRS team)
Talk Outline
•What is an “E+A” galaxy?
•What is known about E+A’s in the local
universe?
•Identification of E+A’s within the 2dFGRS –
selecting a high-fidelity sample
•Properties of our E+A sample(s): clustering,
ENVIRONMENTS, luminosity function
What is an E+A galaxy?
In a spectroscopic survey of galaxies in the
z=0.46 3C295 cluster, Dressler & Gunn
(1983) discovered a number of members with
conspicuous Balmer absorption lines and no
emission lines
Have also become known as: “k+a”,
They showed that this spectral signature could be reproduced
“a+k”, “red-HDS”, “PSG” galaxies
by combining an:
+
E galaxy
=
A star
“E+A”
Interpretation of E+A spectral signature:
Strong Balmer absorption
and blue colors  galaxy
underwent STARBURST
which was halted less
than 1Gyr ago
Objects with weaker
Balmer absorption and
redder colors could also
arise from TRUNCATION
of SF in normal starforming (Sp) galaxies
Couch & Sharples (1987)
But are ‘E+A’ galaxies solely tracers of
cluster galaxy evolution?
Poggianti et al. (1999)
z~0.4
z~0.4
What environments do E+A’s inhabit at low-z?
Zabludoff
et al.
(1996): first major search
Early
Surveys
for E+A’s over all environments at low-z
Las Campanas Redshift Survey
Karl Glazebrook
Key features of Zalbudoff et al. study:
•11,113 galaxy spectra
from the LCRS
analysed
•Identification of
‘E+A’ signature
based on EW
measurements of
[OII]3727 and H,
H, H Balmer lines:
 EW[OII]> 2.5A
 EW(H) > 5.5A
A sample of
21 E+As
identified
(0.2% of
popln)
Main results of Zabludoff et al. study:
•~ 75% of E+As were found to lie in the field, well outside
clusters and rich groups  location within the cluster
environment not a necessary condition for E+A formation!
•5/21 E+A galaxies showed tidal features indicative of
galaxy-galaxy mergers and interactions
“If one mechanism is responsible for E+A formation,
then…” the above two observations “argue that
galaxy-galaxy interactions and mergers are that
mechanism”
Important follow-up to Zabludoff et al. study:
•Norton et al. (2001) – undertook spatially resolved (longslit) spectroscopy of the Z96 E+A sample to measure the
kinematics of the young and old stellar populations:
Concluded galaxies are undergoing a transformation
from gas-rich, star-forming, rotationally supported
disk-dominated galaxies, into gas-poor, quiescent,
pressure-supported, spheroid-dominated galaxies.
•Yang et al. (2004) – obtained HST/WFPC2 high resoln
imaging of the 5 bluest E+A’s in the Z96 sample:
First talk after morning coffee on Friday!!!!
The 2dF Galaxy Redshift Survey
221,000 galaxies sampled over a
~108 Mpc3 volume of the local
universe
a bigger, environmentally –
unbiased sample of E+As, suitable
for statistical studies
(clustering, environment, LF)
Design features of our E+A study:
• Use the 2dFGRS spectral line catalogue (compiled by Ian
Lewis) as our source of spectral line EW measurements.
• Consider only those galaxies with the highest quality
(Q3) spectra and z>0.002
[161,437 gals]
• Select out galaxies with robust [OII]3727 and H, H,
H EW measurements (based on S/N and g.o.f.)
• Identify E+A galaxies in two different ways:
1. Adopt Z96 criteria: EW[OII]>-2.5Å, EW(H)>5.5Å
 “AVERAGE BALMER” [56 gals]
2. Use only the H line: EW(H)>5.5Å, EW[OII]>-2.5Å
”H” sample
[243 gals]
Use a weighted average
Our weighting scheme for determining
<EW(H)>:
EW(H) vs EW(H) for 2dFGRS galaxies
Used the empirically
determined correlations
between EW(H), EW(H),
and EW(H) to convert
our H and H values into
‘effective’ H values, and
then average.
EW(H)=0.50+1.03EW(H)
Caveat: our lowest-z galaxies will suffer from ‘aperture effect’!!
Spectra of typical galaxies in “avg-Balmer”
sample:
Notable for
H generally
being present
only in
ABSORPTION!
Highest fidelity
E+A sample?
Spectra of typical galaxies in “H” sample:
•Generally of
lower S/N
•H emission
present in 60%
of galaxies;
SFR(H)obs 1
[Myr-1]
Population of
dust-obscured
star-forming
galaxies!?
Distribution of our E+A samples within the
EW(H) – color plane:
Broad range
of colors and
hence times
seen after
cessation of
SF; but NO
“red-HDS”
Color of Quiescent
E/S0 galaxy
Morphologies of E+A galaxies:
Objects inspected and (where resolved) morphologically
classified using Supercosmos Sky Survey B, R and I
images.
Galaxies from our
“Average-Balmer”
sample
Galaxies largely spheroid-dominated, with a small number
showing tidal features/disturbed morphology indicative of
recent merger/interaction
“H” E+A sample:
Dominated by disk systems, with yet again some
showing signs of recent merger/interaction
Morphologies of E+As – quantitative
"Average-Balmer"
statistics:
percentage(%)
Spheroid-dominated,
with up to 30%
showing signs of
merger/interaction
50
40
30
20
10
0
E
E/S0
S0
Sa-b
Sc-d
Irr Merg/Int
Sc-d
Irr Merg/Int
"H-delta"
percentage (%)
Includes an
additional population
of late-type disk
galaxies
50
40
30
20
10
0
E
E/S0
S0
Sa-b
The environments of E+A
galaxies
(bench-marked against the entire
2dFGRS galaxy population)
The clustering of E+A’s: spatial correlation
function
Approach: determine the spatial crosscorrelation function, EG, between the E+A
galaxy samples and the rest of the 2dFGRS
catalogue, using cross-pair counts based
estimator:
EG(s) = (nR/nG)[NEG(s)/NER(s)] – 1
[nR = number of randomly distributed points having the
same selection function as 2dFGRS galaxies]
The clustering of E+A’s: spatial correlation
function
Marginal
evidence for
our “Avg
Balmer” E+A’s
being LESS
clustered than
2dFGRS
ensemble
Error bars estimated using ‘jack-knife’ re-sampling
E+A’s residing within or in close proximity
to rich clusters:
•All the known rich clusters of galaxies (from the Abell,
APM, Edinburgh-Durham Catalogues) within the 2dFGRS
survey regions have been identified (and further studied)
by De Propris et al. (2002).
•The transverse separation, Dt, and the radial separation,
Dr, between each E+A galaxy and these clusters was
measured, with the E+A being tagged a ‘cluster’ object if:
Dt<r0 and Dr<[r02+(2/H)2]1/2, where r0=5Mpc, and  is
the cluster velocity dispersion.
Fraction of E+A’s (“avg-Balmer”) identified as ‘cluster’
objects = 11%  most E+A’s reside outside clusters!!
E+A’s in groups?
2dFGRS Group Catalogue
of Eke et al. (2004a)
constructed using a
‘friends-of-friends’
percolation algorithm
~30,000 groups
containing at least 2
members!
Determine whether E+A’s belong to a group (if so,
any preferential type?) or are ‘isolated’
E+A’s in groups?
•Found ~50% of E+A galaxies to be ‘isolated’.
•For the other ~50% residing in groups, the distribution
in group size (as measured by the number of group
members) was statistically no different to randomly
drawn 2dFGRS galaxies.
But group membership a poor indicator of group size,
since visibility of members dependent on redshift;
hence used Eke et al’s (2004b) corrected total group
luminosity as proxy for mass/size:
Groups hosting E+A’s: how luminous?
E+A’s appear to
inhabit groups with
a broad range in
total luminosity,
and with a
distribution no
different to that
of ordinary
2dFGRS galaxies
But do differ to galaxies with passive ‘elliptical’-type spectra!!
The ‘local’ environment of E+A’s:
Explored in 3 different ways:
• Transverse physical separation (in kpc) to the nearest
faint neighbour
• Transverse physical separation (in kpc) to the nearest
bright neighbour
• Local physical surface density defined by the 5
nearest bright neighbours
Definitions:
‘faint’ = bJ corresponds to M b > M*
b + 1 at zE+A
J
J
‘bright’ = bJ corresponds to Mb < M*
+ 1 at zE+A
b
J
J
[photometry taken from Supercosmos Sky Survey]
Distribution in E+A ‘local’ environments:
faint
A K-S test shows that
there is NO statistical
evidence that the
distributions of E+A
galaxy local
environments (solid
histograms) are any
different from
2dFGRS galaxies as a
whole (dashed lines)
bright
Local density
Luminosity function of E+As:
bJ-band LFs
constructed for
our E+A samples
using SSS
photometry and
SWML method
(Efstathiou et
al. 1988)
In an identical way,
constructed LFs for:
 all 2dFGRS galaxies
 gals with ‘elliptical’
spectra
All 2dFGRS gals
2dFGRS ‘ellipticals’
Both E+A samples consistent
with overall 2dFGRS LF
Luminosity function of E+A’s:
However,
struggling with
stats for “avgBalmer” E+A
sample: tried
dropping
<EW(H)>
threshold from
5.5Å to 4.5Å
All 2dFGRS gals
2dFGRS ‘ellipticals’
‘Average-Balmer’ E+A LF
significantly different to that of
the full 2dFGRS sample; more
consistent with 2dFGRS ‘ellipticals’!
Summary:
•Selection: ensuring Balmer line absorption is consistently
strong across H, H and H essential in identifying bona
fide non-star-forming E+A galaxies. Selection based on H
alone leads to inclusion of dusty star-forming galaxies!
•Morphology: E+A’s in the local universe mainly early-type
(E/S0, early-Sp), with ~30% showing signs of recent
mergers/interactions.
•Environment: E+A’s could NOT be distinguished in any way
from the average 2dFGRS galaxy population in terms of
their global and local environments.
•Luminosity Function: has the flatter slope seen for
2dFGRS ‘ellipticals’, consistent with early-type morphology.
•Trigger mechanism: further direct support for merg/int’s
(via morphologies); also 2dFGRS galaxies most likely to be
E+A progenitors are CLOSE PAIRS (Balogh et al. 2003).
Spatially resolved spectroscopy of distant
cluster E+As with GMOS/IFU on Gemini
HST
GMOS
OII
H
R=18.58
Sbc
R=19.68
Sc
Courtesy: Mike Pracy