The Environments of E+A galaxies in the local universe

(further clues from the 2dFGRS)

Environments of Galaxies Meeting: Chania, Crete, Aug 2004

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

They showed that this spectral signature could be reproduced by combining an:

E galaxy A star “E+A”

+ =

Have also become known as: “k+a”, “a+k”, “red-HDS”, “PSG” galaxies

Interpretation of E+A spectral signature:

Couch & Sharples (1987)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 star-forming (Sp) galaxies

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?

Karl Glazebrook

Early SurveysEarly SurveysEarly SurveysEarly Surveys

Las Campanas Redshift Survey

Zabludoff et al. (1996): first major search for E+A’s over all environments at low-z

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 (long-slit) 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 (Q3) 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)>:

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) vs EW(H) for 2dFGRS galaxies

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 [Myr-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 statistics: "Average-Balmer"

0

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E E/S0 S0 Sa-b Sc-d Irr Merg/Int

pe

rce

nta

ge

(%)

"H-delta"

0

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E E/S0 S0 Sa-b Sc-d Irr Merg/Int

perc

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%)

Spheroid-dominated, with up to 30% showing signs of merger/interaction

Includes an additional population of late-type disk galaxies

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 cross-correlation 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

Error bars estimated using ‘jack-knife’ re-sampling

Marginal evidence for our “Avg Balmer” E+A’s being LESS clustered than 2dFGRS ensemble

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<[r0

2+(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!!

2dFGRS Group Catalogue of Eke et

al. (2004a)

constructed using a ‘friends-of-friends’

percolation algorithm

~30,000 groups containing at least 2

members!

E+A’s in groups?

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 > M* + 1 at zE+A

‘bright’ = bJ corresponds to M < M* + 1 at zE+A

[photometry taken from Supercosmos Sky Survey]

bJ bJ

bJ bJ

Distribution in E+A ‘local’ environments:

faint

bright

Local density

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)

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:

All 2dFGRS gals

2dFGRS ‘ellipticals’

However, struggling with stats for “avg-Balmer” E+A sample: tried dropping <EW(H)> threshold from 5.5Å to 4.5Å

‘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).

R=18.58Sbc

R=19.68Sc

OII HGMOSHST

Spatially resolved spectroscopy of distant cluster E+As with GMOS/IFU on Gemini

Courtesy: Mike Pracy