The Environments of E+A galaxies in the local universe ( further clues from the 2dFGRS)

1. 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 BekkiUNSW (+ the 2dfGRS team)

2. 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

3. 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”, “a+k”, “red-HDS”, “PSG” galaxies They showed that this spectral signature could be reproduced by combining an: + = E galaxy A star “E+A”

4. 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

5. 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?

6. Zabludoff et al. (1996): first major search for E+A’s over all environments at low-z Early Surveys Las Campanas Redshift Survey

7. 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)

8. 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”

9. 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!!!!

10. 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)

11. 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: • Adopt Z96 criteria:EW[OII]>-2.5Å, EW(H)>5.5Å •  “AVERAGE BALMER”[56 gals] • Use only the H line: EW(H)>5.5Å, EW[OII]>-2.5Å • ”H” sample[243 gals] Use a weighted average

12. 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’!!

13. Spectra of typical galaxies in “avg-Balmer” sample: Notable for H generally being present only in ABSORPTION! Highest fidelity E+A sample?

14. 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!?

15. 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

16. 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

17. “H” E+A sample: Dominated by disk systems, with yet again some showing signs of recent merger/interaction

18. Morphologies of E+As – quantitative statistics: Spheroid-dominated, with up to 30% showing signs of merger/interaction Includes an additional population of late-type disk galaxies

19. The environments of E+A galaxies (bench-marked against the entire 2dFGRS galaxy population)

20. 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]

21. 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

22. 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!!

23. 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’

24. 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:

25. 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!!

26. 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

27. 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

28. Luminosity function of E+As: bJ-band LFs constructed for our E+A samples using SSS photometry and SWML method (Efstathiou et al. 1988) All 2dFGRS gals 2dFGRS ‘ellipticals’ In an identical way, constructed LFs for: all 2dFGRS galaxies gals with ‘elliptical’ spectra Both E+A samples consistent with overall 2dFGRS LF

29. Luminosity function of E+A’s: However, struggling with stats for “avg-Balmer” 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’!

30. 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).

31. Spatially resolved spectroscopy of distant cluster E+As with GMOS/IFU on Gemini OII GMOS HST H R=18.58Sbc R=19.68Sc Courtesy: Mike Pracy