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The Environments of E+A galaxies in the local universe (further clues from the 2dFGRS) Environments of Galaxies Meeting: Chania, Crete, Aug 2004 Warrick.

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Presentation on theme: "The Environments of E+A galaxies in the local universe (further clues from the 2dFGRS) Environments of Galaxies Meeting: Chania, Crete, Aug 2004 Warrick."— Presentation transcript:

1 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)

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 They showed that this spectral signature could be reproduced by combining an: E galaxyA star“E+A” += Have also become known as: “k+a”, “a+k”, “red-HDS”, “PSG” galaxies

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 What environments do E+A’s inhabit at low-z?

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

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 ~10 8 Mpc 3 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: 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

12 Our weighting scheme for determining : 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  )= EW(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) = (n R /n G )[N EG (s)/N ER (s)] – 1 [n R = number of randomly distributed points having the same selection function as 2dFGRS galaxies]

21 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

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, D t, and the radial separation, D r, between each E+A galaxy and these clusters was measured, with the E+A being tagged a ‘cluster’ object if: D t

23 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’

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 Do E+A’s reside in overdense or under- dense regions? Compared observed counts in spheres centred on each E+A galaxy relative to those predicted from 2dFGRS LF  local over/under-density Repeated this procedure for galaxies drawn at random from 2dFGRS catalogue At all scales, the mean overdensity where E+As are located is, statistically no different to that of 2dFGRS galaxies And would seem to differ from 2dFGRS E-gals, despite morphology!!

27 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’ = b J corresponds to M > M* + 1 at z E+A ‘bright’ = b J corresponds to M < M* + 1 at z E+A [photometry taken from Supercosmos Sky Survey] bJbJ bJbJ bJbJ bJbJ

28 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)

29 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

30 Luminosity function of E+A’s: All 2dFGRS gals 2dFGRS ‘ellipticals’ However, struggling with stats for “avg- Balmer” E+A sample: tried dropping 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’!

31 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).

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

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