Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.eso.org/~hkuntsch/papers/MNRAS_396_1349.pdf Äàòà èçìåíåíèÿ: Fri Mar 5 12:34:55 2010 Äàòà èíäåêñèðîâàíèÿ: Tue Oct 2 08:07:47 2012 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: ï ï ï ï ï ï ï ï

Mon. Not. R. Astron. Soc. 396, 1349­1369 (2009)

doi:10.1111/j.1365-2966.2009.14836.x

The kinematics and spatial distribution of stellar populations in E+A galaxies
Michael B. Pracy,1 Harald Kuntschner,2 Warrick J. Couch,3 Chris Blake, Kenji Bekki4 and Frank Briggs1
1 2 3 4

3

Research School of Astronomy & Astrophysics, The Australian National University, Weston Creek, ACT 2611, Australia Space Telescope European Coordinating Facility, European Southern Observatory, Karl-Schwarzschild Strasse 2, 85748, Garching, Germany Centre for Astrophysics and Supercomputing, Swinburne University of Technology, PO Box 218, Hawthorn, VIC, 3122, Australia School of Physics, University of New South Wales, Sydney NSW 2052, Australia

Accepted 2009 March 26. Received 2009 March 26; in original form 2008 November 18

ABSTRACT

We have used the Gemini Multi-Object Spectragraph (GMOS) instrument on the 8.1-m Gemini-South Telescope to obtain spatially resolved two-colour imaging and integral field unit (IFU) spectroscopy of a sample of 10 nearby (z = 0.04­0.20) `E+A' galaxies selected from the Two Degree Field Galaxy Redshift Survey. These galaxies have been selected to lie in a variety of environments from isolated systems to rich clusters. Surface brightness profiles measured using our imaging data show the isophotal profiles of our sample are generally r 1/4 -like, consistent with a sample dominated by early-type galaxies. Only one galaxy in our sample has an obvious exponential (`disc-like') component in the isophotal profile. This is further underscored by all galaxies having early Hubble-type morphological classifications, and showing a behaviour in the central velocity dispersion-absolute magnitude plane that is consistent with the Faber­Jackson relation, once the transitory brightening that occurs in the E+A phase is corrected for. In addition, two-thirds of our sample shows clear evidence of either ongoing or recent tidal interactions/mergers, as evidenced by the presence of tidal tails and disturbed morphologies. While all the galaxies in our sample have total integrated colours that are relatively blue (in keeping with their E+A status), they show a diversity of colour gradients, possessing central core regions that are either redder, bluer or indistinct in colour relative to their outer regions. The E+A spectra are well fitted by that of a young stellar population, the light from which is so dominant that it is impossible to quantify the presence of the underlying old stellar population. Consistent with other recent findings, there is little evidence for radial gradients in the Balmer absorption line equivalent widths over the central few kiloparsecs (<4 kpc), although we are unable to search for the previously reported radial gradients at larger galactocentric radii due to the limited spatial extent of our IFU data. Kinematically, the most striking property is the significant and unambiguous rotation that is seen in all our E+A galaxies, with it being generally aligned close to the photometric major axis. This is contrary to the findings of Norton et al., who found little or no evidence for rotation in a very similar sample of nearby E+A galaxies. We also clearly demonstrate that our E+A galaxies are, in all but one case, consistent with being `fast rotators', based on their internal angular momentum per unit mass measured as a function of radius and ellipticity. We argue that the combination of disturbed morphologies and significant rotation in these galaxies supports their production via gas-rich galaxy mergers and interactions. The large fraction of fast rotators argues against equal mass mergers being the dominant progenitor to the E+A population. Key words: galaxies: evolution ­ galaxies: formation ­ galaxies: stellar content.

1 I NTR O DUCTION
E-mail: mpracy@mso.anu.edu.au
C

Galaxy mergers and interactions are a fundamental driving force of galaxy evolution. `E+A' galaxies are a crucial part of this
C

2009 The Authors. Journal compilation

2009 RAS


1350

M. B. Pracy et al.
2001), Blake et al. (2004) concluded that their sample was consistent with a major merger origin based on their morphologies, incidence of tidal disruption and luminosity function. In addition, Goto's (2005) study of a sample of low-redshift E+A galaxies selected from the Sloan Digital Sky Survey (SDSS; Abazajian et al. 2004) revealed an excess in the projected galaxy density on small scales surrounding the E+A galaxies providing a strong hint of galaxy­galaxy interactions as a formation mechanism. A more powerful and direct method of inferring the physical mechanism(s) responsible for the E+A galaxy phase is to study the internal properties of individual galaxies. The kinematics and spatial distribution of the stellar populations of a galaxy in the E+A phase hold a wealth of information about the history of its formation and are critical information for discerning which of the candidate mechanisms is responsible. Mergers and tidal interaction are expected to give rise to star formation which is centrally concentrated as gas is funnelled to the Galactic Centre (Noguchi 1988; Barnes & Hernquist 1991; Mihos, Richstone & Bothun 1992; Mihos & Hernquist 1996; Bekki et al. 2005; Hopkins et al. 2009). For an equal mass merger (e.g. two massive spirals), the remnant should settle quickly to the virial plane and be dynamically pressure supported (NGZZ; Bekki et al. 2005). Such mergers can also produce rotating remnants a small fraction of the time, but this requires specific merger configurations (Bekki et al. 2005; Bournaud et al. 2008). In the case of unequal mass mergers and tidal interactions, rotation of the young stellar populations is generally expected (Bekki et al. 2005). In contrast, if star formation is simply truncated abruptly in a `normal' star-forming disc, the young stellar population should be spread throughout the extent of the galaxy and rotation should be present in both the young and old stellar populations (NGZZ; Bekki et al. 2005; Pracy et al. 2005). Spatially resolved spectroscopic studies of E+A galaxies residing in clusters have revealed a young stellar population which is widely spread throughout the galaxy and not confined to the galaxy core; observations have also shown evidence for strong rotation in these galaxies (Franx 1993; Caldwell et al. 1996). Pracy et al. (2005) found diversity in the spatial distribution of the young population in the E+A galaxy population in the intermediate redshift cluster AC114; some E+As have a centrally concentrated young stellar population consistent with a tidal or merger origin and others revealed a more distributed young star component consistent with the truncation of a spiral disc. The small number of examples of spatially resolved spectroscopy of E+A galaxies in the nearby field has generally revealed a central concentration of the young stellar population (NGZZ; Goto et al. 2008) although there exist exceptions (Swinbank et al. 2005). NGZZ used long slit spectroscopy to study the internal kinematics of field E+A galaxies over the central 2­4 kpc. They found little difference in the kinematics of the young and old stellar populations and that in all but a small fraction of cases the E+As showed little or no rotation. They interpret their results as being consistent with E+As being in the midst of a transformation from gas-rich, rotationally supported discs into gas-poor, pressure-supported early-type galaxies. Yamauchi & Goto's (2005) investigation of the 2D colour distributions of E+A galaxies found an excess of galaxies with blue cores (relative to normal earlytype galaxies) and they interpreted their results as being consistent with the galaxies having undergone a centralized starburst caused by a merger or interaction. Yang et al. (2008), using HST imaging found diversity in the internal colour distributions of the LCRS sample, but also found a tendency towards bluer centres which they interpreted as evidence for merger/interaction-induced star formation.
C

picture. They are conspicuous by their unusual spectra (Dressler & Gunn 1983): strong hydrogen Balmer absorption lines (implying a relatively young `A'-type stellar population) superimposed upon an elliptical (`E') galaxy spectrum with no optical emission lines (implying no ongoing star formation). The A-stars must have been produced by a powerful recent episode of star formation, which has been suddenly truncated in the past 1 Gyr (Couch & Sharples 1987; Poggianti et al. 1999). The spectroscopic data are also consistent with the uniform truncation of star formation in a disc, without necessarily requiring a starburst (e.g. Shioya, Bekki & Couch 2004). Observational evidence indicates that the E+A phenomenon may, in part, mark the transitory phase between star-forming disc galaxies and quiescent spheroidal systems (e.g. Caldwell et al. 1996; Zabludoff et al. 1996; Norton et al. 2001, hereafter NGZZ). Understanding the origin of the E+A phase is complicated by the fact that E+A galaxies inhabit a wide range of environments, in a manner that depends upon both redshift and luminosity. In summary, luminous E+A galaxies are commonplace in intermediate redshift clusters, where they were first identified and studied (Dressler & Gunn 1983). In the low-redshift Universe cluster E+As are still frequent, but their luminosities are much lower than their higher-z counterparts (Poggianti et al. 2004). Most luminous E+A galaxies at low redshift are located in the field, although they represent a very low fraction of the overall field galaxy population (Zabludoff et al. 1996). The physical mechanism(s) responsible in triggering intense star formation and subsequently rapidly quenching it remains the key question to better understand the E+A galaxy phase and the role it plays in galaxy evolution. There have been several mechanisms suggested which could give rise to the E+A spectral signature. These include major mergers (Mihos & Hernquist 1996; Bekki et al. 2005), unequal mass mergers (Bekki, Shioya & Couch 2001) and galaxy interactions (Bekki et al. 2005). For E+As galaxies residing in clusters, other plausible mechanisms exist, such as interaction with the strongly varying global cluster tidal field (Bekki 1999), galaxy harassment (Moore, Lake & Katz 1998) or interaction with the hot intra-cluster gas (Gunn & Gott 1972; Dressler & Gunn 1983; Bothun & Dressler 1986). The continuous accretion of cold gas on to galaxies is predicted in galaxy formation models (e.g. Birnboim & Dekel 2003; Keres et al. 2005; Semelin & Combes 2005; Dekel & Birnboim 2006) and could also be a driver of galaxy evolution. Such a mechanism, however, should not induce a starburst, but rather contribute to slow and continuous levels of star formation. Given the large and rapid changes needed to produce the spectral properties of E+A galaxies, and the evidence that a high fraction of these systems are morphologically disturbed, accretion of cold gas from largescale structure is unlikely to be an important mechanism in E+A formation. There is evidence that the E+As in the low-redshift field are the result of merging or tidal interactions between galaxies. Groundbased imaging of a sample of 21 E+A galaxies drawn from the Las Campanas Redshift Survey (LCRS) revealed an increased incidence of tidal features associated with these galaxies implying galaxy interactions or galaxy mergers had taken place (Zabludoff et al. 1996). Their conclusions were later confirmed using highresolution Hubble Space Telescope (HST ) imaging of the LCRS sample (Yang et al. 2008), which found a high incidence of tidal features consistent with mergers or interactions. Samples of E+A galaxies constructed from larger redshift surveys reinforce these ideas. Using a robust sample of E+A galaxies selected from the Two Degree Field Galaxy Redshift Survey (2dFGRS; Colless et al.

2009 The Authors. Journal compilation

C

2009 RAS, MNRAS 396, 1349­1369


The kinematics of E+A galaxies
Table 1. List of target galaxies. Name E+A E+A E+A E+A E+A E+A E+A E+A E+A E+A 1 2 3 4 5 6 7 8 9 10 2dFGRS ID TGS439Z075 TGS271Z130 TGS519Z227 TGS520Z261 TGS480Z208 TGS358Z179 TGS387Z032 TGS539Z123 TGS266Z090 TGS350Z150 00 23 02 02 22 23 02 23 23 23 RA 29 41 33 40 18 59 15 15 10 26 10.97 08.90 10.60 24.27 22.99 29.87 40.52 25.13 46.57 36.76 - - - - - - - - - - Dec. 32 28 33 33 33 30 30 35 28 30 42 55 52 25 02 16 50 12 31 19 34.2 25.4 24.4 50.6 36.7 21.9 54.5 59.1 49.7 27.4 z 0.108 0.082 0.070 0.035 0.101 0.120 0.092 0.196 0.088 0.158 g 17.25 17.83 N/A 17.64 17.80 17.24 17.13 17.71 17.21 17.86 r 16.29 16.75 15.70 16.61 16.77 16.36 16.12 16.75 16.24 16.97 g-r 0.96 1.08 N/A 1.03 1.13 0.88 1.01 0.96 0.97 0.89 - - - - - - - - - - MR 22.34 21.32 21.96 19.38 21.66 21.45 22.17 23.27 21.99 22.52 Re (arsec) 1.44 1.65 1.63 2.57 1.70 1.39 1.60 1.52 1.64 1.23

1351

Environment Isolated Group Cluster Group Isolated Group Group Isolated Cluster Isolated

Notes. Listed are the basic parameters of our E+A sample. Column 1 is an arbitrary ID and column 2 is the 2dfGRS ID. The remainder of the columns are (from left to right): target right ascension and declination (J2000), redshift, g-band magnitude, r magnitude, g - r colour, absolute R magnitude inclusive of a k-correction (Wild et al. 2005), the effective radius measured from the imaging using the IRAF ELLIPSE task, and galaxy environmental classification from Blake et al. (2004).

In this paper, we present high-quality imaging and spatially resolved 2D spectroscopy of a sample of robustly selected E+A galaxies from the 2dFGRS (Blake et al. 2004). These observations consist of deep two-colour imaging with GMOS on the 8.1-m GeminiSouth Telescope to investigate internal colour gradients and to look for faint tidal tails or disturbances which could be symptomatic of a recent interaction or merger. We have also obtained integral field unit (IFU) observations with GMOS in order to study the internal kinematics and line-strength distribution. Throughout, we adopt an = 0.7 and H 0 = 70 km s-1 Mpc-1 cosmology. M = 0.3, 2 O BSER VATIONS AND D AT A REDUCTION 2.1 Sample We have selected 10 relatively bright (bJ 18.4) and nearby (z < 0.2) E+A galaxies from the 2dFGRS E+A catalogue compiled by Blake et al. (2004) for follow-up with high-resolution imaging and spatially resolved spectroscopic observations. The selection criteria used for the Blake et al. (2004) sample were based on that of Zabludoff et al. (1996), specifically a galaxy was required to have [OII] equivalent width of less than 2.5 å in emission and a mean Balmer absorption line strength (based on a weighted combination of the H , H and H lines) of greater than 5.5 å in absorption. These galaxies have already had their morphologies and external environments investigated by Blake et al. (2004) and we have selected our sample to span a wide range of environments. Specifically, four galaxies are `isolated', four are located in groups (i.e. linked to
Table 2. Summary of GMOS observations. Name E+A E+A E+A E+A E+A E+A E+A E+A E+A E+A 1 2 3 4 5 6 7 8 9 10 g exptime (s) 1262 1262 1262 2524 2103 1262 1262 1682 1262 1262 FWHM (arcsec) 0.88 0.92 0.57 1.14 1.16 0.98 0.95 0.95 1.02 1.05 r exptime (s) 1262 1262 1262 2524 1262 1262 1262 1262 1262 1262

other survey objects by a percolation algorithm; Eke et al. 2004) and two galaxies inhabit cluster environments. The details of our target galaxies are summarized in Table 1. 2.2 Imaging 2.2.1 Observations For each galaxy in our sample, we obtained g- and r-band imaging using GMOS on Gemini-South. The imaging was obtained in queue mode between 2005 September 1 and December 4. The imaging consisted of 3 â 420.5 s exposures for each object, in each band, giving a total integration time of 1261.5 s. Longer integrations were obtained for E+A 4andE+A 5. The details of the imaging observations are given in Table 2. The imaging was taken in seeing of 1 arcsec and never worse than 1.2 arcsec, and was sampled with 0.145 arcsec pixels (see Table 2). The final reduced g-band images of each target are shown in the first column of Fig. 1. 2.2.2 Data reduction The imaging was reduced in the standard manner using the IRAF GEMINI package task GIREDUCE to perform bias and flat-field corrections and remove the overscan region. The individual exposures were then combined using the task GEMCOMBINE. Since no standard star calibrations were obtained, photometric zero-points were calculated by matching stars in the field to the SuperCosmos catalogue. The SuperCosmos magnitudes were converted from bJ and rF to g

FWHM (arcsec) 0.76 0.79 1.15 1.08 1.23 1.02 0.96 0.83 0.99 1.00

Spec exptime (s) 4082 4082 4082 N/A 4082 4082 4082 N/A 4082 4082

Observed (å) 4122­5380 4122­5380 4122­5380 N/A 4122­5380 4122­5380 4122­5380 N/A 4122­5380 4122­5380

Rest (å) 3720­4855 3810­4972 3852­5028 N/A 3744­4886 3680­4803 3775­4926 N/A 3788­4945 3559­4646

Notes. Column 1 is galaxy ID. Columns 2 and 3 are the g-band exposure time and seeing. Columns 4 and 5 are the r-band exposure times and the corresponding seeing. Column 6, 7 and 8 are the spectroscopic exposure time, observed spectral wavelength coverage and rest-frame spectral wavelength coverage, respectively. 2009 The Authors. Journal compilation 2009 RAS, MNRAS 396, 1349­1369

C

C


1352

M. B. Pracy et al.

Figure 1. Top to bottom: E+A 1 to E+A 10; left column: g-band image; second column: residual image after subtraction of a model elliptical profile; third column: g - r colour image; right column: colour image of the central 3 arcsec. There is no colour image for E+A 3 because of saturation in the galaxy core in the g-band image as well as poor image quality in the r band (note: the spatial and colour scales vary between columns).
C

2009 The Authors. Journal compilation

C

2009 RAS, MNRAS 396, 1349­1369


The kinematics of E+A galaxies
and r using the filter conversions given in Cross et al. (2004) and references therein. For each GMOS observation, we had between four and 10 stars that were suitable for use in calibration (isolated within a suitable magnitude range). It is known that a small fraction of objects in the SuperCosmos catalogue have spurious magnitudes with errors of the order of 0.5 mag. We therefore excluded the few obvious outliers in calculating the zero-point values. The rms scatter in the derived zero-point for all images was 0.05 mag. This calibration is more than adequate for our purposes since the most important information is contained in the relative colour differences across individual objects rather than their absolute colours. The point spread function (PSF) for each image was measured from stars near to the target galaxy and modelled as an elliptical Gaussian (since the delivered images can have a slight asymmetry). We created colour images by carefully aligning the g- and r-band images and matching the PSFs by convolving each image with an elliptical Gaussian such that the final PSFs of both images were circular Gaussians with final image qualities slightly worse than the worst original image, i.e. final =
x ,y x,y

1353

original

x,y

2

+

2 x,y smooth

,

(1)

where final is chosen to be slightly larger than the sigma of the semimajor axis of the worst seeing image. These matched images were then divided by one another to produce a colour image. Colour images of each galaxy are shown in column 3 of Fig. 1, with a zoomin of the central 3 â 3arcsec2 region shown in the following column. Note that the colour scales vary between the two columns. We could not construct a colour map of E+A 3 because the g-band image is saturated. 2.3 Spectroscopy 2.3.1 Observations Our spectroscopic observation were obtained with the GeminiSouth GMOS spectrograph in IFU mode. The observations were conducted in queue mode between 2005 September 2 and December 6. We used the B600 grating in combination with the g-filter resulting in a spectral coverage of 4100­5380 å with a resolution of 1.9 å. We used the IFU in two-slit mode, which gives a rectangular field of view of 5 â 7 arcsec2 sampled by 1000 â 0.2 arcsec individual lenslets. For each galaxy, we obtained 4 â 1020.5 s dithered exposures, resulting in a total exposure time for each target of 4082 s. Only eight of our 10 targets were observed spectroscopically. A summary of the spectroscopic observations is given in Table 2.

2.3.2 Data reduction The spectroscopic data were reduced using standard IRAF routines. First, the flat-field spectra were overscan-subtracted and trimmed using the IRAF GEMINI package task GIREDUCE. The flat-field fibre spectra were traced, extracted and the individual CCDs mosaiced using the GFEXTRACT task. The task GFRESPONSE was used to calculate the relative fibre throughputs from the flat-field spectra. Arc images were then matched to their temporally nearest flat-field image and reduced in a similar manner but using the flat-field to define the spectroscopic apertures and traces. Following this, the wavelength solution was determined interactively using the GSWAVELENGTH task. The science spectra were overscan-subtracted, trimmed, extracted using the trace of the nearest flat-field image, wavelength calibrated
C

using the appropriate wavelength solution and corrected for variations in the relative fibre throughput using their corresponding response images. At this point, inspection of the extracted 2D spectra revealed several problems. There were throughput discontinuities in the wavelength direction where the different CCDs had been mosaiced together. There was also a discontinuity in the `spatial' direction of the 2D spectra corresponding to the crossover between the two slits. Further, looking at the spectra from sky fibres (and fibres corresponding to the outer part of the science IFU which are expected to have little contribution from the target galaxy) revealed a gradient in throughput with aperture number (spatial axis of the 2D spectrum). This gradient appeared (and had the same sense) in both halves of the 2D spectra corresponding to the two separate slits. These gradients were fitted, in each individual exposure, with a linear function on either side of each discontinuity and then divided out to give an approximately uniform level in the sky spectra. The sky spectra corresponding to each slit were then averaged and sky subtraction was performed on all spaxels.1 The accuracy and systematics in the sky subtraction were checked by examining both the sky-subtracted sky spectra and also (importantly) the sky-subtracted spectra from the outer regions of the target galaxy IFU, which have essentially no signal from the galaxy, and hence are a good indication of how well our reduction and sky subtraction have worked for our galaxy spectra. In general, this procedure worked well, however, small residual offsets and gradients in the sky-subtracted sky spectra remained. At this point, data cubes were constructed for each IFU observation using the task GFCUBE. The data were resampled spatially by this procedure and the resulting data cube had spatial pixels which were square with 0.2 arcsec sides. The data cubes corresponding to individual dithered exposures for each target were shifted and averaged into a single data cube. The original observations were performed with a square dither pattern with each individual exposure offset by 1 arcsec. This dither pattern between exposures was checked a posteriori by collapsing the cubes of individual exposures along the spectral direction and comparing the resulting images. This confirmed the accuracy of the dither pattern to subspaxel accuracy and means any smearing of the image quality during combination will be small compared to the seeing. Therefore, we were able to combine the individual data cubes by shifting an integer number of spaxels (i.e. 5 â 0.2 arcsec spaxels) before combining the spectra using the IRAF task SCOMBINE. Each spectrum was then cleaned of any remaining cosmic rays using the IRAF task LINECLEAN. The residual offsets described above could be seen in the final data cube as additive offsets between different columns of IFU elements and these were removed by subtracting off the average spectrum from the two outermost lenslets from all other lenslets in that column. After this procedure, examination of those spectra in the data cube, which had no discernible signal from the galaxy, revealed a well-behaved sky subtraction with little sign of systematics present. The systematics in the sky subtraction can be quantified by measuring the rms of the mean flux in spaxels expected to have no contribution from the object. This can then be compared with the random noise by measuring the mean of the rms scatter in those spaxels. For our data, the ratio of these is in the range 19 to 26 per cent, implying that the systematics in the sky subtraction are small compared with the random errors. A relative flux calibration was performed using observations of a flux standard star taken

1

A spaxel is the SPAtial piXture ELement of the instrument; in this case, the IFU lenslet array.

2009 The Authors. Journal compilation

C

2009 RAS, MNRAS 396, 1349­1369


1354

M. B. Pracy et al.
angle and ellipticity of each isophotal ellipse to vary freely. In several cases, the tidal/debris structures can be seen more clearly and further towards the galaxy centres. E+A 2 and E+A 5 reveal disc-like structures along their major axes consistent with the shapes of their isophotal profiles. We find an even higher rate of morphological disturbance than the original Zabludoff et al. (1996) study of the LRCS which found five out of 21 (24 per cent) galaxies showed signs of disturbance. This difference is likely due to the limited depth of the Digital Sky Survey (DSS) imaging used. Indeed, many of the tidal features in the imaging presented here are of too low surface brightness to be detected in the DSS which was one of the motivations for acquiring deeper imaging. 3.3 Colour morphologies We constructed 2D g - r colour maps as outlined in Section 2.2.2. These maps are shown in the two rightmost columns in Fig. 1 and show a 60 â 60 arcsec2 map (third column) and a 3 â 3arcsec2 map of the galaxy core (fourth column). The colour distributions in the galaxy cores are varied with the most common property being the presence of a red nuclear core. The cores are red only in a relative sense with respect to the outer parts of the galaxy ­ the absolute colours of the galaxies are blue in agreement with their E+A status. A red core is evident in four of the nine galaxies (E+A 2, E+A 7, E+A 8, E+A 10). Two galaxies have irregular colour structure in their centres (E+A 1, E+A 5) and two little structure at all (E+A 4, E+A 9) whilst one galaxy in our sample shows a clear blue core (E+A 6). This is in contrast to the recent study of Yang et al. (2008) using HST imaging of 21 E+A galaxies of which six had compact blue cores with very steep profiles. These structures, however, are on scales much smaller than our resolution element. Some of these profiles have complicated core colour structures and it is unclear how these colour morphologies would appear at our resolution given the flux and area variation with radius, or even whether the cores would be red or blue [see e.g. EA12, EA02 and EA09 in fig. 6 of Yang et al. (2008)]. While both samples display diversity in colour morphology with examples of blue, red and irregular cores, our sample does appear to have a higher fraction of red core galaxies even if direct comparison is difficult. 3.4 Colour gradients On larger scales, outside the core, we still see a diversity in behaviour. In Fig. 1, several galaxies have negative colour gradients becoming bluer with increasing galactocentric radius (e.g. E+A 7) whilst others have little sign of a gradient at all (e.g. E+A 10) or have large-scale patchiness in their colour distribution (e.g. E+A 9). We use the IRAF ELLIPSE task to examine the nature of the large-scale radial (semimajor axis) colour gradients. This is done by fitting elliptical isophotes to the PSF-matched g- and r-band images at regularly spaced semimajor axis lengths. The results for our sample are shown in Fig. 3. On these scales, we again see variation in the radial colour profile shapes. We use the same classification scheme that Yang et al. (2008) use to classify their large-scale colour gradients by categorizing profiles as positive, negative or flat/variable. This colour gradient classification for each galaxy is listed in Table 3. We classify two galaxies as having negative colour gradients (20 per cent), five to be flat/variable (50 per cent) and three to be positive (30 per cent). These can be compared with Yang et al. (2008) who find 29, 19, and 51 per cent of their sample to be negative, flat/variable and positive, respectively. Note, however,
C

with the same set-up as the science observations, which are done as part of the standard calibrations at Gemini Observatory.

3 P HO T O METRIC CHARA CTERISTICS 3.1 Isophotal profiles We constructed isophotal surface brightness profiles using the IRAF task ELLIPSE. The g-band surface brightness profiles are shown in Fig. 2. The r-band profiles (not shown) are qualitatively similar. The inner parts of the profiles out to 0.8 arcsec are flattened by convolution with the seeing disc. At larger galactocentric radii, the profiles appear like typical early-type galaxies with r 1/4 -like profiles (linear on a r 1/4 horizontal axis). The exception is the isophotal profile of E+A 2 (and to a lesser extent E+A 5) which appears more typical of a disc, i.e. a strong downward concavity in the surface brightness profile. An r 1/4 law fit to the data beyond 1Re is shown as the red line in Fig. 2. The profiles display some irregular structure expected for objects which have disturbed morphologies (see Fig. 1), which is also evidenced by the position angles and ellipticities of the best-fitting ellipse changing with semimajor axis distance. These results are generally consistent with the HST study of Yang et al. (2008) which found E+A galaxies from the Zabludoff et al. (1996) LCRS sample to be predominantly early-type systems but with a greater than normal level of asymmetries.

3.2 Morphological properties Several previous studies have found E+A samples to have a large fraction of morphologically disturbed members (Zabludoff et al. 1996; Blake et al. 2004; Yang et al. 2004) and nearby companion galaxies (Goto 2005). A primary aim of acquiring deep imaging with an 8-m class telescope of our sample was to search for the signatures of interactions and mergers in the form of faint tidal tails and debris. Inspection of the g-band images (see column 1, Fig. 1), as indeed the r-band images of our sample reveals that six out of 10 galaxies in the sample (E+A 1, E+A 4, E+A 7, E+A 8, E+A 9, E+A 10) show either tidal bridges or tails or clearly disturbed morphologies with at least three of these apparently currently undergoing an interaction with a companion (E+A 4, E+A 7, E+A 9). The remaining four galaxies appear like undisturbed early-type systems. Table 3 contains a summary of the morphological properties of the sample. T