Environmental Properties of a Sample of Starburst Galaxies Selected from the 2dFGRS

Matt Owers (UNSW)

Warrick Couch (UNSW)

Chris Blake (UBC)

Michael Pracy (UNSW)

Kenji Bekki (UNSW)

Why Study Starbursts?

• Important stage in the evolution of galaxies.• Interesting mechanisms may be occurring causing

these violent starbursts. (mergers, galaxy harassment etc)

• What environments do these starbursts reside in? (Clusters, groups, infall regions, field/filaments etc)

• Nature or nurture??? • Possible progenitors to E+A or “post-starburst”

galaxies. • Triggering and cessation mechanisms of the

starburst leading to an E+A are unknown.• Replicate the Blake et al. study to try to answer

some of the above questions.

The 2dF Survey• Approximately 225 000 galaxy redshifts.

• Limited in apparent magnitude at bj~19.45.

• Median redshift ~ 0.1• Spectra cover rest frame 3600-8000Å.• Fibre aperture diameter of 2” = 3.7 kpc at z = 0.1

(for h = 0.7 Mpc s km-1, m = 0.3, = 0.7 cosmology) – Not obtaining spectra from the whole galaxy!

• Use the 2dFGRS spectral line catalogue (Lewis et al. 2002)

Selecting Starburst Galaxies: Criteria

• ADC = 1, redshift quality >= 3, no double detections.

• Leaves 162,223 galaxies with good quality spectra to select from.

• Select out emission line galaxies with EW > 5, S/N > 1and Flag >= 4 for H, H, Nandlines.

• Use the BPT (Baldwin, Phillips and Terlevich, 1981) diagram to differentiate between AGN and star forming galaxies.

Star-forming

AGN

Log([O]/H) < 0.61/{Log([N]/H)-0.05}+1.3 (Kauffmann et al. 2003)

Selecting Starburst Galaxies: Criteria

• Excluding AGN leaves 5286 star-forming galaxies.• Need to define a starburst sample.• No flux calibration (D’oh!)• Measure a star formation rate normalised to L* (Lewis et

al., 2002) i.e., * = WHL*– x10-42 Mo s yr-1 erg-1 (Sullivan et al., 2001)

– L* = 1.1x1040 erg s-1 Å-1 (luminosity corresponding to the knee in the r` band luminosity function of Blanton et al, 2001)

– WHis the H equivalent width.

• Define starburst sample as those galaxies with * >= 10 Mo yr-1.

• Gives a sample of 400 “starburst” galaxies with z < 0.17.

Environmental Analysis

• Luminosity Function

• Distance to near neighbours (bright and faint)

• Surface density of bright neighbours

• Group membership

• Cross correlation with remaining 2dFGRS galaxies

• For each of the above, we randomly select galaxy samples from the 2df for comparison to our starburst sample

Luminosity Function

• Cut sample to z range 0.002 < z < 0.3 and apparent mag 14 < bj < 19.2

• Use Stepwise Maximum Likelihood code

• SGP and NGP only

• Compare starburst sample to the whole 2df sample

• Normalise to constant source surface density of Deg-2

Starburst (sgp and ngp)

2dFGRS (sgp and ngp)• Poisson error bars

• Starburst sample are preferentially less luminous objects.

• Need to fit a Schechter function to find M* etc.

• Is our selection criteria biased to low

mass galaxies?

Near Neighbour Study

• Use supercosmos catalogue extraction to find near neighbours within a 5’ radius.

• Nearest faint neighbours:– b*(z)+1< bfn <= 22.5– b*(z) is the apparent magnitude an M* galaxy would

appear as at the redshift of the starburst galaxy of interest. (M*=-20.5, Norberg et al. 2002)

– Measure transverse separation– Compare with random sample using the Kolmogorov-

Smirnov (K-S) statistical test to determine if the 2 samples are drawn from the same parent distribution.

• Nearest bright neighbours:– bbn <= b*(z)+1

Random Sample

Starburst Sample• Near neighbours important

within 20kpc?

• K-S probability of ~ 0 ie, not drawn from the same parent distribution.

• Higher number of faint near neighbours within 20kpc.

Starburst Sample

Random Sample

• Slightly more bright near neighbours within 60kpc

• K-S probability = 1.2x10-4

Surface Density of Bright Near Neighbours

• Measure the projected separation of the fifth bright near neighbour

• Surface density is just 5/d52

– d5 is the distance in Mpc to the 5th nearest bright neighbour

• Compare to a random sample

Starburst Sample

Random Sample

• Slightly higher surface density around starbursts?

• K-S probability = 2.5x10-7.

• Neighbours important in triggering a starburst?

Cross Correlation With remaining 2dFGRS galaxies

• Overdensity={(nrand)/(n2df)*(Ncount)/(Nexpected)} – 1

• Count number of galaxies in the 2dFGRS within ro h-1 Mpc of each starburst galaxy (ro =1,2,3,…15). (Ncount)

• Generate a mock un-clustered random distribution catalogue using publicly available 2df software (Norberg and Cole).

• Count the number of galaxies in the mock catalogue within ro h-1 Mpc of each starburst galaxy (Nexpected).

• Average overdensity for all starbursts and compare to a random and elliptical galaxy sample.

Elliptical Galaxies

Starburst Sample

Random Sample

• Ellipticals inhabit the most dense environments

• Starbursts inhabit less dense environments overall

• Error bars show the error in the mean of the overdensity distribution (σ/sqrt{N})

Cross Correlation with 2dFGRS Group Catalogue

• Select starburst galaxies that appear in the Eke et al. (2004a) 2PIGG catalogue.

• Cut out groups with z > 0.12 due to higher probability of interlopers.

• 54% of the starburst sample reside in a 2PIGG group, whilst 53% of the random sample reside in a 2PIGG group

• Number of members in the group is not a great method of defining group size (due to apparent magnitude cut offs etc.)

• Measure the corrected total group luminosity • Do the same for a random catalogue and compare the

distribution of the group luminosities.

Starburst Sample

Random Sample

• Starbursts reside in less luminous groups

• Not many starbursts in clusters

• Poisson error bars

Conclusions• Our starburst sample contains a high number of low

luminosity systems.• It appears that faint near neighbours within 20 kpc may

play an important role.• Bright near neighbours may also be significant within 60

kpc• Surface density plots for random and starburst samples

are inconsistent.• Starbursts inhabit regions less dense than both the

elliptical and random samples.• Starbursts seem to live in lower luminosity groups, ie.

Lower mass groups.• It is clear that not all of the galaxies in this sample will

evolve into post-starburst systems.

Future Work

• Measure the clustering properties using the spatial cross/auto-correlation function

• Get morphologies from SSS images• Compare sample with late type galaxy

sample• Write ground breaking paper! • Reduce 2.3m long slit spectroscopy. Will

give SFR vs radius, velocity dispersions etc.