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					        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,
  Nandlines.
• Use the BPT (Baldwin, Phillips and
  Terlevich, 1981) diagram to differentiate
  between AGN and star forming galaxies.
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., * = W HL*
    – 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)
    – WHis 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
       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
   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
  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.
   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.
                   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.

				
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posted:6/4/2011
language:English
pages:20