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Exploring the Stellar Populations
of Early-Type Galaxies in the
6dF Galaxy Survey
Philip Lah
Honours Student
h
Supervisors:
Matthew Colless
Heath Jones
6dF Galaxy Survey
• spectroscopic survey of nearby galaxies across the southern sky
• targets are near infrared selected (K band)
• started mid 2001; to end mid 2005; the first public data release
was Dec 2002
• ~13,000 V & R spectra with redshifts; peak z~0.05 (~200 Mpc)
Contour Plot of 6dF Galaxy Distribution
Exploring the Stellar Populations of Early-Type Galaxies in the 6dF Galaxy Survey
Philip Lah, Honours Student
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Early-Type
Late-Type
Early
& Late Type Galaxies
• elliptical & lenticular (S0)
• spiral & irregular
• no evidence of HII emission
regions: no H or [OIII]
emission & have H absorption
• evidence of HII emission
regions: H, [OIII] & H
emission
• dominated by old stars with
no recent star formation
• mixture old & young stars,
ongoing star formation
Exploring the Stellar Populations of Early-Type Galaxies in the 6dF Galaxy Survey
Philip Lah, Honours Student
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Lick Indices
• Lick System: 4000-6200Å at ~9Å FWHM resolution
• common lines used in models above (measured many more)
Exploring the Stellar Populations of Early-Type Galaxies in the 6dF Galaxy Survey
Philip Lah, Honours Student
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Stellar Population Models
•
input age of star formation, metallicity & IMF
•
generate lists Lick indices, colours
•
problem of age / metallicity degeneracy (increase in age same
effect as decrease metallicity)
stellar population models:
 Worthey (1994) – standard model in literature
 Thomas (2003) – includes non-solar abundances (various
models with different [/Fe])
Exploring the Stellar Populations of Early-Type Galaxies in the 6dF Galaxy Survey
Philip Lah, Honours Student
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What am I doing?
I am studying properties of the stellar population of early-type
galaxies using the stellar population models.
In particularly looking for any differences between lenticulars
(S0s) and ellipticals.
What and Why
Also looking for any relationship between luminosity and age or
metallicity.
Why am I doing this?
This work should give clues on how galaxies formed.
Help discriminate between the hierarchal merger and monolithic
collapse formation models.
Exploring the Stellar Populations of Early-Type Galaxies in the 6dF Galaxy Survey
Philip Lah, Honours Student
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Other’s Results
• Moore PhD 2001 - 87 early-type galaxies in Coma cluster – found
ellipticals 2 Gyr older on average than lenticulars with similar
metallicity
• Kuntschner 2000 - sample 22 early-type galaxies in Fornax
cluster found lenticulars younger and similar metallicity
• Jones et al. 1999 no difference ages ellipticals and lenticulars
• Thomas & Kauffmann 1999 - luminous ellipticals are the last to
form in the hierarchical merger model
• Bernardi et al. 2003 - SDSS more massive galaxies are older
Exploring the Stellar Populations of Early-Type Galaxies in the 6dF Galaxy Survey
Philip Lah, Honours Student
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Data Reduction
• measure Lick indices with a program indexf
• problems of lines redshifted into R spectrum & negative points
in Lick bandpasses
• measure S/N & redshift error using repeated observations
• broaden to Lick resolution & continuum divide spectrum
• measure velocity dispersion & correct Lick indices
• measure H (& nearby [NII] doublet)
• calibrate data against other observations
• automate all these processes
Exploring the Stellar Populations of Early-Type Galaxies in the 6dF Galaxy Survey
Philip Lah, Honours Student
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My Sample
10054 galaxies observed once & 1105 galaxies observed twice
11159 observed once & higher S/N of repeated observations
8198 with “good” velocity dispersions (K and G star templates)
4538 my early-type cut (non-starforming) => H > 0.7 Å – 2 and
[OIII]4959 > 0.8 Å – 2 and [OIII]5007 > –0.6 Å – 2
742 Large Sample –cut on error / S/N - HG and [MgFe]  <0.30
(S/N ~20+)
MgFe  
100 Good Sample HG and [MgFe]  <0.15
(S/N ~40+)
Exploring the Stellar Populations of Early-Type Galaxies in the 6dF Galaxy Survey
Mg b  Fe
Fe 5270  Fe 5335
Fe 
2
Philip Lah, Honours Student
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My Sample - Morphology
Classified morphology using 2MASS NIR J, H & K and
SuperCOSMOS Sky Survey optical B & R images
742 Large Sample - ellipticals 54%, lenticular 37%, spirals 9%
100 Good Sample - ellipticals 50%, lenticular 41%, spirals 9%
Exploring the Stellar Populations of Early-Type Galaxies in the 6dF Galaxy Survey
Philip Lah, Honours Student
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Velocity Dispersion & Morphology
• different morphology – different velocity dispersion distribution
• S0s – component from disk rotation at different alignments
Exploring the Stellar Populations of Early-Type Galaxies in the 6dF Galaxy Survey
Philip Lah, Honours Student
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Faber-Jackson Relationship
• L  4 (log slope –4)
black line slope -4
red line ellipticals
Faber Jackson Relationship
y = -4.9 (0.7) x –14 (2)
rms 0.40
red dots - elliptical
blue crosses - lenticular (S0)
black distorted squares - spirals
blue line lenticular (S0)
y = -2.4 (1.5) x –19 (3)
rms 0.59
Exploring the Stellar Populations of Early-Type Galaxies in the 6dF Galaxy Survey
Philip Lah, Honours Student
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Mgb vs. Velocity Dispersion
• black line slope –0.131
same as Colless et al. 1998
(offset different due to
different calibration)
Mgb vs. velocity dispersion
• red line ellipticals
y = 0.22 (0.04) x –0.32
(0.1) rms 0.40
• blue line lenticular (S0)
red dots - elliptical
blue crosses - lenticular (S0)
black distorted squares - spirals
y = 0.099 (0.034) x –0.044
(0.077) rms 0.59
Exploring the Stellar Populations of Early-Type Galaxies in the 6dF Galaxy Survey
Philip Lah, Honours Student
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Trager Data
• 381 galaxies & 38
globular clusters –
re-observations of
objects used in
defining
Lick
indices (absorption
line objects)
• used to calibrate
stellar population
models
• used to calibrate
my data – so that
have
similar
distribution – add
offset
red dots - elliptical
blue crosses - lenticular (S0)
black distorted squares - spirals
Worthey Large
Large Sample
(742 galaxies)
• Error smaller
than Trager data
rms error 0.20 Å
• my results
concentrate on
Good Sample of
100 galaxies
red dots - elliptical
blue crosses - lenticular (S0)
black distorted squares - spirals
Worthey Good
Good Sample
(100 galaxies)
• note: lower HG
points are removed
by quality cut –
probably problems
with Lick index
continuum fit
• rms Error 0.12 Å
Ellipticals Only
50 points
Worthey Elliptical
Lenticular (S0)
41 points
Worthey Lenticular
no statistically
significant
difference
Good Sample (100)
red dots - elliptical
blue crosses - lenticular (S0)
black distorted squares - spirals
[/Fe] = 0.0
Solar abundance
Thomas 0 Large
Good Sample (100)
red dots - elliptical
blue crosses - lenticular (S0)
black distorted squares - spirals
[/Fe] = 0.5
• shifts up and to
left – more shift in
HG
Thomas 5 Large
• my data fits on
both grids
• haven’t looked at
ages & metallicities
from Thomas grid
Ages for Good Sample
• x-axis in log(age) – if plot linear age - fraction is relative constant
• difference with Large Sample – S0’s drop off in age in last bin
Exploring the Stellar Populations of Early-Type Galaxies in the 6dF Galaxy Survey
Philip Lah, Honours Student
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Cumulative Distribution of Ages
• red line ellipticals
blue line lenticulars (S0)
• K-S Statistic
(Kolmogorov-Smirnov)
• probability data drawn
from same distribution
• Lenticular & Elliptical
Ages sample probability
0.92 (Large Sample
0.135)
Exploring the Stellar Populations of Early-Type Galaxies in the 6dF Galaxy Survey
Philip Lah, Honours Student
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Metallicity for Good Sample
• looks like lenticulars slightly more metal rich by eye – probably
more effect of binning limits than real effect
Exploring the Stellar Populations of Early-Type Galaxies in the 6dF Galaxy Survey
Philip Lah, Honours Student
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Cumulative Distribution of [Fe/H]
red ellipticals line
blue lenticulars (S0) line
• Lenticular & Elliptical
Metallicities taken from
same sample Probability
0.63 (from Large Sample
0.037)
Exploring the Stellar Populations of Early-Type Galaxies in the 6dF Galaxy Survey
Philip Lah, Honours Student
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Comparison with Other’s Results
• no variation in metallicity agrees with Moore PhD 2001,
Kuntschner 2000 and Jones 1999
• I detect no age variation unlike Moore and Kuntschner but
agrees with Jones et al. 1999
• reason for difference:
- my sample not from one rich cluster or only from rich
clusters (includes field galaxies) (my data is a magnitude
limited sample)
- large error bars hide any small pattern
Exploring the Stellar Populations of Early-Type Galaxies in the 6dF Galaxy Survey
Philip Lah, Honours Student
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Age & [Fe/H] vs. K magnitude
• K luminosity – good indicator of stellar mass
• no pattern in age or metallicity with K
Exploring the Stellar Populations of Early-Type Galaxies in the 6dF Galaxy Survey
Philip Lah, Honours Student
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Comparison with Other’s Results
• disagree Thomas & Kauffmann 1999 and Bernardi et al. 2003 –
no pattern between age and luminosity / mass
• reason for difference:
- my sample over small magnitude range (~2 mag) for
bright galaxies (magnitude limited sample)
- size of my errors smear out any correlation
- there may not be any correlation
Exploring the Stellar Populations of Early-Type Galaxies in the 6dF Galaxy Survey
Philip Lah, Honours Student
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Field vs.
Cluster
z limit
0.05
Cluster
sizes
rectangles
300 kms-1
by
150 kms-1
6dF Galaxy
Survey Slices
Future Work
• do a field vs. cluster comparison
• use other stellar population models and other indices (like H
and H) for ages and metallicities measurements
• use other indices that are more sensitive to [/Fe] to examine any
differences using Thomas model
• I have H and [NII] equivalent widths measured – an analysis of
these interesting
• do a direct comparison of the stellar population model spectra
and 6dFGS spectra – go beyond Lick indices
• another 6dF Galaxy Survey data release in Dec 2003 – more data
with new gratings on 6dF spectrograph with higher S/N – the
eventually completed survey will have ~13 times more data
Exploring the Stellar Populations of Early-Type Galaxies in the 6dF Galaxy Survey
Philip Lah, Honours Student
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The End