Exploring the Stellar Populations of Early-Type Galaxies in the 6dF Galaxy Survey

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

Philip Lah, Honours StudentExploring the Stellar Populations of Early-Type Galaxies in the 6dF Galaxy Survey 2

Early & Late Type GalaxiesEarly-Type• elliptical & lenticular (S0)

• no evidence of HII emission regions: no H or [OIII] emission & have H absorption

• dominated by old stars with no recent star formation

Late-Type• spiral & irregular

• evidence of HII emission regions: H, [OIII] & H emission

• mixture old & young stars, ongoing star formation


Lick Indices

• Lick System: 4000-6200Å at ~9Å FWHM resolution

• common lines used in models above (measured many more)


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])


What and Why


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.

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.

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


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


My Sample10054 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 - HG and [MgFe] <0.30 (S/N ~20+)

100 Good Sample HG and [MgFe] <0.15 (S/N ~40+)


2

5335Fe5270FeFe

FeMgMgFe b

My Sample - MorphologyClassified 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%


Velocity Dispersion & Morphology• different morphology – different velocity dispersion distribution

• S0s – component from disk rotation at different alignments


Faber Jackson Relationship

Faber-Jackson Relationship


red dots - ellipticalblue crosses - lenticular (S0)black distorted squares - spirals

• L 4 (log slope –4)black line slope -4

red line ellipticals y = -4.9 (0.7) x –14 (2)

rms 0.40

blue line lenticular (S0) y = -2.4 (1.5) x –19 (3)

rms 0.59

Mgb vs. velocity dispersion

• black line slope –0.131 same as Colless et al. 1998 (offset different due to different calibration)

• red line ellipticals

y = 0.22 (0.04) x –0.32 (0.1) rms 0.40

• blue line lenticular (S0)

y = 0.099 (0.034) x –0.044 (0.077) rms 0.59

Mgb vs. Velocity Dispersion



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

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


Worthey Good

Good Sample (100 galaxies)

• note: lower HG points are removed by quality cut – probably problems with Lick index continuum fit

• rms Error 0.12 Å


Worthey Elliptical

Ellipticals Only

50 points

Worthey Lenticular

Lenticular (S0)

41 points

no statistically significant difference

Thomas 0 Large

Good Sample (100)

[/Fe] = 0.0

Solar abundance


Thomas 5 Large

Good Sample (100)

[/Fe] = 0.5

• shifts up and to left – more shift in HG

• 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


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)


Metallicity for Good Sample• looks like lenticulars slightly more metal rich by eye – probably more effect of binning limits than real effect


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)


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


Age & [Fe/H] vs. K magnitude• K luminosity – good indicator of stellar mass

• no pattern in age or metallicity with K


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


6dF Galaxy Survey Slices

Field vs. Cluster

z limit

0.05

Cluster sizes

rectangles

300 kms-1

by

150 kms-1

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


The End

Documents

Exploring the Stellar Populations of Early-Type Galaxies in the 6dF Galaxy Survey