Transcript Colour and Luminosity Trends from the 6dFGS

Luminosity Functions
from the 6dFGS
Heath Jones
ANU/AAO
Background
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Luminosity functions of NIR-selected galaxies are effective tracers
of the stellar mass function of collapsed structures
Light from the near-infrared is dominated by the older and cooler
stars that make up the bulk of the stellar mass.
Early attempts were limited to small sky areas and/or sample sizes
in the hundreds
With the advent of 2MASS, more recent attempts have exploited the
power of wide-field redshift surveys like 2dFGRS, SDSS and ZCAT
Of these, 6dFGS has the largest 2MASS overlap to date
Background
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Luminosity functions of NIR-selected galaxies are effective tracers
of the stellar mass function of collapsed structures
Light from the near-infrared is dominated by the older and cooler
stars that make up the bulk of the stellar mass.
Early attempts were limited to small sky areas and/or sample sizes
in the hundreds
With the advent of 2MASS, more recent attempts have exploited the
power of wide-field redshift surveys like 2dFGRS, SDSS and ZCAT
Of these, 6dFGS has the largest 2MASS overlap to date
Working in the Near-Infrared
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Extinction is minimal at longer wavelengths
Mass-to-light ratios are better constrained in near-infrared
passbands (e.g. Bell & de Jong 2001).
Cosmological k-corrections are small
2MASS affords digital (as opposed to photographic) photometry
over the wide sky areas now spanned by redshift surveys
Stellar Mass Function
Cole et al (2001)
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Does the total stellar
mass in the present-day
universe support cosmic
star formation history
observed at higher
redshift?
Star Formation History
of the Universe
log (Mstars/h-2M)
Sky completeness
K-band
bJ-band
Magnitude Completeness
Galaxies grouped
according to the
completeness of
the field to which
they belong
Total and Isophotal Magnitudes
Total mags (Ktot) are preferred to isophotal (Kiso) because total
luminosity is the physical quantity we ultimately seek
The Ktot mags provided for
the 2MASS XSC become
unreliable at low |b|
However, the Kiso are
reliable, and so we use these
(and the mean surface
brightness within uK20 = 20)
to provide a ‘corrected’ total
magnitude:
Ktot=Kiso - 1.5 exp1.25(uK2020)
Above: (Kiso-Ktot) versus the average surface brightness
Simple exponential disc model (solid) and adopted correction (upper
dashed)
Number Counts
2MASS isophotal magnitudes and 6dFGS total magnitudes
6dF Luminosity Function: The 1/Vmax Method
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1/Vmax straightforward to implement and does not
assume a functional form for the LF (non-parametric)
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Very robust with respect to apparent magnitude
incompleteness ---- good for samples with poorly
characterised magnitude incompleteness functions
However, assumes survey volume is homogeneous
---- biased if the galaxy distribution is clustered
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K-band LF
2MASS +
2dF
 6dFGS K-band LF
~17000
goes ~1.5 to 2 mags
galaxies
better at both the
bright
faint ends
600
sqand
deg
2MASS +
ZCAT
 Agrees with previous
~4000
measures within the
differences between
galaxies
magnitude
systems
7000
sq deg
employed
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2MASS +
The
smaller redshift
SDSS
surveys have larger
~12000
uncertainties about the
galaxies
normalisation
400 sq deg
6dFGS:
~63500 galaxies, 9500 sq deg
K-band LF
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Schechter fit is only a
close fit around M* to
(M*+4)
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Fails to turn over
sufficiently rapidly for the
bright end
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Faint end also drops off
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Simple 3-parameter
function insufficient to
properly characterise the
luminosity distribution
galaxies over this range
of 10,000x in luminosity
V/Vmax statistic
Suppose V(z) as the
survey volume within a
redshift z
zi is redshift of galaxy i
zmax,i is the maximum
redshift that same galaxy
could have and still
satisify the survey
selection criteria
If sample is complete
and of uniform density,
then V(zi)/V(zmax,i) is
uniformly distributed in
the interval 0 to 1
K-band 1/Vmax and STY together
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STY does not need to
assume that the LF is
independent of local
density, therefore is
insensitive to clustering
in the sample
STY does not require
binning
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However, is parametric,
and must assume some
functional form for the LF
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6dFGS STY fit is virtually
identical to Schechter
function fit to 1/Vmax LF
Correction for Virgo and Great Attractor Infall
No infall correction:
Corrected:
Model of Burstein et al (1989)
cz correction goes beyond
10% for galaxies MK> -19
J-band LF: 1/Vmax and STY
General agreement with
2dFGRS+2MASS study of Cole
et al (2001)
J and H-band LF: STY
In general, STY follows Schechter fit to 1/Vmax to high precision
bJ and rF-bands: 1/Vmax
Faint end rises as we move towards optical passbands
Current and Future Work
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StepWise Maximum-Likelihood: Currently working on our SWML fits to
the 6dFGS data. (SWML is a non-parametric maximum-likelihood LF
estimator, that is also insensitive to clustering).
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Normalisation: Want to examine the change in the mean number
density in the 6dFGS over redshift shells of increasing volume.
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Stellar Mass Function: Derive stellar masses for these galaxies from
their NIR photometry, fit the SMF and derive the total stellar mass
content of the local universe.
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Blue and Red Galaxies: Demarcate the sample along lines of extreme
(b-K) colour and examine the LF shape relative to the basic LFs