Transcript Quenched models as Progenitors of Red Sequence Galaxies

A Quenched model Origin of the
Red Sequence
Justin Harker
Sandy Faber
Ricardo Schiavon
EGS Meeting
17 Dec 2005
What are the ages of Red Sequence
Galaxies?
Early type galaxies have fairly uniformly red colors
consistent with old populations
Monolithic collapse: predicts a red sequence
formed at a single epoch
Balmer Line data
Spectral data at low
redshift, however, are
consistent with a
spread in ages, and
cannot be fit by
single-aged
populations formed
at a reasonable epoch
R. Schiavon in prep.
Frosting models
Frosting models: consist
of mostly old stars
formed in an early burst
mixed with younger
population
Characterized by 2
parameters: percent
mass reserved for the
frosting phase (~1-20),
and the e-folding time
(1-8 Gyr) which defines
frosting phase
Quenched models
Characterized by:
-Early quiescent star
formation
-Truncated at
regularly spaced
intervals after
specified epoch.
-Quenching is either
instantaneous or
preceded by a
burst of
enhanced star
formation.
Selection of Quenched Models
We examine quenching
epochs of z=1.5, 2.0
To compute line strengths
and colors of the
composite population,
we take averaged HδF,
U-B of quenched models
that pass a color cut at a
given epoch.
Single Stellar Populations
•
•
•
Filled squares – DEEP-2 data
Open squares – SDSS data
Open stars – RC3 data
Frosting Models
Altering
the total frosting mass affects primarily zero point
in color, line strength
Altering the e-folding timescale for the frosting phase
affects primarily slope of fit
Line strength fit is reasonable for 7.5% frosting mass with
an exponent of 4 Gyr, but colors are rather blue at high
redshift, and you cannot improve color fit without
worsening line strength fit.
Quenched Models
The averaged quenched model is a good fit to
U-B color under all conditions (thanks in part
to the color cut)
Both pure and burst models are a reasonable
fit to data for a quenching epoch of z=2,
z=1.5 produces fairly high Balmer line
strengths at high redshift for the burst models.
Either epoch
RSG Number Density
of quenching
fits the data to
DEEP-2, COMBO17 observe a rise
the number
anin
extent,
z=2
density of RSGs by a factor of 2-4
after z=1
probably
the
safer choice
Quenched models also trace this evolution
in a
way neither SSPs or our frosting models can
Conclusions

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
Balmer line strength data are inconsistent with a
monolithic collapse red sequence
Frosting models cannot simultaneously fit color
and line strength data
Quenched models are able to match the data
accurately, and are suitable for testing against
number density evolution, which they also
match.