A Quenched model Origin of the Red Sequence

1
A Quenched model Origin of the Red Sequence

• Justin Harker
• Sandy Faber
• Ricardo Schiavon
• EGS Meeting
• 17 Dec 2005

2
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

3
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.
4
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

5
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.

6
Selection of Quenched Models

• We examine quenching epochs of z1.5, 2.0
• To compute line strengths and colors of the
  composite population, we take averaged HdF,
• U-B of quenched models that pass a color cut at
  a given epoch.

7
Single Stellar Populations

• Filled squares DEEP-2 data
• Open squares SDSS data
• Open stars RC3 data

8
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.

9
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
z2, z1.5 produces fairly high Balmer line
strengths at high redshift for the burst models.
10
RSG Number Density

• DEEP-2, COMBO17 observe a rise in the number
  density of RSGs by a factor of 2-4 after z1
• Quenched models also trace this evolution in a
  way neither SSPs or our frosting models can

11
Conclusions

• 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.