A Quenched model Origin A Quenched model Origin of the Red Sequenceof the Red Sequence

Justin HarkerJustin HarkerSandy FaberSandy Faber

Ricardo SchiavonRicardo SchiavonEGS MeetingEGS Meeting17 Dec 200517 Dec 2005

What are the ages of Red What are the ages of Red Sequence Galaxies?Sequence Galaxies?

Early type galaxies have fairly Early type galaxies have fairly uniformly red colors consistent with uniformly red colors consistent with old populationsold populations

Monolithic collapse: predicts a red Monolithic collapse: predicts a red sequence formed at a single epochsequence formed at a single epoch

Balmer Line dataBalmer Line dataSpectral data at Spectral data at low redshift, low redshift, however, are however, are consistent with consistent with a spread in a spread in ages, and ages, and cannot be fit by cannot be fit by single-aged single-aged populations populations formed at a formed at a reasonable reasonable epochepochR. Schiavon in prep.R. Schiavon in prep.

Frosting modelsFrosting modelsFrosting models: Frosting models: consist of mostly old consist of mostly old stars formed in an stars formed in an early burst mixed early burst mixed with younger with younger populationpopulationCharacterized by 2 Characterized by 2 parameters: percent parameters: percent mass reserved for mass reserved for the frosting phase the frosting phase (~1-20), and the e-(~1-20), and the e-folding time (1-8 folding time (1-8 Gyr) which defines Gyr) which defines frosting phasefrosting phase

Quenched modelsQuenched modelsCharacterized by:Characterized by:-Early quiescent -Early quiescent

star formationstar formation-Truncated at -Truncated at

regularly regularly spaced intervals spaced intervals after specified after specified epoch.epoch.

-Quenching is -Quenching is either either instantaneous instantaneous or preceded by or preceded by a burst of a burst of enhanced star enhanced star formation.formation.

Selection of Quenched Selection of Quenched ModelsModels

We examine quenching We examine quenching epochs of z=1.5, 2.0epochs of z=1.5, 2.0

To compute line To compute line strengths and colors of strengths and colors of the composite the composite population, we take population, we take averaged Haveraged HδδFF, , U-B of quenched U-B of quenched models that pass a models that pass a color cut at a given color cut at a given epoch.epoch.

Single Stellar Single Stellar PopulationsPopulations

• Filled squares – DEEP-2 dataFilled squares – DEEP-2 data• Open squares – SDSS dataOpen squares – SDSS data• Open stars – RC3 dataOpen stars – RC3 data

Altering the total frosting mass affects Altering the total frosting mass affects primarily zero point in color, line strengthprimarily zero point in color, line strengthAltering the e-folding timescale for the Altering the e-folding timescale for the frosting phase affects primarily slope of fitfrosting phase affects primarily slope of fitLine strength fit is reasonable for 7.5% Line strength fit is reasonable for 7.5% frosting mass with an exponent of 4 Gyr, but frosting mass with an exponent of 4 Gyr, but colors are rather blue at high redshift, and colors are rather blue at high redshift, and you cannot improve color fit without you cannot improve color fit without worsening line strength fit.worsening line strength fit.

Frosting ModelsFrosting Models

Quenched ModelsQuenched Models

The averaged quenched model is a The averaged quenched model is a good fit to U-B color under all good fit to U-B color under all conditions (thanks in part to the conditions (thanks in part to the color cut)color cut)Both pure and burst models are a Both pure and burst models are a reasonable fit to data for a reasonable fit to data for a quenching epoch of z=2, z=1.5 quenching epoch of z=2, z=1.5 produces fairly high Balmer line produces fairly high Balmer line strengths at high redshift for the strengths at high redshift for the burst models.burst models.

RSG Number DensityRSG Number Density

DEEP-2, COMBO17 observe a rise in DEEP-2, COMBO17 observe a rise in the number density of RSGs by a the number density of RSGs by a factor of 2-4 after z=1factor of 2-4 after z=1

Quenched models also trace this Quenched models also trace this evolution in a way neither SSPs or evolution in a way neither SSPs or our frosting models canour frosting models can

Either Either epoch of epoch of quenching quenching fits the data fits the data to an to an extent, z=2 extent, z=2 probably probably the safer the safer choicechoice

ConclusionsConclusions Balmer line strength data are Balmer line strength data are

inconsistent with a monolithic collapse inconsistent with a monolithic collapse red sequencered sequence

Frosting models cannot simultaneously Frosting models cannot simultaneously fit color and line strength datafit color and line strength data

Quenched models are able to match Quenched models are able to match the data accurately, and are suitable the data accurately, and are suitable for testing against number density for testing against number density evolution, which they also match.evolution, which they also match.