Age-dating Luminous Red galaxies

MSc project by Ando Ratsimbazafy1

Supervisor: Prof Catherine Cress1

Collaborators: S. Crawford2; S. Blyth3; E. Olivier2; K. van der Heyden3

1UWC, 2SAAO, 3UCT

MNRAS 2009 (submitted)

Luminous Red Galaxies (LRGs): L > 3L* , massive early-type galaxies, red colours Photometry: old, passively evolving stellar populations Spectroscopy: homogeneous spectral properties Found in clusters

Motivation

Motivation: observation vs. models

Luminosity of LRGs (Almeida et al. 2008 MNRAS 386, 2145)

Observed LRGs

LRGs provide a very good observational sample to test models of galaxy formation and evolution.

Motivation: observation vs. models

Clustering of LRGs (Almeida et al. 2008 MNRAS 386, 2145)

Bower et al.

LRGs & cosmology

Jimenez & Loeb 2002, ApJ 573, 37

Age – dating 2 populations of LRGs

LRGs: homogeneous populations

LRGs form at the same time

Objectives: Validity of the assumptions using LRGs in Millennium Simulation (MS) Generate LRGs spectra using spectral synthesis models Estimate the errors on ages to optimize the experiment using SALT

Identifying LRGs in MS

DM only, N-body simulation.

1010 particles in 500h-1 Mpc3 box.

Semi-analytic modelling of galaxies : de Lucia et al. (2006) & Bower et al. (2006) models.

Properties of galaxies (eg. SFH, colours etc) stored in database.

Identifying LRGs in MS

NOT QUALIFIED

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Using SDSS LRG selection criteria (Eisenstein et al. 2001 AJ, 122, 2267) Using Absolute magnitude cuts (M_V < -23; B-V > 0.81) Age-dating LRGs requires more homogeneous samples & similar SFH

z=0.32 z=0.51z=0.46 z=0.56

Identifying LRGs in MS

Age Age

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z=0.32 z=0.46 z=0.51 z=0.56

Strong peak at a single age: the de Lucia et al. model >>> cosmic chronometers to recover the cosmology in MS No good separation between the age peaks: the Bower et al. model

H(z) from simulated LRGs Using average age of LRGs 0< z <1

Error in H(z) depends only on the error in age

Each pair of snapshots provides H(z) near the redshift interval

Average: H(z) calculated to 1.6% precision at 0.32 < z < 0.51

Fixed fit: H(z) calculated to 1.1% precision at 0.32< z < 0.51

Pair 20: H(z) calculated to 2.8% at z~0.42 Larger error = smaller dz

Smaller error= larger dz

Calculated H(z) vs. expected H(z)

Modelling & age-dating LRG spectraLRG spectra : using SSP library of BC 03, SFH(t), Z(t) from MS

Fitting with SSPs : the offset in the systematic error did not correlate with the age of the Universe

Fitting with model spectra: using its mass-weighted age as fiducial age, the errors were reduced

Observation programme Estimating the error on individual ages of galaxies as a function of SNR and resolution using Monte Carlo simulations of spectral fitting. Estimating the error on the mean age at a given redshift as a function of the number of galaxies observed and the error on ages of individual galaxies. A SNR ~ 10 and an exposure time of ~ 100s are required using RSS on SALT to obtain an age error on an individual galaxy of σage = 0.5 Gyr. To measure H(z) to 3, 5 or 10% >>>T = 184, 72 or 17 hours, observing 840, 327 or 80 galaxies respectively at z = 0.32 and z = 0.51

Summary & outlook LRGs selected with Abs mag cuts in de Lucia et al. model can be used as a cosmic chronometers

H(z) can be calculated from mass-weighted ages to a precision < 3%

SSPs do not accurately recover the ages of individual galaxies, need to use model fitting

Investigation of age-dating of LRGs in more detail by determining which part of the spectrum is the most sensitive, and by exploring different age-dating techniques (e.g. full spectrum versus lick indices, etc.)

The observation of LRGs with SALT in 2010

Constraining H(z) >>> a great deal of information on the evolution of the most massive galaxies at intermediate redshift

Thank you