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Age-dating Luminous Red galaxies

Age-dating Luminous Red galaxies. MNRAS 2009 ( submitted ). MSc project by Ando Ratsimbazafy 1 Supervisor: Prof Catherine Cress 1 Collaborators : S. Crawford 2 ; S. Blyth 3 ; E. Olivier 2 ; K. van der Heyden 3. 1 UWC, 2 SAAO, 3 UCT. Motivation.

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Age-dating Luminous Red galaxies

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  1. Age-dating Luminous Red galaxies MNRAS 2009 (submitted) MSc project by Ando Ratsimbazafy1 Supervisor: Prof Catherine Cress1 Collaborators: S. Crawford2; S. Blyth3; E. Olivier2; K. van der Heyden3 1UWC, 2SAAO, 3UCT

  2. Motivation • 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

  3. Motivation: observation vs. models Observed LRGs Luminosity of LRGs (Almeida et al. 2008 MNRAS 386, 2145) LRGs provide a very good observational sample totest models of galaxy formation and evolution.

  4. Motivation: observation vs. models Bower et al. Clustering of LRGs (Almeida et al. 2008 MNRAS 386, 2145)

  5. LRGs & cosmology • Age – dating 2 populations of LRGs • LRGs: homogeneous populations • LRGs form at the same time Jimenez & Loeb 2002, ApJ 573, 37 • Objectives: • Validity of the assumptions using LRGs in Millennium Simulation (MS) • Generate LRGs spectra using spectral synthesis models • Estimate theerrors on ages to optimize the experiment using SALT

  6. Identifying LRGs in MS • DM only, N-body simulation. • 1010particles 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.

  7. Identifying LRGs in MS NOT QUALIFIED NOT QUALIFIED NOT QUALIFIED z=0.32 z=0.46 z=0.51 z=0.56 • 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

  8. Identifying LRGs in MS NOT QUALIFIED NOT QUALIFIED NOT QUALIFIED z=0.32 z=0.56 z=0.46 z=0.51 Age Age • 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

  9. H(z) from simulated LRGs Calculated H(z) vs. expected H(z) • Using average age of LRGs 0< z <1 • Error in H(z) depends only on the error in age • Each pair of snapshots providesH(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

  10. Modelling & age-dating LRG spectra LRG 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

  11. 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 numberof 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

  12. 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

  13. Thank you

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