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Sandage 1986

HUBBLE TYPES and STAR_FORMATION HISTORIES. Roughly speaking, the Hubble sequence is also a sequence in star formation histories. Sandage 1986. STAR FORMATION HISTORY OF NON-STARFORMING GALAXIES. 1. Star formation from fossil records: why useful and what for.

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Sandage 1986

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  1. HUBBLE TYPES and STAR_FORMATION HISTORIES Roughly speaking, the Hubble sequence is also a sequence in star formation histories. Sandage 1986

  2. STAR FORMATION HISTORY OF NON-STARFORMING GALAXIES 1. Star formation from fossil records: why useful and what for

  3. Indicators of ongoing star-formation activity - Timescales Emission lines < 3 x 107 yrs UV-continuum emission it depends… FIR emission < a few 10^7 (but…) Radio emission as FIR (?) (Could be higher: relativistic electrons have lifetimes ≤ 10^8 yr)

  4. THE IMPORTANCE OF FOSSIL RECORDS: Goal: understanding galaxy formation and evolution Going to higher-z is not always the solution….problem of connecting progenitors and descendants The evolutionary history of a galaxy is written in its stars, therefore in the light they emit – the light of all stars together form the galaxy integrated spectrum A method to reconstruct the past star-formation history

  5. All these techniques are based on one single fact: that STARS OF DIFFERENT AGES HAVE DIFFERENT SPECTRAL ENERGY DISTRIBUTIONS (in shape and lines)

  6. Young populations are bright Young populations are hot Old populations are faint Old populations are cool

  7. SPECTROPHOTOMETRIC MODELS Simply adding up the light of all stars: a Single Stellar Population (SSP) stellar IMF Monochromatic luminosity emitted by a star with mass m, metallicity Z and age T

  8. SPECTROPHOTOMETRIC MODELS Simply adding up the light of all stars: a galaxy (composite spectrum)

  9. The oldest galaxies at any redshift Color-Magnitude sequence: zero-point, slope and scatter passive evolution of stellar populations formed at z>2-3. Slope is primarily driven by mass-metallicity relation. Morphologically (HST)-selected Es and S0s (Bower et al. 1992, Aragon-Salamanca et al. 1993, Rakos et al. 1995, Stanford et al. 1995, 1996, 1997, 1998, Schade et al. 1996, 1997, Ellis et al. 1997, Lopez-Cruz 1997, Kodama et al. 1998, Barger et al. 1998, van Dokkum et al.1998, 1999, 2000, 2001, Gladders et al. 1998, de Propris et al. 1999, Terlevich et al. 1999, 2001, Vazdekis et al. 2001, Andreon 2003, Merluzzi et al. 2003; Rosati et al. 1999, Lubin et al. 2000, Stanford et al. 1998, 2002, Kajisawa et al. 2000, van Dokkum et al. 2000, Blakeslee et al. 2003) Fundamental Plane, Mass-to-Light ratios and Mg-sigma relation (van Dokkum & Franx 1996, Kelson et al. 1997, 2000, 2001, van Dokkum et al. 1998, Bender et al. 1996, 1998, Ziegler & Bender 1997, Ziegler et al. 2001, Holden et al. 2004) Bright-end of K-band (mass) luminosity function (Kodama & Bower 2004, Toft et al. 2004, Strazzullo et al. 2006) Z = 1.24 Blakeslee et al. 2003

  10. The big problem for hierarchical models like CDM: For the biggest galaxies, the halos continue to merge until late times, z~1 or even z~0.5. This is why a picture in which ellipticals were made by merging spirals at late times seemed the “perfect fit.” However, the stars of elliptical galaxies (and all big spheroids = bulges) really are old, and they are enhanced in alpha-elements compared to spirals.The stars in spheroids seem to be uniformly old, very few, or none of them, are young. Dressler

  11. Passive Galaxies: The Classical PictureHomogeneity of Cluster E/S0 U-V Colors z  0.0 z  0.5 (HST) Virgo & Coma: (U-V)o< 0.05 (Bower, Lucey & Ellis 1990, Bower et al 1998) Morphs: <z> = 0.5 sample: (U-V)o< 0.07 (Ellis et al 1997)

  12. Kelson et al. 2001 Ellis et al. 1997 (ApJ 483,582) UV scatter Fundamental plane -0.4 -0.2 0.0 0.2 0.4 V-I (Rest frame U-B) Kuntschner & Davies (1998 MNRAS, 295, L33) Spectral signatures V-I I814

  13. Tight color-luminosity relations: stars are old z  0.5 z  0 U-V (U-V)0 Universal relation for Es and S0s (Sandage & Visvanathan 1978) Scatter dominated by observational errors (Bower et al 1990, Bower et al 1998) (U-V) is sensitive probe of decline rate of MS component (Buzzoni 1989) uniform star formation history: synchronisation of recent activity or old stellar population zF > 3

  14. Unfortunately, degeneracy between age and metallicity -

  15. Young populations are bright Young populations are hot Metal poor populations are hot Old populations are faint Old populations are cool Metal rich populations are cool

  16. Integrated Colors Young populations are BLUE Metal poor populations are BLUE Old populations are RED Metal rich populations are RED but degenerate… From Colors AGE, Z

  17. Spectral Indices Broad Band Colors are affected by the AGE-METALLICITY DEGENERACY Spectral indices have been introduced to overcome this problem

  18. Kuntschner & Davies 1997

  19. Lick Indices Measurement: EW, e.g.: MAG, e.g.:

  20. SSP Lick Indices:an example Metallic line strenghts increase with both AGE and Metallicity Hβ gets weaker as age increases and as Metallicity increases Use combination of metallic and Balmer line strengths to solve the AGE-METALLICITY degeneracy

  21. A Balmer line versus a metallicity indicator….

  22. In this way, obtaining luminosity-weighted ages and metallicity (~epoch of most recent star formation)

  23. Alpha elements overabundance in Es Worthey, Faber & Gonzales 1992: At given Fe index, the data Mg index is stronger than the model predictions Interpreted as a supersolar Mg/Fe ratio Among various possibilities: Short Formation timescales for Es Solution: Find a combination of indices that does not depend on overabundance (eg Thomas et al., Tantalo et al., et al.)

  24. RELATED ISSUES AND PROBLEMS: In practice, galaxies are not SSPs !!! (again, degeneracies…) Dust normally considered negligible in non-star-forming galaxies Emission can get in the way: filling of Balmer lines Slit effects Never trust absolute ages, only relative ones You get what you’ve put in: model limits There is not a “best method” in an absolute sense. It depends on resolution and S/N of the data

  25. EVOLUTION OF S0s NS0/NE 0 0.6 Redshift Dressler et al. 1997 Fasano et al. 2000

  26. Stellar populations as a function of galaxy morphology Reality of E-S0 differences “confirmed” from spectroscopy and colors (Kuntschner & Davies 1998 in Fornax, Terlevich et al. 1999 in Coma, Smail et al. 2001 in A2218, Poggianti et al. 2001 in Coma, Thomas 2002 PhD Leiden ENACS) but not all studies find differences (Ellis et al.1997, Jorgensen 1999, Lewis et al. 2001, Ziegler et al. 2001)

  27. The age of ellipticals Ellipticals in clusters terminated their SFH at high redshift In contrast, a significant fraction of the S0 galaxies finished forming stars more recently Fornax cluster -- Kuntschner & Davies 1998 (also Coma cluster Poggianti et al. 2001b, Abell 2218 Smail et al. 2001)

  28. Poggianti et al. 2001b

  29. Jones, Smail & Couch 2001

  30. Stellar populations as a function of galaxy morphology Reality of E-S0 differences “confirmed” from spectroscopy and colors (Kuntschner & Davies 1998 in Fornax, Terlevich et al. 1999 in Coma, Smail et al. 2001 in A2218, Poggianti et al. 2001 in Coma, Thomas 2002 PhD Leiden ENACS) but not all studies find differences (Ellis et al.1997, Jorgensen 1999, Lewis et al. 2001, Ziegler et al. 2001) : due to delay between evolution of SF and morphology? (Poggianti et al. 1999) – or to the different luminosity distribution of samples? (P. et al. 2001)

  31. Trends with galaxy mass/luminosity Poggianti et al. 2001a

  32. Differences Es vs S0s **not** visible at the brightest magnitudes Poggianti et al. 2001

  33. DOWNSIZING EFFECT MB >-15.6 -17.3 <-18.6 empty circles: lum. wei. age > 9 Gyr z=1-1.5 crosses: 3 < age < 9 Gyr filled circles: age < 3 Gyr z=0.25 Poggianti et al. 2001a

  34. THE BUILD-UP OF THE RED COLOR-MAGNITUDE SEQUENCE ESO Distant Cluster Survey, De Lucia et al. 2004

  35. The build-up of the CM sequence De Lucia et al. 2004 ApJL A deficiency of red galaxies at faint magnitudes compared to Coma -- A synchronous formation of stars in all red sequence galaxies is ruled out -- Most luminous galaxies are the first ones to conclude their SF activity - The more luminous, the older their stellar populations,and the higher the redshift of their last SF Downsizing effect: the star formation histories of galaxies are anti-hierarchical See Tran et al. 2003 & Poggianti et al. 2004 for downsinzing of the post-starburst cluster population

  36. Results: larger galaxies older than smaller ones VIRGO Caldwell et al. (2003) Yamada etal. (2005a) σ Cowie et al. 1996 Kauffmann etal. (2003)

  37. Downsizing-effect Going to lower redshifts, the maximum luminosity/mass of galaxies with significant SF activity progressively decreases. Active star formation in low mass galaxies seems to be (on average) more protracted than in massive galaxies. IN ALL ENVIRONMENTS. The more luminous/massive, the older their stellar populations, the higher the redshift of their last SF activity More massive galaxies on average older, more metal-rich, higher alpha/iron

  38. z > 1-1.5 Epoch of latest star formation in galaxies in the Coma cluster 0.25 < z < 1 z < 0.25 3% 22% 18% 58% 20% 79% IN NUMBER OF GALAXIES IN TOTAL STELLAR MASS

  39. Stars in cluster ellipticals are old, there seems little doubt, and they also appear to be assembled into mature galaxies very early. But, cluster ellipticals are rarer than those in lower-density environments -- are these “field” ellipticals really all that different (stellar age, assembly age) from cluster E’s?

  40. Declining Red Sequence to z=1: Agreement with CDM? Color-photometric z’s in COMBO-17 Red luminosity density COMBO-17 data suggests 3 decline in `red sequence’ luminosity density to z=1: consistent with hierarchical predictions (Bell et al ApJ 608, 752 2004)

  41. Gemini Deep Deep Survey By contrast, the Gemini DD Survey find an abundance of high mass old objects at redshifts z>1 - in seeming contradiction with the COMBO-17 results to z~1? (Can reconcile these contradictory observations if mass assembly is itself mass-dependent) R-K Redshift Glazebrook et al Nature 430, 181 (2004)

  42. Gemini Deep Deep Survey: Spectroscopic Age-dating • 20 red galaxies z~1.5, age 1.2 - 2.3 Gyr, zF=2.4 - 3.3 • Progenitors have SFRs ~ 300-500 M yr-1 (sub-mm gals?) McCarthy et al Ap J 614, L9 (2004)

  43. “Evolved Galaxies at z>1.5 from the Gemini Deep Deep Survey: The Formation Epoch of Massive Stellar Systems”, McCarthy et al. (GDDS), 2004 ApJ, 614, L9 “Conservative age estimates for 20 galaxies with z > 1.3…give a median age of 1.2 Gyr and zf = 2.4. One-quarter of the galaxies have inferred zf > 4. Models restricted to [Fe/H] ~0 give median ages and zf of 2.3 Gyr and 3.3, respectively. These galaxies are among the most massive and contribute 50% of the stellar mass density at 1 < z < 2. …Our results point toward early and rapid formation for a significant fraction of present-day massive galaxies.” Can feedback with hierarchical CDM explain? “Both composite spectra show strong Mg II 2800, Mg I 2852 absorption and broad spectral features due primarily to Fe II absorption. Overlaid in red is a single-burst Bruzual & Charlot spectral synthesis model with an age of 2 Gyr, solar abundances, and a Salpeter IMF cutoff at 120 Msun” “Spectra of evolved GDDS galaxies with z > 1.3. The SDSS LRG composite has been overlaid on each spectrum, and an offset has been applied to each, in steps of 10^-18 ergs cm-2 Å-1”

  44. How much has the red sequence grown since z = 1? Faber et al. (Deep2)-- a factor of 4 growth since z=1, and this includes the brightest (most massive) red galaxies -- need a lot of merging and quenching to accomplish that! Borch et al. (Combo-17) 2006 A&A, 453, 869. “We find that the total stellar mass density of the universe has roughly doubled since z ~ 1…Intriguingly, the integrated stellar mass of blue galaxies with young stars has not significantly changed since z ~ 1…instead, the growth of the total stellar mass density is dominated by the growth of the total mass in the largely passive galaxies on the red sequence.” Cimatti et al. 2006 A&A 453, L29 -- same conclusion, a factor of two growth in the red sequence, and no growth for the massive systems. Brown et al. 2006 astro-ph 0609584 -- NOAO and Spitzer IRAC survey: “…the stellar mass contained within the red galaxy population has roughly doubled over the past 8Gyr. This is consistent with starforming galaxies being transformed into <L* red galaxies by a decline in their star formation rates.” Only passive evolution for >4L* galaxies since z=1. “ While red galaxy mergers have been observed, such mergers do not produce rapid growth of 4L* red galaxy stellar masses between z=1 and the present day.”

  45. Brown et al. and Cimatti et al. emphasize that, if only a factor of two in mass is added to the red sequence since z~1, and it is mainly in lower luminosity (< 1011Msun) galaxies, then simple “running down” of star formation in disk galaxies, turning them red, can account for the growth. A key point to be resolved, and one that may be telling as to how much the hierarchical picture is in trouble.

  46. Post-starburst galaxies: a class apart

  47. EW(Hdelta) (4102 A) > 3 A and no line detected in emission Balmer lines in absorption are best indicators of “recent” SF (some more than others)

  48. ONE STAR Martins et al. 2005

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