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Unveiling the properties of distant stellar populations. Collaborators @ INAF-Osservatorio Astronomico di Teramo SPoT group www.oa-teramo.inaf.it/spot Enzo Brocato Gabriella Raimondo Ilaria Biscardi Other institutions
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Unveiling the properties of distant stellar populations • Collaborators @ INAF-Osservatorio Astronomico di Teramo • SPoT groupwww.oa-teramo.inaf.it/spot • Enzo Brocato • Gabriella Raimondo • Ilaria Biscardi • Other institutions • John P. Blakeslee - Herzberg Institute of Astrophysics, Canada • Massimo Capaccioli - Università degli studi di Napoli, Italy Michele Cantiello INAF Osservatorio Astronomico di Teramo - Italy
Outline • The study of stellar population properties: resolved vs unresolved populations Methods to analyze integrated stellar light • Examples discussed • Extragalactic Globular Clusters: colour bimodality in early type galaxies, simply a change of ingredients? • Surface Brightness Fluctuations: a powerful tool to study field stars in galaxies!
Most of the light emitted by a galaxy comes from its stars Stars in galaxies* studying the properties of stars studying properties of the host galaxy Unresolved stars Resolved stars • Line of sight integrated properties multi-method approach • Photometry (colors, magnitudes, surface brightness profiles, etc.) • Spectroscopy (many features, spectral indexes, etc.) • Other methods… Nice, but unfeasible for distant unresolved stellar populations! Cignoni et al., (2009) Age/metallicity degeneracy Synergy of methods SPoT models Raimondo et al., 2005 and references therein www.oa-teramo.inaf.it/spot Courtesy of ESO website * I won’t care of dust and gas!
Harris et al., 2009 Star Clusters Nearly single age and single metallicity stellar systems • Why we care of star clusters in general - Because a large fraction of stars were born in clusters (Lada & Lada 2003) - Relatively easy to recognize, as lighthouses on a “smooth” background • … and why we care of GCs? - Old simple stellar populations: ideal for the comparison to models - Luminosity Function of GCs is a D.I. - Surface density profiles; • Radial colour profiles; • Sizes (in some cases possible); - … colour histograms Spitler et al., 2006 NGC5866 Cantiello et al., 2007 Cantiello et al., 2007 Larsen et al., 2001
GC colour bimodality Peng et al. (2006) Predicted (Ashman & Zepf, 1992) and observed in bright ellipticals (Elson & Santiago 1996, Geisler et al. 1996, Forbes et al. 1997, Gebhardt & Kissler-Patig et al. 1999, Kundu et al., 2001, Larsen et al., 2001… and a lot more!) Physical origin of bimodal colours? …look at the MW! Bimodal colours Bimodal Metallicity Possible scenarios • Blue/Red age and metallicity gap (dissipative massive merging, e.g., AZ92) • Z-gap alone (Hierarchical formation; e.g., Cotè et al., 1998) both with pros and cons (West et al., 2004) Cotè (1999) Why should Bimodal colours Bimodal Metallicity ?
Bimodal colours Bimodal Metallicity Strictly true only if the colour-metallicity relations are linear …or “nearly” so … is (are) the colour-metallicity relation(s) linear?
GC bimodality try to change an ingredient… Models (or numerical simulations, as you prefer) Peng et al. (2006) Observations Yoon et al. (2006) Cantiello & Blakeslee (2007) Ok with all models (accretion, major merging with intense SF, etc.), but do we really need a BIMODAL [Fe/H]? Earlier indications from: Harris & Harris, 2002 Cohen et al., 2003 … keep in mind the optical-to near IR colours, e.g, (V-K)!
GC bimodality: comparing optical & optical to near-Ir data… Larsen et al. (2005) Larsen et al. (2001) NGC4472 NGC4365 Hempel et al. (2007) Peng et al. (2006) Spitler et al., 2008: bimodal B-L (Spitzer data) in Cen A! Also Sombrero but to few objects available! …but too few objects, biased toward brigh & red GCs
Harris et al. (2006) One last slide on bimodality Observed feature in bimodal GCs systems: blue tilt mainly massive early-types (Harris et al. 2006; Strader et al. 2006; Mieske et al. 2006; Spitler et al. 2006; Cantiello et al. 2007; Peng et al. 2009; etc.) a mass-metallicity relation? Self-enrichment of massive clusters… unexpected but, possibly, observed also in Local Group clusters (MW GCs with multiple sequences NGC2808, NGC1851, w Cen Piotto et al. 2007, Milone et al. 2008) A toy-model: Unimodal Fe/H distribution including ~ self-enrichment +non-linear CMR (Blakeslee et al., 2009, submitted) …blue tilt vs galaxy luminosity New/different ingredient for the recipe of GC systems: not a bimodal [Fe/H], but, a broad unimodal distribution (with some kind of self enrichment)
“Surface Brightness Fluctuations” what? At the beginning: Tonry & Schneider (1988, TS88) a method to derive distances for ellipticals, up to ~20 Mpc Cantiello M. Phd thesis (2004) Now • SBF for star clusters dwarfs, bulges of spirals, galaxies with peculiar morphologies, besides normal ellipticals (Ajhar & Tonry, 1994; Tonry et al., 2001;Cantiello et al., 2005, 2007; Raimondo et al. 2005, 2009; Biscardi et al., 2008) • Distances from few Kpc, up to >100 Mpc: a key method to reduce uncertainties in the distance scale ledder: skip many other indicators And that’s not the whole story
SBF 101 TS88: “mottling” ≡ the ratio of the 2nd to the 1st moment of the stellar luminosity function Mbar = inili2/inili, Mbar ~ mean luminosity of RGB stars Mbar~ constant DM=mbar-Mbar Alas #1) Measuring SBF is non-trivial • Model the galaxy and subtract the model; • Mask all internal (GCs, dust) and external sources (galaxies, stars); • Estimate the amplitude of the fluctuation in the Fourier domain (because the residual image is convolved with the PSF) • Subtract to the total fluc. amplitude a reasonable estimate of the contribution from unexcised sources (background galaxies, GCs, stars, etc.) M32 @ 0.75 Mpc Not easy, but feasible Tonry+ SBF survey ~ 300 galaxies ACSVCS (Cotè et al.) + ACSFCS (Jordan et al.) ~ 150 galaxies and many other measures! N7768 @ 100 Mpc *RGB because stars must be old if you want measure SBF
SBF to study stellar populations? SPoT models Raimondo et al. (2005) Cantiello et al. (2007) measures Alas #2) SBF ~ mean luminosity of RGB stars* Median RGB population change from galaxy to galaxy depending on the history of star formation of the galaxy! Mbar ≠ constant! Mbar=-1.6 + 4.5[(V-I)-1.15], empirically and theoretically well calibrated (Tonry et al., 2001; Jensen et al., 2003; Cantiello et al., 2007; Worthey, 1994; Vazdekis et al., 2001; Raimondo et al., 2005) …why not to use SBF to study stellar populations? SBF magnitudes properties of bright stars in a population Classical colors and mags most populated phase (H-burning MS stars) I-band & near-IR SBF are more sensitive to [Fe/H] variations (because of the RGB sensitivity to metallicity) what impact on the age/metallicity deg.? *More in details: the SBF magnitude is the mean luminosity of the brightest stars in a system. Optical to near-IR: RGBs and AGBs; U and B: HB stars play a key role!
The last slide on SBF: t/Z degeneracy SPoT models The age/metallicity degeneracy… … with SBF colors is partially lifted, if not removed at all! Applications up to now: • SBF colors, few data (Jensen et al. 2003; Cantiello et al. 2007) • SBF gradients: Z-variation preferred to age (Cantiello et al., 2005) • SBF for relatively young resolved stellar systems (for calibration purposes; Raimondo et al., 2005, 2009) Need more optical to near-IR observational data! High [Fe/H] Age Low [Fe/H]
Summary • To deconvolve the information lost in the integration of the light from many (millions!) different stars, the study of distant unresolved stellar systems cannot rely on one single indicator, either spectroscopic or photometric • Precise measurements, and accurate models help in providing reliable constraints to fundamental paramters that govern astrophysical processes Examples shown here: • GCs bimodality: accurate models, and measurements, show that a unimodal [Fe/H] with non-linear CMR & some self-enrichment could explain the observed colour bimodality • SBF colours and gradients: powerful tool to study field stars
…and the future Why SBF and GCs together? • Optical to near-IR colours are ideal to reveal true [Fe/H] bimodalities. Need more (and more accurate) data • Optical to near-IR SBF colours are ideal to partially lift the age-Fe/H degeneracy • The observational requirements for observations of GCs and SBF are nearly the same! …waiting for more data …
Thanks! a view of Gran Sasso mountain from the Observatory of Teramo
Cantiello & Blakeslee (2007) Puzia et al. (2005)
Peng et al. (2006) NGC5128 Beasley et al. (2008) Spitler et al. (2008)
Cohen et al. (1998) Strader et al. (2007)