Unveiling the properties of distant stellar populations

Michele CantielloINAF Osservatorio Astronomico di Teramo - Italy

Collaborators @ INAF-Osservatorio Astronomico di TeramoSPoT group www.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

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!

SPoT models Raimondo et al., 2005 and references thereinwww.oa-teramo.inaf.it/spot

Stars in galaxies*

Resolved stars

* I won’t care of dust and gas!

studying the properties of stars studying properties of the host galaxy

Unresolved stars

Line of sight integrated properties multi-method approach

•Photometry (colors, magnitudes, surface brightness profiles, etc.)

•Spectroscopy (many features, spectral indexes, etc.)

•Other methods…

Age/metallicity degeneracy Synergy of methods

Most of the light emitted by a galaxy comes from its stars

Cignoni et al., (2009)

Nice, but unfeasible for distant

unresolved stellar populations!

Courtesy of ESO website

Star ClustersNearly 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

Harris et al., 2009

Spitler et al., 2006

Cantiello et al., 2007

Larsen et al., 2001

NGC5866Cantiello et al., 2007

GC colour bimodalityPredicted (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 MetallicityPossible 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)

Peng et al. (2006)

Why should Bimodal colours Bimodal Metallicity ?

Bimodal colours Bimodal MetallicityStrictly 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…

Observations Models (or numerical simulations, as you prefer)

Ok with all models (accretion, major

merging with intense SF, etc.), but do we really need a BIMODAL [Fe/H]?

… keep in mind the optical-to near IR colours, e.g, (V-K)!

Peng et al. (2006)

Yoon et al. (2006)Cantiello & Blakeslee (2007)

Earlier indications from:

Harris & Harris, 2002 Cohen et al., 2003

Hempel et al. (2007)

GC bimodality: comparing optical & optical to near-Ir data…

…but too few objects, biased toward brigh & red GCs

Larsen et al. (2001)

NGC4472

Peng et al. (2006)

Larsen et al. (2005)

Spitler et al., 2008: bimodal B-L (Spitzer data) in Cen A! Also Sombrero but to few

objects available!

NGC4365

One last slide on bimodalityObserved 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, 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)

Harris et al. (2006)

“Surface Brightness Fluctuations” what?

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

At the beginning: Tonry & Schneider (1988, TS88) a method to derive distances for ellipticals, up to ~20 Mpc

Cantiello M. Phd thesis (2004)

SBF 101TS88: “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.)

*RGB because stars must be old if you want measure SBF

Not easy, but feasible Tonry+ SBF survey ~ 300 galaxies ACSVCS (Cotè et al.) + ACSFCS (Jordan et al.) ~ 150 galaxies and many other measures!

SBF to study stellar populations?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)

*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!

SBF magnitudes properties of bright stars in a populationClassical 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.?

…why not to use SBF to study stellar populations?

SPoT models Raimondo et al. (2005)Cantiello et al. (2007) measures

The last slide on SBF: t/Z degeneracy

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!

SPoT models

Low [Fe/H]

High [Fe/H]

Age

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?1) Optical to near-IR colours are ideal to

reveal true [Fe/H] bimodalities. Need more (and more accurate) data

2) Optical to near-IR SBF colours are ideal to partially lift the age-Fe/H degeneracy

3) 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

Puzia et al. (2005)

Cantiello & Blakeslee (2007)

Peng et al. (2006)NGC5128 Beasley et al. (2008)

Spitler et al. (2008)

Cohen et al. (1998)

Strader et al. (2007)