Distances and Stellar Population properties using the SBF method
Michele Cantiello, Gabriella Raimondo, Enzo Brocato, Ilaria Biscardi
aa r X i v : . [ a s t r o - ph ] D ec Mem. S.A.It. Vol. 75, 282 c (cid:13) SAIt 2008
Memorie della
Distan es and Stellar Population propertiesusing the SBF method
M. Cantiello, G. Raimondo, E. Brocato and I. Biscardi
INAF – Osservatorio Astronomico di Teramo, Via Maggini snc, I-64100 Teramo, Italye-mail: [email protected]
Abstract.
We present some results on the study of stellar population properties and dis-tances of galaxies using the SBF technique. The applications summarized here show thatthe Surface Brightness Fluctuations (SBF) method is able to i ) provide accurate distancesof resolved and unresolved stellar systems from ∼
10 Kpc to ∼
150 Mpc, and ii ) to reliablyconstrain the physical properties (e.g. age and metallicity) of unresolved stellar systems. Key words.
Galaxies: stellar content — Galaxies: distances
1. Introduction
The knowledge of the complex evolution of thestellar component in high redshift galaxies re-lies on how detailed is our understanding ofthe history of stars in nearby galaxies. The bestway to trace back the star formation episodesin a galaxy is to study resolved stars. However,this is possible only for few nearby objects,tipically using the best observational facilitiesto date, and only for the brightest stars in thepopulation.Given such limitations several techniqueshave been proposed to disentangle the proper-ties of unresolved stellar systems, one of theseis the Surface Brightness Fluctuations method(Tonry & Schneider 1988, SBF hereafter). TheSBF technique was introduced as a distanceindicator for elliptical galaxies within ∼ Send o ff print requests to : M. Cantiello objects: ellipticals, bulges of spirals, dwarf el-lipticals, globular clusters, etc. In addition, it iswell accepted that SBF magnitudes and colorsrepresent a potential tool to analyze in detailsthe physical and chemical properties of unre-solved and resolved stellar systems.In this paper we will briefly discuss someapplications of the SBF method done at theINAF-Observatory of Teramo by the SPoT group.
2. SBF by the SPoT Group
To derive distances from measured SBF mag-nitudes one needs a calibration of absolute SBFversus one integrated color (Tonry et al. 1990).The calibration is usually derived following an empirical approach (Tonry et al. 2001), whichsu ff ers for the usual drawbacks: one needs toknow in advance the distance of few objects / SPoT. Cantiello et al.: Distances and Stellar Populations with SBF 283 in order to derive the zeropoint of the calibra-tion; the calibration can be obtained for one fil-ter at a time, etc. To avoid these problems onecan derive theoretical calibrations by means ofstellar population models.Taking advantage of the expertise by mem-bers of our group in the numerical sinthesysof stellar populations (e.g. Brocato et al. 1999,2000), we derived SBF models for stellar sys-tems in the age range ∼
50 Myr - 15 Gyr, formetallicity [Fe / H] from ∼ − . z ′ usingthe SPoT models agree nicely with the empir-ical ones (e.g. Biscardi et al. 2008). Also, thecomparison of near–IR models with availabledata has shown that the general behaviour ofobservations is well reproduced.It is worth to emphasize that our approachto derive SBF models is original with respect toother methods. Our technique, in fact, is basedon the synthesis of Simple Stellar Populations(SSP) and it has two main advantages (seeRaimondo et al. (2005), Sect. 3). First, multi-band Color-Magnitude Diagrams, integratedcolors and luminosity functions are availablein addition to SBF models. Thus, models aretested against the observed SBF magnitudes and also versus other astronomical observablesof resolved and unresolved systems. Second,the SPoT code allows the user to set manydi ff erent input parameters (stellar tracks, massloss, IMF, atmosphere models, etc.). As a con-sequence, the sensitivity of SBF to variousproperties of the stellar population can be ana-lyzed.As an example, in our first release of SBFmodels we showed that the properties of hotevolved stars in old stellar systems (HB, Hot-HB, Post-AGB) can be explored using SBFmagnitudes in bands like B, or U. The futureclass of detectors with increased e ffi ciency inthe blue bands - like WFC3 on board of HST,or the WSO satellite (Pagano et al. 2007) - will be of great interest for SBF applications to thewavelength interval below 5000 Å. As mentioned above the SBF was proposed asa method to measure distances. To estimate thedistance of a galaxy the SBF magnitude and,usually, the integrated ( V − I ) color are ob-tained in one single annulus. However, it is rea-sonable to expect that SBF variations can beobserved within a single galaxy.In 2003 we started a campaign aimed at thedetection and study of SBF gradients in galax-ies. For this purpose we have developed a pro-cedure optimized to reveal radial variations ofSBF magnitudes. In our first study of SBF gra-dients - based on I-band images of eight ellip-ticals observed with ACS - we found that thepresence of SBF gradients seems to be corre-lated with the mass of the galaxy. In particu-lar, less massive objects do not show signifi-cant gradients (Cantiello et al. 2005).The presence of SBF gradients, though notunexpected, represents a further piece of in-formation to study the evolutionary path ofgalaxies. Moreover, gradients do not dependon the distance of the object, so any compar-ison with models is free from this heavy uncer-tainty. Comparing data with SPoT models wehave pointed out that the amplitude of the SBFversus color gradient seems to be dominatedby a [Fe / H] variation along the radius of thegalaxy, rather than to age variations. Such re-sult appears even more meaningful if one con-siders that the only galaxy in our sample with agradient likely dominated by age variations isNGC 1344, a galaxy that shows morphologicalirregularities possibly related to a recent gravi-tational interaction.This study has been recently extended,again using ACS data, both in the I- and V-band (Cantiello et al. 2007). The study com-prises 14 galaxies spanning a large interval oftotal magnitudes ( ∼
10 mag), and with very dif-ferent properties. The new data confirm the re-sults found in our previous study, and suggestthat a larger database of SBF measurementswill provide a valuable tool to analyze stars ingalaxies.
84 M. Cantiello et al.: Distances and Stellar Populations with SBF
Fig. 1.
Upper panel: Color-color diagram obtained using the optical-to-near–IR integrated colors V-K andI-K. The metallicity is color coded - [Fe / H] = − . − . − . − .
3, 0 . + . ff erently from upper panel, models with di ff erent chemical compositions are well separated from eachother. Note that the magnitude interval span by the x– and y–axis is the same in both panels. In general, our study on the use of SBF as atracer of stellar population properties - via SBFgradients, absolute SBF magnitudes, or opticalSBF colors - demonstrated that the metallicityof the dominant stellar component can be bet-ter confined with respect to age, unless opticalto near–IR SBF magnitudes are coupled (seenext section).Besides the exploration of the propertiesof unresolved stellar populations, we have alsotaken into account the chance to measure SBFfor the study of resolved stellar systems, andgalaxies at large distances. In the first case wehave carried out a dedicated study on star clus-ters in the Magellanic Clouds, providing thefirst optical SBF measurements for di ff erentMC star clusters, and analysing in details theiroptical and near–IR SBF properties throughdata to models comparisons (Raimondo et al.2005). More recently, we measured SBF mag-nitudes for four distant ellipticals observedwith ACS. The large distances of the ob-jects observed allowed us to estimate H - seeBiscardi et al.’s article in this volume, and ref-erences therein. To date the study of unresolved stellar pop-ulations using SBF has been carried out us-ing three di ff erent approaches: i ) absolute SBFmagnitudes; ii ) optical SBF colors; iii ) SBFgradients. However, since the main applicationof SBF is to derive distances, none of the ap-plications existing in literature is optimized forstellar population studies.In Fig. 1 we show two color-color panels,both obtained using the SPoT models for agesbetween 1.5 and 14 Gyr, and metallicity [Fe / H]from − . ff ects models. On the contrary,the lower panel shows that optical-to-near–IRSBF color models in the age and metallicityregimes considered are well separated, and adata to models comparison will help to put sub-stantial constraints to the metallicity of the sys- . Cantiello et al.: Distances and Stellar Populations with SBF 285 tem. At the same time, thanks to the large sep-aration between models at fixed [Fe / H], theseSBF-color panels can also be used to con-strain the age of the dominant stellar compo-nent within an interval better confined with re-spect to, e.g., integrated colors.The applications of the SBF-color tech-nique described above are to date limited to op-tical colors, or refer to inhomogeneous sets ofoptical and near–IR SBF measurements (e.g.Jensen et al. 2003). In all present applications,however, the comparison of data with modelsconfirm the reliability of the technique pro-posed and of models, and supports the poten-tial of SBF colors to explore the physical andchemical properties of unresolved stellar pop-ulations.
3. Conclusions and Futureperspectives
The SBF technique is to date one of the mostreliable distance indicators for elliptical galax-ies. However, to our point of view, this tech-nique is underestimated with respect to its realpotentiality.Concerning distance measurements, theSBF method is able to provide distances fromfew Kpc up to ∼
150 Mpc with present ob-serving facilities, and possibly to much largerdistances with future instrumentations. Suchunique characteristic gives the SBF the poten-tial to cover the distance scale ladder from lo-cal to low redshift ( z ≤ .
05) distances. Thus,SBF measurements, coupled with a reliablecalibration of absolute SBF magnitudes, pro-vide a great opportunity to substantially reducethe systematic uncertainty that a ff ects the cos-mological distance scale.With regard to stellar population analysis,there are only few studies dedicated to thistopic based on the SBF method. However, it isnow evident that SBF can greatly improve ourunderstanding of the properties of unresolvedstellar systems. We presented SPoT models ina specific SBF optical-to-near–IR color planeshowing the potential of SBF colors to sub-stantially remove the age-metallicity degener-acy. Future application of this technique - pos-sibly coupled with the measure SBF-color gra- dients attainable with the next generation opti-cal and near–IR large FoV detectors - will pro-vide significant constraints to the knowledgeon the formation and evolution of the stellarcomponent in low-redshift galaxies. Acknowledgements.
It is a pleasure to aknowledgeJ. P. Blakeslee and S. Mei for their helpful contribu-tion to the topic of SBF and SBF gradients measure-ments.