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The Carina Dwarf Spheroidal Galaxy: a Goldmine for Cosmology and Stellar Astrophysics

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The Carina Dwarf Spheroidal Galaxy: a Goldmine for Cosmology and Stellar Astrophysics Astronomical Science The Carina Dwarf Spheroidal Galaxy: A Goldmine for Cosmology and Stellar Astrophysics Peter B. Stetson1 support previous photometric evidence Carina plays a key role among the LG Matteo Monelli2, 3 that both the old (12 Gyr) and the inter- dSph galaxies for many reasons: Michele Fabrizio4 mediate-age (4–6 Gyr) subpopulations i) it is relatively close, and its central Alistair Walker5 show a limited spread in chemical com- density is modest; Giuseppe Bono4, 6, 7 position. The use of the Carina galaxy ii) it shows multiple star formation epi­ Roberto Buonanno4, 8 field as a possible standard stellar field sodes, separated in time and in Filippina Caputo6 is outlined. the amount of stellar mass involved Santi Cassisi9 (Monelli et al., 2003); Carlo Corsi6 iii) it hosts a broad variety of variable Massimo Dall’Ora10 Introduction stars ranging from low- and intermedi­ Scilla Degl’Innocenti11, 12 ate-mass hydrogen-burning dwarf Patrick François13 Dwarf galaxies play a fundamental role Cepheids to low- and intermediate- Ivan Ferraro6 in several astrophysical problems. Current mass helium-burning stars (Anomalous Roberto Gilmozzi7 cosmological simulations predict dwarf Cepheids; Dall’Ora et al., 2003); Giacinto Iannicola6 satellite populations significantly larger iv) high resolution spectra are available Thibault Merle14 than the number of dwarfs observed near for fewer than a dozen bright red giants Mario Nonino15 giant spirals like the Milky Way and M31. (RGs). The mean metallicity is [Fe/H] = Adriano Pietrinferni9 This discrepancy is known as the missing –1.69 with a spread of 0.5 (Koch et al., Pier Prada Moroni11, 12 satellite problem and challenges the 2008); Luigi Pulone6 current most popular cosmological model: v) calcium triplet measurements based Martino Romaniello7 the Lambda Cold Dark Matter paradigm on medium resolution spectra are also Frederic Thévenin14 (Madau et al., 2008). However, it is not available for a large sample (437) of yet known whether this discrepancy is RG stars (Koch et al., 2006) with a due to limitations in the theoretical mod­ peak at [Fe/H] = –1.72, but ranges from 1 DAO/HIA/NRC, Victoria, Canada elling or to observational bias at the –2.5 to –0.5. Using the same spectra, 2 IAC, Tenerife, Spain faint end of the galaxy luminosity function but different selection criteria, Helmi 3 Univ. La Laguna, Tenerife, Spain (Kravtsov, 2010). et al. (2006) found a metallicity distri­ 4 Univ. Tor Vergata, Roma, Italy bution from about –2.3 to –1.3. 5 NOAO/CTIO, La Serena, Chile The distribution of dwarf galaxies in 6 INAF–OAR, Monte Porzio Catone, Italy multi-dimensional parameter space indi­ However, we are facing a stark discrep­ 7 ESO cates that they follow very tight relations. ancy between the spread in metallicity 8 ASDC. Frascati, Italy In particular, Prada & Burkert (2002) suggested by spectroscopic measure­ 9 INAF–OACTe, Teramo, Italy suggested that Local Group (LG) dwarf ments and photometric metallicity indica­ 10 INAF–OAC, Napoli, Italy galaxies show strong correlations (Fun­ tors. Deep and accurate colour–magni­ 11 Univ. Pisa, Pisa, Italy damental Line [FL]) between mass-to- tude diagrams (CMDs) of Carina indicate 12 INFN, Pisa, Italy light (M/L) ratio, surface brightness and that the width in colour of the red giant 13 Obs. de Paris-Meudon, Paris, France metallicity (Z). They explained the corre­­ branch (RGB) is quite limited (Monelli 14 Obs. Côte d’Azur, Nice, France lation between M/L ratio and Z using a et al., 2003; Harbeck et al., 2001). This 15 INAF–OAT, Trieste, Italy simple chemical enrichment model where evidence was further supported in a the hot metal­enriched gas, at the end recent inves tigation by Bono et al. (2010) of the star formation epoch, is lost via employing a large number of multiband We present deep and precise multiband galactic winds (Mac Low & Ferrara, 1999; (U, B, V, I) CCD images from ground- (U, B, V, I) optical data for the Carina Woo et al., 2008). Furthermore, the FL of based telescopes covering the entire dwarf spheroidal galaxy. Data were col- dwarf irregulars (dIs) in a five-dimensional body of the galaxy. lected using three different ground- parameter space (total mass, surface based telescopes (4-metre Blanco brightness, rotation velocity, metallicity CTIO, MPG/ESO 2.2-metre, 1.5-metre and star formation rate) is linear and tight, Photometric datasets and photometric CTIO) and cover the entire body of and extends the scaling relations of calibration the galaxy. We discuss the reliability of giant late-type galaxies to lower mass. the absolute photometric zero-point On the other hand, the FL of dwarf sphe­ The data were collected in various calibrations of images collected with roidals (dSphs) in a four-dimensional observing runs between December 1992 large-format and mosaic CCD cameras. parameter space (total mass, surface and January 2005. They include images The typical precision for the B- , V- brightness, rotation velocity and Z) is also from three telescopes: the Cerro Tololo and I-bands is better than 0.01 mag. linear and very tight, but is not an ex­­tra­ Inter-American Observatory (CTIO) The U-band is a little worse. A prelimi- polation of the scaling relations of giant 1.5-metre telescope with a single Tektro­ nary comparison with scaled-solar early-type galaxies. nix 2K CCD, the CTIO 4-metre Blanco and α-enhanced evolutionary predic- telescope with the Mosaic II camera, and tions covering a broad range of ages the MPG/ESO 2.2-metre telescope with and chemical compositions seems to the Wide Field Imager (WFI) camera (both 32 The Messenger 144 – June 2011 proprietary and archival data). The data nificant part of his career attempting to repeatability of the observations. For our discussed here represent 4152 individual ensure that measurements of very faint measurements we were able to impose CCD images with essentially complete stars obtained with CCDs on large tele­ the additional condition that each star coverage of the central regions of Carina scopes are on the same photometric shows no evidence of intrinsic variation at (about 40 by 55 arcminutes, lacking only system as measurements of very bright a level greater than 0.05 mag root mean those regions obliterated by bright fore­ stars made with photo multipliers on small square (rms), when the data from all filters ground stars). They were obtained in four telescopes (Stetson, 2000; 2005)1. are considered. photometric bands (B and I with the 1.5-metre telescope, and UBVI with both The question of whether the photometry The left panels of Figure 1 show the mag­ the 4-metre and 2.2-metre telescopes). discussed here is on the standard system nitude residuals (in the sense published The resulting colour image is shown on breaks down into two parts: minus ours) plotted versus apparent p. 20. i) the extent to which the CCD photome­ visual magnitude, while the right panels try, in general, is on the standard sys­ of Figure 1 plot the same residuals The photometric data were reduced tem; and against the B–I colour. The differences using the DAOPHOT/ALLFRAME package ii) the extent to which the photometry for show a systematic departure of a few (Stetson, 1987; 1994). Because of the Carina, in particular, is on the same hundredths of a magnitude between our multiple chips and pointings, any given system as the rest of the photometry. photometric system and the standard star may have up to 17 calibrated meas­ As of now, 2076 datasets have been system for stars brighter than approxi­ urements in U, 156 in B, 207 in V, and homogeneously analysed through our mately V = 9. We infer that the stars with 70 in I. A total of 205 338 individual stars software; among these, 1591 are V ≤ 9 were simply too bright to be meas­ were catalogued and measured. Among considered to be of photometric qual­ ured with our equipment, and we were these, 72 595 have photometric measure­ ity, and 485 have been analysed in unduly optimistic about these measure­ ments in all four filters, while 129 230 non-photometric mode (viz. data taken ments. We therefore advise readers to have at least V and either B–V or V–I. The under non-photometric conditions). ignore Stetson’s results for standard stars remainder, either extremely faint (i.e., with V ≤ 9 and rely entirely upon the detectable in only one filter) or located Figure 1 shows the magnitude differ­ previously published photometric indices near the periphery, have astrometry and ences between the calibrated standard­ of such stars instead. Figure 1 demon­ instrumental magnitudes only. system magnitudes from our own ob­­ strates that our photometry matches the servations of the fundamental photometric definitive published results with a high The fundamental standard system of standards, as compared to the pub- degree of fidelity over a dynamic range of photometric magnitudes was established lished values for the same stars. In order ~600 in flux (9 <~ V <~16) and over the decades ago using photoelectric pho­ to restrict the comparison to stars full available range of stellar temperature. tometers on small telescopes — mostly with the most reliable data, we consider Quantitatively, the weighted mean dif- of apertures 0.4–0.9 metres (e.g., Land­ only magnitudes based on at least five ferences between the published results olt, 1973; 1988; 1992; Graham, 1982; and independent measurements obtained on and ours are +0.0012 ± 0.0028, +0.0016 references therein to the establishment photometric occasions, and having ± 0.0013, −0.0007 ± 0.0009 and −0.0025 of UBV by Johnson and RI by Cousins). standard errors of the mean not greater ± 0.0013 mag in U, B, V and I, respec­ One of the authors (PBS) has spent a sig­ than 0.02 mag based on the internal tively, based upon 109, 209, 220 and Figure 1.
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