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Clues to the Formation of the Milky Way's Thick Disk

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Clues to the Formation of the Milky Way's Thick Disk A&A 579, A5 (2015) Astronomy DOI: 10.1051/0004-6361/201425459 & c ESO 2015 Astrophysics Clues to the formation of the Milky Way’s thick disk M. Haywood1, P. Di Matteo1, O. Snaith1;?, and M. D. Lehnert2 1 GEPI, Observatoire de Paris, CNRS, Universite´ Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France e-mail: [email protected] 2 Institut d’Astrophysique de Paris, CNRS UMR 7095, Universite´ Pierre et Marie Curie, 98bis Bd Arago, 75014 Paris, France Received 4 December 2014 / Accepted 25 March 2015 ABSTRACT We analyze the chemical properties of a set of solar vicinity stars, and show that the small dispersion in abundances of α-elements at all ages provides evidence that the star formation history (SFH) has been uniform throughout the thick disk. In the context of long- timescale infall models, we suggest that this result points either to a limited dependence of the gas accretion on the Galactic radius in the inner disk (R < 10 kpc) or to a decoupling of the accretion history and SFH due to other processes governing the interstellar medium in the early disk, suggesting that infall cannot be a determining parameter of the chemical evolution at these epochs. We argue that these results and other recent observational constraints – namely the lack of radial metallicity gradient and the non-evolving scale length of the thick disk – are better explained if the early disk is viewed as a pre-assembled gaseous system, with most of the gas settled before significant star formation took place – formally the equivalent of a closed-box model. In either case, these results point to a weak or non-existent inside-out formation history in the thick disk or in the first 3–5 Gyr of the formation of the Galaxy. We argue, however, that the growing importance of an external disk whose chemical properties are distinct from those of the inner disk would give the impression of an inside-out growth process when seen through snapshots at different epochs. The progressive, continuous process usually invoked may not have actually existed in the Milky Way. Key words. Galaxy: abundances – Galaxy: evolution – Galaxy: disk – Galaxy: formation 1. Introduction substantial fraction of the mass of galaxies and because it may account for the main increase in the metal enrichment of galaxies The cosmic star formation history (SFH) shows that galaxies since the formation of the first stars. These views are opposite to went through their most vigorous phase of star formation at red- the standard chemical evolution scenario based on long-term in- shift z = 1:5–3 (e.g., Madau & Dickinson 2014). Structures fall (e.g., Chiappini et al. 1997; Fraternali 2014) and the recogni- thought to have formed at this epoch in disk galaxies are clas- tion that the solar vicinity contains too few low-metallicity dwarf sical bulges and thick disks. The stellar content of the nearby stars compared to the “closed-box” model. In other words, ob- universe in environments with average density is dominated by servations of the solar vicinity would suggest that the enrichment pure disk galaxies (Kormendy et al. 2010; Fisher & Drory 2010; rate has been faster than the star formation rate (SFR). Infall Laurikainen et al. 2014). This suggests that classical bulges rep- models have been introduced in order to solve this problem by resent only a minor mass component of galaxies. Thick disks, limiting the dilution of ejected metals to a disk parsimoniously however, are found to be ubiquitous, and are detected on nearly provisioned with gas, and by boosting the increase of the metal- all nearby edge-on disks galaxies (Comerón et al. 2011). Recent licity at early times without creating too many metal-poor stars. results (Ness et al. 2013a,b; Di Matteo et al. 2014, 2015) have Hence, these scenarios leave no room for a substantial compo- confirmed the view that the Milky Way (MW) is also a pure disk nent of intermediate metallicity in the MW. This credo has been galaxy (Shen et al. 2010; Kunder et al. 2012), i.e., it has no sub- questioned by a new scheme presented in Haywood et al.(2013), stantial classical bulge. Furthermore, measurements of its SFH Snaith et al.(2014, 2015), and Haywood(2014a,b). have shown that during the peak of star formation in the uni- Another important feature of the standard chemical evolu- verse, the MW was a star-bursting galaxy and was forming its tion scenario is to make the accretion of gas radially dependent, thick disk (Snaith et al. 2014, 2015; Lehnert et al. 2014). The with slower accretion outward, in order to mimic an inside-out structural parameters of this population, with a short scale length formation. Inner regions form their stars first, because gas den- (Bensby et al. 2011; Cheng et al. 2012a; Bovy et al. 2012) sug- sity increases faster, while outer regions grow at later times and gest that it constitutes most of the mass in the MW’s central re- at a slower pace. Because of the coupling assumed between ac- gion (Ness et al. 2013a,b; Di Matteo et al. 2014, 2015; Haywood cretion and SFR in these models, a natural signature of such a et al., in prep.). scheme would therefore be that SFHs vary with distance from This emerging scheme suggests that the formation of the the Galactic center. Direct measurement of the star formation thick disk is a crucial phase in the formation of disks (see history outside the solar vicinity is not feasible currently; how- Lehnert et al. 2014) because this population could represent a ever, we know that a given SFH leaves specific signatures on the radial and temporal dependence of the chemical abundances ? Current address: Department of Physics & Astronomy, University in stars (Haywood et al. 2013; Snaith et al. 2014, 2015). Since of Alabama, Tuscaloosa, Albama, USA. thick disk stars observed in the solar vicinity were born over a Article published by EDP Sciences A5, page 1 of7 A&A 579, A5 (2015) 14.0 wide range of galactocentric distances (see Sect.2), their chem- 0.4 ical properties should therefore reflect the variety in the radial 12.6 11.2 SFHs of the thick disk, if any. 0.3 In the following, we show that these chemical properties are 9.8 8.4 in fact remarkably homogeneous at all ages in the thick disk, 0.2 7.0 leaving little room for significant variation in the SFH of this /Fe] a [ Age [Gyr] 5.6 population as a function of radius. The outline of the paper is as 0.1 follows: in the following section we briefly describe our data, in 4.2 2.8 Sect.3 we describe the model and present our results, and we 0.0 discuss these results in Sect.4. 1.4 0.0 −0.1 −1.5 −1.0 −0.5 0.0 0.5 2. Data [Fe/H] We use the sample of Adibekyan et al.(2012) already analyzed Fig. 1. [α/Fe]–[Fe/H] distribution at the solar vicinity for our sample. in Haywood et al.(2013). The sample contains 363 stars with age The colors (indicated in the legend) and size (larger implies older) of determinations, to which we added the 36 stars from the sample the symbols indicate the age of the stars. The stars below and to the of Nissen & Schuster(2010) for which we could determine ages. left of the line are either accreted stars according to Nissen & Schuster Ages for this last sample were determined in the same manner (2010; for those objects that have [Fe/H] < −0:7 dex), or metal-poor as those from the Adibekyan et al.(2012) sample in Haywood thin disk stars. They are represented by empty circles in Fig.2, and removed from the age-alpha distribution in Fig.3. et al.(2013). However, as reported by Schuster et al.(2012), the effective temperature of the stars in Nissen & Schuster may have been underestimated. We checked this statement by com- paring the 11 Hipparcos stars in common between the two sam- 0.4 G G ples. All these objects have a lower temperature in Nissen & G 0.40 G G GG G G0.21 Schuster(2010) than in Adibekyan et al.(2012), with a mean G GG GG 0.3 G G G GG GG GG GGGGG 0.02 G G GGGG G G offset of –75 K, which confirms the statement of Schuster et al. GG G G GGGG G G G GGGG −0.17 HR 3138 G G G (2012). Following their recommendation, we added +100 K to 0.2 G GG e] G G G GGG G G −0.36 F G GG G / G G G G G the Nissen & Schuster(2010) values. G G G G G G G e/H] −0.55 G G GG G F G G [ [ G G GG GG G G G GG GGGG GGGG G −0.74 0.1 G G G G In Haywood et al.(2013) the spectroscopic scale of G G GGG G G GGGGGGG GGG GG G G G GGGGGGG G GG GGG GGG GG GGGGGGGGG GGG GGGGGGGGGGGGG G −0.93 Adibekyan et al.(2012) was compared to photometric temper- GG GGGGGGGGGGGGGGGGGGGGGGGG GGGGGGG GG GGGGGGGGGGGGGGGGGGGGGG GGG GG GGG GGGGGGGGGGGGGGGGG GG G −1.12 0.0 G G GGGG atures using the T − V − K calibration of Casagrande et al. G GGGG GGG eff G G G −1.31 G (2010), from which we deduced that the two scales are off G −1.50 by 56 K.
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