In this letter we examine the evolution of the radial metallicity gradient
induced by secular processes, in the disk of an $N$-body Milky Way-like galaxy.
We assign a [Fe/H] value to each particle of the simulation according to an
initial, cosmologically motivated, radial chemical distribution and let the
disk dynamically evolve for 6 Gyr. This direct approach allows us to take into
account only the effects of dynamical evolution and to gauge how and to what
extent they affect the initial chemical conditions. The initial [Fe/H]
distribution increases with R in the inner disk up to R ~ 10 kpc and decreases
for larger R. We find that the initial chemical profile does not undergo major
transformations after 6 Gyr of dynamical evolution. The final radial chemical
gradients predicted by the model in the solar neighborhood are positive and of
the same order of those recently observed in the Milky Way thick disk.
  We conclude that: 1) the spatial chemical imprint at the time of disk
formation is not washed out by secular dynamical processes, and 2) the observed
radial gradient may be the dynamical relic of a thick disk originated from a
stellar population showing a positive chemical radial gradient in the inner
regions.Comment: 10 pages, 5 figures, Accepted for publication on Astrophysical
  Journal Letter

Curir, A.

Diaferio, A.

Fiorentin, P. Re

Lattanzi, M. G.

Serra, A. L.

Spagna, A.

The Astrophysical Journal

English

arXiv

Serra, Ana Laura

Diaferio, Antonaldo

Institutional Research Information System University of Turin

The Astrophysical Journal Letters, 784:L24 (5pp), 2014 April 1 doi:10.1088/2041-8205/784/2/L24C© 2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A.THE RADIAL METALLICITY GRADIENTS IN THE MILKY WAY THICK DISK AS FOSSILSIGNATURES OF A PRIMORDIAL CHEMICAL DISTRIBUTIONA. Curir1, A. L. Serra1,2,3, A. Spagna1, M. G. Lattanzi1, P. Re Fiorentin1, and A. Diaferio2,31 INAF-Osservatorio Astrofisico di Torino, Italy; curir@oato.inaf.it2 Dipartimento di Fisica, Universita` di Torino, Torino, Italy3 Istituto Nazionale di Fisica Nucleare (INFN), Sezione di Torino, Torino, ItalyReceived 2013 December 6; accepted 2014 February 17; published 2014 March 14ABSTRACTIn this Letter we examine the evolution of the radial metallicity gradient induced by secular processes, in the diskof an N-body Milky Way-like galaxy. We assign a [Fe/H] value to each particle of the simulation according to aninitial, cosmologically motivated, radial chemical distribution and let the disk dynamically evolve for ∼6 Gyr. Thisdirect approach allows us to take into account only the effects of dynamical evolution and to gauge how and towhat extent they affect the initial chemical conditions. The initial [Fe/H] distribution increases with R in the innerdisk up to R ≈ 10 kpc and decreases for larger R. We find that the initial chemical profile does not undergo majortransformations after ∼6 Gyr of dynamical evolution. The final radial chemical gradients predicted by the modelin the solar neighborhood are positive and of the same order as those recently observed in the Milky Way thickdisk. We conclude that (1) the spatial chemical imprint at the time of disk formation is not washed out by seculardynamical processes and (2) the observed radial gradient may be the dynamical relic of a thick disk originated froma stellar population showing a positive chemical radial gradient in the inner regions.Key words: Galaxy: abundances – Galaxy: disk – Galaxy: evolution – methods: numerical – solar neighborhoodOnline-only material: color figures1. INTRODUCTIONObservations suggest that the negative radial metallicitygradient, d[Fe/H]/dR, measured in the solar neighborhoodclose to the plane, flattens or even attains positive values ifwe select stars at higher |z| from the galactic plane or withhigh [α/Fe] abundances (Allende Prieto et al. 2006; Boeche &the RAVE Collaboration 2013; Anders et al. 2013; Hayden et al.2013). This effect can be a signature of a positive radial gradientin the thick disk population, as derived by Marsakov & Borkova(2005) and by Carrell et al. (2012), who analyzed kinematicallyselected thick disk samples. Alternatively, a similar feature canbe simply produced by the variation of the fraction of thin diskand thick disk stars due to their different scale-height and scale-length (Anders et al. 2013). This scenario is indeed consistentwith the short scale length, hR ∼ 2 kpc, recently estimatedfor α-enhanced stars by Bensby et al. (2011) and Cheng et al.(2012).Furthermore, there is observational evidence indicating thatthe Milky Way (MW) is a barred galaxy. Non-axisymmetricstructures in the disk (such as bars and spiral arms) drive galaxyevolution: they redistribute angular momentum among thedifferent galactic components and can enhance radial migrationinside the disk. The amount of impact that these secularprocesses can have on the formation of the thick disk and on itschemical and kinematical properties is still a matter of debate(Rosˇkar et al. 2008; Loebman et al. 2011; Brunetti et al. 2011;Minchev et al. 2012; Kubryk et al. 2013).This work is aimed at analyzing whether dynamical effectscan be responsible for the non-negative radial metallicity gra-dients observed today in the MW thick disk, and suggests thatsuch gradients can be regarded as a cosmological feature.We use an N-body disk that develops a bar and whosefinal population displays a radial metallicity gradient; this isobtained from an initial axisymmetric disk with a positive radialmetallicity gradient for R < 10 kpc, which has dynamicallyevolved for 6.1 Gyr. This secular evolution model was recentlyproven (Curir et al. 2012) to reproduce the observed velocity-metallicity correlation in the thick disk (Spagna et al. 2010; Leeet al. 2011). In this Letter, we show that the positive slope of thechemical radial gradient in the MW thick disk observed todayin the solar neighborhood might be a signature remaining fromthe primordial population of disk stars.2. NUMERICAL SIMULATIONSAND CHEMICAL TAGGINGThe simulation used in this work was presented in Curiret al. (2012); it is an exponential disk with structural parametersappropriate for an MW-like dark matter (DM) halo at redshiftz = 0. The disk, having a mass of 5.6 × 1010 M and scalelength h = 3.5 kpc, is embedded in a Navarro–Frenk–WhiteDM halo (Navarro et al. 1997) with 107 particles havingmass Mvir = 1012 M and concentration Cvir = 7.4 (seeCurir et al. 2012 for details). We ran the simulation usingthe public parallel treecode GADGET2 (Springel 2005) onthe cluster matrix at the CASPUR (Consorzio Interuniversitarioper le Applicazioni del Supercalcolo) consortium, Rome. Oursimulation can be considered as an evolving thick disk model,consistent with the scenario where the thick disk is originatedfrom gas accreted in a chaotic period at high redshift and withshort star formation timescales (Brook et al. 2004, 2012). Theinitial radial metallicity distribution of the disk can be seen as thelink between our isolated simulated disk and the cosmologicalscenario, as we explain in the following paragraphs.In order to avoid the effect of relaxation of the initialconditions, we let the dynamical system evolve for 1 Gyr. Thenthe time is set to zero and each particle in the disk is tagged witha [Fe/H] label, according to the initial radial function shown inFigure 1. This function is based on Model S2IT from Spitoni1The Astrophysical Journal Letters, 784:L24 (5pp), 2014 April 1 Curir et al.0 5 10 15 20R (kpc)-0.60-0.55-0.50-0.45-0.40-0.35[Fe/H] (dex)0 Gyr1.85 Gyr3.72 Gyr6.1 GyrFigure 1. Median radial chemical distribution at the beginning of the dynamical simulations (dot-dashed line), at ≈ 1.9 Gyr (solid line), 3.7 Gyr (dashed line), and6.1 Gyr (dotted line). The points are the median values of the data over bins 0.2 kpc wide.(A color version of this figure is available in the online journal.)& Matteucci (2011). We assign metallicities with a dispersionσ[Fe/H] = 0.1 dex, which represents the variation of the medianchemical profile over 1 Gyr, according to Spitoni & Matteucci(2011). Regarding the metallicity distribution as a function ofthe height, z, from the galactic plane, no initial vertical gradienthas been included in our simulation.The positive slope of the function for R < 10 kpc is justifiedwith an inside-out disk formation model: at early epochs theefficiency of chemical enrichment in the inner regions of the diskis low due to a large amount of infalling primordial gas. Thisscenario is consistent with recent results published by Cresciet al. (2010), who found evidence for an “inverse” (i.e., positive)metallicity gradient in a sample of galaxies with stellar mass>3 × 109 M, at z ∼ 3. They conclude that this is produced bythe accretion of primordial gas (Mott et al. 2013), which dilutedthe abundance of elements heavier than helium in the center ofthe galaxies.The simulation produces a bar at around 2 Gyr whichpersists throughout. The radial migration, illustrated in Figure 2,redistributes the angular momenta of the orbits inside andoutside the bar. In Figure 2 we can see to what extent the radiusof each particle in the simulation has changed from Ri, at the startof the dynamical time, to Rf : 1.9, 3.7 or 6.1 Gyr later. The starswith Rf /Ri < 1 have moved toward inner regions, whereas theother stars have migrated further away. Thus, while the particlesin the inner parts have a tendency to go toward the center, theouter regions of the disk are expanding. As for the distributionperpendicular to the plane of the disk, we observe a similarbehavior: the internal regions tend to compact with time along|z|, whereas, in the external regions the disk particles increasetheir median |zf /zi | (e.g., Loebman et al. 2011; Rosˇkar et al.2013). The profile |zf /zi | at 6 Gyr is the result of the formationof a boxy bulge, probably due to buckling instabilities, althoughfurther analysis is needed in order to provide a full consistentexplanation.3. RESULTS AND DISCUSSIONFigure 1 shows the median radial chemical distribution at fourdifferent dynamical epochs: t = 0, 1.9, 3.7 and 6.1 Gyr. Theradial migration flattens out the initial chemical profile in thedisk outer region, while it does not induce major modificationsin the inner regions. The peak in the metallicity distribution,initially at 9.9 kpc, shifts to 10.7 kpc after ∼6 Gyr; alsoFigure 2. Migration of particles. Top panel: profiles of the median ratio Rf/Ri,where Ri is the radius of a particle at initial time and Rf is that of the sameparticle at 1.85 Gyr (solid line), 3.7 Gyr (dashed line) or 6.1 Gyr (dotted line).Bottom panel: profiles of the median ratio |zf/zi|, at the same times. The binningrange over the x-axis is 0.5 kpc in both panels.(A color version of this figure is available in the online journal.)the value of the [Fe/H] peak varies from an initial valueof −0.40 dex kpc−1 to −0.42 dex kpc−1. These relativelysmall changes point out the fact that dynamics alone can onlymarginally affect the initial radial chemical distribution.Figure 3 shows the radial chemical distribution of the sim-ulated disk particles after 6.1 Gyr of dynamical evolution.The particles are taken within a few kpc from the solar cir-cle (between 7 kpc and 10.5 kpc), at different intervals of |z|.We find a positive slope that slightly decreases with |z| fromd[Fe/H]/dR = 0.0157±0.0002 dex kpc−1 for |z| < 0.5 kpc to0.0112±0.0007 dex kpc−1 in the range 2.5 kpc < |z| < 3.0 kpc(the squares in Figure 4). We have measured d[Fe/H]/dR bycalculating the median of the chemical distribution in bins of0.05 kpc in R and fitting a straight line to those medians, takinginto account the 1σ width of the distribution of the data in each2The Astrophysical Journal Letters, 784:L24 (5pp), 2014 April 1 Curir et al.Figure 3. Radial chemical distribution of the disk stars in a few kiloparsecs of the MW solar circle, i.e., 7 kpc < R < 10.5 kpc, after 6.1 Gyr of dynamical evolution.The different panels refers to different |z| ranges. The black dashed lines are the best fit to the median values of [Fe/H] in each bin in R (of 0.05 kpc).(A color version of this figure is available in the online journal.)bin; the gradient uncertainties are taken as the 1σ error of the fitto the medians.Recently, Carrell et al. (2012) examined the metallicitydistribution (in R and |z|) in the Galactic thick disk, using F, Gand K dwarf stars selected from Sloan Digital Sky Survey DR8.We compare our simulation with their sample of kinematicallyselected thick disk stars. Here we consider only the resultsderived by Carrell et al. (2012) for |z| > 1.5 kpc, in orderto minimize the contamination of thin disk stars (Bensby et al.2011).Carrell et al. (2012) measured the metallicity gradient in theradial directions for stars in the annular region (7.0 kpc < R <10.5 kpc from the galactic center) and at several distances fromthe galactic plane. For thick disk stars they obtained an increas-ing d[Fe/H]/dR with |z|, from 0.025 ± 0.007 dex kpc−1 for1.5 kpc< |z| < 2 kpc to 0.041 ± 0.016 dex kpc−1 for 2.5 kpc <|z| < 3.0 kpc (Isochrone Distance Method). These results areconsistent, within 1σ uncertainties, with a constant positiveslope of ≈0.032 dex kpc−1 for the Isochrone Distance Methodand a slope of ≈0.023 dex kpc−1 for the Photometric DistanceMethod.A mild positive gradient of 0.010 ± 0.005 dex kpc−1 was alsomeasured by Hayden et al. (2013), among α-enhanced stars withTeff > 4200 K located between |z| = 1 kpc and |z| = 2 kpc. Ourresults suggest a mildly decreasing positive slope of the orderof ∼0.013 dex kpc−1.The evolution of the radial gradients in the solar neighborhoodis illustrated in Figure 5. The initial radial gradients are given3The Astrophysical Journal Letters, 784:L24 (5pp), 2014 April 1 Curir et al.Figure 4. Radial metallicity gradients of the disk stars with 7 kpc < R < 10.5 kpc. The squares are our estimates of the metallicity gradients at 6.1 Gyr from thedynamical N-body simulation. The abscissa of each point is the mean height of the particles in the respective bin (broad 0.5 kpc in R or |z|). The error bars on thevertical axis are 3σ width of the distribution, as 1σ errors would be hardly appreciated in this figure. Circles and diamonds are the gradients for |z| > 1.5 kpc measuredby Carrell et al. (2012) through the Isochrone Distance and the Photometric Distance, respectively.(A color version of this figure is available in the online journal.)0 2 4 6 8Time (Gyr)0.0080.0100.0120.0140.0160.0180.0200.022d[Fe/H]/dR (dex kpc-1 )0.0<|z|<0.50.5<|z|<1.01.0<|z|<1.51.5<|z|<2.02.0<|z|<2.52.5<|z|<3.0Figure 5. Evolution of the radial gradient in the solar annulus in several layers in |z|, namely 0 kpc < |z| < 0.5 kpc (empty circles), 0.5 kpc < |z| < 1.0 kpc (filledcircles), 1.0 kpc < |z| < 1.5 kpc (diamonds), 1.5 kpc < |z| < 2.0 kpc (triangles), 2.0 kpc < |z| < 2.5 kpc (squares), 2.5 kpc < |z| < 3.0 kpc (stars).(A color version of this figure is available in the online journal.)by the model we use to assign the metallicities and, as thereis no initial dependence on |z|, the radial gradients at different|z| have approximately the same values. These profiles reach asubstantial stability between t ∼ 1.8 and 6 Gyr, for 0 kpc <|z| < 2.5 kpc.After 6 Gyr of evolution of our model galaxy, we donot see any obvious trend in the vertical metallicity gradi-ents as a function of radial distance from the Galactic cen-ter. We find a significant, but tiny, positive d[Fe/H]/d|z| of0.0031 ± 0.0003 dex kpc−1 for the stars in the radial and verti-cal ranges 7.0 kpc < R < 10.5 kpc and 1 kpc < |z| < 3 kpc. Onthe other hand Carrell et al. (2012) measured d[Fe/H]/d|z| =−0.113 ± 0.010 dex kpc−1 for the whole sample of thick diskstars in the same spatial region. Hayden et al. (2013) measured avalue of −0.22 ± 0.02 dex kpc−1 for d[M/H]/d|z| in the range7 kpc < R < 9 kpc and of −0.13 ± 0.04 dex kpc−1 in the range9 kpc < R < 11 kpc.4. CONCLUSIONSThe crucial role of a positive slope d[Fe/H]/dR in theinner disk for the primordial chemical distribution to producea positive rotation-metallicity correlation in the thick disk(Spagna et al. 2010; Lee et al. 2011) was already shown inCurir et al. (2012). In this work we have analyzed how thesame cosmologically motivated chemical distribution allows usto constrain the influence of the dynamical evolution on themetallicity gradients observed today in the MW thick disk.We have drawn the radial and vertical chemical distributionsfrom the same simulated MW galaxy used in Curir et al. (2012),which includes a positive radial chemical gradient in the innerdisk. At 6.1 Gyr, within the solar neighborhood (7.0 kpc <R < 10.5 kpc), we still obtain a positive slope for theradial chemical distribution in the inner region. This slopedecreases in going from |z| < 0.5 kpc to the region4The Astrophysical Journal Letters, 784:L24 (5pp), 2014 April 1 Curir et al.2.5 kpc< |z| < 3 kpc (d[Fe/H]/dR = 0.016 ± 0.002 dex kpc−1and d[Fe/H]/dR = 0.011 ± 0.007 dex kpc−1, respectively).The radial chemical gradients predicted by the model are atmost within a factor of two of those observed by Carrell et al.(2012) with the Photometric Distance Method and by Haydenet al. (2013). Our final positive radial gradients are a direct con-sequence of our initial conditions, that include a positive radialgradient in the inner disk.As shown in Figure 5, the radial gradients in the solar neigh-borhood are of the same order from 1.8 Gyr to 6.1 Gyr, indepen-dently of the layer in |z| (although some redistribution appearsto take place). Such a stability seen between 2 and 6 Gyr is con-sistent with the stability of the rotation-metallicity correlationpointed out by Curir et al. (2012) in the same time range.Furthermore, the vertical gradient in the final configurationof our simulation is very close to zero. This originates fromthe fact that we do not impose any dependence on |z| in theinitial chemical distribution. The lack of a |z| dependence isalso responsible from the lack of a trend for the radial gradientsto increase with |z|.We conclude that the secular disk evolution does not appear tobe able to modify significantly the disk global chemical profiles,suggesting that the radial and vertical chemical distribution ofthe thick disk is likely to be a fossil signature of the originaldistribution.Our results indicate that, if the positive radial metallicitygradient in the solar neighborhood will be confirmed by theobservations of the MW thick disk, this is consistent with athick disk population showing an early positive [Fe/H] radialgradient in the inner disk (R < 10 kpc) and negative in the outerdisk (R > 10 kpc). In the context of the inside-out formationscenarios of the galactic disk (Spitoni & Matteucci 2011; Mottet al. 2013), such a gradient “inversion” derives from the stronginfall of primordial gas that can occur at early times in the innerdisk.The authors thank R. Drimmel for accurate insightful com-ments.We are grateful to F. Matteucci and E. Spitoni for providing uswith the data of their chemical gradient model for a primordialdisk. A.C., P.R.F., A.S. acknowledge the financial support ofthe PRIN-MIUR 2012, grant No 1.05.01.97.02, “Chemical anddynamical evolution of our Galaxy and of the galaxies of theLocal Group.”A.L.S. and A.D. acknowledge partial support from the INFNgrant InDark and from the grant Progetti di Ateneo/CSP TOCall2 2012 0011 of the Universita` di Torino.M.G.L. and A.L.S. acknowledge partial support from MIURand the Italian Space Agency through Contract No. I/058/10/0(The Italian participation in the Gaia Mission).REFERENCESAllende Prieto, C., Beers, T. C., Wilhelm, R., et al. 2006, ApJ, 636, 804Anders, F., Chiappini, C., Santiago, B. X., et al. 2013, arXiv:1311.4549Bensby, T., Alves-Brito, A., Oey, M. S., Yong, D., & Mele´ndez, J. 2011, ApJL,735, L46Boeche, C. the RAVE Collaboration 2013, arXiv:1309.6460Brook, C. 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The radial metallicity gradients in the Milky Way thick disk as fossil signatures of a primordial chemical distribution

https://iris.unito.it/bitstream/2318/154759/1/apjl_784_2_24.pdf

The radial metallicity gradients in the Milky Way thick disk as fossil
  signatures of a primordial chemical distribution

The radial metallicity gradients in the Milky Way thick disk as fossil signatures of a primordial chemical distribution

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