The Milky Way without X: An alternative interpretation of the double red clump in the Galactic bulge
aa r X i v : . [ a s t r o - ph . GA ] A ug Mon. Not. R. Astron. Soc. , 1–7 (2015) Printed 31 July 2018 (MN L A TEX style file v2.2)
The Milky Way without X: An alternative interpretationof the double red clump in the Galactic bulge
Young-Wook Lee ⋆ , Seok-Joo Joo † , and Chul Chung Center for Galaxy Evolution Research and Department of Astronomy, Yonsei University, Seoul 120-749, Korea
Accepted 2015 August 22. Received 2015 August 7; in original form 2015 June 15
ABSTRACT
The presence of two red clumps (RCs) in high latitude fields of the Milky Waybulge is interpreted as evidence for an X-shaped structure originated from the barinstability. Here we show, however, that this double RC phenomenon is more likely tobe another manifestation of multiple populations observed in globular clusters (GCs)in the metal-rich regime. As in the bulge GC Terzan 5, the helium enhanced secondgeneration stars in the classical bulge component of the Milky Way are placed on thebright RC, which is about 0.5 mag brighter than the normal RC originated from thefirst generation stars, producing the observed double RC. In a composite bulge, wherea classical bulge can coexist with a boxy pseudo bulge, our models can also reproducekey observations, such as the dependence of the double RC feature on metallicity andGalactic latitude and longitude. If confirmed by Gaia trigonometric parallax distances,this would indicate that the Milky Way bar is not sufficiently buckled to form theX-shaped structure in the bulge, and suggest that the early-type galaxies would besimilarly prevailed by super-helium-rich subpopulation.
Key words:
Galaxy: bulge — Galaxy: structure — Galaxy: formation — galaxies: elliptical andlenticular, cD — globular clusters: general — stars: horizontal-branch
As the nearest early-type system, the Milky Way bulgeprovides a unique opportunity to study the details of itsresolved stellar populations. Five years ago, the presenceof double red clump (RC) was discovered in the higherlatitude ( | b | > . ◦ ) fields of the Milky Way bulge fromwide-field photometric surveys (McWilliam & Zoccali 2010;Nataf et al. 2010; Saito et al. 2012). These and follow-up ob-servations have established that (1) the two RCs have almostidentical mean colours, (2) the double RC feature is only ev-ident among metal-rich stars, while metal-poor populationsshow only faint RC, (3) the separation between the two RCsis vanished at low Galactic latitudes, and (4) the relativestrength of the two RCs changes strongly with longitude(Ness et al. 2012, 2013; Nataf et al. 2015; Uttenthaler et al.2012; Rojas-Arriagada et al. 2014). These observations arewidely accepted as evidence for the X-shaped bulge origi-nated from the disk and bar instabilities, which led to theconsensus that even higher latitude region of the MilkyWay bulge has more characteristics of a “pseudo bulge”, ⋆ E-mail: [email protected] † E-mail: [email protected] rather than a classical bulge (McWilliam & Zoccali 2010;Saito et al. 2011; Li & Shen 2012; Wegg & Gerhard 2013;Nataf et al. 2015, and references therein). Without the crucial distance information, however,an alternative interpretation, in which the brighter RCis representing an intrinsically brighter subpopulationrather than a distance effect, must be explored in de-tail to see whether it can equally reproduce the keyobservations described above. It is now well establishedthat most globular clusters (GCs) host multiple pop-ulations with helium and light-elements enhanced sec-ond and third generation stars, for which the origin canbe traced back to the chemical enrichments and pollu-tions by asymptotic-giant-branch (AGB) stars and/or fast-rotating massive-stars (FRMS) (D’Antona & Caloi 2004;Lee et al. 2005; Decressin et al. 2007; Ventura & D’Antona2009; Carretta et al. 2009; Gratton et al. 2012, and refer- Although the X-shaped structures are not uncommon amongextragalactic bulges (see, e.g., Bureau et al. 2006), they are usu-ally very faint and require significant image processing to revealthem, in contrast with the apparently strong signal claimed forthe Milky Way.c (cid:13)
Y.-W. Lee, S.-J. Joo, and C. Chung ences therein). Some massive GCs, such as ω Cen andTerzan 5, also show evidence for supernovae enrichment(Lee et al. 1999; Piotto et al. 2005; Marino et al. 2009;J.-W. Lee et al. 2009; Ferraro et al. 2009; Lim et al. 2015).Chemical enrichments and pollutions similar to these mighthave affected stellar populations in the bulge as well. Fur-thermore, Renzini (1994) proposed that bulge populationsmight be helium-enriched due to their super-solar metallicityand the helium enrichment parameter (∆
Y / ∆ Z ) between 2and 3, with an average helium content of 0.31 – 0.35.In general, super-helium-rich subpopulations are placedon the bluer horizontal-branch (HB) during the corehelium-burning phase (Lee et al. 2005; Gratton et al. 2012;Joo & Lee 2013; Marino et al. 2014; Jang et al. 2014). How-ever, when the metallicity is high enough as in the bulge, thestrong metallicity effect overwhelms the helium effect, andthe metal-rich counterpart would be placed on the brighterRC. This was correctly pointed out by D’Antona et al.(2010) in their investigation of the double RC observed inthe metal-rich bulge GC Terzan 5 (Ferraro et al. 2009). Thedouble RC observed in Terzan 5 is very analogous to thatin the bulge in terms of the magnitude difference, metallic-ity, and population ratio. Both McWilliam & Zoccali (2010)and Nataf et al. (2010) noted this similarity, but they dis-missed this possibility in the bulge mostly based on the factthat the two RCs in Terzan 5, unlike the double RC in thebulge, also show the difference in colour. The colour of theRC stars, however, can vary with metallicity, helium abun-dance, or age, similarly to the case of more metal-poor HBstars (Lee et al. 1994, see Figure 1 below).Spectroscopic observations of red-giant-branch (RGB)and RC stars in the bulge field show a large range in metal-licity, − ≤ [Fe / H] ≤ .
5, with a mean metallicity at almostsolar (Zoccali et al. 2008; Babusiaux et al. 2010; Hill et al.2011; Ness et al. 2013; Rojas-Arriagada et al. 2014). Themetallicity distribution functions (MDFs) derived fromthese observations show bimodality with metal-rich andmetal-poor populations. A metallicity gradient is also ob-served in the bulge along the minor axis, in the sense thatthe fraction of the metal-rich population is decreased at highGalactic latitudes. Gonzalez et al. (2011) further showedthat the α -elements enhancement is decreased at highermetallicities (see also Uttenthaler et al. 2012; Johnson et al.2014). Many of these observations and recent theoreticalstudies have suggested a possibility of a composite bulgein the Milky Way, in which a classical bulge (CB) can co-exist with a boxy pseudo bulge originated from the bar(Babusiaux et al. 2010; Hill et al. 2011; Erwin et al. 2015;Rojas-Arriagada et al. 2014; Zoccali et al. 2014; Saha 2015;Saha et al. 2015). In this composite bulge, low Galactic lati-tude ( | b | < ◦ ) fields would be dominated by the most metal-rich bar population, while relatively metal-poor CB popula-tion becomes more and more important at higher latitudes(Babusiaux et al. 2010; Gonzalez et al. 2011; Obreja et al.2013; Rojas-Arriagada et al. 2014). The purpose of this pa-per is to show that, in such a composite bulge, the multiplepopulation models can equally explain the presence of thedouble RC and other key observations in the Milky Waybulge. The RC stars are metal-rich counterpart of core helium-burning HB stars in metal-poor GCs. In order to study theRC populations in the bulge, we have therefore constructed aseries of synthetic HB models for the metal-rich populations,following the techniques developed by Lee et al. (1994) andas updated by Joo & Lee (2013). Our models are based onthe Yonsei-Yale (Y ) HB evolutionary tracks and isochroneswith enhanced helium abundance (Han et al. 2009), all con-structed under the assumption that [ α/ Fe] = 0 .
3. Follow-ing our recent investigations for the halo GCs with multiplepopulations (Jang et al. 2014), Reimers (1977) mass-loss pa-rameter η was adopted to be 0.40, and the mass dispersionon the RC was adopted to be σ M = 0 . M ⊙ for each sub-population.In our multiple population models for the CB com-ponent of the bulge, the faint RC (fRC) is produced byfirst-generation stars (G1), which were assumed to followthe standard helium enrichment parameter (∆Y / ∆Z = 2,Y = 0 .
23 + Z(∆Y / ∆Z)), while the brighter RC (bRC)is populated by super-helium-rich (Y = 0.39) second-generation stars (G2). The choice of Y = 0.39 for G2 is basedon the empirical constraint from super-helium-rich subpop-ulations in ω Cen (Joo & Lee 2013), which is also compara-ble with the theoretical predictions for the helium contentin the ejecta of both super-AGB stars and type II super-novae (Ventura et al. 2013; Woosley & Weaver 1995). Notefurther that this choice of helium abundances for G1 and G2(Y = 0.27 & 0.39 at Z = 0.02), in the mean (Y = 0.33), isconsistent with the suggestion from the analysis of the redgiant-branch bump of the Milky Way bulge (Renzini 1994;Nataf et al. 2013). Spectroscopy of RC stars in the bulge(De Propris et al. 2011; Ness et al. 2012; Uttenthaler et al.2012) indicates that stars in bRC are somewhat more metal-rich (0.12 – 0.23 dex) than those in fRC, and therefore themetallicity difference between G2 and G1 was assumed tobe ∆[Fe / H] = 0 . J − K, K ) colour magnitude diagram(CMD). The models are presented at four different metal-licity regimes relevant to the Milky Way bulge to illustratethe sensitivity of the double RC feature on metallicity. Itis clear from the models in panel (b) that the separationin magnitude between the two RCs ( ∼ c (cid:13) , 1–7 he MW without X Figure 1.
Our population models for the two RCs from G1 and G2 in the ( J − K, K ) CMDs. The models are presented at four differentmetallicity regimes to illustrate the sensitivity of the double RC feature on metallicity. The helium abundance for G1 follows the standardcase (∆Y / ∆Z = 2, Y = 0 .
23 + 2Z), while that for G2 is adopted to be Y = 0.39. Panels (a) – (d) are models for [ α/ Fe] = 0 .
3, whilepanels (e) – (h) are for [ α/ Fe] = 0 .
0. Crosses represent some RR Lyrae variables produced by G2. be observed in the relatively metal-poor population. This ismainly because helium-rich stars evolve more rapidly, andhence have lower masses at given age (see Lee et al. 1994,2005). Interestingly, the most metal-rich models in panel(a) show that the separation between the two RCs would bediminished or vanished in super-metal-rich ([Fe / H] ≥ . / ∆Z = 2, becomes very helium-rich (Y ≈ .
33) atsuper-metal-rich regime, increasing the luminosity of fRC.In a two-component composite bulge, this most metal-richregime would be more relevant to the bar component.The helium enhanced evolutionary tracks adopted inour modeling were constructed under the assumption that[ α/ Fe] = 0 . / H] regimes. In the bulge fields, how-ever, the α -enhancement is observed to be decreased withincreasing [Fe / H] (Gonzalez et al. 2011; Uttenthaler et al.2012; Ness et al. 2013; Johnson et al. 2014). In order to in-vestigate the effect of [ α/ Fe], our α -enhanced evolution-ary tracks are rescaled back to [ α/ Fe] = 0 . α/ Fe] = 0 .
0. At solar metallicity ([Fe / H] ≈ . α/ Fe] = 0 .
0) models are fainter by ∼ α -enhanced ([ α/ Fe] = 0 .
3) models, but themagnitude difference between the two RCs is little affected.Furthermore, according to Ness et al. (2013, see their Fig-ure 19), the difference in [ α/ Fe] between the two dominantcomponents (at [Fe / H] ≈ +0 .
15 and − .
25) in the bulge isobserved to be only ∼ α/ Fe] on the magnitude difference between the two RCswould be negligible.In panel (c) of Figure 2, our single metallicity models inFigure 1 (for [ α/ Fe] = 0 .
3) are combined to generate a com-posite model for the CB component of the bulge. For this,models in the metallicity range of − . ≤ [Fe / H] ≤ +0 . / H] = − .
1. Inthese models, the magnitude difference between the two RCsis ∆ K = ∆ I ≈ .
43 mag, while the colour difference is neg-ligible, ∆( J − K ) ≈ .
00, which are in reasonable agree- c (cid:13) , 1–7 Y.-W. Lee, S.-J. Joo, and C. Chung ments with the observed differences (see Figure 3). Sim-ilar results are obtained even if the peak metallicity andage are varied by ∼± . ∼± For com-parison, panel (a) shows our model for the GC Terzan 5,where the metallicities for G1 and G2 were adopted fromOriglia et al. (2011), and the helium abundance for G2 wasadopted to be Y = 0.35. Ages for G1 and G2 are identi-cal to those adopted for the CB models (i.e., 12 & 10 Gyr,respectively). Our models show that a larger difference inmetal abundance (∆[Fe / H] = 0 . ∼ ∼ / H] = +0 .
15 and σ [Fe / H] =0 .
10 dex is adopted. As described above, this super-metal-rich G1, following the standard ∆Y / ∆Z = 2, becomes rela-tively helium-rich (Y ≈ K ≈ − . ∼ ∼ / H] ≥ − .
4) populations as suggested from the observa-tions (Ness et al. 2012; Uttenthaler et al. 2012). Our modelsfor the metal-poor populations in the bulge show only fRC(see Figure 1 lower panel), because, unless very metal-rich([Fe / H] ≥ − . Likewise, if a MDF with two Gaussian peaks at [Fe / H] = − . .
15 (Babusiaux et al. 2010; Ness et al. 2013) is adoptedroughly in the same metallicity range, the magnitude and colourdifferences are varied by only ∼ Figure 2.
Same as Figure 1, but for the metal-rich GC Terzan 5and the two components (bar and CB) in the Milky Way bulge.Models for the bar and CB are superimposed in panel (d) toillustrate the composite bulge at low Galactic latitudes, wherethe population ratio between the bar and CB is adopted to be2:1. Photometric errors, differential reddening and some deptheffects ( σ K ≈ . σ J − K ≈ .
033 mag) are included in oursimulations.
Figure 3.
Same as Figure 2, but our models for the CB arecompared with the observed CMD for the high latitude ( b = − ◦ )field of the bulge (2MASS data from Skrutskie et al. 2006; Figure2 of McWilliam & Zoccali 2010). Note that the observed CMD iscontaminated by the foreground disk stars at ( J − K ) ≈ . (cid:13) , 1–7 he MW without X low |b| N l = 0 (cid:176) , |b| » (cid:176) −0.6 0.0 +0.6 D K maghigh |b|
N bRC fRC 0.51.00.0 +0.6 +l N 0.0 +0.6 −l Figure 4.
Schematic diagrams illustrating the latitude (vertical panels) and longitude (horizontal panels) dependences of the double RCfeature in a two-component bulge model. The mono-modal distribution in gray is for the bar component, while the bimodal distributionis for the CB component. The two components are assumed to be equally populated for three horizontal panels, while the fraction of thebar component decreases with increasing lbl in three vertical panels. Simulations for the sum of the two components (with 3 × stars)are represented as blue histograms, and the tick marks in vertical panels indicate the RC peak positions suggested from the Gaussiandecomposition (see the text). tion from the bar component is decreased/vanished, whilethe CB component (with G1 and G2) becomes more andmore important, producing a larger magnitude differencebetween the two RCs with increasing | b | . Finally, in a two-component bulge with a spheroidal CB embedded in a tiltedbar (pseudo bulge), the longitude dependence of the RC lu-minosity function can be reproduced in our model as well.As also illustrated in Figure 4 (three horizontal panels), theobserved luminosity function, in our scenario, is the sum ofa CB component (with double RC) and a metal-rich barcomponent (with single RC). Towards negative longitudesthe bar component is placed at the far side of the bulge andtherefore fRC becomes gradually more populated, while theopposite effect is anticipated towards positive longitudes.Most of these observations were obtained in 5 ◦ < | b | < ◦ fields (see Figure 11 of Nataf et al. 2015), where the CBcomponent is expected to be comparable to the bar compo-nent in terms of population ratio. More detailed populationmodels that fully take into account the observed MDF andits dependence on Galactic latitude and longitude will bepresented in our forthcoming paper (in preparation), wherewe will also compare the observed RC luminosity functionswith the models constructed under various combinations ofinput parameters. The kinematics of RC and giant stars in the bulge can pro-vide further constraints on our models. The radial velocity( V r ) measurements show that the bulge rotates cylindrically (Ness et al. 2013; Zoccali et al. 2014), which is a strong ev-idence for the bar at low latitude fields ( | b | < ◦ ). For thehigher latitudes, however, this is not necessarily inconsis-tent with our CB dominated scenario in this region of thebulge, because, for example, Saha et al. (2015) showed thatan initially non-rotating CB could absorb a significant frac-tion of the angular momentum from the bar within a fewGyr (see also Zoccali et al. 2014). The observed kinematicsof stars in the two RCs are also consistent with our sce-nario. At b = − ◦ , V´asquez et al. (2013) found an excessof stars moving towards the Sun in the bRC, while thatreceding from the Sun is observed in the fRC. In our com-posite bulge scenario, this can be understood by the depth( ∼ ≈ V r , in the mean, between bRC and fRC.At higher latitudes ( b = − − ◦ ), however, there isno difference in V r between the two RCs (De Propris et al.2011; Uttenthaler et al. 2012; Rojas-Arriagada et al. 2014),which is consistent with our CB (with multiple populations)dominated scenario in these regions of the bulge.One challenge to our multiple population model forthe bulge is to understand the population ratio of G2 (He-enhanced) which is observed to be comparable to that ofG1 (He-normal). In the metal-poor GCs, it is now well es-tablished that the formation of super-helium-rich subpop-ulation requires specific conditions in the central region of c (cid:13) , 1–7 Y.-W. Lee, S.-J. Joo, and C. Chung a massive proto-GC (see. e.g., D’Ercole et al. 2008), mainlyto fulfill the apparently very extreme helium to metal en-richment (∆Y / ∆Z > / H] ≈ .
0) sys-tem like the bulge, where the formation of super-helium-richG2 requires only relatively modest enrichment parameter(∆Y / ∆Z ≈ − M ⊙ (Maeder1992; Timmes et al. 1995). In addition to supernovae, AGBstars and FRMS would have also provided helium to G2,and therefore helium-rich G2 could have been formed ubiq-uitously in the metal-rich “building blocks” like Terzan 5.While the details of chemical evolution in the bulge andTerzan 5 require further investigations (see D’Antona et al.2010; Bekki 2012), it appears therefore that both G1 and G2were effectively provided to the CB component of the MilkyWay when merging and disruption of these building blockswere much more active in the early phase of the Milky Wayformation.The distance measurements for the two RCs from soonto be released Gaia trigonometric parallax data would pro-vide the most direct test for the suggested model. If the twoRCs are due to the X-shaped structure, ∼ | b | > . ◦ ). Furthermore, ifthe split RC is mostly due to the effect of multiple popu-lations, rather than a distance effect, the previous studieson the structure of the Milky Way bulge and bar based onRC, high latitude fields in particular, should be reexaminedin the new paradigm. Finally, our result for the CB com-ponent of the Milky Way bulge, the nearest early-type sys-tem, would also suggest that the early-type galaxies wouldbe similarly prevailed by super-helium-rich subpopulation.If ∼
50% of stars in early-type galaxies are helium and lightelements enhanced G2, this would have profound impactson the interpretation of integrated spectra by employingpopulation synthesis models, such as the origins of the Naenhanced giant elliptical galaxies (van Dokkum & Conroy2010; Jeong et al. 2013) and the UV upturn phenomenon(Chung et al. 2011).
ACKNOWLEDGMENTS
We thank the referee for a number of helpful suggestionswhich led to several improvements in the manuscript. Wealso thank Sang-Il Han for his assistance in obtaining theobserved data for Figure 3. Support for this work was pro-vided by the National Research Foundation of Korea to theCenter for Galaxy Evolution Research.
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