On the Origin of Dynamically Cold Rings Around the Milky Way

On the Origin of Dynamically Cold Rings around the Milky Way

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Citation Younger, Joshua D., Gurtina Besla, T. J. Cox, Lars Hernquist, Brant Robertson, and Beth Willman. 2008. “On the Origin of Dynamically Cold Rings around the Milky Way.” The Astrophysical Journal 676 (1): L21–24. https://doi.org/10.1086/587099.

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Terms of Use This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Open Access Policy Articles, as set forth at http:// nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#OAP arXiv:0802.0872v1 [astro-ph] 7 Feb 2008 ohaD Younger, D. Joshua 60637 tet hcg,I 60637 Eas IL 933 Chicago, Chicago, Street, of University Astrophysics, and Astronomy tet abig,M 02138 MA Cambridge, Street, ik–dnmclycl ihalwecnrct ri – orbit eccentricity low a the with from cold distinct stud- kinematically dynamically Spectroscopic is – it radial kpc. disk that galactocentric 20 shown a and have ies 15 at between lies of and distance longitude galactic in one l 05,20) h R subtends MRi The 2003; al. 2007). et 2005b, (Ibata al. surveys photo- et optical followed–up Conn 2006; been several (2004), has by (Grillmair and al. metrically data 2006), et 2006) al. SDSS Martin et Mi- al. Belokurov and updated et Two Skrutskie (2003) using the al. reanalyzed from et MRi (2MASS: Rocha-Pinto infrared the Survey by the detection, (SDSS: All–Sky in initial Survey cron this identified Sky Since been Digital has 2000). Sloan al. the et York from (MRi), Monoceros data in structure using ring–like coherent 2007). a al. tified al. et of et Kazantzidis Quinn heating 1996; 1992; dynamical al. Ostriker the et & Walker Toth and 1993; (e.g., , disk “ex- al. 2007) stellar Peñarrubia et and al. the 2005; et 2005; al. 2007) Ibata et al. 2006; (Ibata al. et disks et Younger outer (Erwin 2006; tended” al. Trujillo “antitruncated” et & Bell Pohlen 2005; so–called build–up Johnston & the Bullock 2007), including: (e.g. halos of galaxies, stellar variety external of a and to MW in connected phenomena the kinematic galax- are and satellite structural mergers stellar smaller 1993; observable minor In accreting These by Cole up mass ies. & 2000). build their (Lacey (MW) Way al. of Milky et much the occurrence like Somerville galaxies scenario, frequent 1999; this Kolatt a & Somerville are ers 7 6 5 4 3 2 rpittpstuigL using 2018 typeset 28, Preprint November version Draft 1 noronglx,Nwege l 20)rcnl iden- recently (2002) al. et Newberg galaxy, own our In merg- which in universe, hierarchical a in live We lyFellow Clay Fellow Spitzer nioFriIstt,54 ot li vne hcg,IL Chicago, Avenue, Ellis South 5640 Insitute, Fermi Enrico al nttt o omlgclPyisadDprmn of Department and Physics Cosmological for Institute Kavli Fellow Foundation Keck [email protected] avr–mtsna etrfrAtohsc,6 Garden 60 Astrophysics, for Center Harvard–Smithsonian fnal yaial odrn–iefaue nlwecnrct orbit exam low–eccentricity we on encounters features the ring–like that cold of find co dynamically We more nearly are works. of conditions previous initial in These considered encounter. flyby eccentricity, tla ouaino h ooeo ig hrfr,w n hta that find we Way. headings: Milky Therefore, fo Subject the for scenario around ring. appealing features cosmologically like th Monoceros a to represents the galaxy similar satellite of quantitatively and population qualitatively stellar both are they Moreover, epeetaseai o h rdcino yaial odrnsar rings cold dynamically of production the for scenario a present We ∼ 2 NTEOII FDNMCLYCL IG RUDTEMLYWAY MILKY THE AROUND RINGS COLD DYNAMICALLY OF ORIGIN THE ON − y i h ia epneo h rmr aayt h ls passa close the to galaxy primary the of response tidal the via Gyr 4 1. 1,2 A INTRODUCTION T E utn Besla, Gurtina tl mltajv 08/22/09 v. emulateapj style X aay ieaisaddnmc,glx:srcue aais inte galaxies: structure, galaxy: dynamics, and kinematics galaxy: tutr,methods: structure, 1 .J Cox, J. T. rf eso oebr2,2018 28, November version Draft n ∼ > bd simulations –body 56th t 100 1,3 ABSTRACT asHernquist, Lars ◦ 05 07.Tems ucsflo hs oesar- with models companion these mass of low al. Peñarrubia very successful et a 2005; most 2003; for The al. al. gue et et Martin (Helmi 2007). orbit 2005; 2005, coplanar low–mass al. a nearly et of Meza a disruption tidal on via satellite formed 2006). be 2005, could al. tures et Martin 2005a; al. et Conn 2003; n scmoe rmrl flwmtliiysaswith stars low–metallicity of primarily − composed is and IM Srne enus 03 otnd( sub– softened – a 2003) incorporate Hernquist medium & also interstellar (Springel (Schmidt the We (ISM) Law of model Schmidt multi–phase local 1998). resolution observed Hernquist Kennicutt & the formation, Springel 1959; fit star and to of cooling en- tuned radiative include the formalism We using (2002). conserving code Hydrodynamics) tropy Particle (Smooth lc oe htaceegsadrlaeiorpcthermal isotropic release and gas accrete that holes black ∼ 0 omdwith formed etgt h rgeigeeti eal n ics its discuss and in- detail, we MRi. here in the first to event but relevance was triggering (2007), satel- scenario the al. This small et vestigate a Kazantzidis orbit. with eccentricity by high encounter suggested a flyby on a structures lite MW: ring–like cold the dynamically around forming for have anism may models of these many conditions. initial MRi, reproducing satel- MRi unappealing There- the at a the cosmologically of successful such form 2008). characteristics are al. that the to they et unlikely time Boylan-Kolchin while 2007; is fore, in al. it circularized et very result have (Besla is a could interactions as lite dy- such The and for eccentric. long timescale highly of friction orbits be the likely namical that are found 2007). galaxies have satellite 2006) 2005, Burkert 2005; al. & Peñarrubia (Benson et Khochfar simulations 2005; cosmological recent al. However, et (Martin bit 1 . / 1 5 uhta h aswihe S eprtr is temperature ISM mass–weighted the that such 25) ueia oeighssgetdta iia struc- similar that suggested has modeling Numerical h iuain rsne nti td eeper- were study this in presented simulations The ihti nmn,w rps natraiemech- alternative an propose we mind, in this With 00o erycrua ( circular nearly a on 1000 10 . 6 4 ∼ < . 5 n ikprilsrpeetn supermassive representing particles sink and – K [Fe 1 / rn Robertson, Brant H] Gadget2 mlgclymtvtdta those than motivated smologically mn ieaial itntring– distinct kinematically rming ∼ < n eeial rdc series a produce generically ine udteMlyWyvaahigh– a via Way Milky the ound itiuin ieais and kinematics, distribution, e htpritoe timescales over persist that s − ihecnrct yyb a by flyby eccentricity high 2. 0 . Caee l 03 an tal. et Yanny 2003; al. et (Crane 4 SIMULATIONS Srne 05,a N–Body/SPH an 2005), (Springel eo h satellite. the of ge atos galaxies: ractions, 4,5,6 e 0 = ehWillman Beth & . )pord or- prograde 1) M s /M q MW EoS 1,7 = ∼ 2

Fig. 1.— The evolution of the projected stellar mass density, colored according to a logarithmic scale. The panels are 70 kpc on a side, and the simulation time is printed in the upper left hand corner in units of Gyr. The color-bar indicates the projected K–band surface brightness in units of mag arcsec−2, assuming a constant M/L ratio of 2 in solar units. feedback to self–regulate their growth (Springel et al. wrap around the primary galaxy as it continues to re- 2005b). volve, forming a set of concentric ring–like features. The progenitor galaxy models were constructed fol- Star formation induced by the merger is concentrated lowing Springel et al. (2005a), to which we refer the in the nucleus of the primary galaxy (see e.g., Hernquist reader for details. The primary galaxy (total mass of 1989; Mihos & Hernquist 1994, 1996; Hernquist & Mihos 12 −1 M200 = 10 h M⊙) is analogous to the MW with a ba- 1995), making these rings primarily disk stars. They are 6 ronic mass fraction of mb =0.05. It was realized with 10 furthermore dynamically cold and kinematically distinct halo particles in a Hernquist (1990) profile with a concen- from typical disk stars (see Figure 3), and slowly disperse tration of c = 9 as motivated by cosmological simulations on a timescale of ∼ 2−4 Gyr (or several rotation periods) (Bullock et al. 2001), and 4 × 105 stellar disk (80% of owing to phase mixing of the collisionless stellar parti- the baryonic mass) and 2 × 105 gas particles (20% of the cles (Binney & Tremaine 1987). Because these rings are baryonic mass). The satellite galaxy had a total mass of generated via gravitational interactions, they are largely 10 −1 insensitive to the gas content or structural parameters of M =5 × 10 h M⊙ (M /M ∼ 20) and an iden- 200 PG SG the satellite galaxy. And because the flyby is effectively tical baryonic mass fraction. It was realized with 105 an impulse interaction, they will still be formed even if halo particles in a Hernquist (1990) profile with a con- the satellite is disrupted during the encounter. centration of c = 18, again motivated by cosmological simulations, and 2 × 104 stellar disk (50% of the bary- 4. RELEVANCE TO THE MONOCEROS RING onic mass) and 4×104 gas particles (50% of the baryonic The ring features that are produced by the flyby inter- mass). They were placed on a parabolic encounter with action are similar in several ways to the MRi first iden- −1 Rp = 5h kpc perigalactic radius (∼ 1 scale length), tified by Newberg et al. (2002). They provide a good consistent with the results of cosmological simulations match to the kinematics and location of the ring, and (Benson 2005; Khochfar & Burkert 2006). are roughly consistent with metallically measurements of 3. MRi stars. And, cosmological simulations indicate that DISCUSSION such interactions are common for MW–sized halos over The interaction is summarized in Figures 1 and the timescale for ring formation (i.e., within the past ∼ 4 2. After the initial close passage, resonances be- Gyr; Stewart et al. 2007). Therefore, while we do not tween the orbital frequency of the satellite and pri- claim to have modeled the MRi in detail, based on these mary galaxy disk particles (“stars”) excite coplanar tidal similarities we propose a flyby interaction as a possible arms (Toomre & Toomre 1972). These features then formation scenario. Rings Around the Milky Way 3

Fig. 2.— Three diﬀerent projections (X–Y, Z–Y, and X–Z) of the K–band surface brightness (left; again assuming a constant M/L ratio of 2 in solar units) and gas surface mass density (right) at t ≈ 3 Gyr. Panels are 50 kpc on a side in the X and Y directions, and 20 kpc in the Z direction. The solid line corresponds to one of the ring structures as roughly the same galactocentric location (Rgac = 19 kpc) as the MRi.

Fig. 3.— Phase space diagram for the stellar particles at t ≈ 3 Gyr, showing the orbital frequency (Ω) as a fraction of that expected from a circular orbit at that radius (Ωk) as a function of galactocentric radius (Rgac). The left panel shows the density of points in gray-scale, with linear contours overlaid to guide the eye and a dashed line at the location indicated in Figure 2. The right hand panel is color coded by the radial velocity dispersion (σ(vR)) in each cell, with the same contours overlaid. The MRi forms a coherent structure over ∼ that our simulations produce a ring in the right galac- 100◦ in galactic longitude from approximately 15–20 tocentric radial range that is approximately circularly kpc from the galactic center (Newberg et al. 2002; supported – 0.9 ∼< Ω/Ωk ∼< 1.1, where Ω is the orbital Ibata et al. 2003; Rocha-Pinto et al. 2003; Martin et al. frequency and Ωk is that expected from circular motion 2004; Conn et al. 2005b, 2007). Kinematically, it is dis- – with a similarly low radial velocity dispersion (see Fig- tinguished from MW disk stars by its low radial velocity ure 3). This ring also extends −4 ∼< z ∼< 4 kpc above dispersion (∼ 25 km s−1); the MRi is dynamically cold and below the galactic plane, which is consistent with (Crane et al. 2003; Yanny et al. 2003; Conn et al. 2005a; detections such as those of Conn et al. (2007). Although Martin et al. 2005, 2006). Proper motion measurements8 it is unclear what fraction of the ring would be identified 8 also suggest that the ring stars are in low–eccentricity or- in observations, it contains ∼< 1% ( ∼< 5 × 10 M⊙) of the bits (e.g., Crane et al. 2003; Yanny et al. 2003). We find total stellar mass of the disk, which is consistent with the mass estimates of Yanny et al. (2003). 8 Crane et al. (2003) measure an overall circular velocity of A disk origin for MRi stars is also broadly consistent ∼ 100 km s−1. However, we note (as in Peñarrubia et al. 2005) with their observed stellar population. Several spectro- that the accuracy of such measurements in physical units is lim- ≈ ≈ scopic studies (e.g., Yanny et al. 2003) have found mean ited to ∆vperp 4.74R⊙∆µ where R⊙ 8 kpc is the galacto- − ± centric radius of the sun and ∆µ ∼ 3 − 4 mas yr−1 is the typical metallicities of [Fe/H] = 1.6 0.3, while Crane et al. proper motion measurement error. We have therefore chosen to (2003) find a higher mean metallicity of [Fe/H] ≈ −0.4 interpret these measurements as being generally consistent with using a different tracer population. This suggests a pri- low–eccentricity orbits. marily metal–poor stellar population with a spread in 4

the satellite galaxy that produced the MRi is either (1) hidden from view, at low galactic latitude and/or on the other side of the galaxy, or (2) it is lower mass with 1/100 ∼< MSG/MPG ∼< 1/20. Therefore, it is not unrea- sonable to speculate that Leo I, which had at least one passage through the MW disk within the past 2–4 Gyr (Sohn et al. 2007), could have excited the MRi we see to- day. Moreover, it is likely that this interaction stripped a significant fraction of its mass9, making current dynamical estimates (MLeo I/MMW ∼ 1/1000; Mateo et al. 2007) a lower limit on its mass at the time of the interaction. Furthermore, simulations indicate that there are likely ∼> several sub–haloes in the MW halo with 1/100 ∼< MSH /MPG ∼< 1/20 on orbits that take then through the disk (Moore et al. 1999; Giocoli et al. 2007; Kazantzidis et al. 2007). Since galaxy formation may be inefficient in low mass haloes (e.g., Haiman et al. 1997; Barkana & Loeb 1999; Kravtsov et al. 2004), they may be significantly underluminous and therefore difficult to identify. Fig. 4.— Three projections (X–Y, Z–Y, and X–Z) of the K–band Finally, there have been suggestions in the literature surface brightness (again assuming a constant M/L ratio of 2 in (Crane et al. 2003; Frinchaboy et al. 2004) that some solar units) at the same simulation time as Figures 2 and 3 for a globular clusters (GCs) may be associated with the MRi. MSG/MP G ≈ 1/100 interaction with identical orbital parameters. This is also potentially in conflict with our modeling. metallicities, and possibly multiple epochs of star forma- However, we note that their physical association with tion. Our modeling indicates that the MRi may have the MRi is somewhat speculative. Alternatively, there formed from outer disk stars moved outwards by tidal are ∼several nearly coplanar GCs at roughly the same interactions with the satellite galaxy, which is consis- radius (Harris 1996) as progenitor MRi stars in our sim- tent with this metal–poor stellar population (Luck et al. ulations that they could have also been moved into the 2006; Yong et al. 2006). Furthermore, the tidal inter- rings by the interaction. action moves a significant supply of cold gas into the same ring structures (see Figure 2), which provides the 5. CONCLUSION raw material for subsequent epochs of star formation. We present a scenario for the production of dynami- While the density–dependent prescription in our simu- cally cold rings around a disk galaxy such as the MW lations does not accurately capture some modes of star via a prograde flyby encounter with a satellite galaxy. formation owing to the limitations of the SPH method, Tidal arms excited during close passage coalesce and prescriptions that include an approximate treatment of wrap around the disk of the primary galaxy. These kinds shocks (e.g., Barnes 2004) may show these multiple star of interactions are more cosmologically likely than the formation episodes. nearly circular orbits presented by Martin et al. (2005) While ring features in our simulations capture many and Peñarrubia et al. (2005, 2007), while dynamical fric- of the properties of the MRi, there are some observa- tion is insufficient to circularize the orbit of such a low tions which potentially conflict with our proposal. In mass companion. Our modeling shows rings with simi- particular, the satellite in our simulations is roughly the lar spatial distribution and kinematics to the MRi. The same mass as the LMC, and by ∼ 2 Gyr after the in- ∼ disk origin of MRi stars is furthermore broadly consis- teraction would be at a galactocentric distance of 250 tent with observed stellar populations. Therefore, we kpc. Such a massive object is well within the detec- find that a flyby encounter represents a more cosmolog- tion threshold of current surveys (e.g., Willman et al. ically appealing scenario for the production of the MRi 2002; Koposov et al. 2007); Leo I – a far lower mass and other dynamically cold rings around the MW. MW companion – was detected at comparable distance (Caputo et al. 1999; Bellazzini et al. 2004; Mateo et al. 2007). However, dynamically cold ring structures are Thanks to the anonymous referee for helpful com- produced generically in flyby encounters, and similar ments, and to M. Ashby, J. Bullock, S. Dutta, P.. F. features are present in both a lower–inclination inter- Hopkins, C. Hayward, and S. Bush for valuable discus- action (i = 10◦) and for a lower mass satellite galaxy sions. This work was supported in part by a grant from (MSG/MPG ∼ 1/100; Figure 4). It is thus possible that the W. M. Keck foundation.

9 In our simulations, both the 1/20 and 1/100 encounters ∼ passage through the disk is likely to remove a signiﬁcant fraction stripped 50% of the satellite mass. While the eﬃciency of this of the satellite’s total mass. process is sensitive to both the initial structure of the satellite and the orbital parameters of the interaction, this suggests that a a

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