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					Sagittarius, New Outer Halo Tidal Streams



              QuickTime™ an d a Animation decomp ressor are needed to see th is pictu re.




and the LMC as a Dwarf Galaxy “Cue-ball”

   Steven Majewski - University of Virginia


    Vienna                                                                              UT 11 April 2005
       The Sagittarius Dwarf Spheroidal (dSph)




                      Figure by Wyse, Gilmore & Franx (1997)

• Discovered by Ibata, Gilmore, Irwin (1994).
• Seemed to be archetype dwarf galaxy satellite merger.
Even with rather modest early data constraints…




                                  Dinescu et al. (2002)
   Even with rather modest early data constraints…
relatively good/consistent models of debris shape found:




            Ibata & Lewis (1998)    Johnston et al. (1999)




                                    Martinez-Delgado et al.
              Ibata et al. (2001)           (2004)
             Sagittarius in the Infrared:
All Stars in Two Micron All Sky Survey (2MASS)




  2MASS is ideal for Sgr study: - Reduced dust effects
                                - All sky
                                - Sgr filled with M giants
           2MASS All-Sky M Giants

KS = 11
                   tail
                            Sgr
                            core     tail

KS = 12



            LMC + SMC                RA

- Sgr is primary source (>80%) of high halo (|ZGC| > 20 kpc) M giants
- Though filled with M giants, Magellanic Clouds show no M giant tails
Can Fit Sgr + Milky Way Interaction Models
     to Full Sky M Giant Spatial Data
         e.g., Law, Johnston & Majewski (2005)
Aided by Radial Velocities Sagittarius Stream Stars
          Majewski et al. (2004), and papers in prep.




       Data collected on the
       Swope 1-m, CTIO 1.5-m, KPNO 2.1-m, Bok 2.3-m,
       Du Pont 2.5-m, CAHA 3.5-m, KPNO 4-m
 Best Fitting Sgr + Milky Way Interaction Models
Generally agreement on basic character of Sgr orbit, e.g.:
• Computer model satellite:
   velocity = 326 km/s, Period ~ 0.8 Gyr, apo:peri ~ 57:13 kpc
• But, 2MASS M giants appear to trace ~ 2.5 orbits (~2.0 Gyr)
   of Sgr debris (…then end?)




 Law, Johnston & Majewski (2005)
Despite well matching models that generally agree
               between groups …

   … several (old and new) mysteries remain.


      (1) Model mismatches with data
      (2) Several timing problems
                   Leading arm velocity problem
• Law, Johnston & Majewski (2005) point out difficulties
accounting for slow Sgr leading arm radial velocities.
     vGSR (km/s)




                        leading arm               trailing arm




                          L, orbital longitude (deg)
           Leading Arm Velocity Problem
• Prolate MW halo “solves” RV discrepancy (Helmi 2004).
• Changes Sgr orbit from near head-on collision with present
  solar position to one that passes over us (Law et al. 2004).


                  prolate             oblate
                                           Law, Johnston & Majewski (2004)

                                             Prolate    (q = 1.25)
vGSR (km/s)




                                             Spherical (q = 1.0)




                                             Oblate      (q = 0.9)




              L, orbital longitude (deg)    Fhalo = v2halo ln(R2 + [z2/q2] + d2)
BUT: Orbital Plane Precession in a Flattened Potential
                 Fhalo = v2halo ln(R2 + [z2/q2] + d2)

                                                    Projection of Sgr orbit
                                                    onto Galactic plane for
                                                    +/-2 radial orbits

                                                         past orbit
                                                         (“trailing debris”)

                                                         future orbit
                                                         (“leading debris”)

                                                         amount of planar
                                                         precession in
                                                         4 radial orbits
Johnston, Law & Majewski (2005): Precession of Sgr debris:
• Gives only slightly oblate halo to ~50 kpc (q ~ 0.92 +/- 0.2).
• Strongly rules out prolate (5s): Precesses Sgr backwards.




                           ~50 kpc




• But leaves problem of leading arm velocities unresolved.
    NASA’s SIM will help us unravel
    true orbits of Sgr leading arm stars.

Proposed solutions to RV
discrepancy (Law, Johnston & Majewski 2005):
 Consider Sgr orbital evolution ?
  • induced by evolution of Milky Way potential?
      • Needs to be large in recent times.
      • Affects orbit of Sgr and debris similarly.
  • large lump encounters?
      • Seemed unlikely- needs sudden change
         in orbital character…but see later
  • dynamical friction?
     Testing the Dynamical Friction Hypothesis
               Hummels, Johnston, Majewski & Law

• Evolve model Sgr core (105 particles ) back from present
    RGC ~ 16 kpc, VLOS ~ 171 km/s, MSgr ~ 2-6 x 108 Msun
      and observe leading arm velocity trend.
• Live MW halo (106 particles, NFW potential) where
     mass loss and DF occur naturally
     as function of satellite mass.
• GyrfalcON treecode (Dehnen 2000)
• To match old (NGP) leading
     arm RVs nearby and
     younger debris requires
     Sgr pericenter to decay
     from ~20 kpc to 14 kpc in
     last ~2-3 orbits.
     Testing the Dynamical Friction Hypothesis
                Hummels, Johnston, Majewski & Law

                                           mass evolution
• Two Sgr models
   – 2 x 1010 Msun
   – 2 x 109 Msun



                                       orbital radius evolution
• Low mass progenitor
has too little mass for
significant orbital decay.
      Testing the Dynamical Friction Hypothesis
                    Hummels, Johnston, Majewski & Law
• Model debris ages
   orb = 0 (present peri)
   orb = -1
   orb = -2
   orb = -3
                                                        2 x 1010 Msun
• Maybe can fit recent
debris…
• … but can’t
  simultaneously get
  velocity trend and
  cold stream.
                                                        2 x 109 Msun
• DF doesn’t seem to
  work.
               Timing Problems
(A) How is it (after numerous orbits) we just
   happen to be seeing what looks like critical
   Sgr disruption now?


 • Velazqueze & White (1995):
       “Not too surprising, since it must happen
       at some point to any satellite, and Sgr
       probably endured enough periGalacticons
       to reduce binding energy to critical stability”
                 Timing Problems
(B) How has relatively small Sgr survived a Hubble
    time in this orbit?

   E.g., most models run for only several Gyr.

• Fine tuning of, or large (M/L ~100) DM content in,
      Sgr         (e.g., Ibata et al. 1997, Ibata & Lewis 1998)

       But Sgr M/L appears to be more modest
          (M/L ~ 10-15)
                    (Majewski et al. 2003, Zaggia et al. 2004,
                    Law et al. 2005)
               Timing Problems
(B) How has relatively small Sgr survived a Hubble
    time in this orbit?



 • Sgr is a tidal dwarf formed in major merger
            (e.g., Gomez-Flechoso et al. 1999)

       No evidence for rest of merger.
                Timing Problems
(B) How has relatively small Sgr survived a Hubble
    time in this orbit?
 • Sgr not always in present orbit
   • Just fell in (Koribalski et al. 1994)
       ruled out by discovered streams
   • Dynamical friction (Jiang & Binney 2000, Zhao 2004)
       requires large mass loss quickly
            (Colpi et al. 1999, Law et al. 2004, Hummels et al.)

   • Deflection off subhalo lump, e.g., LMC
            (Ibata & Lewis 1998, Zhao 1998)
 But Sgr-LMC encounter unlikely?:
  • 4% probability at < 5 kpc (Ibata & Lewis 1998)
  • much less (Zhao 1998)


      Zhao (1998)

       Sagittarius
       LMC
       SMC


However,
  • Sgr orbit now much better constrained 
  • Problem reduced primarily to LMC orbit (i.e., m)
         Galactic Billiards?
                                   Sagittarius
                                   LMC
                                   Separation


                                    Animation by
                                    David Nidever
                                       & SRM
                                   (Univ. Virginia)
      QuickTime™ and a
      GIF decompressor
are neede d to see this picture.



                                     (LMC m
                                    optimized
                                      for HI
                                     position
                                   and velocity)
(C) M giant timing problems
• M giants formed in higher metallicity stellar populations
• Sarajedini & Layden (1995) Age-Metallicity Relation for Sgr
   • Old dynamical timing dilemma: Why are M giant arms so long?
                                ~M giants form




• Bellazzini et al. (2006): New CMD+MDF find Sgr enriched to
solar metallicity by 6 Gyr ago (also Lanfranchi & Matteucci 2004)
    New timing dilemma: Why aren’t M giant arms longer?!
Recall:
M giants apparently trace ~ 2.0 Gyr (~2.5 orbits) of debris.
  • Where are older arms?
  • Timing here is very interesting!
 (D) Metallicity “Timing” Problem
Echelle spectroscopy of stars in Sgr core:
  Smecker-Hane & McWilliam (2002),
  Monaco et al. (2005), Chou et al. (2006)
Metallicity Distribution Function spans [Fe/H] ~ -1.6 to solar




                                -1.5         -1.0       -0.5   0.0
                                               [Fe/H]
  (D) Metallicity “Timing” Problem
Chou, SRM, Smith, Cunha (2006):
   Strong [Fe/H] variation along stream
    • D<[Fe/H]> ~ 1 dex in ~2 Gyr of dynamical evolution
    • From AMR most/all these stars are >6 Gyr old
    • Suggests dramatic loss of binding energy in last ~2 Gyr
      of a Sgr progenitor with initial metallicity gradient
                             -1.5     -1.0       -0.5   0.0




                                        [Fe/H]
  (D) Metallicity “Timing” Problem
Chou, SRM, Smith, Cunha (2006):
   Strong [Fe/H] variation along stream
    • D<[Fe/H]> ~ 1 dex in ~2 Gyr of dynamical evolution
    • From AMR most/all these stars are >6 Gyr old
    • Suggests dramatic loss of binding energy in last ~2 Gyr
      of a Sgr progenitor with initial metallicity gradient
  For example, -0.5 dex gradient in Sculptor dSph (Tolstoy et al. 2004):
          Probing the Limits of 2MASS
w/ Pakzad, Nidever, Ivezic, Prada, Johnston, Hummels, Law & Skrutskie


                                                       M giants with
                                                       1 < J-K < 1.1

                                                          K<14.2


                        K mag
         Even Longer Sagittarius Arms?
w/ Pakzad, Nidever, Ivezic, Prada, Johnston, Hummels, Law & Skrutskie


                                                       M giants with
                                                       1 < J-K < 1.1

                                                          K<14.2
Correspondence of 2MASS M giants and SDSS RR Lyrae
                  Ivezic et al. (2004)




 Suggests that even more distant structures in M giant maps rea
   Newberg et al. (2004):       Clewley et al. (2005):
~90 kpc SDSS MSTO feature   SDSS moving group 6 BHB stars
  Even Longer Sagittarius Arms?
       Data                Current Best-Fit Model




Model mismatch a signature of orbital evolution?
 Radial Velocity Data of Outer Halo M giants
         (from KPNO 4-m, CTIO 4-m, CAHA 3.5-m)




• Keplerian RV trend very suggestive of a tidal stream.
• Spatial and velocity data together provide useful constraints.
• Spatial and velocity data consistent with tidal arms from a
  parent object on a more circular orbit with RGC ~ 70 kpc.
• ... But only debris from before 2 Gyr ago needed…
• … and no “dSph core-like” structure is found where expected.
            Outer debris
            progenitor orbit




RGC (kpc)

                               Current Sgr
                               orbit



                t (Gyr)
                   LMC-Sgr Collision?
• Suggests a scenario where outer debris is Sgr, which was “nudged”
     from higher to lower orbit by LMC interaction ~2 Gyr ago.
• Interestingly close to time of near passage of LMC and Sgr.
                LMC-Sgr Collision?
• Sum of M giant data fits picture whereby Sgr “nudged” from
       higher to lower orbit by LMC interaction ~2 Gyr ago.
 ZGC (kpc)




                                RGC (kpc)


             XGC (kpc)                         t (Gyr)
Inner and Outer Sagittarius?
   Pakzad et al. poster 142.05




                 QuickTime™ and a
                None decompressor
          are neede d to see this picture.




                                             Animation by David Law & SRM
                                             Caltech/University of Virginia
                   LMC-Sgr Collision?
• Revisit “Could it Happen??” (Nidever, SRM & Johnston)

• 3-body model:
      Milky Way: static Johnston, Spergel & Hernquist (‘95) potential
      LMC: - 1 kpc softened Plummer model
            - 2 x 1010 Msun (explained later…)
            - dynamical friction (analytically)
            - current observed radial velocity
            - proper motion variable
      SGR: point mass = 7.5x108 Msun

• Run backwards from present configuration varying
      proper motion…
                    LMC-Sgr Collision?
• Look for closest LMC-Sgr approach.
• Look for closest match to putative, stitched old + new Sgr orbit.




                x




    • Minima occur at ~same proper motion.
    • Similar to Jones et al. (1994) m (corrected for LMC rotation)
              (x gives ~match to HI stream plane and RVs used earlier)
             Galactic Billiards?
• Nidever,
Majewski
    &
Johnston:

 3-body
 collision
 starting                 QuickTime™ an d a
                          GIF decompressor
3 Gyr ago           are need ed to see this p icture .
                    Galactic Billiards?
• Nidever, Majewski & Johnston: 3body + “stitched orbit” comparison




• Next step is a full N-body simulation.
Sgr “bumped” from higher to lower orbit by LMC?
      Additional support & problems scenario solves:
• Newly found outer debris in similar plane as “inner” Sgr debris.
• M giants are rare population in dSphs:
   • Almost all halo M giants in inner halo are Sgr.
   • Would need coincidence of second dSph with M giants in
       similar orbital plane.
• No bright M giant dSph core corresponding to the outer debris found.
• Known orbits of LMC and Sgr indicate a close encounter 2 Gyr ago:
   • Matches age inner tails end, longer tails expected if older M giants.
   • LMC had starburst about then.
• Sudden orbital evolution via LMC collision also may explain:
       1) how Sgr survived despite destructive orbit.
       2) apparent dramatic recent change in Sgr binding energy.
       3) >2 Gyr age Sgr debris with “problem” velocities?
… and much more speculatively…
• Nidever et al. (in prep.)
 Leiden-Argentine-Bonn
       HI Survey
   • Age of HI Magellanic
   Stream (MS) of order
   several Gyr old.

   • One of pair of Mag Stream
   filaments can be traced
   back to 30 Dor hotspot…
  … which is at LMC radius
  ~2 kpc = impact parameter
  in model 3-body collision.
                  The LMC ``Cue Ball”
• Is the putative Sgr-LMC collision unique?
• Is LMC active perturber of dwarf galaxy “Oort Cloud”?
   • Most dSphs spend most of their lives RGC > 100 kpc,
      but with significant overlap of (evolving) LMC orbit
      (Rapo< 200).
   • Polar orbits preferred (although selection effect).
   • Other meetings near the Galactic poles might be expected
      (past and/or present) and promote late infall?
• Evidence LMC is significantly bigger cue ball than
      previously measured…
12 deg2 (>6 King radii) Carina dSph Study
               Munoz et al. (2005)
              Magellan+MIKE echelle
                  spectroscopy



                   Carina




         Moving group 21 stars: sv = 10 km/s
           Extreme RV, similar to LMC
Linking fields

                 23 deg = 20 kpc
Minimum rt??
                     Main Points
• New debris stream(s) in outer halo (40-110 kpc) found.
• Sum of M giant streams matches toy models of a Sgr that
      transitions from outer to inner orbit ~2 Gyr ago.
• 2 Gyr coincides with close passage of LMC and Sgr
      and several other LMC/Sgr events.
• Scenario resolves (or portentially resolves) a number
      of mysteries with regard to the Sgr system.
• LMC stars found to RLMC = 23 deg = 20 kpc.
     If bound, LMC larger, more massive perturbing
     “cue-ball” than previously thought.

				
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posted:3/11/2013
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