2. and the LMC as a Dwarf Galaxy “Cue-ball”
Sagittarius, New Outer Halo Tidal Streams
and the LMC as a Dwarf Galaxy “Cue-ball” ok
Steven Majewski - University of Virginia
Vienna
UT 11 April 2005

3. The Sagittarius Dwarf Spheroidal (dSph)
At larger mass scales we now also have the Sgr dwarf spheroidal galaxy, which was discovered in 1994 by Ibata and collaborators and was immediately recognized to be a system in tidal duress. An unfortunate aspect of the Sgr system is that the core is situation behind the Galactic bulge
Where the foreground dust and stars make it difficult to study in the optical.
Figure by Wyse, Gilmore & Franx (1997)
Discovered by Ibata, Gilmore, Irwin (1994).
Seemed to be archetype dwarf galaxy satellite merger.

4. Even with rather modest early data constraints…
Fortunately, the Two Micron All Sky Survey turns out to be IDEAL for studying the Sgr system because of the reduced effects of foreground dust at infrared wavelengths. In addition, it is an all sky digital survey. And finally, Sgr is filled with M giant stars, which are particularly bright at infrared wavelengths.
Dinescu et al. (2002)

5. 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.
(2004)
Ibata et al. (2001)

6. 2MASS is ideal for Sgr study: - Reduced dust effects - All sky
Sagittarius in the Infrared: All Stars in Two Micron All Sky Survey (2MASS)
Fortunately, the Two Micron All Sky Survey turns out to be IDEAL for studying the Sgr system because of the reduced effects of foreground dust at infrared wavelengths. In addition, it is an all sky digital survey. And finally, Sgr is filled with M giant stars, which are particularly bright at infrared wavelengths.
2MASS is ideal for Sgr study: - Reduced dust effects
- All sky
- Sgr filled with M giants

7. 2MASS All-Sky M Giants KS = 11 tail DEC Sgr core tail KS = 12
When we did so, we were stunned to see not only the Sgr core stand out, but also a pair of extensive arcs of M giants extending outward, as shown in this all sky view of the Galactic M giant distribution. Here we have whited out the numerous M giants from the metal-rich Galactic disk, and shown two cycles of the sky to demonstrate the continuity of the features. Sgr turns out to be primary… Mag Clouds no tails…
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

8. Can Fit Sgr + Milky Way Interaction Models to Full Sky M Giant Spatial Data e.g., Law, Johnston & Majewski (2005)
This past year we have published what we think is the best fitting model given the current constraints….velocity, period, apo:peri.
The 2MASS data appear to trace what appears to be about 2.5 orbits or 2 Gyr worth of Sgr debris, then apparently end.

9. Aided by Radial Velocities Sagittarius Stream Stars
Majewski et al. (2004), and papers in prep.
Aided by extensive work to collect RVs over the past few years. These M giant RV show more or less the expected Keplerian trends one would expect to see in a system circling the Galaxy in an ELLIPTICAL orbit.
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

10. 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?)
This past year we have published what we think is the best fitting model given the current constraints….velocity, period, apo:peri.
The 2MASS data appear to trace what appears to be about 2.5 orbits or 2 Gyr worth of Sgr debris, then apparently end.
Law, Johnston & Majewski (2005)

11. Model mismatches with data Several timing problems
Despite well matching models that generally agree between groups … … several (old and new) mysteries remain.
Model mismatches with data
Several timing problems
These newly mapped tidal tails provide interesting NEW OPPORTNUIITIES for probing the Milky Way’s gravitational potential. …

12. Leading arm velocity problem
Law, Johnston & Majewski (2005) point out difficulties accounting for slow Sgr leading arm radial velocities.
vGSR (km/s)
In my earlier discussion of matching the Sgr models to the radial velocities I slyly swept under the rug one problem that our otherwise very will-fitting model has: Namely, that we have had DIFFICULTY ACCOUNTING for the RVs of the leading arm velocities WITH SPHERICAL OR OBLATE halo models. The velocities of the stars are too slow compared to predictions of the model.
leading arm
trailing arm
L, orbital longitude (deg)

13. 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
…this puts more of the space velocity of the debris in the UNKNOWN PROPER MOTIONS and less in the measured radial velocities.

14. Fhalo = v2halo ln(R2 + [z2/q2] + d2)
Law, Johnston & Majewski (2004)
Prolate (q = 1.25) Spherical (q = 1.0) Oblate (q = 0.9)
vGSR (km/s)
The effect on the expected velocities can be seen in our models here.
Fhalo = v2halo ln(R2 + [z2/q2] + d2)
L, orbital longitude (deg)

15. 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 radial orbits

16. Johnston, Law & Majewski (2005): Precession of Sgr debris:
Gives only slightly oblate halo to ~50 kpc (q ~ /- 0.2).
Strongly rules out prolate (5s): Precesses Sgr backwards.
~50 kpc
In our own analysis of the orbital plane precession using the largest sample of Sgr data we find…
But leaves problem of leading arm velocities unresolved.

17. Proposed solutions to RV discrepancy (Law, Johnston & Majewski 2005):
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?

18. 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.

19. Testing the Dynamical Friction Hypothesis
Hummels, Johnston, Majewski & Law
mass evolution
Two Sgr models
2 x 1010 Msun
2 x 109 Msun
Low mass progenitor has too little mass for significant orbital decay.
orbital radius evolution

20. Testing the Dynamical Friction Hypothesis
Hummels, Johnston, Majewski & Law
Model debris ages orb = 0 (present peri) orb = orb = orb = -3
Maybe can fit recent debris…
2 x 1010 Msun
… but can’t simultaneously get velocity trend and cold stream.
DF doesn’t seem to work.
2 x 109 Msun

21. 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”
These newly mapped tidal tails provide interesting NEW OPPORTNUIITIES for probing the Milky Way’s gravitational potential. …

22. 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)
These newly mapped tidal tails provide interesting NEW OPPORTNUIITIES for probing the Milky Way’s gravitational potential. …

23. 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.
These newly mapped tidal tails provide interesting NEW OPPORTNUIITIES for probing the Milky Way’s gravitational potential. …

24. 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)
These newly mapped tidal tails provide interesting NEW OPPORTNUIITIES for probing the Milky Way’s gravitational potential. …

25. ® 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)

26. Galactic Billiards? Sagittarius LMC Separation
Animation by
David Nidever & SRM (Univ. Virginia)
In fact, some orbital evolution would seem to be INEVITABLE….
It turns out that Sgr and the Magellanic Clouds orbit in almost perpendicular orbits and have overlapping ranges of Galactocentric distance.
Integrating their current orbits backwards…
(LMC m optimized for HI position and velocity)

27. ( 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?!

28. M giants apparently trace ~ 2.0 Gyr (~2.5 orbits) of debris.
Recall:
M giants apparently trace ~ 2.0 Gyr (~2.5 orbits) of debris.
Where are older arms?
Timing here is very interesting!
Isn’t it interesting that the Sgr debris we have seen and modeled so far appears to end at about this time? Is there more going on here? We believe that the answer is YES!

29. ( 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
Isn’t it interesting that the Sgr debris we have seen and modeled so far appears to end at about this time? Is there more going on here? We believe that the answer is YES!
-1.5
-1.0
-0.5
0.0
[Fe/H]

30. ( 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
Isn’t it interesting that the Sgr debris we have seen and modeled so far appears to end at about this time? Is there more going on here? We believe that the answer is YES!
[Fe/H]

31. ( 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):
Isn’t it interesting that the Sgr debris we have seen and modeled so far appears to end at about this time? Is there more going on here? We believe that the answer is YES!

32. 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
Here is a picture of the 2MASS M giant catalogue, plotted in magnitudes.
Stars around the edge of this picture have K magnitudes of about 13.5, near where the 2MASS photometry starts to have large uncertainties.

33. Even Longer Sagittarius Arms?
w/ Pakzad, Nidever, Ivezic, Prada, Johnston, Hummels, Law & Skrutskie
M giants with
1 < J-K < 1.1 K<14.2
What’s interesting is that we see a ring of fainter sources lying near the Sgr plane and at the limit of 2MASS and originally I was inclined to believe this was noise. However a correspondence of these sources to RR Lyrae Stars in the SDSS suggested that they were real M giants lying as far as 100 kpc away. This configuration also seems to be a continuation of the original Sgr system I have been talking about.
…well beyond the known Sgr material but apparently connected to it.

34. Correspondence of 2MASS M giants and SDSS RR Lyrae Ivezic et al. (2004)
Suggests that even more distant structures in M giant maps real.

35. Newberg et al. (2004): ~90 kpc SDSS MSTO feature
Clewley et al. (2005): SDSS moving group 6 BHB stars
CHECK Other claims here -- Clewly?

36. Even Longer Sagittarius Arms?
Data Current Best-Fit Model
But if it IS Sgr debris, it is not in the right place according to our previous best fitting model which suggests that older debris…
Model mismatch a signature of orbital evolution?

37. 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.

38. Spatial and velocity data consistent with tidal arms from a
parent object on a more circular orbit with RGC ~ 70 kpc.

39. ... But only debris from before 2 Gyr ago needed…
… and no “dSph core-like” structure is found where expected.
Need version with other debris

40. Outer debris progenitor orbit
RGC (kpc)
Current Sgr orbit
t (Gyr)

41. 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.
anim

42. 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)

43. Inner and Outer Sagittarius?
Pakzad et al. poster
Animation by David Law & SRM Caltech/University of Virginia

44. LMC-Sgr Collision? 3-body model:
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…

45. LMC-Sgr Collision? Look for closest LMC-Sgr approach.
Look for closest match to putative, stitched old + new Sgr orbit. x
(x is ~match to HI plane and RVs at 1.4, -0.2, nearest point is Jones et al. )
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)

46. 3-body collision starting 3 Gyr ago
Galactic Billiards?
Nidever,
Majewski & Johnston:
3-body collision starting 3 Gyr ago
LMC cue ball hitting Sgr on a bank shot

47. Galactic Billiards?
Nidever, Majewski & Johnston: 3body + “stitched orbit” comparison
Next step is a full N-body simulation.

48. 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?
Check paper comments

49. Nidever et al. (in prep.) Leiden-Argentine-Bonn HI Survey
… 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.

50. 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…
What about LMC side of the equation?
See Nidever poster on HI
Note no M giant streams despite huge M giant pop.

51. 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

52. Linking fields
23 deg = 20 kpc

53. Minimum rt??

54. 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.