Sagittarius, New Outer Halo Tidal Streams

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

Steven Majewski - University of Virginia

Vienna UT 11 April 2005

QuickTime™ and aAnimation decompressorare needed to see this picture.

• Discovered by Ibata, Gilmore, Irwin (1994). • Seemed to be archetype dwarf galaxy satellite merger.

The Sagittarius Dwarf Spheroidal (dSph)

Figure by Wyse, Gilmore & Franx (1997)

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)

Ibata et al. (2001)

Johnston et al. (1999)

Martinez-Delgado et al. (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

RA

DE

C

KS = 11

KS = 12

Sgr core

LMC + SMC

tail

tail

- 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 Modelsto Full Sky M Giant Spatial Data

e.g., Law, Johnston & Majewski (2005)

Aided by Radial Velocities Sagittarius Stream Stars

Data collected on theSwope 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

Majewski et al. (2004), and papers in prep.

Best Fitting Sgr + Milky Way Interaction ModelsGenerally 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 agreebetween groups …

… several (old and new) mysteries remain.

(1) Model mismatches with data

(2) Several timing problems

• Law, Johnston & Majewski (2005) point out difficultiesaccounting for slow Sgr leading arm radial velocities.

trailing armleading arm

Leading arm velocity problem

, orbital longitude (deg)

v GSR

(km

/s)

• 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).

Leading Arm Velocity Problem

prolate oblate

Prolate (q = 1.25)

Spherical (q = 1.0)

Oblate (q = 0.9)

Law, Johnston & Majewski (2004)

, orbital longitude (deg)

v

GSR

(km

/s)

halo = v2halo ln(R2 + [z2/q2] + d2)

BUT: Orbital Plane Precession in a Flattened Potential

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

past orbit (“trailing debris”) future orbit (“leading debris”)

amount of planar precession in 4 radial orbits

halo = v2halo ln(R2 + [z2/q2] + d2)

~50 kpc

Johnston, Law & Majewski (2005): Precession of Sgr debris: • Gives only slightly oblate halo to ~50 kpc (q ~ 0.92 +/- 0.2).

• But leaves problem of leading arm velocities unresolved.

• Strongly rules out prolate (5): Precesses Sgr backwards.

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?

NASA’s SIM will help us unravel true orbits of Sgr leading arm stars.

Testing the Dynamical Friction HypothesisHummels, 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 HypothesisHummels, Johnston, Majewski & Law

• Two Sgr models– 2 x 1010 Msun

– 2 x 109 Msun

• Low mass progenitor has too little mass for significant orbital decay.

mass evolution

orbital radius evolution

Testing the Dynamical Friction HypothesisHummels, Johnston, Majewski & Law

• Model debris ages orb = 0 (present peri) orb = -1 orb = -2 orb = -3

• Maybe can fit recentdebris…

2 x 1010 Msun

2 x 109 Msun

• … but can’t simultaneously get velocity trend and cold stream.• 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)

Zhao (1998)

SagittariusLMCSMC

But Sgr-LMC encounter unlikely?:• 4% probability at < 5 kpc (Ibata & Lewis 1998)• much less (Zhao 1998)

However, • Sgr orbit now much better constrained• Problem reduced primarily to LMC orbit (i.e., )

Galactic Billiards?

QuickTime™ and aGIF decompressor

are needed to see this picture.

SagittariusLMCSeparation

Animation by David Nidever

& SRM (Univ. Virginia)

(LMC optimized

for HI position

and velocity)

( C) M giant timing problems

• Bellazzini et al. (2006): New CMD+MDF find Sgr enriched to solar metallicity by 6 Gyr ago (also Lanfranchi & Matteucci 2004)

• 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

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!

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

( D) Metallicity “Timing” Problem

[Fe/H]0.0-1.0 -0.5-1.5

Chou, SRM, Smith, Cunha (2006): Strong [Fe/H] variation along stream• [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

( D) Metallicity “Timing” Problem

[Fe/H]

0.0-1.0 -0.5-1.5

( D) Metallicity “Timing” Problem

For example, -0.5 dex gradient in Sculptor dSph (Tolstoy et al. 2004):

Chou, SRM, Smith, Cunha (2006): Strong [Fe/H] variation along stream• [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

Probing the Limits of 2MASS

K mag

M giants with1 < J-K < 1.1

K<14.2

w/ Pakzad, Nidever, Ivezic, Prada, Johnston, Hummels, Law & Skrutskie

Even Longer Sagittarius Arms?

M giants with1 < J-K < 1.1

K<14.2

w/ Pakzad, Nidever, Ivezic, Prada, Johnston, Hummels, Law & Skrutskie

Correspondence of 2MASS M giants and SDSS RR LyraeIvezic et al. (2004)

Suggests that even more distant structures in M giant maps real.

Newberg et al. (2004): ~90 kpc SDSS MSTO feature

Clewley et al. (2005): SDSS moving group 6 BHB stars

Even Longer Sagittarius Arms?

Model mismatch a signature of orbital evolution?

Data Current Best-Fit Model

• Keplerian RV trend very suggestive of a tidal stream.• Spatial and velocity data together provide useful constraints.

Radial Velocity Data of Outer Halo M giants(from KPNO 4-m, CTIO 4-m, CAHA 3.5-m)

• 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.

t (Gyr)

RG

C (

kpc)

Current Sgr orbit

Outer debris progenitor orbit

• 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?

QuickTime™ and aGIF decompressor

are needed to see this picture.

• Sum of M giant data fits picture whereby Sgr “nudged” from higher to lower orbit by LMC interaction ~2 Gyr ago.

LMC-Sgr Collision?

t (Gyr)XGC (kpc)

ZG

C (

kpc)

RG

C (

kpc)

Animation by David Law & SRM Caltech/University of Virginia

Inner and Outer Sagittarius?Pakzad et al. poster 142.05

QuickTime™ and aNone decompressor

are needed to see this picture.

• 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.

LMC-Sgr Collision?

x

(x gives ~match to HI stream plane and RVs used earlier)

• Minima occur at ~same proper motion.• Similar to Jones et al. (1994) (corrected for LMC rotation)

• Nidever, Majewski

& Johnston:

3-body collisionstarting

3 Gyr ago

Galactic Billiards?

• Nidever, Majewski & Johnston: 3body + “stitched orbit” comparisonGalactic Billiards?

• Next step is a full N-body simulation.

Sgr “bumped” from higher to lower orbit by LMC?• 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?

Additional support & problems scenario solves:

• Nidever et al. (in prep.) Leiden-Argentine-Bonn

HI Survey

… and much more speculatively…

• Age of HI MagellanicStream (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 parameterin model 3-body collision.

• 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 thanpreviously measured…

The LMC ``Cue Ball”

12 deg2 (>6 King radii) Carina dSph StudyMunoz et al. (2005)

Magellan+MIKE echellespectroscopy

Carina

Moving group 21 stars: v = 10 km/sExtreme RV, similar to LMC

23 deg = 20 kpc

Linking fields

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 Sgrand several other LMC/Sgr events.

• Scenario resolves (or portentially resolves) a numberof 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.