New Insights Into the Sagittarius Stream

New insights into the Sagittarius stream

EWASS, Turku July 8th, 2013

Martin C. Smith Shanghai Astronomical Observatory http://hubble.shao.ac.cn/~msmith/ Sagittarius dwarf spheroidal(ish)

• Since its discovery in 1994, the Sagittarius dwarf spheroidal has been hugely important for understanding cosmology in our back yard

Ibata et al (1994) Sagittarius dwarf spheroidal(ish)

• Since its discovery in 1994, the Sagittarius dwarf spheroidal has been hugely important for understanding cosmology in our back yard • Its tidal tails have been traced around our entire galaxy

Ibata et al (1994)

160 A. Helmi

Southern Arc

Sgr Core 45 Northern 0 Fluff

-45

δ 45

0 Northern -45 Arm

180 90 0 270 180 90 0 270 180 LMC SMC Majewski et al (2003) α Fig. 6 2MASS revealing the streams from Sgr. Smoothed maps of the sky for point sources selected according to 11 Ks 12 and 1.00 < J Ks < 1.05 (top), and 12 Ks 13 and 1.05 < J Ks < 1.15 (bottom). Two≤ cycles≤ around the sky are− shown to demonstrate the≤ continuity≤ of features. (From− Majewski et al. 2003. Courtesy of Steve Majewski. Reproduced with permission of the AAS)

database on the basis of their colors) were able to map the streams 360o around the sky as shown in Fig. 6. The vast extent of the Sgr dwarf streams has been used to constrain the shape of the Galactic gravitational potential, albeit with conﬂicting results (Helmi 2004b; Johnston et al. 2005; Fellhauer et al. 2006). In the years to follow, many of the structures discovered in the halo would be linked to the Sgr debris (Newberg et al. 2002; Vivas et al. 2001; Martínez-Delgado et al. 2004). An exception to this is the Monoceros ring (Yanny et al. 2003; Ibata et al. 2003), a low-latitude stream towards the anticenter of the Galaxy, that spans about 100o in longitude at a nearly constant distance. It is somewhat of a semantic distinction to state that this structure is part of the stellar halo, or of the Galactic thin or thick disks. As yet, the nature of the feature and of the Canis Major overdensity in a similar location at low Galactic latitude (Martin et al. 2004), are highly debated. Possible interpretations are that it is debris from an accreted satellite (e.g. Helmi et al. 2003; Martin et al. 2004; Peñarrubia et al. 2005; Martínez-Delgado et al. 2005), an overdensity associated to the Galactic warp (Momany et al. 2004, 2006), or even a projection effect caused by looking along the nearby Norma-Cygnus spiral arm (Moitinho et al. 2006). Recently, Grillmair (2006a) has mapped the structure of the Monoceros overdensity using SDSS, and showed that it consists, at least in part, of a set of very narrow low-latitude substructures (see Fig. 7), which can easily be explained by the models of Peñarrubia et al. (2005). It is worthwhile mentioning here that such a satellite may well perturb the Galactic disk, and induce departures from axial symmetry or overdensities such as e.g. Canis Major. Furthermore, the Sgr dwarf leading tail also appears to be crossing the Galactic plane at roughly the same location and distance as the ring (Newberg et al. 2007), and may well be responsible for some of the substructures observed.

123 Sagittarius dwarf spheroidal(ish)

• Since its discovery in 1994, the Sagittarius dwarf spheroidal has been hugely important for understanding cosmology in our back yard • Its tidal tails have been traced around our entire galaxy

Ibata et al (1994)

160 A. Helmi

Southern Arc

Sgr Core 45 Northern 0 Fluff

-45

δ 45

0 Northern -45 Arm

123 Sagittarius dwarf spheroidal(ish)

• Since its discovery in 1994, the Sagittarius dwarf spheroidal has been hugely important for understanding cosmology in our back yard • Its tidal tails have been traced around our entire galaxy • This is a wonderful probe of the Milky Way’s potential!

Ibata et al (1994)

160 A. Helmi

Southern Arc

Sgr Core 45 Northern 0 Fluff

-45

δ 45

0 Northern -45 Arm

Law & Majewski (2010) 180 90 0 270 180 90 0 270 180 LMC SMC Majewski et al (2003) α Fig. 6 2MASS revealing the streams from Sgr. Smoothed maps of the sky for point sources selected according to 11 Ks 12 and 1.00 < J Ks < 1.05 (top), and 12 Ks 13 and 1.05 < J Ks < 1.15 (bottom). Two≤ cycles≤ around the sky are− shown to demonstrate the≤ continuity≤ of features. (From− Majewski et al. 2003. Courtesy of Steve Majewski. Reproduced with permission of the AAS)

123 Along comes SDSS...

• The most spectacular recent images have come from the Sloan Digital Sky Survey (SDSS), which traced the density of main-sequence turn-oﬀ stars in the halo

Sagittarius Stream

Belokurov et al. (2006) Along comes SDSS...

• The most spectacular recent images have come from the Sloan Digital Sky Survey (SDSS), which traced the density of main-sequence turn-oﬀ stars in the halo

• More recently the streams have been traced in exquisite detail across the whole sky

• The secondary stream appears to accompany the entire orbit 2

Koposov et al. (2012)

Fig. 1.— Density of MSTO stars with 0 star. The dashed red line is the projection of the Sgr orbital plane, as deﬁned in Majewski et al. (2003), and the blue dotted line shows the outline of the comparison ﬁeld as discussed in the text.

al. 1998, 2006; York et al. 2000) archives as well as new profiles and stellar populations of the streams. Where measurements made public as part of the new Data Re- the streams cross Stripe 82, we can take advantage of the lease 8 (DR8)(Aihara et al. 2011; Eisenstein et al. 2011). coadded photometry (Annis et al. 2011), which reaches Crucially, this dataset now includes significant coverage ∼ 2magnitudesdeeperthanthesingleepochSDSSmea- of the southern Galactic hemisphere not available to Be- surements. We use photometric metallicities to demon- lokurov et al. (2006). strate that the two streams have different chemical prop- The paper is arranged as follows. We extend the ‘Field erties. Untangling the substructure is considerably com- of Streams’ plot (Belokurov et al. 2006) to the south in plicated by the existence of a further stream, already Section 2. This shows immediately that the Sgr stream – noticed by Newberg et al. (2009) and dubbed the Cetus in the somewhat misleading nomenclature of our earlier stream. This is studied in Section 4 using blue straggler paper – is bifurcated. Everywhere we look, in both the (BS) and blue horizontal branch (BHBs) stars. south and the north, there is evidence for what appears 2. to be two streams. In Section 3, we use starcounts and THE STELLAR HALO IN THE SOUTH Hess diagrams (Hess 1924) to characterise the density To study substructure in the Galactic stellar halo, we select old and moderately metal-poor stars with the sim- The Astrophysical Journal,750:80(9pp),2012May1 Koposov et al. Along comes SDSS...

• The most spectacular recent images have come from the Sloan Digital Sky Survey (SDSS), which traced the density of main-sequence turn-oﬀ stars in the halo

• More recently the streams have been traced in exquisite detail across the whole sky

• The secondary stream appears to accompany the entire orbit 2

Figure 2. Left: density of MSTO stars (with the same color–magnitude selection as for Figure 1) across the Sgr stream in the south (90◦ < Λ < 120◦) (solid line) and the density of 2MASS M giants in the same region (dotted line). A constant background (around 7000 stars per bin) has been subtracted from the histograms. Right panel: the density of MSTO stars in different slices across the stream from 70◦ < Λ < 80◦ to 110◦ < Λ < 120◦. Although the secondary stream seems to have the same offset from the main stream at different Λ, this is not actually the case as shown in Figure 3.

Koposov et al. (2012)

Figure 4. Background-subtracted Hess diagram of the Sgr stream in the area deﬁned by 100◦ < Λ < 110◦ and 5◦

we show the Hess diagram of the Sgr streams in the range To construct the Hess diagrams, we make use of the fact 100◦ < Λ < 110◦ and 5◦

Fig. 1.— Density of MSTO stars with 0

• Analyse a sample of SDSS spectroscopy in the trailing arm, including BHBs, BSs, RGBs

Fig. 1.— Density of MSTO stars with 0

0 • Analyse a sample of SDSS spectroscopy in the trailing arm, including BHBs, BSs, RGBs -50

• Using kinematics we can -100

determine the total mass of the Vgsr [km/s]

progenitor galaxy -150 2 -200 120 110 100 90 [deg]

Fig. 1.— Density of MSTO stars with 0

• Analyse a sample of SDSS RV Dispersion 10 spectroscopy in the trailing arm, 25 including BHBs, BSs, RGBs Age <= 2 Gyr 20 Sagittarius

• Using kinematics we can 15 determine the total mass of the 10

progenitor galaxy RV Dispersion 5 140 120 100 80 60 40 20 sun [deg]

-50

-100 Vgsr [km/s]

-150

-200 120 110 100 90 [deg] Zhu & Smith (in prep) The mass of Sagittarius

• Analyse a sample of SDSS RV Dispersion 10 spectroscopy in the trailing arm, 25 including BHBs, BSs, RGBs Age <= 2 Gyr 20 Sagittarius

• Using kinematics we can 15 determine the total mass of the 10

progenitor galaxy RV Dispersion 5 • Our analysis suggests upward 234140 120 LAW100 & MAJEWSKI80 60 40 20 Vol. 714 9 [deg] 4.2. Sgr Leading Arm Properties revision to around 3x10 M⊙, sun Although both the leading and trailing tidal tails are traced i.e. the galaxy is much larger by the M-giant population, the location of Sgr slightly past pericenter along its orbit in combination with foreshortening of than originally thought the orbital rosettes caused by our location 8 kpc from the center of the Milky Way means that while the trailing arm extends for 150 across the southern Galactic hemisphere the leading arm ∼ ◦ is clearly0 visible only for 75◦ (a significant section of which is obscured as it passes behind∼ the Galactic disk/bulge). The • Similar conclusions found in exact track of leading Sgr debris as it passes through the north Galactic-50 cap toward the anticenter is therefore uncertain in the Niederste-Ostholt (2010), 2MASS data since the M-giant tracer population is no longer which found that 70% of as prominent in this older section of the stream. In contrast, SDSS-100 imaging is sensitive to older Sgr populations, and clearly the luminosity in the tails! shows (Belokurov et al. 2006;Newbergetal.2007;Coleetal. Law & Majewski 2008Vgsr [km/s] ) the Sgr leading arm arcing through the northern Galactic cap toward re-entry into the Galactic disk in the direction of the (2010) anticenter.-150 Belokurov et al. (2006)initiallysuggestedthattheremaybe abifurcationintheangularpositionoftheSgrstreaminthe -200 northern Galactic120 hemisphere,110 with100 a high surface-brightness90 “A” stream representing the primary [deg] wrap of the leading tidal arm passing through the coordinates (α, δ) (160 , 20 ) and a Figure 4. Velocity dispersion σv along the Sgr trailing arm (filled boxes) as ∼ ◦ ◦ afunctionofthefinalboundSgrmassMSgr for 160 N-body simulations lower surface-brightness “B” stream representing a secondary ∼ with different realizations of the Galactic potential, Sgr initial mass MSgr,0,and wrap of the trailing tidal arm offset to higher declination around interaction time. The hatched area and dashed line indicate the observational 1 (α, δ) (160◦, 30◦)(seealsoFellhaueretal.2006). We adopt range σv 8.3 0.9kms− measured from high-resolution spectra by Monaco ∼ et al. (2007=). The± solid curve represents a polynomial fit to the simulated data. the positions of the “A”-stream survey fields of Belokurov et al. (2006)alongthishighsurface-brightnessbranchasour constraint on the orbital path of leading Sgr tidal debris. These integrated along reasonable orbits for various lengths of time in 17 pointings are tabulated in Table 1.NotethatBelokurovetal. different realizations of the Milky Way gravitational potential. (2006) adopted a photometric distance scale calibrated against The resulting relation between the final bound Sgr mass MSgr and adistancetoSgrof25kpc.ThedistanceslistedinTable1 have the velocity dispersion σ is shown in Figure 4.Extrapolating been rescaled to our adopted DSgr 28 kpc by multiplying by v = from the best-fit polynomial to the curve, we conclude that a afactorof1.12. +1.3 8 We note that the dynamical origin of the lower surface- present bound mass MSgr 2.5 1.0 10 M produces the best fit to the observed velocity= dispersion.− × Adopting$ a typical brightness B stream is uncertain: recent observational data mass-loss history (generally similar along reasonable orbits in presented by Yanny et al. (2009)indicatethattheAand the range of Galactic potentials explored here), this implies that Bstreamshaveindistinguishabletrendsindistance,radial +3.6 8 velocity, metallicity, and mix of stellar types. This suggests the original mass of Sgr was MSgr,0 6.4 2.4 10 M if it has been orbiting in the Galactic potential= for− 8Gyr.Weadopt× $ that the A and B branches represent debris stripped from Sgr 8 ∼ MSgr,0 6.4 10 M ,correspondingtoaninitialSgrscale at similar times, and that the bifurcation may likely be within = × $ length r0 0.85 kpc (see Equation (7)). asinglephaseoftidaldebris,originatinginsubstructurewithin = the initial Sgr dwarf (see discussion in Section 5.2). Since we do not attempt to constrain the internal structure of the Sgr dwarf 4. CONSTRAINING THE PROPERTIES OF THE MILKY (we focus here on exploring the effects of triaxiality in the WAY HALO gravitational potential of the Milky Way, and such an effort is beyond the scope of this contribution), we therefore do not We have now constrained the properties of Sgr itself as func- explicitly constrain our model to match the B branch of the tions of the Galactic parameters φ,q1, and qz. In the following bifurcation. subsections, we go on to explore within which of these poten- Numerically, we define the statistic tials the observational characteristics of the tidal streams are 2 1 (δmodel[i] δobs[i]) best reproduced. χ 2 − (11) δ = n 3 σ 2 δ i δ − ! 4.1. Sgr Trailing Arm Properties describing the accuracy with which the declination of the N-body model traces the A stream of Belokurov et al. (2006), As defined in Section 3.3,weusethestatisticχv,trail to and quantify the fit of a given N-body simulation to the trailing arm (w w )2 χ 2 δ,model δ,obs (12) velocity trend. While χv,trail has already been minimized for a w −2 = σw given choice of φ,q1,qz by determining the orbital velocity of the satellite, the precise minimum value can vary slightly describing the accuracy with which the angular width of the between different halo realizations and we therefore also use it N-body model matches the SDSS data at a right ascension as a constraint on the properties of the Milky Way. α 180 (the non-spherical gravitational potential gives rise to = ◦ Zhu & Smith (in prep) The Astrophysical Journal,750:80(9pp),2012May1 Koposov et al. Secondary stream

• It has lower velocity dispersion

15 [km/s]

gsr 10

Figure 1. Density of MSTO stars with 0

3 Zhu & Smith (in prep) The Astrophysical Journal,750:80(9pp),2012May1 Koposov et al. Secondary stream

• It has lower velocity dispersion • Hints of lower metallicity

15 80 RGB

BHB [km/s] BS 60 gsr 10

Figure 1. Density of MSTO stars with 0

3 Zhu & Smith (in prep) The Astrophysical Journal,750:80(9pp),2012May1 Koposov et al. Secondary stream

• It has lower velocity dispersion • Hints of lower metallicity Blue stragglers L = 86 L = 92 -0.5 L = 100 L = 106 L = 108 -1.0 [Fe/H]

-1.5 80 RGB BHB -15 -10 -5 0 5 10 15 BS 60 B [deg] -1.0 Figure 1. RedDensity of MSTOgiant stars with branch 0

[Fe/H] -1.3density proﬁle, and the behavior of centroids argue in favor of do not always appear to be fully consistent. Distances to the the hypothesis of two streams. stream in the south still rely on the comprehensive study of M giants extracted from the 2MASS data set (e.g., Majewski et al. -1.43.2. Color–Magnitude Diagrams and Distance Gradients 2003). 0 Here, we will rely on the SDSS photometric data and -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 Distances to many different parts of the Sgr stream have been concentrate on the area in the southern Galactic hemisphere -1.5measured in the past using various stellar tracers: carbon stars where the stream is imaged contiguously. Our aim is to construct [Fe/H] (e.g., Totten & Irwin 1998), BHBs (e.g., Yanny et al. 2000; clean Hess diagrams of the two streams so as to analyze Newberg-15 et al. 2003),-10 subgiant branch stars-5 (e.g., Belokurov0 their stellar5 populations. Distances10 or, more accurately,15 relative et al. 2006), red-clump stars (e.g., Correnti et al. 2010), and distances along the stream are needed. If uncorrected for RR Lyrae variables (e.g., Prior et al. 2009;Watkinsetal.B [deg]distance gradients, the features in our Hess diagrams lose 2009). However, when combined to provide as continuous a sharpness. Here, we will use red clump and subgiant stars as coverage of the stream as possible, the results of these methods distance indicators.

3 Zhu & Smith (in prep) The Astrophysical Journal,750:80(9pp),2012May1 Koposov et al. Secondary stream

• It has lower velocity dispersion • Hints of lower metallicity Blue stragglers L = 86 • Fewer blue stragglers? L = 92 -0.5 L = 100 L = 106 • Looks like the companion L = 108 stream is from a smaller, -1.0 [Fe/H] less-enriched system -1.5 3.0 RGB halo -15 -10 -5 0 5 10 15 2.5 BS [Fe/H]>-2.2 B [deg]

2.0 -1.0 Figure 1. RedDensity of MSTOgiant stars with branch 0

[Fe/H] -1.3density proﬁle, and the behavior of centroids argue in favor of do not always appear to be fully consistent. Distances to the the hypothesis of two streams. stream in the south still rely on the comprehensive study of M 0.5 giants extracted from the 2MASS data set (e.g., Majewski et al. -1.43.2. Color–Magnitude Diagrams and Distance Gradients 2003). Here, we will rely on the SDSS photometric data and 0.0 Distances to many different parts of the Sgr stream have been concentrate on the area in the southern Galactic hemisphere -1.5measured in the past using various stellar tracers: carbon stars where the stream is imaged contiguously. Our aim is to construct -15 -10 -5 0 5 10 15 (e.g., Totten & Irwin 1998), BHBs (e.g., Yanny et al. 2000; clean Hess diagrams of the two streams so as to analyze Newberg-15 et al. 2003),-10 subgiant branch stars-5 (e.g., Belokurov0 their stellar5 populations. Distances10 or, more accurately,15 relative B [deg] et al. 2006), red-clump stars (e.g., Correnti et al. 2010), and distances along the stream are needed. If uncorrected for RR Lyrae variables (e.g., Prior et al. 2009;Watkinsetal.B [deg]distance gradients, the features in our Hess diagrams lose 2009). However, when combined to provide as continuous a sharpness. Here, we will use red clump and subgiant stars as coverage of the stream as possible, the results of these methods distance indicators.

3 Precession of the Sagittarius stream 3

Sky density of MSTO stars 20 Belokurov et al. (2013) Precession10 of the stream

0 [deg] • O ~ B • New detection of distant debris from the trailing arm -10

-20 0 100 200 300 ~ ΛO • [deg] Line-of-sight distances of BHB stars

[mag] 18 BHB (m-M) 16

BHB Leading MS/BS Monoceros BHB Cetus BS Leading BHB Trailing BS Trailing 14 0 100 200 300 ~ ΛO • [deg] Line-of-sight velocities of giant stars

200

100 Leading Trailing

[km/s] 0 GSR V -100

-200

0 100 200 300 ~ ΛO • [deg]

Figure 2. Sagittarius stream tomography with multiple tracers. Darker regions correspond to enhanced stellar density. Top: Sky density of MSTO stars in the Sgr stream coordinate system similar to that deﬁned by Majewski et al. (2003). Red (blue) lines show the range of latitude B used to select stars for Middle (Bottom) panel. Middle: Density of stream stars in the plane of Sgr stream longitude Λ˜ ! and distance modulus. For this plot, BHB candidate stars ◦ ◦ with −12 < B˜! < 12 are selected using the criteria of Yanny et al. (2000), while the distances are assigned according to eq. 7 of Deason et al. (2011). ◦ ◦ In the North, the Sgr leading tail is clearly seen at 40 < Λ˜ ! < 120 in both BHBs (17

Figure 1 shows the properties of the candidate BHB stars in 11 turning blue in u − g at around g − r ∼−0.35.Asisobviousfrom Galactic star clusters analysed by An et al. (2008). The left panel of the middle panel of this Figure, the absolute magnitude of a BHB the Figure conﬁrms the effectiveness of the colour cuts suggested varies with its g − r color, as governed by changing temperature. by Yanny et al. (2000). BHBs from clusters with different stellar At small negative values of g − r, Mg ∼ 0.5.However,onmov- populations essentially lie on top of each other forming a tight lo- ing bluewards, the Horizontal Branch tips and by g − r = −0.35, cus going bluewards in g − r at constant u − g and eventually the absolute magnitude is at least half a magnitude fainter. The be-

"c 2012 RAS, MNRAS 000,1–16 Precession of the Sagittarius stream 3

Sky density of MSTO stars 20

0 [deg] • O ~ B

-10

-20 0 100 200 300 ~ ΛO • [deg] Line-of-sight distances of BHB stars Belokurov et al. (2013) Precession20 of the stream

[mag] 18 BHB (m-M) • New detection16 of distant debris from the trailing arm BHB Leading MS/BS Monoceros BHB Cetus BS Leading BHB Trailing BS Trailing 14 0 100 200 300 ~ ΛO • [deg] Line-of-sight velocities of giant stars

200

100 Leading Trailing Precession of the Sagittarius stream 3

[km/s] 0 GSR

V Sky density of MSTO stars -100 20

10 -200

0 0 100 200 300 [deg]

• O ~ • [deg] ~ B ΛO

Figure 2. Sagittarius-10 stream tomography with multiple tracers. Darker regions correspond to enhanced stellar density. Top: Sky density of MSTO stars in the Sgr stream coordinate system similar to that deﬁned by Majewski et al. (2003). Red (blue) lines show the range of latitude B used to select stars for Middle (Bottom) panel. Middle: Density of stream stars in the plane of Sgr stream longitude Λ˜ ! and distance modulus. For this plot, BHB candidate stars ◦ -20 ◦ with −12 < B˜! < 012 are selected using the criteria100 of Yanny et al. (2000), while200 the distances are assigned according300 to eq. 7 of Deason et al. (2011). ~ ◦ ◦ΛO • [deg] In the North, the Sgr leading tail is clearly seen at 40 < Λ˜ ! < 120 in both BHBs (17

[mag] 18 BHB Figure 1 shows the properties of the candidate BHB stars in 11 turning blue in u − g at around g − r ∼−0.35.Asisobviousfrom (m-M) Galactic star clusters16 analysed by An et al. (2008). The left panel of the middle panel of this Figure, the absolute magnitude of a BHB the Figure conﬁrms the effectiveness of the colour cuts suggested varies with its g − r color, as governed by changing temperature. BHB Leading MS/BS Monoceros BHB Cetus by Yanny et al. (2000). BHBsBS Leading from clusters with different steBHBllar Trailing At smallBS negativeTrailing values of g − r, Mg ∼ 0.5.However,onmov- 14 populations essentially0 lie on top of each other100 forming a tight lo- ing200 bluewards, the Horizontal300 Branch tips and by g − r = −0.35, ~ cus going bluewards in g − r at constant u − g and eventually ΛO • [deg] the absolute magnitude is at least half a magnitude fainter. The be- Line-of-sight velocities of giant stars

"c 2012 RAS, MNRAS200 000,1–16

100 Leading Trailing

[km/s] 0 GSR V -100

-200

0 100 200 300 ~ ΛO • [deg]

"c 2012 RAS, MNRAS 000,1–16 Belokurov et al. (2013)

Precession of the Sagittarius stream 7

Precession of the stream ~ ΛO • 0 100 200 300 120 Heliocentric distances, kpc BHB distance, leading SGB distance, Branch A 100 SGB distance, Branch C BHB distance, trailing • New detection of distant debris from the trailing arm RC distance, trailing 80 NGC 2419 Sgr dSph 60

0 120 Galactocentric distances, kpc Gaussian fit log-normal fit 100

20 Leading apo-centre Trailing apo-centre 0 200 LOS GSR velocities, km/s

100

-100

-200

0 100 200 300 ~ ΛO •

Figure 6. Distance and velocity measurements of the Sgr stream.Top: Violet (red) data-points with error bars show the centroid oftheheliocentricdistance of the stream debris at given longitude Λ˜ ! for the leading (trailing) tail. Blue (magenta) ﬁlled circles (squares) are SGB-based Branch A (C) distance measurements from Belokurov et al. (2006) increased by 0.15 mag to match the BHB signal. Orange data-points with error-bars are RGB-based distance measurements from Koposov et al. (2012) increased by 0.35 magtocorrectforthereddeningtowardstheprogenitor.Ablackstarmarksthelocationofthe globular cluster NGC 2419. Middle: Galactocentric stream distances. The stream is assumed to beatB =0◦ everywhere. While this will bias the run of ◦ distances for the individual branches of the leading arm at Λ˜ ! > 150 ,thisisaveryreasonableapproximationforthedebrisaround both apo-centres. Violet and red solid curves show the log-normal ﬁts to the data, whileblacksolidcurvesrepresentpureGaussianmodels.Dottedlines mark the location of the leading and trailing apo-centres. Bottom: Measurements of the line-of-sight velocity VGSR along the stream. The velocity centroids are those based on the SDSS giants stars as presented in Tables 3, 4 and 5. Note that the stream velocity appears to go through zero in the vicinity of the apo-centre.

The available kinematics of the trailing tail lend support toourin- close to the peri-centre, which for the trailing tail seems tolienot terpretation. The lower panel of Figure 6 shows that as the debris too far from the point of the crossing with the leading tail. ◦ reach the apo-centre at ˜ ! ,theLOSvelocitychangessign Λ ∼ 170 If the above interpretation is correct, then the Sgr trailingde- and goes through zero. The maximum LOS velocity is reached at a bris are ﬂung out as far as 100 kpc away from the Galactic centre. point where the line of sight is best aligned with the stream. In the This in turn implies a difference of ∼ 50 kpc between the lead- trailing arm velocity data, there is a clear indication of theplateau ◦ ing and the trailing apo-centres, which is not predicted by any of at ˜ ! .TheLOSvelocityisthenexpectedtodroptozero Λ ∼ 135 the current Sgr disruption models. While the orbital precession is

!c 2012 RAS, MNRAS 000,1–16 Belokurov et al. (2013)

Precession of the Sagittarius stream 7

0 120 Galactocentric distances, kpc Gaussian fit log-normal fit 100

20 Leading apo-centre Trailing apo-centre 0 200 LOS GSR velocities, km/s

100

-100

-200

0 100 200 300 ~ ΛO •

!c 2012 RAS, MNRAS 000,1–16 Belokurov et al. (2013) Precession of the stream

• New detection of distant debris from the trailing arm • What does this mean for the model, which was Precession of the Sagittarius stream 7

not built to ﬁt this detection? ~ ΛO • 0 100 200 300 120 Heliocentric distances, kpc BHB distance, leading SGB distance, Branch A 100 SGB distance, Branch C BHB distance, trailing RC distance, trailing 80 NGC 2419 Sgr dSph Precession60 of the Sagittarius stream 11

0 120 100 100 Galactocentric distances, kpc Gaussian fit log-normal fit 100

50 50 40 NGC 2419

20 [kpc] [kpc] Leading apo-centre Trailing apo-centre

SGR SGR 0

Y Y 200 LOS GSR velocities, km/s 0 0 Sun 100 Sgr 0

-100

-50 -50 -200

0 100 200 300 ~ -50 0 50 100 -50 0 50 100 ΛO • XSGR [kpc] Figure 6. Distance andX velocitySGR measurements[kpc] of the Sgr stream.Top: Violet (red) data-points with error bars show the centroid oftheheliocentricdistance of the stream debris at given longitude Λ˜ ! for the leading (trailing) tail. Blue (magenta) filled circles (squares) are SGB-based Branch A (C) distance measurements from Belokurov et al. (2006) increased by 0.15 mag to match the BHB signal. Orange data-points with error-bars are RGB-based distance measurements from Koposov et al. (2012) increased by 0.35 magtocorrectforthereddeningtowardstheprogenitor.Ablackstarmarksthelocationofthe Figure 10. Stream precession in the plane of the Sgr orbit. The plane chosen has its pole at Galactocentricglobular cluster NGC 2419. Middle:◦ andGalactocentric stream− distances.◦.Allsymbols, The stream is assumed to beatB =0◦ everywhere. While this will bias the run of lGC =275 bGC = 14 ◦ distances for the individual branches◦ of the leading arm at Λ˜ ! > 150 ,thisisaveryreasonableapproximationforthedebrisaround both apo-centres. Violet colours and curves are identical to those in Figure 6. Left: Note that the actual Galactocentric orbitaland precession red solid curves of show93 the log-normalis slightly fits to lower the data, thanwhileblacksolidcurvesrepresentpureGaussianmodels.Dotted the difference lines mark the location of the between the heliocentric apo-centre phases from Figure 6. Right: Comparison with the Sgr disruptionleading model and by trailing Law apo-centres. & MajewskiBottom: Measurements (2010) ofshown the line-of-sight as grey-scale velocity VGSR along the stream. The velocity centroids are those based on the SDSS giants stars as presented in Tables 3, 4 and 5. Note that the stream velocity appears to go through zero in the vicinity of the apo-centre. ◦ ˜T ◦ density. Note that in the logarithmic halo used in the model, the orbital precession is ∼ 120 and the trailing apo-centre lies in the Galactic disk at BGC ∼ 0 T ˜T ◦ T and distance of R ∼ 65 kpc. This should be contrasted with the new measurement of BGC ∼ 23 andThe availableR =102 kinematics.5 ± of the2. trailing5 kpc. tail lend support toourin- close to the peri-centre, which for the trailing tail seems tolienot terpretation. The lower panel of Figure 6 shows that as the debris too far from the point of the crossing with the leading tail. ◦ reach the apo-centre at ˜ ! ,theLOSvelocitychangessign Λ ∼ 170 If the above interpretation is correct, then the Sgr trailingde- and goes through zero. The maximum LOS velocity is reached at a bris are flung out as far as 100 kpc away from the Galactic centre. point where the line of sight is best aligned with the stream. In the This in turn implies a difference of ∼ 50 kpc between the lead- trailing arm velocity data, there is a clear indication of theplateau ◦ ing and the trailing apo-centres, which is not predicted by any of at ˜ ! .TheLOSvelocityisthenexpectedtodroptozero 200 Λ ∼ 135 the current Sgr disruption models. While the orbital precession is

!c 2012 RAS, MNRAS 000,1–16

100

GSR 0 V

-100 Leading Trailing North Trailing South L&M 2010 Leading typical stream FWHM in L&M 2010 model -200 L&M 2010 Trailing

0 100 200 300 ~ ΛO •

Figure 11. Measurements of the Sgr stream radial velocity (colored datapoints)andthepredictionsoftheLaw&Majewski(2010)disruption model (filled and empty circles). In each bin of Λ˜ ",thecentroidofthemodeldebrisisgivenbythemedianparticle velocity. The agreement between the data and the model is perfect in the South. The kinematics of the leading tail in the North is also reproduced fairly well, although there seems to be a systematic offset of order of ∼20 km s−1.Thisoffsetishoweversmallerthanthebreadthofthemodeltidal debris as indicated by the stand-alone black error-bar. The most significant ◦ ◦ discrepancy is between the distant trailing tail data (red) and the model prediction (filled circles) in the range 130 < Λ˜ " < 190 .

4THEPRECESSIONOFTHESAGITTARIUSDEBRIS plerian potentials, the precession angle is 0◦ to ensure the orbits are closed after one period, while in logarithmic haloes, the precession 4.1 Orbital precession is ∼ 120◦.Orbitsintheouterregionsofsphericalgalaxiesshould posses a precession rate somewhere between 0◦ and ∼ 120◦.The The rate with which an orbit precesses in a spherically symmetric precession rate is not solely dependent on the mass decay ratein gravitational potential depends primarily on how quickly the mass the host, it is also a weak function of the orbital energy and an- generating the gravity ﬁeld decays with radius. For example,inKe-

#c 2012 RAS, MNRAS 000,1–16 Belokurov et al. (2013) Precession of the stream

• New detection of distant debris from the trailing arm • What does this mean for the model, which was Precession of the Sagittarius stream 7

not built to fit this detection? ~ ΛO • 0 100 200 300 120 Heliocentric distances, kpc BHB distance, leading SGB distance, Branch A 100 SGB distance, Branch C • This amount of precession is inconsistent with logarithmic halo profilesBHB distance,and trailing RC distance, trailing 80 NGC 2419 requires a much steeper fall-off in density Sgr dSph Precession of the Sagittarius stream 11 Precession60 of the Sagittarius stream 11

0 120 100 100 100 Galactocentric distances, kpc Gaussian fit log-normal fit 100

50 50 50 40 NGC 2419 NGC 2419 20 [kpc] [kpc] [kpc] [kpc] Leading apo-centre Trailing apo-centre SGR SGR

SGR SGR 0 Y Y Y Y 200 LOS GSR velocities, km/s 0 0 0 Sun Sun 100 Sgr Sgr 0

-100

-50 -50 -50 -200

0 100 200 300 ~ -50 0 50 100 -50 0 50 100 -50 0 50 100 ΛO • XSGR [kpc] XSGR [kpc] Figure 6. Distance andX velocitySGR measurements[kpc] of the Sgr stream.Top: Violet (red) data-points with error bars show the centroid oftheheliocentricdistance of the stream debris at given longitude Λ˜ ! for the leading (trailing) tail. Blue (magenta) filled circles (squares) are SGB-based Branch A (C) distance measurements from Belokurov et al. (2006) increased by 0.15 mag to match the BHB signal. Orange data-points with error-bars are RGB-based distance measurements from Koposov et al. (2012) increased by 0.35 magtocorrectforthereddeningtowardstheprogenitor.Ablackstarmarksthelocationofthe Figure 10. Stream precession in the plane of◦ the Sgr orbit. The plane◦ chosen has its pole at Galactocentricglobular cluster NGC 2419. Middle:◦ andGalactocentric stream− distances.◦.Allsymbols, The stream is assumed to beatB =0◦ everywhere. While this will bias the run of Figure 10. Stream precession in the plane of the Sgr orbit. The plane chosen has its pole at Galactocentric lGC =275 and bGC = −14 .Allsymbols, lGC =275 bGC = 14 ◦ ◦ distances for the individual branches◦ of the leading arm at Λ˜ ! > 150 ,thisisaveryreasonableapproximationforthedebrisaround both apo-centres. Violet colours and curves are identical to those in Figure 6. Left: Note that the actualcolours Galactocentric and curves areorbital identical precession to those of 93 in Figureis slightly 6. Left: lowerNote than that the the difference actual Galactocentric orbitaland precession red solid curves of show93 the log-normalis slightly fits to lower the data, thanwhileblacksolidcurvesrepresentpureGaussianmodels.Dotted the difference lines mark the location of the between the heliocentric apo-centre phases from Figure 6. Right: Comparison with the Sgr disruptionleading model and by trailing Law apo-centres. & MajewskiBottom: Measurements (2010) ofshown the line-of-sight as grey-scale velocity VGSR along the stream. The velocity centroids are those based on the between the heliocentric apo-centre phases from Figure 6. Right: Comparison with the Sgr disruption model by Law & Majewski (2010) shown as grey-scale SDSS giants stars as presented in Tables 3, 4 and 5. Note that the stream velocity appears to go through zero in the vicinity of the apo-centre. ◦ ˜T ◦ ◦ ˜T ◦ density. Note that in the logarithmic halo used in the model, the orbital precessiondensity. is Note∼ 120 thatand in the the logarithmic trailing apo-centre halo used lies in in the the model, Galacticthe disk orbital at B precessionGC ∼ 0 is ∼ 120 and the trailing apo-centre lies in the Galactic disk at BGC ∼ 0 T ˜T T ◦ T ˜T ◦ T and distance of R ∼ 65 kpc. This should be contrasted with the new measurementand distance of B ofGCR ∼∼2365andkpc.R This=102 should.5 be± contrasted2.5 kpc. with the new measurement of BGC ∼ 23 andThe availableR =102 kinematics.5 ± of the2. trailing5 kpc. tail lend support toourin- close to the peri-centre, which for the trailing tail seems tolienot terpretation. The lower panel of Figure 6 shows that as the debris too far from the point of the crossing with the leading tail. ◦ reach the apo-centre at ˜ ! ,theLOSvelocitychangessign Λ ∼ 170 If the above interpretation is correct, then the Sgr trailingde- and goes through zero. The maximum LOS velocity is reached at a bris are flung out as far as 100 kpc away from the Galactic centre. point where the line of sight is best aligned with the stream. In the This in turn implies a difference of ∼ 50 kpc between the lead- trailing arm velocity data, there is a clear indication of theplateau ◦ ing and the trailing apo-centres, which is not predicted by any of at ˜ ! .TheLOSvelocityisthenexpectedtodroptozero 200 200 Λ ∼ 135 the current Sgr disruption models. While the orbital precession is !c 2012 RAS, MNRAS 000,1–16

100 100 GSR GSR 0 0 V V

-100 Leading -100 Leading Trailing North Trailing North Trailing South Trailing South L&M 2010 Leading typicalL&M 2010 stream Leading FWHM in L&M 2010 model typical stream FWHM in L&M 2010 model -200 L&M 2010 Trailing -200 L&M 2010 Trailing

0 100 2000 300100 200 300 ~ ~ ΛO • ΛO •

Figure 11. Measurements of the Sgr stream radial velocity (colored datapoints)andthepredictionsoftheLaw&Majewski(2010)disFigure 11. Measurements of the Sgr stream radial velocity (coloredruption modeldatapoints)andthepredictionsoftheLaw&Majewski(2010)dis (filled ruption model (filled and empty circles). In each bin of Λ˜ ",thecentroidofthemodeldebrisisgivenbythemedianpartiand empty circles). In eachcle velocity. bin of Λ˜ The",thecentroidofthemodeldebrisisgivenbythemedianparti agreement between the data and the model cle velocity. The agreement between the data and the model is perfect in the South. The kinematics of the leading tail in the North is alsois perfect reproduced in the fairly South. well, The although kinematics there of see thems leading to be atail systematic in the North offset is alsoof order reproduced fairly well, although there seems to be a systematic offset of order of ∼20 km s−1.Thisoffsetishoweversmallerthanthebreadthofthemodeloftidal∼20 debris km s− as1.Thisoffsetishoweversmallerthanthebreadthofthemodel indicated by the stand-alone black error-bar. The most significanttidal debris as indicated by the stand-alone black error-bar. The most significant ◦ ◦ ◦ ◦ discrepancy is between the distant trailing tail data (red) and the model predictiondiscrepancy (filled is circles) between in the distantrange 130 trailing< Λ tail˜ " data< 190 (red). and the model prediction (filled circles) in the range 130 < Λ˜ " < 190 .

4THEPRECESSIONOFTHESAGITTARIUSDEBRIS 4THEPRECESSIONOFTHESAGITTARIUSDEBRISplerian potentials, the precession angle is 0◦ to ensure the orbits are plerian potentials, the precession angle is 0◦ to ensure the orbits are closed after one period, while in logarithmic haloes, the precession closed after one period, while in logarithmic haloes, the precession ◦ 4.1 Orbital precession 4.1 Orbitalis ∼ 120 precession◦.Orbitsintheouterregionsofsphericalgalaxiesshouldis ∼ 120 .Orbitsintheouterregionsofsphericalgalaxiesshould posses a precession rate somewhere between 0◦ and ∼ 120◦.Theposses a precession rate somewhere between 0◦ and ∼ 120◦.The The rate with which an orbit precesses in a spherically symmetric The rateprecession with which rate an is orbit not solely precesses dependent in a spherically on the mass symme decaytric rateinprecession rate is not solely dependent on the mass decay ratein gravitational potential depends primarily on how quickly the mass gravitationalthe host, potential it is also depends a weak primarily function on of how the quickly orbital energy the mass and an- the host, it is also a weak function of the orbital energy and an- generating the gravity ﬁeld decays with radius. For example,inKe-generating the gravity ﬁeld decays with radius. For example,inKe-

#c 2012 RAS, MNRAS 000,1–16 #c 2012 RAS, MNRAS 000,1–16 What have we learnt from Sagittarius?

• Large surveys like SDSS have allowed us to draw many insights from the Sagittarius system

• The velocity dispersion suggests

that the progenitor was a pretty RV Dispersion 10 massive galaxy, potentially similar 25 Age <= 2 Gyr in size to the SMC 20 Sagittarius 15 • There is a companion stream, likely 10 RV Dispersion from a smaller system (related to 5 140 120 100 80 60 40 20 NGC2419?), with smaller dispersion sun [deg] and lower metallicities than the main stream

• Precession of the stream tells us that the halo proﬁle is probably steeply declining with radius