New insights into the Sagittarius stream

Transcription

1 New insights into the Sagittarius stream EWASS, Turku July 8th, 213 Martin C. Smith Shanghai Astronomical Observatory

2 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)

3 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) Southern Arc 45 Sgr Core Northern Fluff -45 δ Northern Arm LMC SMC α Majewski et al (23) Fig. 6 2MASS revealing the streams from Sgr. Smoothed maps of the sky for point sources selected accord-

4 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) Southern Arc 45 Sgr Core Northern Fluff -45 δ Northern Arm LMC SMC α Majewski et al (23) Fig. 6 2MASS revealing the streams from Sgr. Smoothed maps of the sky for point sources selected accord-

5 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) Southern Arc 45 Sgr Core Northern Fluff -45 δ Northern Arm Law & Majewski (21) 18 Majewski et al (23) LMC SMC α Fig. 6 2MASS revealing the streams from Sgr. Smoothed maps of the sky for point sources selected accord-

6 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. (26)

7 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. (212)

8 The Astrophysical Journal, 75:8 (9pp), 212 May 1 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 the density of 2MASS M giants in the same region (dotted line). A constant background (aro panel: the density of MSTO stars in different slices across the stream from 7 < Λ < 8 t same offset from the main stream at different Λ, this is not actually the case as shown in Figu Koposov et al. (212)

9 Zhu & Smith (in prep) The mass of Sagittarius Analyse a sample of SDSS spectroscopy in the trailing arm, including BHBs, BSs, RGBs 2

10 Zhu & Smith (in prep) The mass of Sagittarius Analyse a sample of SDSS spectroscopy in the trailing arm, including BHBs, BSs, RGBs Vgsr [km/s] Using kinematics we can determine the total mass of the progenitor galaxy [deg] 9

11 Zhu & Smith (in prep) The mass of Sagittarius Analyse a sample of SDSS spectroscopy in the trailing arm, including BHBs, BSs, RGBs Using kinematics we can determine the total mass of the progenitor galaxy RV Dispersion Age <= 2 Gyr Sagittarius sun [deg] -5 Vgsr [km/s] [deg]

12 Zhu & Smith (in prep) The mass of Sagittarius Analyse a sample of SDSS spectroscopy in the trailing arm, including BHBs, BSs, RGBs Using kinematics we can determine the total mass of the progenitor galaxy Our analysis suggests upward revision to around 3x1 9 M, i.e. the galaxy is much larger than originally thought RV Dispersion Age <= 2 Gyr Sagittarius sun [deg] Similar conclusions found in Niederste-Ostholt (21), which found that 7% of the luminosity in the tails! Law & Majewski (21) Vgsr [km/s] [deg]

13 Zhu & Smith (in prep) Secondary stream It has lower velocity dispersion 2 gsr [km/s] Figure 1. Density of MSTO stars with <g i<.7 and19.5 <i<22 from the SDSS DR8 on the sky in different coordinate systems. The top panel sho the map in right ascension and declination, the middle panel in Galactic coordinates, while the bottom panel in a coordinate system (Λ, B) aligned with the orbi Sagittarius, as defined in Majewski et al. (23). Several stellar streams are clearly visible, the most prominent of which is the one originating from the Sgr dsph. T Sgr stream dominating the area around the North Galactic Cap has been seen in the previous SDSS data releases. While some pieces of the southern stream have b revealed before, the new data give a much more complete picture. Similarly to the tail in the North, the tail in the South appears to have a fainter extension at one s (at higher B). The present location of Sgr dwarf is marked by a red star. The dashed red line is the projection of the Sgr orbital plane, as defined in Majewski et -15-1(23), and the blue dotted -5line shows the outline of the comparison field as discussed 5 in the text (A color version of this figure is available in the online journal.) B [deg] density profile, and the behavior of centroids argue in favor of the hypothesis of two streams Color Magnitude Diagrams and Distance Gradients Distances to many different parts of the Sgr stream have been measured in the past using various stellar tracers: carbon stars (e.g., Totten & Irwin 1998), BHBs (e.g., Yanny et al. 2; Newberg et al. 23), subgiant branch stars (e.g., Belokurov et al. 26), red-clump stars (e.g., Correnti et al. 21), and RR Lyrae variables (e.g., Prior et al. 29; Watkins et al. 29). However, when combined to provide as continuous a coverage of the stream as possible, the results of these methods do not always appear to be fully consistent. Distances to stream in the south still rely on the comprehensive study of giants extracted from the 2MASS data set (e.g., Majewski et 23). Here, we will rely on the SDSS photometric data a concentrate on the area in the southern Galactic hemisph where the stream is imaged contiguously. Our aim is to constru clean Hess diagrams of the two streams so as to analy their stellar populations. Distances or, more accurately, relat distances along the stream are needed. If uncorrected distance gradients, the features in our Hess diagrams lo sharpness. Here, we will use red clump and subgiant stars distance indicators.

14 Zhu & Smith (in prep) Secondary stream It has lower velocity dispersion Hints of lower metallicity RGB BHB BS gsr [km/s] [Fe/H] 5 Figure 1. Density of MSTO stars with <g i<.7 and19.5 <i<22 from the SDSS DR8 on the sky in different coordinate systems. The top panel sho the map in right ascension and declination, the middle panel in Galactic coordinates, while the bottom panel in a coordinate system (Λ, B) aligned with the orbi Sagittarius, as defined in Majewski et al. (23). Several stellar streams are clearly visible, the most prominent of which is the one originating from the Sgr dsph. T Sgr stream dominating the area around the North Galactic Cap has been seen in the previous SDSS data releases. While some pieces of the southern stream have b revealed before, the new data give a much more complete picture. Similarly to the tail in the North, the tail in the South appears to have a fainter extension at one s (at higher B). The present location of Sgr dwarf is marked by a red star. The dashed red line is the projection of the Sgr orbital plane, as defined in Majewski et -15-1(23), and the blue dotted -5line shows the outline of the comparison field as discussed 5 in the text (A color version of this figure is available in the online journal.) B [deg] density profile, and the behavior of centroids argue in favor of the hypothesis of two streams Color Magnitude Diagrams and Distance Gradients Distances to many different parts of the Sgr stream have been measured in the past using various stellar tracers: carbon stars (e.g., Totten & Irwin 1998), BHBs (e.g., Yanny et al. 2; Newberg et al. 23), subgiant branch stars (e.g., Belokurov et al. 26), red-clump stars (e.g., Correnti et al. 21), and RR Lyrae variables (e.g., Prior et al. 29; Watkins et al. 29). However, when combined to provide as continuous a coverage of the stream as possible, the results of these methods do not always appear to be fully consistent. Distances to stream in the south still rely on the comprehensive study of giants extracted from the 2MASS data set (e.g., Majewski et 23). Here, we will rely on the SDSS photometric data a concentrate on the area in the southern Galactic hemisph where the stream is imaged contiguously. Our aim is to constru clean Hess diagrams of the two streams so as to analy their stellar populations. Distances or, more accurately, relat distances along the stream are needed. If uncorrected distance gradients, the features in our Hess diagrams lo sharpness. Here, we will use red clump and subgiant stars distance indicators.

15 Zhu & Smith (in prep) Secondary stream It has lower velocity dispersion Hints of lower metallicity [Fe/H] Blue stragglers L = 86 L = 92 L = 1 L = 16 L = RGB BHB BS B [deg] [Fe/H] [Fe/H] -1. Figure 1. Density of MSTO stars with <g i<.7 and19.5 <i<22 from the SDSS DR8 on the sky in different coordinate systems. The top panel sho the map in right ascension and declination, the middle panel in Galactic coordinates, while the bottom panel in a coordinate system (Λ, B) aligned with the orbi Sagittarius, as defined in Majewski et al. (23). Several stellar streams are clearly visible, the most prominent of which is the one originating from the Sgr dsph. T Sgr stream dominating the area around the North Galactic Cap has been seen in the previous SDSS data releases. While some pieces of the southern stream have b revealed before, the new data give a much more complete picture. Similarly to the tail in the North, the tail in the South appears to have a fainter extension at one s (at higher B). The present location of Sgr dwarf is marked by a red star. The dashed red line is the projection of the Sgr orbital plane, as defined in Majewski et (23), and the blue dotted line shows the outline of the comparison field as discussed in the text. (A color version of this figure is available in the online journal.) density profile, and the behavior of centroids argue in favor of the hypothesis of two streams Red giant branch 3.2. Color Magnitude Diagrams and Distance Gradients Distances to many different parts of the Sgr stream have been measured in the past using various stellar tracers: carbon stars (e.g., Totten & Irwin 1998), BHBs (e.g., Yanny et al. 2; Newberg et al. 23), subgiant branch stars (e.g., Belokurov et al. 26), red-clump stars (e.g., Correnti et al. 21), and RR Lyrae variables (e.g., Prior et al. 29; Watkins et al. 29). However, when combined to provide as continuous a coverage of the stream as possible, the results of these methods do not always appear to be fully consistent. Distances to stream in the south still rely on the comprehensive study of giants extracted from the 2MASS data set (e.g., Majewski et 23). Here, we will rely on the SDSS photometric data a concentrate on the area in the southern Galactic hemisph where the stream is imaged contiguously. Our aim is to constru clean Hess diagrams of the two streams so as to analy their stellar populations. Distances or, more accurately, relat distances along the stream are needed. If uncorrected distance gradients, the features in our Hess diagrams lo sharpness. Here, we will use red clump and subgiant stars distance indicators B [deg]

16 Zhu & Smith (in prep) Secondary stream It has lower velocity dispersion Hints of lower metallicity Fewer blue stragglers? Looks like the companion stream is from a smaller, less-enriched system [Fe/H] Blue stragglers L = 86 L = 92 L = 1 L = 16 L = RGB halo BS [Fe/H]> B [deg] Sgr/halo ratio [Fe/H] B [deg] -1. Figure 1. Density of MSTO stars with <g i<.7 and19.5 <i<22 from the SDSS DR8 on the sky in different coordinate systems. The top panel sho the map in right ascension and declination, the middle panel in Galactic coordinates, while the bottom panel in a coordinate system (Λ, B) aligned with the orbi Sagittarius, as defined in Majewski et al. (23). Several stellar streams are clearly visible, the most prominent of which is the one originating from the Sgr dsph. T Sgr stream dominating the area around the North Galactic Cap has been seen in the previous SDSS data releases. While some pieces of the southern stream have b revealed before, the new data give a much more complete picture. Similarly to the tail in the North, the tail in the South appears to have a fainter extension at one s (at higher B). The present location of Sgr dwarf is marked by a red star. The dashed red line is the projection of the Sgr orbital plane, as defined in Majewski et (23), and the blue dotted line shows the outline of the comparison field as discussed in the text. (A color version of this figure is available in the online journal.) density profile, and the behavior of centroids argue in favor of the hypothesis of two streams Red giant branch 3.2. Color Magnitude Diagrams and Distance Gradients Distances to many different parts of the Sgr stream have been measured in the past using various stellar tracers: carbon stars (e.g., Totten & Irwin 1998), BHBs (e.g., Yanny et al. 2; Newberg et al. 23), subgiant branch stars (e.g., Belokurov et al. 26), red-clump stars (e.g., Correnti et al. 21), and RR Lyrae variables (e.g., Prior et al. 29; Watkins et al. 29). However, when combined to provide as continuous a coverage of the stream as possible, the results of these methods do not always appear to be fully consistent. Distances to stream in the south still rely on the comprehensive study of giants extracted from the 2MASS data set (e.g., Majewski et 23). Here, we will rely on the SDSS photometric data a concentrate on the area in the southern Galactic hemisph where the stream is imaged contiguously. Our aim is to constru clean Hess diagrams of the two streams so as to analy their stellar populations. Distances or, more accurately, relat distances along the stream are needed. If uncorrected distance gradients, the features in our Hess diagrams lo sharpness. Here, we will use red clump and subgiant stars distance indicators B [deg]

17 Belokurov et al. (213) Precession of the stream New detection of distant debris from the trailing arm Line-of-sight distances of BHB stars 2 (m-m) BHB [mag] BHB Leading BS Leading MS/BS Monoceros BHB Trailing BHB Cetus BS Trailing ~ Λ O [deg]

18 Belokurov et al. (213) Precession of the stream New detection of distant debris from the trailing arm Line-of-sight velocities of giant stars 2 1 V GSR [km/s] Leading Trailing ~ Λ O [deg] Line-of-sight distances of BHB stars 2 (m-m) BHB [mag] BHB Leading BS Leading MS/BS Monoceros BHB Trailing BHB Cetus BS Trailing ~ Λ O [deg]

19 Belokurov et al. (213) Precession of the stream New detection of distant debris from the trailing arm 12 Galactocentric distances, kpc 1 Gaussian fit log-normal fit Leading apo-centre 2 LOS GSR velocities, km/s Trailing apo-centre ~ Λ O

20 Belokurov et al. (213) Precession of the stream New detection of distant debris from the trailing arm What does this mean for the model, which was not built to fit this detection? Law & Majewski (21) 12 Galactocentric distances, kpc 1 Gaussian fit log-normal fit Leading apo-centre 2 LOS GSR velocities, km/s Trailing apo-centre ~ Λ O

21 Belokurov et al. (213) Precession of the stream New detection of distant debris from the trailing arm What does this mean for the model, which was not built to fit this detection? Precession of the Sagittarius stream Galactocentric distances, kpc 1 Gaussian fit log-normal fit 8 Y SGR [kpc] 5 Sgr Sun NGC 2419 Y SGR [kpc] Leading apo-centre 2 LOS GSR velocities, km/s Trailing apo-centre X SGR [kpc] ~ X SGR [kpc] Λ O Figure 1. Stream precession in the plane of the Sgr orbit. The plane chosen has its pole at Galactocentric l GC =275 and b GC = 14.Allsymbols,

22 Belokurov et al. (213) Precession of the stream New detection of distant debris from the trailing arm What does this mean for the model, which was not built to fit this detection? This amount of precession is inconsistent with logarithmic halo profiles and requires a much steeper fall-off in density Precession of the Sagittarius stream 11 Precession of the Sagittarius stream Galactocentric distances, kpc 1 Gaussian fit log-normal fit 8 9 Y SGR [kpc] 5 Sgr Sun NGC 2419 Y SGR [kpc] Leading apo-centre 2 LOS GSR velocities, km/s Trailing apo-centre X SGR [kpc] ~ X SGR [kpc] Λ O plane chosen Figure has its 1. pole Stream at Galactocentric precession in the l plane of the Sgr orbit. The plane chosen has its pole at Galactocentric l GC =275 and b GC = 14 GC =275 and b GC = 14.Allsymbols,.Allsymbols,

23 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 massive galaxy, potentially similar in size to the SMC There is a companion stream, likely from a smaller system (related to NGC2419?), with smaller dispersion and lower metallicities than the main stream RV Dispersion Age <= 2 Gyr Sagittarius sun [deg] Precession of the stream tells us that the halo profile is probably steeply declining with radius