J/AJ/113/634 The survival of Sagittarius dwarf galaxy (Ibata+ 1997)
The kinematics, orbit, and survival of the Sagittarius dwarf spheroidal galaxy
Ibata R.A., Wyse R.F.G., Gilmore G., Irwin M.J., Suntzeff N.B.
<Astron. J. 113, 634 (1997)>
=1997AJ....113..634I 1997AJ....113..634I
ADC_Keywords: Galaxies, photometry ; Radial velocities
Abstract:
The Sagittarius dwarf spheroidal galaxy, the closest satellite galaxy
of the Milky Way, has survived for many orbits about the Galaxy.
Extent numerical calculations modeled this galaxy as a system with a
centrally-concentrated mass profile, following the light, and found
that it should lose more than one-half of its mass every 2-4 orbits
and be completely disrupted long before now. Apparently the
Sagittarius dwarf spheroidal, and by implication other dSph galaxies,
do not have a centrally-concentrated profile for their dark matter. We
develop a model in which the stars of the Sgr dwarf are embedded in a
constant-density dark matter halo, representing the core of a
tidally-limited system, and show that this is consistent with its
survival. We present new photometric and kinematic observations of the
Sagittarius dwarf spheroidal and show these data are consistent with
this explanation for the continued existence of this galaxy. The
Sagittarius dwarf is being tidally distorted and is tidally limited,
but is not disrupted as yet. The corresponding minimum total mass is
109M☉, while the central mass to visual light ratio is ∼50 in
Solar units. Our new photographic photometry allows the detection of
main-sequence stars of the Sagittarius dwarf over an area of
22x8°. The Sagittarius dwarf is prolate, with axis ratios ∼3:1:1.
For an adopted distance of 16±2kpc from the Galactic center on the
opposite side of the Galaxy to the Sun, the major axis is ≳9kpc long
and is aligned approximately normal to the plane of the Milky Way
Galaxy, roughly following the coordinate line l=5°. The central
velocity dispersion of giant stars which are members of the
Sagittarius dwarf is 11.4±0.7km/s and is consistent with being
constant over the face of the galaxy. The gradient in mean
line-of-sight velocity with position along the major axis, dv/db, is
∼0km/s/degree in the central regions and increases in amplitude to
dv/db=-3km/s/degree over the outermost three degrees for which we have
data. A first measurement of the proper motion of the Sagittarius
dwarf determines the component of its space velocity parallel to its
major axis to be 250±90km/s, directed towards the Galactic Plane. We
model these kinematic data to determine the orbit of the Sagittarius
dwarf. Our best fit model has an orbital period of ≲1Gyr and has the
Sagittarius dwarf spheroidal close to perigalacticon. This period is
shorter, by about a factor of ≳10, than the age of the bulk of its
stellar population.
(Copyright) 1997 American Astronomical Society.
Description:
Spectroscopic data for three observing runs on the 3.9m
Anglo-Australian Telescope (AAT) determine the mean kinematics of the
Sagittarius dwarf. Table2a presents the data from the third AAT run
(1994 August 1-4).
Complementary observations (table2b) were obtained with the ARGUS
multi-fiber setup at the 4m Cerro Tololo Interamerican Observatory
(CTIO) on 1994 August 2-6 feeding the bench-mounted echelle
spectrograph. The Blue Air Schmidt camera, 5186Å filter and
1200*200 CCD were used for the echelle observations.
Table1: Physical parameters of the globular clusters of the Sagittarius
dwarf spheroidal:
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Cluster ra (1950) dec (1950) [Fe/H] RVGal (km/s) T (Gyr) MV
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M54 18 51 51.2 -30 32 34 -1.55 ±0.10 172 ±1 -10.0
Ter 7 19 14 26.2 -34 44 54 -0.4 -->-1 186 ±4 9--12 -6.0
Ter 8 19 38 30.3 -34 07 05 -1.99 ±0.08 158 ±8 16--19 -5.0
Arp 2 19 25 33.9 -30 27 26 -1.70 ±0.11 154 ±10 13--14 -5.3
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table2a.dat 52 231 Radial velocity data from 1994 AAT run
table2b.dat 67 272 Radial velocity data from 1994 CTIO run
table3.dat 52 12 Mean velocities and velocity dispersions in the
observed Sgr fields
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See Also:
I/267 : The APM-North Catalogue (McMahon+, 2000)
Byte-by-byte Description of file: table2a.dat
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Bytes Format Units Label Explanations
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1- 2 I2 h RAh Hour of Right Ascension (B1950)
4- 5 I2 min RAm Minte of Right Ascension (B1950)
7- 11 F5.2 s RAs Second of Right Ascension (B1950)
14 A1 --- DE- Sign declination (always blank)
15- 16 I2 deg DEd Degree of Declination (B1950)
18- 19 I2 arcmin DEm Arcminute of Declination (B1950)
21- 24 F4.1 arcsec DEs Arcsecond of Declination (B1950)
26- 32 F7.2 mag Rmag R magnitude (G1)
34- 39 F6.2 mag B-R BJ-R color index (G1)
41- 48 F8.0 km/s HRV CTIO heliocentric radial velocity
52 A1 --- o2 [yn] 'y' if observed by both CTIO and AAT (G2)
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Byte-by-byte Description of file: table2b.dat
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Bytes Format Units Label Explanations
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1- 2 I2 h RAh Hour of Right Ascension (B1950)
4- 5 I2 min RAm Minute of Right Ascension (B1950)
7- 11 F5.2 s RAs Second of Right Ascension (B1950)
14 A1 --- DE- Sign declination (always blank)
15- 16 I2 deg DEd Degree of Declination (B1950)
18- 19 I2 arcmin DEm Arcminute of DecDeclination (B1950)
21- 24 F4.1 arcsec DEs Arcsecond of Declination (B1950)
26- 32 F7.2 mag Rmag R magnitude (G1)
34- 39 F6.2 mag B-R BJ-R color index (G1)
41- 48 F8.0 km/s HRV AAT heliocentric radial velocity
50- 56 F7.2 --- R(TD) Tonry-Davis (1979AJ.....84.1511T 1979AJ.....84.1511T)
cross-correlation R value
58- 63 F6.2 km/s DV ?=99.99 ΔV value (1)
67 A1 --- o2 [yn] 'y' if observed by both CTIO and AAT (G2)
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Note (1): Some spectroscopic integrations were divided into two exposures.
Where applicable, the column DV lists the absolute value of the
velocity difference between measurements on the two exposures.
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Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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1- 2 A2 --- Field Field designation (F1 to f8)
4- 7 A4 --- Inst Instrument (CTIO or AAT) (G2)
9- 13 F5.3 deg GLON Galactic longitude
15- 21 F7.3 deg GLAT Galactic latitude
23- 29 F7.3 km/s <RVGal> Mean galactocentric radial velocity
31- 35 F5.3 km/s e_<RVGal> rms uncertainty on <RVGal>
37- 42 F6.3 km/s sigmaV Velocity dispersion
44- 48 F5.3 km/s e_sigmaV rms uncertainty on sigmaV
50- 52 I3 --- Nstar Nunber of stars
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Global notes:
Note (G1): APM photometry.
Note (G2): Telescopes are:
CTIO = Cerro Tololo Interamerican Observatory;
AAT = 3.9m Anglo-Australian Telescope.
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Nomenclature note:
In Tables 2a and 2b, objects are <[IWG97] HHMMSS.ss-DDMMSS> in Simbad.
History:
AAS CD-ROM series, Volume 8, 1997 Lee Brotzman [ADS] 04-Apr-1997
(End) [CDS] 03-Jul-1997