CDROM/AJ/V113/P0634
The Survival of Sagittarius (Ibata+ 1997)
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The Kinematics, Orbit, and Survival of the Sagittarius Dwarf Spheroidal
Galaxy
Rodrigo A. Ibata, Rosemary F. G. Wyse, Gerard Gilmore, Michael J.
Irwin,
and Nicholas B. Suntzeff
<1997, AJ, 113, 634>
=1997AJ....113..634I
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Abstract:
The Sagittarius dwarf spheroidal galaxy, the closest satellite galaxy
of the Milky Way, has survived for many orbits about the Galaxy. Extant
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
10^9 Msun, 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 22 x 8
deg. The Sagittarius dwarf is prolate, with axis ratios ~3:1:1. For an
adopted distance of 16+/-2 kpc from the Galactic center on the opposite
side of the Galaxy to the Sun, the major axis is ~>9 kpc long and is
aligned approximately normal to the plane of the Milky Way Galaxy,
roughly following the coordinate line l=5 deg. The central velocity
dispersion of giant stars which are members of the Sagittarius dwarf is
11.4+/-0.7 km/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 ~0 km/s/degree in the central regions
and increases in amplitude to dv/db=-3 km/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+/-90 km/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 ~<1 Gyr 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:
Tables 1 and 3 were not noted as being included on the AAS CD-ROM
Series
in the printed paper, but are included here in AASTeX table form.
File Summary:
------------------------------------------------------------------------------File Name
Lrecl
Records
Explanations
------------------------------------------------------------------------------table1.tex
100
25
Physical parameters of the globular
clusters
of the Sagittarius dwarf spheroidal
(AASTeX)
table2a.dat
52
231
Radial velocity data from 1994 AAT run
table2a.tex
91
256
AASTeX version of table2a.dat
table2b.dat
67
272
Radial velocity data from 1994 CTIO run
table2b.tex
109
300
AASTeX version of table2b.dat
table3.tex
83
33
Mean velocities and velocity dispersions
in the
observed fields (AASTeX)
------------------------------------------------------------------------------Byte-by-byte Description of file: table2a.dat
------------------------------------------------------------------------------Bytes Format Units
Label
Explanations
------------------------------------------------------------------------------1- 2
I2
h
RAh
Right ascension 1950
3- 5
I3
min
RAm
R.A.
6-11
F6.2
s
RAs
R.A.
12-13
2X
----Blank
14
A1
--DESign declination (always blank)
15-16
I2
deg
DEd
Declination 1950
17-19
I3
arcmin DEm
Dec.
20-24
F5.1
arcsec DEs
Dec.
25-32
F8.2
mag
R
R magnitude
33-39
F7.2
mag
B_J-R
B_J - R color
40-48
F9.0
km/s
V_helio
Heliocentric radial velocity
49-51
3X
----Blank
52
A1
--CTIO
[yn] 'y' if observed in CTIO run,
else 'n'
------------------------------------------------------------------------------Byte-by-byte Description of file: table2b.dat
------------------------------------------------------------------------------Bytes Format Units
Label
Explanations
------------------------------------------------------------------------------1- 2
I2
h
RAh
Right ascension 1950
3- 5
I3
min
RAm
R.A.
6-11
F6.2
s
RAs
R.A.
12-13
2X
----Blank
14
A1
--DESign declination (always blank)
15-16
I2
deg
DEd
Declination 1950
17-19
I3
arcmin DEm
Dec.
20-24
F5.1
arcsec DEs
Dec.
25-32
F8.2
mag
R
R magnitude
33-39
F7.2
mag
B_J-R
B_J - R color
40-48
F9.0
km/s
V_helio
Heliocentric radial velocity
49-56
F8.2
--R_TR
Tonry-Davis cross-correlation R value
57-63
F7.2
km/s
dV
[]?=99.99 Delta V value
64-66
3X
----Blank
67
A1
--AAT
[yn] 'y' if observed in AAT run, else
'n'
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(End)
Lee Brotzman [ADS] 04Apr-1997