of 6
TRIANGULUM II: POSSIBLY A VERY DENSE ULTRA-FAINT DWARF GALAXY
*
Evan N. Kirby
1
, Judith G. Cohen
1
, Joshua D. Simon
2
, and Puragra Guhathakurta
3
1
California Institute of Technology, 1200 E. California Boulevard, MC 249-17, Pasadena, CA 91125, USA
2
Observatories of the Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, CA 91101, USA
3
UCO
/
Lick Observatory and Department of Astronomy and Astrophysics, University of California, 1156 High Street, Santa Cruz, CA 95064, USA
Received 2015 October 12; accepted 2015 November 3; published 2015 November 16
ABSTRACT
Laevens et al. recently discovered Triangulum
II
(
Tri
II
)
, a satellite of the Milky Way. Its Galactocentric distance is
36 kpc, and its luminosity is only
L
4
50
. Using Keck
/
DEIMOS, we measured the radial velocities of six member
stars within 1
2 of the center of Tri
II, and we found a velocity dispersion of
s
=
-
+
-
5.1 km s
v
1.4
4.0
1
. We also
measured the metallicities of three stars and found a range of 0.8 dex in
[
Fe
/
H
]
. The velocity and metallicity
dispersions identify Tri
II as a dark matter-dominated galaxy. The galaxy is moving very quickly toward the
Galactic center
(
=-
-
v
262 km s
GSR
1
)
. Although it might be in the process of being tidally disrupted as it
approaches pericenter, there is no strong evidence for disruption in our data set. The ellipticity is low, and the mean
velocity,
á
ñ=- 
-
v
382.1 2.9 km s
helio
1
, rules out an association with the Triangulum
–
Andromeda substructure
or the Pan-Andromeda Archaeological Survey stellar stream. If Tri
II is in dynamical equilibrium, then it would
have a mass-to-light ratio of
-
+-
ML
3
600
,
2100
3500
1
☉
☉
the highest of any non-disrupting galaxy
(
those for which
dynamical mass estimates are reliable
)
. The density within the 3D half-light radius would be
-
+
-
M
4
.8
pc ,
3.5
8.1
3
☉
even
higher than Segue
1. Hence, Tri
II is an excellent candidate for the indirect detection of dark matter annihilation.
Key words:
galaxies: abundances
–
galaxies: dwarf
–
Local Group
1. INTRODUCTION
The Sloan Digital Sky Survey
(
SDSS; Abazajian et al.
2009
)
revolutionized Local Group astronomy in the last decade by
discovering more than a dozen new dwarf galaxies around the
Milky Way
(
MW
)
. We are now in the midst of another
revolution. The Panoramic Survey Telescope and Rapid
Response System
(
Pan-STARRS; Kaiser et al.
2010
)
, the Dark
Energy Survey
(
DES, Flaugher et al.
2012
)
, and other Dark
Energy Camera imaging surveys have discovered more than 20
previously unknown MW satellites
(
e.g., Bechtol et al.
2015
;
Kim & Jerjen
2015
; Laevens et al.
2015a
)
. The greater
photometric depth and expanded sky coverage of Pan-
STARRS and DES over SDSS has enabled the discovery of
many new satellites with luminosities less than
L
10
4
☉
and also
satellites more distant than 200 kpc.
Dwarf galaxy candidates are discovered through imaging,
but their identi
fi
cation as galaxies or star clusters is made
secure through spectroscopy
(
e.g., Willman & Strader
2012
)
.A
candidate can be considered a galaxy if it shows evidence for
dark matter, including a velocity dispersion in excess of what
would be expected from stellar mass alone or a dispersion in
stellar metallicity, which indicates chemical self-enrichment.
Spectroscopy of satellites discovered in the last two years has
already con
fi
rmed
fi
ve new galaxies and one globular cluster
(
Kirby et al.
2015b
; Koposov et al.
2015
; Martin et al.
2015
;
Simon et al.
2015
; Walker et al.
2015
)
.
Laevens et al.
(
2015b
)
discovered Triangulum
II
(
Tri
II
)
in
Pan-STARRS images. Its luminosity
(
L
4
50
)
and 2D half-
light radius
(
34
pc
)
are comparable to Segue
1, the faintest
galaxy known
(
Belokurov et al.
2007
; Geha et al.
2009
; Simon
et al.
2011
)
. Laevens et al.
(
2015a
)
suggested that Tri
II could
be associated with the Triangulum
–
Andromeda halo substruc-
ture
(
Majewski et al.
2004
)
or the Pan-Andromeda Archae-
ological Survey
(
PAndAS
)
stream
(
Martin et al.
2014
)
. If so,
then it could be one of the progenitors of that tidal debris.
However, spatial coincidence is not suf
fi
cient evidence for the
association. The velocity of the progenitor should also match
that of the debris.
We obtained spectra of stars in Tri
II in order to learn about
its origin and identity. The velocity dispersion can identify it as
a galaxy or a star cluster, and the mean velocity can support or
disprove an association with stellar debris. We describe our
observations in Section
2
and our measurements of velocities
and metallicities in Section
3
. We consider whether Tri
II is in
dynamical equilibrium or tidally disrupting in Section
4
.
Finally, we discuss the nature of Tri
II and its importance for
the study of dark matter in Section
5
.
2. OBSERVATIONS
2.1. Imaging
We imaged Tri
II with Keck
/
LRIS
(
Oke et al.
1995
)
on
2015 July 15. We obtained simultaneous 10 s exposures with
V
and
I
fi
lters in the blue and red channels, respectively. We also
obtained 10 s exposures of the photometric standard
fi
eld
PG0231 in the same
fi
lters. We performed aperture photometry
on both
fi
elds using SExtractor
(
Bertin & Arnouts
1996
)
. The
photometric zeropoint was determined by
fi
nding the offsets
between our instrumental magnitudes and P.B.
Stetson
ʼ
s
calibrated magnitudes in PG0231.
4
We discarded resolved
galaxies by eliminating objects with
class
_
star
<
0.5.
The Astrophysical Journal Letters,
814:L7
(
6pp
)
, 2015 November 20
doi:10.1088
/
2041-8205
/
814
/
1
/
L7
© 2015. The American Astronomical Society. All rights reserved.
*
The data presented herein were obtained at the W.
M.
Keck Observatory,
which is operated as a scienti
fi
c partnership among the California Institute of
Technology, the University of California and the National Aeronautics and
Space Administration. The Observatory was made possible by the generous
fi
nancial support of the W.
M.
Keck Foundation.
4
http:
//
www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca
/
en
/
community
/
STETSON
/
standards
/
1
2.2. Spectroscopy
We designed a slitmask for Keck
/
DEIMOS
(
Faber
et al.
2003
)
from the LRIS photometry. We selected 19 stars
for spectroscopy based on their locations in the color
–
magnitude diagram
(
CMD; Figure
1
(
a
))
. Stars near the sub-
giant and RGB tracks
—
the
“
ridgeline
”—
of the metal-poor
globular cluster M92
(
Clem
2006
)
were considered for
spectroscopy. However, the
fi
eld is dense enough that we
could not target every star. When slitmask design constraints
forced a choice among several stars, we chose to target the
brightest star. Figure
1
(
b
)
shows the 17 spectroscopic targets
with signal-to-noise ratio
(
S
/
N
)
suf
fi
cient for velocity mea-
surements
(
see Section
3
)
.
We obtained 61 minutes of DEIMOS exposures in 1
6
seeing on 2015 October 6. The poor seeing resulted in low S
/
N
for the fainter stars. We obtained 52 minutes of exposures in
0
9 seeing on 2015 October 7. The spectra from the
fi
rst night
were used only to compare with velocity measurements from
the second night
(
Section
3.1
)
. The measurements of velocity
and metallicity dispersions for Tri
II are based only on data
from 2015 October 7.
We reduced the data with the
spec2d
software
(
Cooper
et al.
2012
; Newman et al.
2013
)
with modi
fi
cations described
by Kirby et al.
(
2015a
,
2015b
)
. Among other improvements,
the 2D wavelength solution was improved by tracing the sky
lines along the slit, and the extraction was improved by taking
into account differential atmospheric refraction along the slit.
3. SPECTROSCOPIC MEASUREMENTS
3.1. Radial Velocities and Metallicities
We measured heliocentric radial velocities
(
v
helio
)
and
metallicities
(
[
Fe
/
H
]
)
in a manner identical to Kirby et al.
(
2015b
)
, who based their analysis on Simon & Geha
(
2007
)
.
Radial velocities were computed by
fi
nding the velocity that
minimized the
c
2
between the observed spectrum and eight
template spectra observed with DEIMOS. The template
spectrum with the lowest
χ
2
was used. We corrected the
velocity shift due slit mis-centering by measuring the observed
wavelength of telluric absorption in the stellar spectrum
(
e.g.,
Sohn et al.
2007
)
. We computed errors due to random noise by
fi
nding the standard deviation of the velocities of 10
3
Monte
Carlo realizations of the spectrum. The total error,
d
v
,
was
calculated by adding the random error in quadrature with a
systematic error of 1.49 km s
−
1
. The systematic error includes
sources of uncertainty that cannot be attributed to random
noise, such as uncorrected spectrograph
fl
exure or small errors
in the wavelength solution. The magnitude of the systematic
error was calculated by Kirby et al.
(
2015b
)
by comparing
repeated measurements of the same stars.
We tested our estimate of velocity errors by comparing the
low-S
/
N measurements of
v
helio
from 2015 October 6 to the
high-S
/
N measurements from 2015 October 7. Of the six stars
we determined to be members
(
Section
3.2
)
,
fi
ve velocities
were measurable with data from the
fi
rst night. We computed
dd
-+
vv vv
helio,1
helio,2
1
2
2
2
(
)
for each pair of measurements
of the member stars. The variance of this quantity should be 1 if
we estimated errors properly. We measured it to be
1.3 0.6.
Hence, the error estimates are reasonable.
We measured effective temperatures
(
T
eff
)
and metallicities
for member stars with suf
fi
cient S
/
N in the same manner as
Kirby et al.
(
2008
,
2010
)
. First, we divided the spectrum by a
polynomial
fi
t to regions of the spectrum free of absorption
lines. Next, we estimated temperatures and surface gravities
(
g
log
)
by
fi
tting Yonsei-Yale theoretical isochrones
(
Demar-
que et al.
2004
)
to observed stellar colors and magnitudes.
Then, we used these parameters and an initial guess of
[
Fe
/
H
]
=
−
1.5 to construct a synthetic spectrum at the
observed spectrum
ʼ
s resolution. This spectrum was linearly
interpolated from Kirby et al.
ʼ
s
(
2010
)
synthetic spectral grid.
In order to minimize
c
2
between the observed and synthetic
spectra, we changed the synthetic spectrum
ʼ
s
T
eff
and
[
Fe
/
H
]
but held
g
log
fi
xed. The measured values of
T
eff
and
[
Fe
/
H
]
Figure 1.
(
a
)
Color
–
magnitude diagram from LRIS photometry, showing spectroscopic members
(
large blue points
)
and non-members
(
red crosses
)
. The cyan line
shows the ridgeline of the metal-poor globular cluster M92
(
Clem
2006
)
.
(
b
)
The map of spectroscopic targets shown as distances from the center of Tri
II. The
DEIMOS slitmask outline is shown in green. The full slitmask extends beyond the bounds of the
fi
gure because the
fi
eld of view of LRIS
—
from which we selected
DEIMOS targets
—
is smaller than for DEIMOS.
2
The Astrophysical Journal Letters,
814:L7
(
6pp
)
, 2015 November 20
Kirby et al.
are those of the synthetic spectrum with the minimum
χ
2
. The
error on
[
Fe
/
H
]
is the appropriate diagonal term of the
covariance matrix added in quadrature with a systematic error
of 0.11 dex, which Kirby et al.
(
2010
)
determined from repeat
measurements. We kept
[
Fe
/
H
]
measurements of the three stars
with uncertainties less than 0.5 dex and discarded the others.
These three stars lie in the range
-
<<-
/
3FeH
2
[]
with a
mean of
á
ñ=- 
/
Fe H
2.50 0.08.
[]
Tri
II has the lowest
measured mean metallicity of any galaxy except Segue
1
(
Frebel et al.
2014
)
and Reticulum
II
(
Simon et al.
2015
;
Walker et al.
2015
)
. However, the metallicity measurements for
Tri
II are based on only three stars. While the standard error of
the mean is
0
.08
dex, the mean metallicity of a larger sample
could be substantially different.
Table
1
lists the radial velocities for all the stars we observed
with DEIMOS except two stars with spectra that were too noisy
to identify any absorption lines. Temperatures and metallicities
are given for the three member stars with suf
fi
cient quality for
those measurements.
3.2. Membership and Velocity Dispersion
We identi
fi
ed a peak in the velocity distribution of the
observed stars around
−
380 km s
−
1
. We took stars within
50 km s
−
1
of this peak as the initial member list. We measured
the velocity dispersion
(
σ
v
)
of these six stars in the same
manner as Kirby et al.
(
2014
,
2015b
)
, who based their analysis
on Walker et al.
(
2006
)
. We estimated
s
v
via maximum
likelihood. A Monte Carlo Markov chain
(
MCMC
)
with 10
7
trials explored the parameter space of mean velocity
(
á
ñ
v
helio
)
and
σ
v
. We quote the values corresponding to the peaks of the
probability distributions as the measurements of
á
ñ
v
helio
and
σ
v
.
The asymmetric 1
σ
con
fi
dence interval on
σ
v
is the range on
either side of the mean value that bounds 68.3% of the trials.
We measured
á
ñ=- 
-
v
382.1 2.9 km s
helio
1
and
s
=
-
+
-
5.1 km s
v
1.4
4.0
1
. All six candidate member stars are within
s
1.1
v
of
á
ñ
v
.
helio
Furthermore, all six stars are close to the M92
ridgeline in Figure
1
(
a
)
, indicating that they pass a CMD
membership cut. None of the six stars shows a strong
Na
I
8190 doublet, which would have indicated that the star
is a foreground dwarf.
Figure
2
shows the spectra of the six member stars in
observed wavelength. Ca
II
8542 appears at a different
observed wavelength for every star, showing that we have
resolved the velocity dispersion of Tri
II.
Other than Tri
II, only three galaxies with
<
L
L
10
4
☉
have
published measurements of
s
>
-
5kms
v
1
: Boötes
II
(
Koch
et al.
2009
)
, Pisces
II
(
Kirby et al.
2015b
)
, and Ursa Major
II
(
Simon & Geha
2007
)
. Koch et al. measured
s
=
-
10.5 7.4 km s
v
1
for Boötes
II, but more recent mea-
surements have found a smaller dispersion
(
M.
Geha et al.
2015, in preparation
)
and the presence of at least one binary
that in
fl
ates the apparent dispersion
(
Ji et al.
2015
)
.
Even if all of the stars are members, some of them might be
binaries. The orbital velocity of the binary would arti
fi
cially
in
fl
ate our measurement of
σ
v
for the galaxy. We tested the
robustness of our measurement of
s
v
by jackknife resampling.
We recalculated
σ
v
for each of the six subsets of member stars
formed by removing one star. All of the probability distribu-
tions are well separated from zero. The minimum velocity
dispersion, calculated by removing star 65, is
-
+
-
2
.8 km s
1.7
4.0
1
.
The jackknife error, calculated as the standard deviation of
σ
v
for all of the jackknife trials, is
-
1.2 km s
1
, somewhat smaller
than the error calculated from the MCMC distribution.
4. DYNAMICAL EQUILIBRIUM
Table
2
gives some characteristics of Tri
II, including the
mass within the 3D half-light radius
(
M
;
12
Wolf et al.
2010
)
.
This quantity and its associated quantities, mass-to-light ratio
ML
V
12
[
()]
and density
(
r
12
)
within the 3D half-light radius,
presume that the galaxy is spherically symmetric and in
dynamical equilibrium. However, the velocity dispersion
accurately re
fl
ects the mass even in the presence of moderate
tidal forces
(
Oh et al.
1995
)
. If the velocities of the stars we
measured are very heavily affected by tides, then these
quantities are not meaningful. We now consider whether the
center of Tri
II is in dynamical equilibrium.
Table 1
Target List
ID
R.A.
(
J2000
)
Decl.
(
J2000
)
V
0
-
VI
0
()
S
/
N
a
v
helio
Member?
T
eff
g
log
[
Fe
/
H
]
(
mag
)(
mag
)(
Å
−
1
)(
km s
−
1
)(
K
)(
cm s
−
2
)
166
02 13 11.42
+
36 12 33.0
17.99
0.85
115
11.0
±
5.9
N
KKK
174
02 13 12.85
+
36 11 20.2
18.06
0.64
102
−
100.8
±
1.6
N
KKK
177
02 13 13.21
+
36 11 35.5
19.73
0.69
36
−
280.7
±
2.5
N
KKK
126
02 13 14.11
+
36 12 22.9
16.81
0.97
127
2.4
±
1.5
N
KKK
128
02 13 14.21
+
36 09 51.4
19.78
0.80
24
−
384.9
±
3.2
Y
5292
3.17
−
2.72
±
0.39
127
02 13 14.33
+
36 13 04.3
19.31
0.71
54
−
86.0
±
1.8
N
KKK
116
02 13 15.92
+
36 10 16.0
20.26
0.71
26
−
377.6
±
3.7
Y
KKK
113
02 13 16.16
+
36 11 16.3
18.97
0.78
72
−
57.6
±
1.6
N
KKK
111
02 13 16.42
+
36 13 01.5
18.86
0.64
66
−
62.0
±
1.9
N
KKK
106
02 13 16.51
+
36 10 45.9
17.10
0.99
219
−
382.3
±
1.5
Y
4922
1.88
−
2.86
±
0.11
100
02 13 18.03
+
36 12 33.2
18.64
0.87
87
−
38.0
±
1.6
N
KKK
91
02 13 19.28
+
36 11 33.4
20.14
0.78
29
−
386.0
±
3.1
Y
KKK
84
02 13 19.69
+
36 11 15.3
19.22
0.77
60
−
66.4
±
1.7
N
KKK
82
02 13 19.87
+
36 12 12.4
18.41
0.86
101
−
177.0
±
1.6
N
KKK
76
02 13 20.55
+
36 09 46.7
20.72
0.61
17
−
389.7
±
3.0
Y
KKK
65
02 13 21.48
+
36 09 57.6
18.85
0.85
81
−
374.5
±
1.7
Y
5169
2.74
−
2.04
±
0.13
45
02 13 24.21
+
36 10 15.3
17.07
0.85
152
−
62.1
±
1.5
N
KKK
Note.
a
To convert to S
/
N per pixel, multiply by 0.57.
3
The Astrophysical Journal Letters,
814:L7
(
6pp
)
, 2015 November 20
Kirby et al.
The MW exerts the maximum tidal shear on satellite galaxies
at their pericenters
(
e.g., Mayer et al.
2001
)
. It is more likely to
fi
nd a tidally disrupting galaxy close to the Galactic center than
far from it. Tri
II is only
=
D
36 2 kpc
GC
from the Galactic
center
(
Laevens et al.
2015b
)
. It is also rapidly approaching its
pericenter. Assuming a solar orbital velocity of 220 km s
−
1
, the
velocity of Tri
II relative to the Galactic standard of rest
(
GSR
)
is
-
-
262 km s
1
. The fact that Tri
II is approaching pericenter
rather than receding from it is consistent with its imminent tidal
disruption. However, its large velocity limits the time that it
will spend near pericenter and consequently reduces the total
tidal effect.
A tidally disrupting galaxy could have a high ellipticity. The
ellipticity of Tri
II is
=
-
+
0.21
0.21
0.17
(
Laevens et al.
2015b
)
.In
contrast with presently disrupting galaxies, like Sagittarius
(
=
0.65;
Majewski et al.
2003
)
, Tri
II is not obviously
elliptical.
Along with high ellipticity, ongoing tidal disruption could
cause a non-Gaussian velocity distribution. A Shapiro
–
Wilk
test gives a
p
value of 0.87. A completely Gaussian distribution
would have a
p
value of 1. Therefore, there is no evidence for
non-Gaussianity in the velocity distribution. Of course, our
small sample size limits the signi
fi
cance of this result.
Furthermore, we have measured only line of sight velocities,
and even a tidally disrupting system may have normally
distributed velocities along some lines of sight.
We estimated a lower limit to the tidal radius assuming
that
M
1
/
2
is the entire mass of Tri
II. The Roche limit for
a
fl
uid satellite is
~
r
DMM
0.4
,
tidal
GC 1 2 MW
13
()
where
»
MM
10
MW
12
☉
is the MW
ʼ
s mass. Under these assumptions,
»
r
140
tidal
pc for Tri
II, or about three times its 3D half-light
radius
(
4
/
3 of the 2D, projected half-light radius
)
. The tidal
radius would shrink to the same value as the 3D half-light
radius at
∼
12 kpc from the Galactic center. Therefore, all of the
stars we observed in Tri
II are presently insulated from Galactic
tides. Although this estimate of tidal radius presumes that the
velocity dispersion re
fl
ects the present mass, simulations
suggest that the velocity dispersion is a good indicator of the
instantaneous mass except for a short time after pericenter,
even for systems experiencing signi
fi
cant tidal stripping
(
Oh
et al.
1995
; Muñoz et al.
2008
; Peñarrubia et al.
2009
)
.
Laevens et al.
(
2015b
)
noted the possible association of
Tri
II with the Triangulum
–
Andromeda halo substructure
(
Majewski et al.
2004
)
or the PAndAS stream
(
Martin et al.
2014
)
. Association with such halo debris might indicate that the
galaxy is being disrupted and that it is the source of the debris.
However, Deason et al.
(
2014
)
measured the GSR velocities of
these structures as 30
–
70 km s
−
1
, which is roughly 300 km s
−
1
different from
v
GSR
for Tri
II. Therefore, there is no presently
known stream that could be associated with Tri
II. This does
not prove that Tri
II is in dynamical equilibrium, but it does
show that, if Tri
II is being tidally disrupted, it is not the source
of the Triangulum
–
Andromeda or PAndAS stellar debris.
The luminosity
–
metallicity relation
(
LZR; Figure
3
(
a
))
is a
diagnostic of tidal stripping. The LZR for classical dwarf
Figure 2.
Small regions of DEIMOS spectra of the six member stars shown
against observed wavelength. The full spectrum
—
much wider than shown here
—
is used for the velocity measurement. The dashed red line shows the
observed wavelength of Ca
II
8542 at the mean geocentric velocity of Tri
II.
The blue dotted lines show the observed wavelength of Ca
II
8542 for each
star, and the blue whiskers indicate the
s
1
uncertainty of the observed
centroid of the absorption line for each star. The spectra are ordered from the
lowest to highest radial velocity. The shift of Ca
II
8542 is apparent even
by eye.
Table 2
Properties of Triangulum
II
Property
Value
N
member
6
LL
log
V
(
)
☉
2.65
±
0.20
r
h
-
+
3
.9
0.9
1.1
arcmin
r
h
-
+
3
4
8
9
pc
á
ñ
v
helio
−
382.1
±
2.9 km s
−
1
v
GSR
−
262 km s
−
1
s
v
-
+
5
.1
1.4
4.0
km s
−
1
MM
log
12
()
☉
a
-
+
5
.9
0.2
0.
4
ML
V
12
()
a
,
b
-
+
3
600
2100
3500
-
M
L
1
☉
☉
r
12
a
,
c
-
+
4
.8
3.5
8.1
-
M
pc
3
☉
áñ
/
Fe H
[]
−
2.50
±
0.08
Notes.
a
These quantities presume that Tri
II is in dynamical equilibrium.
b
Mass-to-light ratio within the half-light radius, calculated as
s
=
-
M
Gr
4
v
h
12
1
2
(
Wolf et al.
2010
)
.
c
Density within the half-light radius.
References.
The measurements of
L
log
V
and
r
h
come from Laevens et al.
(
2015b
)
.
4
The Astrophysical Journal Letters,
814:L7
(
6pp
)
, 2015 November 20
Kirby et al.
galaxies is very tight, with an rms of only 0.13 dex
(
Kirby et al.
2011
; Kirby et al.
2013b
)
. If a galaxy initially conforms to the
LZR, then tidal stripping will decrease its luminosity while
keeping its average metallicity roughly constant. This corre-
sponds to a leftward move in Figure
3
(
a
)
. Some of the galaxies
with
L
L
10 ,
4
☉
especially Segue
2
(
Kirby et al.
2013a
)
, lie
signi
fi
cantly to the left of the LZR. On the other hand, Tri
II is
consistent with the LZR. Therefore, any tidal stripping that
already happened is likely to have been mild.
5. DISCUSSION
Tri
II satis
fi
es the de
fi
nition of
“
galaxy
”
given by Willman &
Strader
(
2012
)
. The velocity dispersion is much too large to be
explained by stars alone. We also found a large dispersion in
metallicity. The stars span 0.8 dex in
[
Fe
/
H
]
, which is evidence
for chemical self-enrichment. The present mass of stars alone
would not have been enough to retain supernova ejecta. Hence,
without substantial mass loss, the velocity and metallicity
dispersions are evidence for a large amount of dark matter.
It is unclear whether Tri
II is in dynamical equilibrium. With
a total luminosity of only
L
4
50
, the galaxy has very few stars
available to measure its shape very precisely. Even fewer stars
are available for spectroscopy. Therefore, resolving this
question will be very dif
fi
cult. Regardless, we now consider
Tri
II
ʼ
s place among the MW satellite population under the
presumption of dynamical equilibrium.
Figures
3
(
b
)
–
(
d
)
show the trends of
M
,
12
ML
,
V
12
(
)
and
r
12
with luminosity. Tri
II has the largest mass-to-light ratio
(
-
+-
ML
3
600
2100
3500
1
☉
☉
)
of any galaxy except Boötes
III, whose
tidal disruption is nearly complete
(
Carlin et al.
2009
; Grillmair
2009
)
. If Tri
II is in dynamical equilibrium, then it is the most
dark-matter dominated galaxy known.
The
fi
ve galaxies with
r
>
-
M
1pc
12
3
☉
in order from
densest to least dense are Willman
1, Horologium
I, Tri
II,
Segue
1, and Pisces
II. Four of these galaxies comprise the
least luminous galaxies with measured velocity dispersions.
This correlation could arise because these galaxies also have
the smallest half-light radii. If galaxies
’
mass pro
fi
les peak in
the center, then smaller galaxies will be observed to have larger
r
,
12
even if their total masses and mass pro
fi
les are identical.
The correlation between
r
12
and
L
V
could also arise because
less massive galaxies are more susceptible to tidal stripping.
Hence, the measurement of
r
12
would be invalid because the
galaxies are not in equilibrium. Willman
1 has a velocity
distribution that does not seem consistent with dynamical
equilibrium
(
Willman et al.
2011
)
. The velocity dispersions of
Horologium
I, Pisces
II, and Tri
II were all measured from 5
–
7
stars
(
Kirby et al.
2015b
; Koposov et al.
2015
, and this work
)
.
Hence, Segue
1
(
Simon et al.
2011
)
remains the galaxy with
the most secure measurement of a very high central density.
In summary, we measured
s
=
-
+
-
5.1 km s
v
1.4
4.0
1
for Tri
II.
The present measurements cannot determine whether the
galaxy is in dynamical equilibrium or being tidally disrupted.
However, the possibility that it is in equilibrium is very
exciting. Tri
II would be the most dark-matter dominated
galaxy known, and it would be an excellent candidate for the
indirect, gamma-ray detection of dark matter annihilation. The
annihilation signal scales as
r
,
2
which makes very dense
galaxies
—
possibly including Tri
II
—
the best prospects for
detection.
We thank Gina Duggan for obtaining LRIS images, Emily
Cunningham for helpful statistics advice, and the anonymous
referee for helpful feedback. P.G. acknowledges support from
NSF grants AST-1010039 and AST-1412648. We are grateful
to the many people who have worked to make the Keck
Telescope and its instruments a reality and to operate and
maintain the Keck Observatory. The authors wish to extend
special thanks to those of Hawaiian ancestry on whose sacred
mountain we are privileged to be guests. Without their
generous hospitality, none of the observations presented herein
would have been possible.
Facilities:
Keck:I
(
LRIS
)
, Keck:II
(
DEIMOS
)
.
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