arXiv:1510.03856v3 [astro-ph.GA] 12 Nov 2015
Accepted to ApJL on 2015 November 3
Preprint typeset using L
A
T
E
X style emulateapj v. 5/2/11
TRIANGULUM II: POSSIBLY A VERY DENSE ULTRA-FAINT DWARF GALA
XY
Evan N. Kirby
1
, Judith G. Cohen
1
, Joshua D. Simon
2
, Puragra Guhathakurta
3
Accepted to ApJL on 2015 November 3
ABSTRACT
Laevens et al. recently discovered Triangulum II, a satellite of the M
ilky Way. Its Galactocentric distance is 36 kpc,
and its luminosity is only 450
L
⊙
. Using Keck/DEIMOS, we measured the radial velocities of six membe
r stars within
1.2
′
of the center of Triangulum II, and we found a velocity dispersion of
σ
v
= 5
.
1
+4
.
0
−
1
.
4
km s
−
1
. We also measured
the metallicities of three stars and found a range of 0.8 dex in [Fe/H]. T
he velocity and metallicity dispersions
identify Triangulum II as a dark matter-dominated galaxy. The galax
y is moving very quickly toward the Galactic
center (
v
GSR
=
−
262 km s
−
1
). Although it might be in the process of being tidally disrupted as it app
roaches
pericenter, there is no strong evidence for disruption in our data s
et. The ellipticity is low, and the mean velocity,
h
v
helio
i
=
−
382
.
1
±
2
.
9 km s
−
1
, rules out an association with the Triangulum–Andromeda substruc
ture or the Pan-
Andromeda Archaeological Survey (PAndAS) stellar stream. If Tr
iangulum II is in dynamical equilibrium, then it
would have a mass-to-light ratio of 3600
+3500
−
2100
M
⊙
L
−
1
⊙
, the highest of any non-disrupting galaxy (those for which
dynamical mass estimates are reliable). The density within the 3-D ha
lf-light radius would be 4
.
8
+8
.
1
−
3
.
5
M
⊙
pc
−
3
,
even higher than Segue 1. Hence, Triangulum II is an excellent candid
ate for the indirect detection of dark matter
annihilation.
Subject headings:
galaxies: dwarf — Local Group — galaxies: abundances
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 Sur-
vey (DES, Flaugher et al. 2012), and other Dark En-
ergy Camera (DECam) imaging surveys have discovered
more than 20 previously unknown MW satellites (e.g.,
Laevens et al. 2015b; Bechtol et al. 2015; Kim & Jerjen
2015). The greater photometric depth and expanded sky
coverage of Pan-STARRS and DES over SDSS has en-
abled the discovery of many new satellites with lumi-
nosities less than 10
4
L
⊙
and also satellites more distant
than 200 kpc.
Dwarf galaxy candidates are discovered through
imaging, but their identification as galaxies or star
clusters is made secure through spectroscopy (e.g.,
Willman & Strader 2012). A candidate can be consid-
ered a galaxy if it shows evidence for dark matter, in-
cluding a velocity dispersion in excess of what would be
expected from stellar mass alone or a dispersion in stel-
lar metallicity, which indicates chemical self-enrichment.
Spectroscopy of satellites discovered in the last two
*
The data presented herein were obtained at the W. M. Keck
Observatory, which is operated as a scientific partnership a
mong
the California Institute of Technology, the University of C
ali-
fornia and the National Aeronautics and Space Administrati
on.
The Observatory was made possible by the generous financial
support of the W. M. Keck Foundation.
1
California Institute of Technology, 1200 E. California Blv
d.,
MC 249-17, Pasadena, CA 91125, USA
2
Observatories of the Carnegie Institution of Washington, 8
13
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
years has already confirmed five new galaxies and one
globular cluster (Simon et al. 2015; Walker et al. 2015;
Koposov et al. 2015; Kirby et al. 2015a; Martin et al.
2015).
Laevens et al. (2015a) discovered Triangulum II
(Tri II) in Pan-STARRS images. Its luminosity
(450
L
⊙
) and 2-D 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. suggested that Tri II could be
associated with the Triangulum–Andromeda halo sub-
structure (Majewski et al. 2004) or the Pan-Andromeda
Archaeological 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-
ficient 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 ve-
locity 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 expo-
sures with
V
and
I
filters in the blue and red channels,
respectively. We also obtained 10 s exposures of the pho-
tometric standard field PG0231 in the same filters. We
performed aperture photometry on both fields using SEx-
tractor (Bertin & Arnouts 1996). The photometric zero-
point was determined by finding the offsets between our
2
Kirby et al.
instrumental magnitudes and P.B. Stetson’s calibrated
magnitudes in PG0231.
4
We discarded resolved galaxies
by eliminating objects with
class
star
<
0
.
5.
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, Fig-
ure 1a). 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 field 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 1b shows the 17 spectroscopic targets with
S/N sufficient for velocity measurements (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 first night were used only to compare with ve-
locity 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-
fications described by Kirby et al. (2015a,b). Among
other improvements, the 2-D wavelength solution was
improved by tracing the sky lines along the slit, and the
extraction was improved by taking into account differen-
tial 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. (2015a), who based their analysis on
Simon & Geha (2007). Radial velocities were computed
by finding the velocity that minimized the
χ
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 tel-
luric absorption in the stellar spectrum (e.g., Sohn et al.
2007). We computed errors due to random noise by find-
ing the standard deviation of the velocities of 10
3
Monte
Carlo realizations of the spectrum. The total error,
δv
,
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 at-
tributed to random noise, such as uncorrected spectro-
graph flexure or small errors in the wavelength solution.
The magnitude of the systematic error was calculated
by Kirby et al. (2015a) by comparing repeated measure-
ments 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),
five velocities were measurable with data from the first
4
http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/en/comm
unity/STETSON/standards/
night. We computed (
v
helio
,
1
−
v
helio
,
2
)
/
√
δv
2
1
+
δv
2
2
for
each pair of measurements of the member stars. The
variance of this quantity should be 1 if we estimated er-
rors properly. We measured it to be 1
.
3
±
0
.
6. Hence,
the error estimates are reasonable.
We measured effective temperatures (
T
eff
) and metal-
licities for member stars with sufficient S/N in the same
manner as Kirby et al. (2008, 2010). First, we divided
the spectrum by a polynomial fit to regions of the spec-
trum free of absorption lines. Next, we estimated tem-
peratures and surface gravities (log
g
) by fitting Yonsei-
Yale theoretical isochrones (Demarque et al. 2004) to ob-
served 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 spec-
trum’s resolution. This spectrum was linearly interpo-
lated from Kirby et al.’s (2010) synthetic spectral grid.
In order to minimize
χ
2
between the observed and syn-
thetic spectra, we changed the synthetic spectrum’s
T
eff
and [Fe/H] but held log
g
fixed. The measured val-
ues of
T
eff
and [Fe/H] are those of the synthetic spec-
trum 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 mea-
surements. 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
−
3
<
[Fe
/
H]
<
−
2
with a mean of
h
[Fe
/
H]
i
=
−
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 ob-
served with DEIMOS except two stars with spectra that
were too noisy to identify any absorption lines. Temper-
atures and metallicities are given for the three member
stars with sufficient quality for those measurements.
3.2.
Membership and Velocity Dispersion
We identified 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, 2015a),
who based their analysis on Walker et al. (2006). We
estimated
σ
v
via maximum likelihood. A Monte Carlo
Markov chain (MCMC) with 10
7
trials explored the pa-
rameter space of mean velocity (
h
v
helio
i
) and
σ
v
. We
quote the values corresponding to the peaks of the prob-
ability distributions as the measurements of
h
v
helio
i
and
σ
v
. The asymmetric 1
σ
confidence interval on
σ
v
is the
range on either side of the mean value that bounds 68.3%
of the trials.
We measured
h
v
helio
i
=
−
382
.
1
±
2
.
9 km s
−
1
and
σ
v
= 5
.
1
+4
.
0
−
1
.
4
km s
−
1
. All six candidate member stars
are within 1
.
1
σ
v
of
h
v
helio
i
. Furthermore, all six stars
are close to the M92 ridgeline in Figure 1a, indicating
that they pass a CMD membership cut. None of the six
Triangulum II
3
0.2
0.4
0.6
0.8
1.0
1.2
1.4
(
V
−
I
)
0
22
21
20
19
18
17
16
V
0
(a)
3
2
1
0
−1
−2
−3
∆
RA (arcmin)
−3
−2
−1
0
1
2
3
∆
Dec (arcmin)
(b)
not observed
non−member
member
Figure 1.
(
a
) Color–magnitude diagram from LRIS photometry, showing sp
ectroscopic 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 slitm
ask outline is shown in green. The full slitmask extends beyo
nd the bounds
of the figure because the field of view of LRIS—from which we sel
ected DEIMOS targets—is smaller than for DEIMOS.
Table 1
Target List
ID RA (J2000) Dec (J2000)
V
0
(
V
−
I
)
0
S/N
a
v
helio
Member?
T
eff
log
g
[Fe/H]
(mag) (mag) (
̊
A
−
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
· · ·
· · ·
· · ·
174 02 13 12.85 +36 11 20.2 18.06
0.64
102
−
100
.
8
±
1
.
6
N
· · ·
· · ·
· · ·
177 02 13 13.21 +36 11 35.5 19.73
0.69
36
−
280
.
7
±
2
.
5
N
· · ·
· · ·
· · ·
126 02 13 14.11 +36 12 22.9 16.81
0.97
127
2
.
4
±
1
.
5
N
· · ·
· · ·
· · ·
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
· · ·
· · ·
· · ·
116 02 13 15.92 +36 10 16.0 20.26
0.71
26
−
377
.
6
±
3
.
7
Y
· · ·
· · ·
· · ·
113 02 13 16.16 +36 11 16.3 18.97
0.78
72
−
57
.
6
±
1
.
6
N
· · ·
· · ·
· · ·
111 02 13 16.42 +36 13 01.5 18.86
0.64
66
−
62
.
0
±
1
.
9
N
· · ·
· · ·
· · ·
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
· · ·
· · ·
· · ·
91 02 13 19.28 +36 11 33.4 20.14
0.78
29
−
386
.
0
±
3
.
1
Y
· · ·
· · ·
· · ·
84 02 13 19.69 +36 11 15.3 19.22
0.77
60
−
66
.
4
±
1
.
7
N
· · ·
· · ·
· · ·
82 02 13 19.87 +36 12 12.4 18.41
0.86
101
−
177
.
0
±
1
.
6
N
· · ·
· · ·
· · ·
76 02 13 20.55 +36 09 46.7 20.72
0.61
17
−
389
.
7
±
3
.
0
Y
· · ·
· · ·
· · ·
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
· · ·
· · ·
· · ·
a
To convert to S/N per pixel, multiply by 0.57.
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 <
10
4
L
⊙
have published measurements of
σ
v
>
5 km s
−
1
:
Bo ̈otes II (Koch et al. 2009), Pisces II (Kirby et al.
2015a), and Ursa Major II (Simon & Geha 2007).
Koch et al. measured
σ
v
= 10
.
5
±
7
.
4 km s
−
1
for
Bo ̈otes II, but more recent measurements have found
a smaller dispersion (M. Geha et al., in prep.) and the
presence of at least one binary that inflates 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 artificially inflate our measurement of
σ
v
for the
galaxy. We tested the robustness of our measurement of
σ
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 distributions are well
separated from zero. The minimum velocity dispersion,
calculated by removing star 65, is 2
.
8
+4
.
0
−
1
.
7
km s
−
1
. The
jackknife error, calculated as the standard deviation of
σ
v
for all of the jackknife trials, is 1
.
2 km s
−
1
, some-
what smaller than the error calculated from the MCMC
distribution.
4
Kirby et al.
0.0
0.5
1.0
star 76
v
helio
=
−
389.7
±
3.0 km s
−
1
0.0
0.5
1.0
star 91
v
helio
=
−
386.0
±
3.1 km s
−
1
0.0
0.5
1.0
star 128
v
helio
=
−
384.9
±
3.2 km s
−
1
0.0
0.5
1.0
star 106
v
helio
=
−
382.3
±
1.5 km s
−
1
0.0
0.5
1.0
star 116
v
helio
=
−
377.6
±
3.7 km s
−
1
8526
8528
8530
8532
8534
8536
observed wavelength (Å)
0.0
0.5
1.0
star 65
v
helio
=
−
374.5
±
1.7 km s
−
1
normalized flux
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 measure-
ment. 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
±
1
σ
uncertainty of the
observed centroid of the absorption line for each star. The s
pectra
are ordered from the lowest to highest radial velocity. The s
hift of
Ca
ii
8542 is apparent even by eye.
4.
DYNAMICAL EQUILIBRIUM
Table 2 gives some characteristics of Tri II, includ-
ing the mass within the 3-D half-light radius (
M
1
/
2
,
Wolf et al. 2010). This quantity and its associated
quantities, mass-to-light ratio [(
M/L
V
)
1
/
2
] and density
(
ρ
1
/
2
) within the 3-D half-light radius, presume that the
galaxy is spherically symmetric and in dynamical equi-
librium. However, the velocity dispersion accurately re-
flects 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.
The MW exerts the maximum tidal shear on satel-
lite galaxies at their pericenters (e.g., Mayer et al. 2001).
It is more likely to find a tidally disrupting galaxy
close to the Galactic center than far from it. Tri II
Table 2
Properties of Triangulum II
Property
Value
N
member
6
log(
L
V
/L
⊙
)
2
.
65
±
0
.
20
r
h
3
.
9
+1
.
1
−
0
.
9
arcmin
r
h
34
+9
−
8
pc
h
v
helio
i
−
382
.
1
±
2
.
9 km s
−
1
v
GSR
−
262 km s
−
1
σ
v
5
.
1
+4
.
0
−
1
.
4
km s
−
1
log(
M
1
/
2
/M
⊙
)
a
5
.
9
+0
.
4
−
0
.
2
(
M/L
V
)
1
/
2
a,b
3600
+3500
−
2100
M
⊙
L
−
1
⊙
ρ
1
/
2
a,c
4
.
8
+8
.
1
−
3
.
5
M
⊙
pc
−
3
h
[Fe
/
H]
i
−
2
.
50
±
0
.
08
References
. — The measurements of
log
L
V
and
r
h
come from Laevens et al.
(2015a).
a
These quantities presume that Tri II is in
dynamical equilibrium.
b
Mass-to-light ratio within the half-light
radius, calculated as
M
1
/
2
= 4
G
−
1
σ
2
v
r
h
(Wolf et al. 2010).
c
Density within the half-light radius.
is only
D
GC
= 36
±
2 kpc from the Galactic cen-
ter (Laevens et al. 2015a). It is also rapidly approach-
ing 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. How-
ever, 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
ǫ
= 0
.
21
+0
.
17
−
0
.
21
(Laevens et al.
2015a). In contrast with presently disrupting galaxies,
like Sagittarius (
ǫ
= 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. There-
fore, there is no evidence for non-Gaussianity in the ve-
locity distribution. Of course, our small sample size lim-
its the significance of this result. Furthermore, we have
measured only line-of-sight velocities, and even a tidally
disrupting system may have normally distributed veloc-
ities 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 fluid satellite is
r
tidal
∼
0
.
4
D
GC
(
M
1
/
2
/M
MW
)
1
/
3
,
where
M
MW
≈
10
12
M
⊙
is the MW’s mass. Under
these assumptions,
r
tidal
≈
140 pc for Tri II, or about
three times its 3-D half-light radius (4
/
3 of the 2-D, pro-
jected half-light radius). The tidal radius would shrink
to the same value as the 3-D half-light radius at
∼
12 kpc
from the Galactic center. Therefore, all of the stars we
observed in Tri II are presently insulated from Galac-
tic tides. Although this estimate of tidal radius pre-
sumes that the velocity dispersion reflects the present
mass, simulations suggest that the velocity dispersion is
Triangulum II
5
−2
−1
〈
[Fe/H]
〉
TriII
(a)
6
7
8
log
M
1/2
(
M
sun
)
TriII
(b)
1
2
3
4
log (
M
/
L
V
)
1/2
(
M
sun
L
sun
−
1
)
TriII
(c)
3
4
5
6
7
log
L (L
sun
)
−2
−1
0
1
log
ρ
1/2
(
M
sun
pc
−
3
)
TriII
(d)
Figure 3.
(
a
) Luminosity–metallicity relation for MW satellite
galaxies. The dashed line shows the linear fit to the galaxies
ex-
cept Tri II (Kirby et al. 2013b, 2015a; Frebel et al. 2014), an
d the
dotted lines show the rms dispersion about the fit. (
b
) Masses
of MW satellite galaxies within their 3-D half-light radii a
ssum-
ing dynamical equilibrium. Data are from McConnachie (2012
,
and references therein), Simon et al. (2015), Koposov et al.
(2015),
and Kirby et al. (2015a). (
c
) Mass-to-light ratios and (
d
) densities
within the 3-D half-light radii. Bo ̈otes II and III are not sh
own
because their published velocity dispersions do not reflect
their
dynamical masses.
a good indicator of the instantaneous mass except for a
short time after pericenter, even for systems experiencing
significant tidal stripping (Oh et al. 1995; Mu ̃noz et al.
2008; Pe ̃narrubia et al. 2009).
Laevens et al. (2015a) noted the possible association
of Tri II with the Triangulum–Andromeda halo sub-
structure (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 3a)
is a diagnostic of tidal stripping. The LZR for classical
dwarf galaxies is very tight, with an rms of only 0.13 dex
(Kirby et al. 2011, 2013b). If a galaxy initially conforms
to the LZR, then tidal stripping will decrease its lumi-
nosity while keeping its average metallicity roughly con-
stant. This corresponds to a leftward move in Figure 3a.
Some of the galaxies with
L
.
10
4
L
⊙
, especially Segue 2
(Kirby et al. 2013a), lie significantly to the left of the
LZR. On the other hand, Tri II is consistent with the
LZR. Therefore, any tidal stripping that already hap-
pened is likely to have been mild.
5.
DISCUSSION
Tri II satisfies the definition 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 metallic-
ity 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 450
L
⊙
, the galaxy has
very few stars available to measure its shape very pre-
cisely. Even fewer stars are available for spectroscopy.
Therefore, resolving this question will be very difficult.
Regardless, we now consider Tri II’s place among the
MW satellite population under the presumption of dy-
namical equilibrium.
Figures 3b–d show the trends of
M
1
/
2
, (
M/L
V
)
1
/
2
,
and
ρ
1
/
2
with luminosity. Tri II has the largest mass-
to-light ratio (3600
+3500
−
2100
M
⊙
L
−
1
⊙
) of any galaxy ex-
cept Bo ̈otes III, whose tidal disruption is nearly com-
plete (Grillmair 2009; Carlin et al. 2009). If Tri II is in
dynamical equilibrium, then it is the most dark-matter
dominated galaxy known.
The five galaxies with
ρ
1
/
2
>
1
M
⊙
pc
−
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 ve-
locity dispersions. This correlation could arise because
these galaxies also have the smallest half-light radii. If
galaxies’ mass profiles peak in the center, then smaller
galaxies will be observed to have larger
ρ
1
/
2
, even if their
total masses and mass profiles are identical. The cor-
relation between
ρ
1
/
2
and
L
V
could also arise because
less massive galaxies are more susceptible to tidal strip-
ping. Hence, the measurement of
ρ
1
/
2
would be invalid
because the galaxies are not in equilibrium. Willman 1
has a velocity distribution that does not seem consis-
tent 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 (Koposov et al.
2015; Kirby et al. 2015a, 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
σ
v
= 5
.
1
+4
.
0
−
1
.
4
km s
−
1
for
Tri II. The present measurements cannot determine
whether the galaxy is in dynamical equilibrium or be-
ing tidally disrupted. However, the possibility that it is
6
Kirby et al.
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 detec-
tion of dark matter annihilation. The annihilation signal
scales as
ρ
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. PG acknowl-
edges 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 Observa-
tory. 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 hospi-
tality, none of the observations presented herein would
have been possible.
Facility:
Keck:I (LRIS), Keck:II (DEIMOS)
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