Table Of ContentDraft version February 16, 2017
PreprinttypesetusingLATEXstyleemulateapjv.12/16/11
THE NEAREST ULTRA DIFFUSE GALAXY: UGC2162
Ignacio Trujillo1,2, Javier Roman1,2, Mercedes Filho1,2 and Jorge Sa´nchez Almeida1,2
Draft version February 16, 2017
ABSTRACT
We describe the structural, stellar population and gas properties of the nearest Ultra Diffuse Galaxy
(UDG) discovered so far: UGC2162 (z=0.00392; R =1.7(±0.2) kpc; µ (0)=24.4±0.1 mag/arcsec2;
e,g g
g-i=0.33±0.02). Thisgalaxy, locatedatadistanceof12.3(±1.7)Mpc, isamemberoftheM77group.
7 UGC2162 has a stellar mass of ∼2(+2)×107 M and is embedded within a cloud of HI gas ∼10 times
1 −1 (cid:12)
moremassive: ∼1.9(±0.6)×108 M . UsingthewidthofitsHIlineasadynamicalproxy,theenclosed
0 (cid:12)
2 mass within the inner R∼5 kpc is ∼4.6(±0.8)×109 M(cid:12) (i.e. M/L∼200). The estimated virial mass
b from the cumulative mass curve is ∼8(±2)×1010 M(cid:12). Ultra-deep imaging from the IAC Stripe82
Legacy Project show that the galaxy is irregular and has many star forming knots, with a gas-phase
e
metallicity around one-third of the solar value. Its estimated Star Formation Rate (SFR) is ∼0.01
F
M /yr. This SFR would double the stellar mass of the object in ∼2 Gyr. If the object were to
(cid:12)
4 stop forming stars at this moment, after a passive evolution, its surface brightness would become
1 extremely faint: µ (0)∼27 mag/arcsec2 and its size would remain large R ∼ 1.8 kpc. Such faintness
g e,g
would make it almost undetectable to most present-day surveys. This suggests that there could be
] an important population of M ∼107 M ”dark galaxies” in rich environments (depleted of HI gas)
A (cid:63) (cid:12)
waiting to be discovered by current and future ultra-deep surveys.
G
Keywords: galaxies: dwarf — galaxies: evolution — galaxies: structure
.
h
p
1. INTRODUCTION are found in groups (e.g. Rom´an & Trujillo 2016b). Are
-
o In the last few years there has been a renewed interest alltheseUDGsconnectedevolutively? Recently, Rom´an
r in the study of extended low-surface brightness galax- & Trujillo (2016b) have suggested a scenario where all
t this diversity could be understood if UDG progenitors
s ies (Impey et al. 1988; Bothun et al. 1991; Dalcanton
a were born in the field, processed by groups, and ended
et al. 1997; Caldwell 2006). The discovery of dozens of
[ their lives inhabiting clusters. To answer all the above
these objects in the Coma Cluster (coined UDGs by van
questions and shed more light on the nature of UDGs,
2 Dokkum et al. 2015) has been followed by a large num-
it would be extremely useful to have the opportunity to
v ber of detections in other clusters (Koda et al. 2015; Mi-
probe,infulldetail,thepropertiesofaclose(D<15Mpc)
4 hos et al. 2015; Mun˜oz et al. 2015; van der Burg et al.
UDG. This would give us the opportunity to explore its
0 2016; Rom´an & Trujillo 2016a), groups (Rom´an & Tru-
individual stars. In particular, it would be extremely
8 jillo2016b;SmithCastellietal.2016;Merrittetal.2016)
3 and in the field (Mart´ınez-Delgado et al. 2016). The low useful to have some information about the gas content
0 stellar mass (107-108 M ) of these objects together with of one of these galaxies. In this work, we present the
. (cid:12) serendipitousdiscoveryofaverynearbyUDG:thegalaxy
1 their large size (Re >1.5 kpc) have opened a number of UGC2162. This galaxy is located in the M77 group (at
0 questionsabouttheultimatenatureofthesegalaxies: are
only12.3Mpcdistancefromus)andhasHIobservations.
7 UDGs“failed”galaxies(i.e. dotheyinhabitdarkmatter
Thisproximityallowsustohaveasuperbspatialresolu-
1 haloslargerthanthoseexpectedaccordingtotheirstellar
tionof60pc/arcsec. Inthiswork, weconductadetailed
: masscontent;vanDokkumetal.2015;Beasley&Trujillo
v analysisofthecharacteristicsofthisgalaxyandconfront
2016)? What is the role of environment? Are the prop-
i theobservationaldatawiththetheoreticalexpectations.
X erties of UDGs produced by their interaction with dense
As we will show, this galaxy is quite rich in HI gas and
environments (Yozin & Bekki 2015)? Are they simply
r is currently forming stars at a rate of 0.01 M /yr. If
a the high-spin tail of normal dwarf galaxies (Amorisco & (cid:12)
this galaxy were suddenly depleted of its gas, it would
Loeb2016)? AreUDGsproducedbyfeedback-drivengas
evolve into a red (g-i∼0.8) object with R ∼1.8 kpc and
outflows and subsequent dark matter and stellar expan- e
µ (0)∼27 mag/arcsec2. All of these are characteristics
sion (Di Cintio et al. 2017)? g
of the population of the faintest UDGs currently found
Observations indicate that UDGs are a heterogeneous
in rich clusters (Mihos et al. 2015; Beasley et al. 2016).
populationofdwarfgalaxies. Someofthemarerelatively
red (g-i∼0.8), have spheroidal shapes, and inhabit rich
2. DATA
galaxy clusters (e.g. van Dokkum et al. 2015), whereas
otherUDGsareblue(g-i∼0.4),haveirregularshapesand UGC2162 (R.A.=02h40m23.1s and
Dec=+01d13m45s) is located within the IAC Stripe82
Legacy Survey (Fliri & Trujillo 2016). The galaxy has a
[email protected] spectroscopic redshift of z=0.00392. The IAC Stripe82
1Instituto de Astrof´ısica de Canarias, Calle V´ıa Lactea, La
datasetisacarefulnewco-additionoftheSDSSStripe82
Laguna,Tenerife,Spain
2University of La Laguna. Avda. Astrof´ısico Fco. S´anchez, data with the aim of preserving the faintest surface
LaLaguna,Tenerife,Spain brightness structures. The pixel scale of these images
2 Trujillo et al.
is 0.396 arcsec and the average seeing is 1 arcsec. The galaxy has been estimated, it is possible, using the ex-
following work is based on the rectified images of this pected cumulative mass curve, to have an estimation of
dataset (http://www.iac.es/proyecto/stripe82/). The the total virial mass of the dark matter halo. We follow
meanlimitingsurfacebrightnessofthisdataco-addition the same approach as in Beasley et al. (2016). In that
is 29.1, 28.6, and 28.1 mag arcsec−2 in the g, r, and i work, the authors compare the cumulative mass distri-
bands respectively (3σ in boxes of 10×10 arcsec). To butionfromtheEAGLEsimulation(Schalleretal.2015)
put this data into context, they are ∼1.2 mag deeper withtheobserveddynamicalmassoftheirgalaxywithin
than the Dragonfly images used to explore UDGs in a given radius (see their Fig. 4, right panel). From that
Coma (van Dokkum et al. 2015) and similar to Koda et comparison they infer a virial mass for the dark mat-
al. (2015). ter halo. Using our measurement of the enclosed mass
UGC2162 is located in the vicinity of M77 ∼4.6(±0.8)×109 M within the inner R∼5 kpc, we esti-
(cid:12)
(R.A.=02h42m40.7s, Dec=00d00m48s; z=0.00379). Its mateavirialM masssimilartothatfoundbyBeasley
200
projected radial separation to this galaxy is 1.3684 de- et al. (2016) for their galaxy (i.e., ∼8(±2)×1010 M ).
(cid:12)
grees. A redshift independent measurement of the dis-
tance to M77 (Tully et al. 2009) locates this galaxy at 3. STRUCTURAL AND STELLAR POPULATION
a distance of D=12.3(±1.7) Mpc. Because this is the PROPERTIES OF UGC2162
most massive galaxy of the group, we use its distance as
To obtain the structural properties of UGC2162 we
a reliable measurement for the distance of UGC21623.
have used the code IMFIT (Erwin 2015). The surface
At that distance, the projected radial separation from
brightness distribution of the galaxy in each band was
UGC2162 to M77 is 293.8(±40.6) kpc and 1 arcsec cor-
modeled using a single S´ersic component. The S´ersic
respondsto60(±8)pc. M77isthecentralmemberofthe
model was convolved with the PSF of the image. The
M77 Group. This is a small group of galaxies that also
IAC Stripe82 Legacy Survey provides, for each band, a
harbors NGC 1055, NGC 1073, UGC 2275, UGC 2302,
PSF representative of the local (0.5×0.5 deg) conditions
UGCA 44, and Markarian 600.
oftheimage. Tohaveafirstestimateofthespatialcoor-
Fig. 1 shows a color image of UGC2162 as seen in the
dinatesofthesource,thepositionangleandtheeffective
IACStripe82images. UGC2162appearstobeanirregu-
radius,weuseSExtractor. Thesevaluesareusedlateras
lar galaxy, in fact, it has been morphologically classified
input parameters for IMFIT. In addition, we mask the
as Im (de Vaucouleurs et al. 1991). The depth of the
closest sources surrounding our galaxy (see Fig. 1).
Stripe82 image allows us to see that the inner star form-
We derived the structural parameters of the galaxy
ing region of the galaxy is surrounded by an extended
in the g, r, and i bands. In all of these bands, the
disk-like structure.
structural parameters of the galaxy were very simi-
UGC2162hasbeenobservedbyHIPASS(HIParkesAll
lar. We obtained R =28 arcsec (which is equivalent
SkySurvey;Meyeretal.2004)withaspectralresolution e
to 1.7(±0.2) kpc). The central surface brightnesses
of18km/s. TheHIPASSsurveydetectsatthepositionof
were µ (0)=24.4±0.1 mag/arcsec2, µ (0)=24.2±0.1
thegalaxyaHIlinewitharadialvelocitypeakof1171.9 g r
mag/arcsec2, and µ (0)=24.1±0.1 mag/arcsec2. These
km/s (in agreement with the velocity recession of its op- i
valueshavebeencorrectedforGalacticreddening(0.117,
ticalcounterpart: 1175±3km/s). TheHIlinefluxpeaks
0.081, and 0.060 in the g, r, and i bands respectively;
at0.089JyandhasanintegratedHIfluxdensity(S )of
HI
Schlafly & Finkbeiner 2011). The S´ersic index in all the
5.4Jykm/s. TheintegratedHIfluxdensitycorresponds
bandswasaroundn=0.9. Thetotalapparentmagnitudes
toaHImassM =2.36×105×D2×S =1.9(±0.6)×108
HI HI were g=16.1 mag, r=15.9 mag, and i=15.8 mag.
M (see, e.g. Filho et al. 2013). One can also use the HI
(cid:12) Using the global color of the galaxy and its abso-
line width W =89.7 km/s to infer a dynamical mass
20 lute magnitude, we can have a rough estimate of its
M =2.326×105 ×(W/2)2 ×r withintheHIradius
dyn HI stellar mass. We follow the recipe by Roediger &
(r ). To have an estimation of the dynamical mass, it
HI Courteau (2015) (assuming a Chabrier IMF), using the
is necessary to correct the line width W for the incli-
20 g-i color and the absolute magnitude in the r-band
nation i of the galaxy, i.e. W= W /sini. We estimate
20 (M =-14.6(±0.3) mag). We obtain a stellar mass of
theinclinationusingtheaxisratioofthe27mag/arcsec2 r
∼2(+2)×107 M .
isophote (g band). This isophote is still bright enough, −1 (cid:12)
UGC2162 has an SDSS spectrum (Plate=1070;
but sufficiently far away from the central part of the
Fiber=450; MJD=52591) located at the coordinates:
galaxy,toproduceareliableestimationoftheshapeofits
R.A.=40.09751 deg and Dec=1.22476 deg (see Fig. 2).
outerdisk. Weobtainanaxisratiob/a=0.7. Thistrans-
The spatial location of the SDSS spectrum is indicated
lates into an inclination (under the assumption of a thin
in Fig. 1. This region corresponds to the brightest
disk) of i∼45 degrees, and consequently, W=128 km/s.
knot of star formation of the galaxy. This knot has
We assume r to be three times the optical R ra-
HI 25
a radius of 1.2 arcsec (Petrosian mag at 90%) and a
dius (see, e.g. Filho et al. 2013). For our galaxy, R =26
25
magnitude in the r band of only 21.9 mag. Using the
arcsec=1.6(±0.3) kpc (measured in g-band). With these
ratio N2≡[NII]λ6583/Hα from the SDSS spectrum and
values,weestimateM =4.6(±0.8)×109 M withinthe
dyn (cid:12) the calibration by Pettini & Pagel (2004), we have es-
inner R∼5 kpc.
timated an oxygen abundance for the star-forming ion-
Once a dynamical mass in the inner region of the
ized gas 12+log(O/H)=8.22±0.07, which corresponds to
one-third of the solar abundance. The gas of UGC2162
3Inwhatfollows,wewillconsidertheuncertaintyinthedistance
is fairly metallic for its mass and magnitude, since the
toM77asthemainsourceoferroratestimatingalltheremaining
quantities which depend on that distance. These errors will be galaxyisahigh-metallicityoutlierofthemass-metallicity
enclosedwithinparenthesistoindicatetheirorigin. relation and the magnitude-metallicity relation worked
The nearest ultra diffuse galaxy 3
Figure 1. Left panel: g,r,i IAC Stripe82 composite image centered on UGC2162. The spatial location of the SDSS spectrum of this
galaxy is indicated with white ticks. Right panel: g-i color map of UGC2162. The central irregular region is located on top of a more
roundedextendeddisk-likestructure. Thewhitecirclesarethemaskedregionsusedinthiswork.
out by Berg et al. (2012). We also checked the spectrum interesting exercise to understand how our object would
for the presence of [OIII]λ4363, which appears in low look in the future.
metallicity objects (e.g. S´anchez Almeida et al. 2016). To model the color and structural evolution of
The line is not in the spectrum, which is consistent with UGC2162, we have used its present-day g-r color map
the moderate metallicity inferred from N2. Since the anditscurrentg,r,andisurfacebrightnessdistributions.
SDSS spectrum is quite noisy, the estimated O abun- Then, we have estimated how every pixel of the images
dance should be regarded as an upper limit. would look if we make their colors evolve passively for
Using the Hα flux (uncorrected for extinction since 6 Gyr. To quantify the color change and the dimming
Hβ does not seem to be reddened), the distance to the in surface brightness of every pixel, we have used the
source, and the recipe of Kennicutt (1998), we have esti- Vazdekis et al. (2015) models assuming a Kroupa IMF.
mated the SFR and the surface SFR of the bright knot, Due to this passive evolution, the galaxy would not only
which turn out to be SFR=8.7(±1.2)×10−5 M /yr and change its global color (becoming g-i=0.77) but would
(cid:12)
Σ =3.4(±0.5)×10−3 M /yr/kpc2. Assuming that also get dimmer (by ∼2.6 mag/arcsec2). The result of
SFR (cid:12)
thegalaxyhas100suchstar-formingknots(reasonablein this evolution is illustrated in Fig. 3. After 6 Gyr of
viewoftheshapeandsizeofthegalaxy)thetotalSFRof passive evolution, the galaxy would have µ (0)=27±0.1
g
thegalaxywouldbeSFR=8.7(±1.2)×10−3 M /yr. This mag/arcsec2 and R = 1.8(±0.2) kpc. Its profile shape
(cid:12) e
value is consistent with the value around 10−2 M /yr wouldnotchangedramatically, anditspassivelyevolved
(cid:12)
workedoutbyHunter&Elmegreen(2004)forthisobject S´ersicindexn∼0.8wouldbesimilartoitsoriginalvalue.
usingHαimaging. Usingtheabovevaluewecanderivea With these characteristics, the galaxy would resemble
specific SFR for UGC2162: sSFR∼5(±0.7)×10−10 yr−1. closely the faintest UDG galaxies discovered so far in
IftheSFRofUGC2162wereconstant, thegalaxywould the Virgo cluster (Mihos et al. 2015). In fact, consid-
double its stellar mass in ∼2 Gyr. ering the virial mass of UGC2162, in an eventual fu-
ture, this galaxy could look very similar to VCC1287,
4. THE FUTURE OF UGC2162 a very low surface brightness UDG galaxy (µg(0)=26.7
mag/arcsec2, R = 2.4 kpc, g-i=0.83, M ∼3×107 M )
UGC2162 is currently located at (a projected separa- e (cid:63) (cid:12)
inhabiting the Virgo cluster (Beasley et al. 2016).
tion of) ∼300 kpc from M77. Due to its large amount of
HI gas (∼10 times larger than its stellar mass), we can
5. DISCUSSION AND CONCLUSIONS
speculate that UGC2162 is undergoing its first infall to
the M77 galaxy group. When a galaxy like UGC2162 The structural and stellar population properties of
falls into a group environment, it suffers a number of UGC2162 seem to fit very well within a scheme where
physical mechanisms that eventually will quench its star theUDGsfoundbothoutsideandinsideclustersarejust
formation. These mechanisms can be either slow (of the different evolutionary stages of the same type of objects
order of a few gigayears) due to gas strangulation (see, (Rom´an & Trujillo 2016b). In this view, UDGs outside
e.g.McCarthyetal.2008)ortheycanberelativelyrapid clusters would be simply the progenitors of the redder
iftheyareproducedbyrampressurestripping(e.g.Wang UDGs found in rich clusters. The link between both
etal.2007). Followingrecentsimulations(Yozin&Bekki typesofUDGswouldbeanevolutionduetotheremoval
2015), one can assume that gas rich dwarf galaxies will of their gas produced by the infall of these galaxies in
be depleted of gas 6 Gyr after their first infall into typi- rich environments. If this picture is correct UDGs out-
calgroupsofgalaxies(1013−13.5 M ). Motivatedbythis side dense environments should be dwarf galaxies with a
(cid:12)
number,wesimulatehowourgalaxywouldlookin6Gyr large HI content. In fact, low-surface brightness dwarf
timeiftheobjectweretostopformingstarsandfollowed galaxies with a large amount of HI seem to be fairly
a passive evolution. Naturally, this is an oversimplifica- common in the field (e.g. Skillman et al. 2013; James
tionoftheactualevolutionofthegalaxy,butitcanbean et al. 2015; Hirschauer et al. 2016; Sanchez Almeida et
4 Trujillo et al.
Figure 2. PiecewiseSDSS-DR12spectrumofUGC2162(theblacksolidline)andoftheextremelymetalpoorgalaxyIZw18(theredsolid
line),thelaterincludedforreference. Themainlinesarelabeled. (a)RegionaroundHα. Theratiobetween[NII]6583andHαisusedto
estimate the gas-phase metallicity. [NII] is very small in metal-poor systems (see the red solid line). The flux in Hα is used as a proxy
forthepresentSFR.(b)Regionaround[OIII]4363. Thislineshowsupinmetalpoorsystems,butisalwaysverysmall(seetheredsolid
line). (c)Regionaround[OIII]5007,whichincludesHβ. ThespectrumofIZw18hasbeenscaledsothatithasthesamefluxin[OIII]5007
asUGC2162. Theunitsofthefluxaresharedbythethreepanelsandaregiveninpanel(c). Wavelengthsarein˚A.
al. 2017). On the contrary, those UDGs found in the ing UDGs is key to have a comprehensive picture about
richestenvironmentsshouldbedepletedofHIgasdueto the nature of these objects and how to connect their dif-
the removal of this component. ferent evolutionary stages.
UGC2162 also teaches us an important lesson about UGC2162hasmanyofthestructuralandstellarpopu-
UDGsandhowtheseobjectsareobservationallyselected. lation properties expected if the large size of this galaxy
This galaxy has a relatively low stellar mass (∼107 M ) is the result of feedback-driven gas outflows (Di Cintio
(cid:12)
compared to the general population of UDGs, which et al. 2017). For instance, UGC2162 has a large amount
peaks at ∼108 M (e.g. Rom´an & Trujillo 2016a). This of HI gas and it is currently forming stars as the cosmo-
(cid:12)
issue is connected to the way UDGs are defined. UDGs logical simulations predicted. In addition, Di Cintio et
areobservationallyselectedusingtheirsurfacebrightness al. (2017) simulations are able to predict the dwarf-like
(µ(0)>24 mag/arcsec2) and size (R >1.5 kpc). This ob- halo mass of this galaxy, as well as its stellar mass, gas
e
servational definition immediately biases the selection of mass, S´ersic index, effective radius, absolute magnitude,
the galaxies depending on their stellar populations. The SFR,irregularappearance,andoff-center-starformation
redderUDGswillbemoremassiveandolder,whereasthe episodes. According to these simulations, these galax-
bluerUDGswillbeyoungerandwithlowerstellarmass. ies would not be at all rare and they will be found in
This implies that if one wants to connect the observed abundance outside clusters. In fact, many of these have
UDGs with their progenitors or with their descendants, already been detected (Rom´an & Trujillo 2016a) in low
it is important to take this into account. For instance, densityenvironments. Ifthepicturesketchedbythecos-
most of the progenitors of massive and red UDGs found mological simulations is correct, a large number of the
in rich clusters would not satisfy the observational cri- descendants of UDGs found in low density environments
teria to be classified as UDGs. These progenitors would wouldbefoundinrichclustershavingthefollowingchar-
have central surface brightnesses brighter than µ(0)=24 acteristics: M (cid:38)107M ,µ (0)(cid:38)27mag/arcsec2,R >1.5
(cid:63) (cid:12) g e
mag/arcsec2 and would have been classified as regular kpc,n(cid:46)1,g-i∼0.8andlowHIgascontent. Theseobjects
(blue) dwarf galaxies. On the other hand, those blue would be hard to find even for current deep surveys. In
UDGs that have been discovered outside clusters (as is fact, UDGswith µg(0)(cid:38)27mag/arcsec2 have onlybere-
the caseof UGC2162)wouldevolveinto thelessmassive portedbyMihosetal.(2015),whiletheremainingUDGs
(and red) UDGs found in rich clusters (as is the case of foundinrichclustershaveallbeenfoundwithµg(0)<27
VCC1287). Accounting for this selection effect in select- mag/arcsec2 (e.g. van Dokkum et al. 2015; Koda et al.
The nearest ultra diffuse galaxy 5
Figure 3. Left column: Theg,r,andipresent-daysurfacebrightnessprofilesofUGC2162andtheirpassiveevolutionafter6Gyr. The
lowerpaneldisplaystheg-rcolorradialprofiles. After6Gyrofpassiveevolution,thegalaxywouldgetsignificantlydimmer,redder,and
maintainasimilarsize. Theverticaldashedlinesshowthepositionoftheeffectiveradiusforthepresent-dayUGC2162anditspotential
future evolution. Right column: a color composite of how UGC2162 looks today and how the galaxy would eventually look in the future
(after6Gyrofpassiveevolution). Thecontoursindicatethepositionofthe(gband)26mag/arcsec2 (upperpanel)and28.5mag/arcsec2
isophotes(lowerpanel).
2015;Mun˜ozetal.2015;vanderBurgetal.2016;Rom´an Martin et al. (2016) discuss the possibility that the And
& Trujillo 2016a). The existence of a large number of XIX large extention could be produced by the gravita-
“dark” M ∼107 M extended galaxies in rich clusters tionaltidesofM31. Theothertwogalaxies(thoughwith
(cid:63) (cid:12)
is a natural prediction of cosmological simulations if the stellarmassesaround107 M orlarger)arebeingtidally
(cid:12)
above evolutionary picture for the UDGs is correct. disrupted. In this sense, their large effective radii are
Finally, it is worth noting that there are a number a consequence of the ongoing disruption process. Com-
of extremely low-surface brightness galaxies at a dis- paredtothepreviousobjects,thefactthatUGC2162has
tance closer than UGC2162 that technically satisfy the agasreservoirisstrongevidencethatitisnotdiffuseand
criteria to be considered UDGs (i.e. R >1.5 kpc and extended because it is being tidally disrupted. For this
e
µ(0)>24 mag/arcsec2). These objects are: a) a satellite reason,UGC2162iscurrentlythenearestnottidallydis-
of M31, Andromeda XIX (R >1.7 kpc and µ(0)=29.3 rupted UDG known, whose large size is probably due to
e
mag/arcsec2; McConnachie et al. 2008; Martin et al. an internal origin alone.
2016), b) a satellite of our own galaxy, the Sagittarius
dwarf (Re=1.6 kpc and µ(0)=25.2 mag/arcsec2; Ibata We thank the referee for a report. We would also like
et al. 1994; Majewski et al. 2003) and c) a satellite of tothankMichelleCollinsforinterestinginsightsintothe
the galaxy NGC4449 located at 3.8 Mpc, NGC4449B population of extremely diffuse Local Group galaxies.
(Re=2.7 kpc and µ(0)=25.5 mag/arcsec2; Mart´ınez- WethankMichaelBeasleyandChrisBrookfortheiruse-
Delgado et al. 2012; Rich et al. 2012). Andromeda XIX ful comments during the development of this work. The
has a very low mass (MV=-9.3 and σ=4.7 km/s; Collins authors of this paper acknowledge support from grant
et al. 2013) and it is significantly less massive than the AYA2013-48226-C3-1-P from the Spanish Ministry of
populationofUDGsthathasbeendiscussedintheliter- Economy and Competitiveness (MINECO). J.R. thanks
ature (which are a factor of ∼100 times more massive). the Spanish Ministry of Economy and Competitiveness
For this reason, And XIX cannot be considered to be (MINECO)forfinancinghisPhDthroughanFPIgrant.
representative of the population of UDGs originally dis-
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