The Carnegie-Chicago Hubble Program: Discovery of the Most Distant Ultra-faint Dwarf Galaxy in the Local Universe

0.42 MB·

by  Myung Gyoon Lee

Draftversion January16,2017
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
THE CARNEGIE-CHICAGO HUBBLE PROGRAM: DISCOVERY OF THE MOST DISTANT ULTRA-FAINT
DWARF GALAXY IN THE LOCAL UNIVERSE
Myung Gyoon Lee1,†, In Sung Jang1,2, Rachael Beaton3, Mark Seibert3, Giuseppe Bono4, and Barry Madore3
Draft version January 16, 2017
ABSTRACT
7 Ultra-faint dwarf galaxies (UFDs) are the faintest known galaxies and due to their incredibly low
1 surface brightness, it is difficult to find them beyond the Local Group. We report a serendipitous
0 discovery of an UFD, Fornax UFD1, in the outskirts of NGC 1316, a giant galaxy in the Fornax
2 cluster. The new galaxy is located at a projected radius of 55 kpc in the south-east of NGC 1316.

n This UFD is found as a small group of resolved stars in the Hubble Space Telescope images of a
a halo field of NGC 1316, obtained as part of the Carnegie-Chicago Hubble Program. Resolved stars
J in this galaxy are consistent with being mostly metal-poor red giant branch (RGB) stars. Applying

2 the tip of the RGB method to the mean magnitude of the two brightest RGB stars, we estimate the
1 distance to this galaxy, 19.0±1.3 Mpc. Fornax UFD1 is probably a member of the Fornax cluster.
The color-magnitude diagram of these stars is matched by a 12 Gyr isochrone with low metallicity

] ([Fe/H] ≈ −2.4). Total magnitude and effective radius of Fornax UFD1 are MV ≈ −7.6±0.2 mag
A and r = 146±9 pc, which are similar to those of Virgo UFD1 that was discovered recently in the
eff
G intracluster field of Virgo by Jang & Lee (2014). Fornax UFD1 is the most distant known UFD that
is confirmed by resolvedstars. This indicates that UFDs are ubiquitous and that more UFDs remain
.
h to be discovered in the Fornax cluster.
p Subject headings: galaxies: dwarf — galaxies: clusters: individual (Fornax UFD1) — galaxies: stellar
- content — galaxies: abundances
o
r
t
s 1. INTRODUCTION to the size of UFDs.
a
UFDs may be the most abundant type of galaxies
[ Ultra-faint dwarf galaxies (UFDs) are the faintest
in the universe, but they are revealing today only a
known galaxies (McConnachie 2012; Belokurov 2013;
1 tiny tip of a giant iceberg to our current observa-
Martin et al. 2016). Some of them are even fainter than
v tional facilities (McConnachie 2012; Sand et al. 2012;
typical globular clusters, albeit they are much more ex-
5 Belokurov et al. 2014; Laevens et al. 2014; Jang & Lee
tended than the latter. Dwarf galaxies fainter than
6 M = −8.0 mag and larger than r (effective radius) 2014; Dark Energy Survey Collaboration et al. 2016).
4 V eff They were first discovered, as a small overdensity of
= 20 pc are often called as UFDs (Bechtol et al. 2015).
3 resolved red giant stars, around the Milky Way Galaxy
Theyareopticallyfeeble,buttheyaredominatedbydark
0
from the Sloan Digital Sky Survey (SDSS) data, and
. matter, having very high values of mass to light ratios
1 (Simon & Geha 2007; McConnachie 2012). Thus nearby more of them were found later around M31 in the Local
0 Group (Willman et al. 2005a,b; McConnachie 2012;
UFDs may be ideal laboratories to explore the parti-
7 Sand et al. 2012; Belokurov et al. 2014; Laevens et al.
cle properties of dark matter (e.g., Calabrese & Spergel
1 2014; Martin et al. 2016). With recent advent of the
(2016)). Stellar populations in UFDs are mostly old
: Dark Energy Camera on CTIO 4m telescope, as many
v metal-poor stars. Brown et al. (2014) found, from deep
as 17 new UFDs have been discovered around the
i photometry of six UFDs around the Milky Way Galaxy,
X Milky Way Galaxy and the Large Magellanic Cloud
that formation of the stars in these galaxies was fin-
r ished by 11.6 Gyr ago (z ≈ 3). They suggested that (LMC) in 2015 (Bechtol et al. 2015; Koposov et al.
a 2015; Kim et al. 2015; Drlica-Wagner et al. 2015;
this early quenching of star formation might have been
Dark Energy Survey Collaboration et al. 2016). It is
due to global processes such as the reionization of the
expected that more UFDs will be discovered in the
universe. Thus, UFDs are considered to be fossils of
Local Group soon from current and future wide field
the first galaxies (Bovill & Ricotti 2011a,b; Jang & Lee
surveys(e.g.,LSST,PANStarrs,etc). UFDsintheLocal
2014; Ricotti et al. 2016). Recently Ricotti et al. (2016)
Group can play an important role to help resolving the
suggested, from high resolution simulations of the first
well-knownmissingsatellitedwarfproblem(Moore et al.
galaxies,that some UFDs may be the remnants of a few
1999)andunderstanding the nature ofthe firstgalaxies.
distinct star clusters that were dissolved and expanded
However, it is very difficult to find UFDs outside the
Local Group with the current observational facilities.
†[email protected]
1DepartmentofPhysicsandAstronomy,SeoulNationalUni- Therefore little is known about the UFDs outside the
versity,Korea Local Group.
2Leibniz-Institut fu¨r Astrophysik Potsdam (AIP), An der Surprisingly, Jang & Lee (2014) discovered one UFD,
Sternwarte16,D-14482,Potsdam,Germany VirgoUFD1,intheintraclusterfieldoftheVirgocluster,
3The Observatories of the Carnegie Institution of Washing-
using deep imagesof the Hubble Space Telescope (HST)
ton,Pasadena, CA91101, USA
4Universit`a di Roma Tor Vergata, Via della Ricerca Scien- available in the MAST archive. Virgo UFD1 was found
tifica,1 00133,Roma,Italy as a resolvedstellar overdensity. Virgo UFD1 is the first
2 Lee et al. 2016
(a) NGC 1316 (b) ACS field
(c) Fornax UFD1
5’ 5"
28 kpc 454pc
Fig.1.— (a)LocationoftheHSTfield(square)intheinvertedcolorimageoftheinnerregionofNGC1316providedintheESOarchive,
embedded inthegrayscalemapofR-bandimagegivenbyRichtleretal.(2012). Northisup, andeast totheleft. Asmallgalaxyinthe
northofNGC1316isNGC1317, adwarfgalaxy. Thepositionofthenew UFDisdenoted byaredcircle. (b)Acolor imageoftheHST
field. (c)A15′′×15′′ sectionoftheHSTfieldcenteredonFornaxUFD1. Notethatmostresolvedstarsinthecentralregionareredgiants
inthisdwarfgalaxy.
UFD found beyond the Local Group. Total magnitude (Lee et al. 1993), and detailed analysis of the data is
andeffective radiusofVirgoUFD1 areM ≈−6.5±0.2 given in Jang et al. (2017). We use photometry of the
V
mag and r =81±7 pc, and its central surface bright- point sources derived from these images by Jang & Lee
eff
nessisaslowasµ =26.37±0.05magarcsec−2. Metal- (2017). The magnitudes of the point sources in the im-
V,0
licity of the stars in this galaxy is estimated from their ages were derived using DAOPHOT (Stetson 1994). In
colors, to be, on average,very low, [Fe/H] =−2.4±0.4. this study we adopteda distance to NGC 1316basedon
VirgoUFD1 islocatedatthe distanceof16.4±0.4Mpc, the TRGB, given in Jang et al. (2017): (m −M) =
0
being the most distant UFD at the time of discovery. 31.45±0.05(ran)±0.06(sys) and d = 19.5±0.5 Mpc.
Later Crnojevi´c et al. (2016) reported detection of one ForegroundextinctionandreddeningvaluestowardNGC
UFD in the remote halo of NGC 5128 (Cen A), a mas- 1316 are small, A = 0.057 and (E(V − I) = 0.026
V
sive elliptical galaxy at the distance of 3.8 Mpc. They (Schlafly & Finkbeiner 2011).
usedtheimagesobtainedaspartofthePanoramicImag- Figure 1(a) displays the location of the HST field
ing Survey of Centaurus and Sculptor (PISCeS) at the in the B,V,R and Hα color image of the inner region
Magellan 6.5 m telescope. of NGC 1316 provided by the ESO archive, embedded
In this Letter we report a serendipitous discovery of in the gray-scale map of the deep R-band image given
an UFD, Fornax UFD1, breaking the distance record by Richtler et al. (2012). A smaller spiral galaxy in the
of Virgo UFD1. It is located in the outskirts of NGC northofNGC 1316is NGC 1317(SAB(r)a). This image
1316,agiantgalaxyintheFornaxcluster. FornaxUFD1 shows various substructures including shells and ripples
was detected as a small group of resolved stars in the aroundNGC 1316,which areprobably mergerremnants
HST/ACS images for a halo field of NGC 1316, ob- (Schweizer 1980; Richtler et al. 2012). The HST field is
tained as part of the Carnegie-ChicagoHubble Program locatedat9.′5 south-eastof the NGC 1316center,where
(CCHP) (Beaton et al. 2016). diffuse stellar light of NGC 1316 is barely visible. It is
expectedthat this HST field wouldbe dominatedby old
2. DATA halo stars in NGC 1316. A color image of the HST field
is also displayed in Figure 1(b).
The CCHP is an ongoing program to estimate the
value of the Hubble constant (H0) with high precision 3. RESULTS
using the population II distance indicators, using RR
3.1. Discovery of a New UFD
Lyrae and the tip of the red giant branch (TRGB)
(Beaton et al.2016). AspartoftheCCHP,deepF606W First,wesearchedforanydwarfgalaxieswithlowsur-
and F814W images of a halo field in the outskirt of face brightness in the HST images of NGC 1316, which
NGC 1316 using HST/ACS were obtained. Total ex- would appear to be similar to the morphology of Virgo
posure times for F606W and F814W are 14,676s and UFD1 (see Fig. 1 in Jang & Lee (2014)). Since the For-
24,396s, respectively. These data were used for esti- nax cluster is only slightly more distant than the Virgo
mation of the distance to NGC 1316 using the TRGB cluster, it is expected that some of the brightest stars in
Fornax Ultra-faint Dwarf Galaxy 3
26
(a) Fornax UFD1 (b) Control Field (c) UFD1 only (1) (d) UFD1 only (2)
N stars: 19 N stars: 302 N stars: 15 N stars: 15
Area: 12.6"2 Area: ×50 of (a) Area: 12.6"2 Area: 12.6"2
27
I
28
29
1 2 3 1 2 3 1 2 3 1 2 3
V-I V-I V-I V-I
26
(e) Virgo UFD1 (f) Control Field (g) UFD1 only (1) (h) UFD1 only (2)
N stars: 30 N stars: 284 N stars: 24 N stars: 25
Area: 28.3"2 Area: ×50 of (e) Area: 28.3"2 Area: 28.3"2
27
I
28
29
1 2 3 1 2 3 1 2 3 1 2 3
V-I V-I V-I V-I
Fig.2.— (Upper panels) I −(V −I) CMDs of the resolved stars (a) at r ≤ 2′′ of the Fornax UFD1 and (b) in the control field at
5′.′0<r≤15′.′0. Theareaofthecontrolfieldis50timesbiggerthantheUFD1field. MeanvaluesoftheDAOPHOTerrorsaredenotedby
theerrorbars. (c)and(d)ThenetCMDsoftheUFD1fieldobtainedafterstatisticalsubtractionofthefieldcontributionintwodifferent
realizations. Curvedlinesdenotethe12Gyrstellarisochronesfor[Fe/H]=−2.4to−0.4instepsof0.2providedbytheDartmouthgroup
(Dotter etal. 2008), shifted in magnitude and color according to (m−M)0 =31.37 and E(V −I)=0.026. (Lower panels) Same as the
UpperpanelsbutforVirgoUFD1.
FornaxUFDs, ifany,wouldbe resolvedinthedeepHST biggerthanthatofthe FornaxUFD1 field. TheCMDof
images. the point sources in the control field shows a broad red
Through visual inspection of the images, we noticed giant branch (RGB), and these point sources are mostly
the presence of a small group of faint resolved point RGB stars in the halo of NGC 1316 (Jang et al. 2017).
sources embedded in diffuse stellar light in the south- We used statistical subtraction to remove the contribu-
ern part of the HST field (at the position of the small tion of the foreground and background sources from the
redcircle inFigure 1(b)). Azoom-inimageofthe field CMD in (a), using the CMD of the control field. For
centeredonthenewsysteminFigure1(c)showsclearly this process we used bins with 0.3 mag in both magni-
a faint fuzzy system embedding a small number of re- tudeandcolor,whichissimilartothemeanerrorsofthe
′′
solvedstars,thediameterofwhichisabout4 . Thisnew RGB stars. Figure 2(c) and (d) showtwo realizations
system looks much fainter and smaller than any other of the control field subtraction. Both CMDs look very
typical dwarf galaxies in the Fornax cluster (Ferguson similar in general. In the lower panels of Figure 2, we
1989; Mieske et al. 2007; Mun˜oz et al. 2015). This sys- plotted similar CMDs but for Virgo UFD1 (Jang & Lee
tem turns out to be, indeed, a new UFD, named as For- 2014) for comparison.
nax UFD1, as shown below. Fornax UFD1 is located Thereare12resolvedstarsbrighterthanI =28.5mag
10.′1(∼55kpc)south-eastoftheNGC1316centerinthe inthenetCMDs(Figure2(c)and(d)). Thenumberof
sky. fieldcontaminantswithI <28.5magintheareacovered
bytheUFDfieldisestimatedtobe4.8±0.3,whilethere
3.2. Color-magnitude Diagrams of the Resolved Stars are15 stars in the areaofUFD. Thus the net number of
in Fornax UFD1 the member stars inthe areaof the UFD is estimatedto
be 10±4, which is consistent with the number derived
Figure 2(a) displays the color-magnitude diagrams
afterCMDdecontamination. Eightofthe12starshavea
(CMDs)oftheresolvedpointsourcesinacircularregion
withr ≤2′′ whereristheprojectedradialdistancefrom narrowrangeofcolorswith0.7<(V −I)<1.4,showing
a vertical sequence, while three of them show much red-
thecenterofFornaxUFD1. WeplottedalsotheCMDof
the control field in the annular region at 5′′ < r ≤ 15′′, der colors than the others. Therefore, these eight stars
are consideredto show the RGB of Fornax UFD1, while
inFigure 2(b). Theareaofthiscontrolfieldis50times
4 Lee et al. 2016
r [pc] r [pc]
10 100 10 100
V=23.8, M =-7.6
V
24 n=0.49±0.07 24
I
e
] d
2c u
e it
s 26 n 26
c n =0.54±0.06 g
ar V a
ag/ l m pc)
m 28 ta 28 6
[ o 4
T 1
µ (
I 6
1. "
30 V 30 =
eff
r
0.1 1.0 0.1 1.0
r [arcsec] r [arcsec]
Fig.3.— The radial profiles of the V (open circles) and I-band (open triangles) surface brightness (a) and cumulative integrated
magnitudes(b)ofFornaxUFD1. Thearrowsdenotethelocationofthecontrolregion. Thebandsrepresenttheerrors. Filledsquareswith
graybands denote the radialnumber densityprofileoftheresolvedredgiantstars(27<I <28.5magand0.0<(V −I)<2.0). Dashed
linesin(a)representaS´ersiclawfitforeachband. TheV-bandeffective radiusofFornaxUFD1ismarkedbyaverticallinein(b).
threeredderstarsareprobablyforegroundorbackground Weconstructedradialprofilesofsurfacebrightnessand
sources. cumulative integrated magnitude of Fornax UFD1 using
Because of the small sample of the RGB stars in the IRAF/ELLIPSE, as plotted in Figure 3(a) and (b).
CMD, it is not easy to derive an accurate distance to We masked out cosmic rays and background galaxies in
Fornax UFD1. We estimate approximately the distance the original image, and ran the ELLIPSE task adopting
to this UFD, assuming that the mean magnitude of the asmoothingoptionof3×3pixelbinning. Theradialpro-
twobrightestblueRGBstarsintheCMD,correspondsto filesofsurfacebrightnessinFigure 3showaflatcorein
the magnitude of the TRGB. The mean magnitude and theinnerregionatr <1′′anddeclinerapidlyintheouter
colorofthesetwostarsareI =27.38±0.14magand(V− region, showing that this UFD is a dynamically relaxed
I)=1.19±0.03. Thismagnitudevalueis0.24magfainter system like typical old star clusters. The radial profiles
than the value of Virgo UFD1, I = 27.14± 0.04 mag ofsurfacebrightnessforr <4′′ arefittedwellbyaSersic
(Jang & Lee 2014) (see the lower panels in Figure 2). law (S´ersic 1963) with an index n = 0.54±0.06 for V-
ApplyingtheTRGBcalibrationinJang & Lee (2017)to bandandn=0.49±0.07forI-band. Theeffectiveradius
thisTRGBmagnitude,wederiveavalueforthedistance measuredattheV-bandofthisgalaxyisr =1′.′6±0′.′1
eff
to Fornax UFD1, d = 19.0 ± 1.3 Mpc ((m − M) = (146±9 pc). This value is much larger than that of the
0
31.37±0.15). This value is consistent with the distance typical globular clusters, r ≈ 3 pc. Its central surface
eff
to NGC 1316. This showsthat FornaxUFD1 is likely to brightness for V and I-bands corrected for foreground
belong to Fornax. extinction are derived to be µ = 25.46± 0.06 mag
V,0
In Figure 2 we overlayed 12 Gyr isochrones for arcsec−2 and µ = 24.66±0.05 mag arcsec−2, respec-
I,0
[α/Fe]=0.2 (Fabrizio et al. 2015) and [Fe/H] = −2.4 to tively. We derived the radial number density profile of
–0.4intheDartmouthmodel(Dotter et al.2008),which theRGBstarswithI <28.5magand0<(V −I)<2.0,
were shifted according to the distance and foreground choosing an annular region at r ≈ 20′′ to subtract the
reddening for Fornax UFD1. Although the number of fieldcontribution. Thisprofileatr<5′′ issimilartothe
stars is small, the RGB of UFD1 is, on average, bluer surface brightness profiles, as shown in Figure 3.
and narrower compared with the RGB of NGC 1316 in The integrated color of Fornax UFD1 for the aper-
thecontrolfield. Thisshowsthatthemeanmetallicityof ture of r is estimated to be (V − I) ≈ 0.98 ± 0.05
eff
this new UFD is much lowerthan that of the NGC 1316 ((V −I) ≈ 0.95±0.05). The integrated color of For-
0
halo. The CMD of these RGB stars is matched roughly nax UFD1 is consistent with those of early-type dwarf
by the RGB partofthe isochronesforlow metallicity. It galaxiesinFornax(Mieske et al.2007),andthatofVirgo
isnotedthattheRGBofFornaxUFD1isevenbluerthan UFD1 (Jang & Lee 2014).
the RGB of the lowest metallicity with [Fe/H] ≈ −2.4. The radial profiles of V and I-band integrated mag-
We estimate roughly the mean metallicity of the RGB nitude of this galaxy become approximately constant at
stars to be [Fe/H] ≈−2.4±0.4. r = 5′′. The integrated magnitude for r = 5′′ is esti-
matedto be V =23.8±0.2magandI =22.5±0.2mag.
3.3. Basic Parameters of Fornax UFD1
These values can be considered to be the total magni-
Fornax Ultra-faint Dwarf Galaxy 5
(a) (b)
10000 15 E&ll i Bp tui cl ga less UCDs
]
2
c
1000 e
s
c
c] ar
p Ellipticals / 20
[ g
r eff100 BUuClDgess ma
[
MW GCs
V
LG satellites 0,
µ
10 LG UFDs 25
DWs in nearby galaxies
DWs in the Fornax cluster
Virgo UFD1
1
Fornax UFD1
30
-25 -20 -15 -10 -5 0 -25 -20 -15 -10 -5 0
M M
V V
Fig.4.—(a)Theeffective radiusversusabsoluteV total magnitude oftheFornaxUFD1(largeredstarletsymbol)incomparisonwith
those for other stellar systems. We also plotted the Virgo UFD1 by the small yellow starlet symbol. (b) The V-band central surface
brightnessversusabsoluteV totalmagnitudeoftheFornaxUFD1. Circlesandlenticularsymbolsareforthegiantellipticalsandbulgesin
spiralgalaxies,downwardtrianglesfortheultracompactdwarfs(UCDs),pentagonsfortheMilkyWayglobularclusters,squaresandempty
lozenges for the Local Group satellites and UFDs, and upward triangles for the dwarfs in M81 and M106 and the low surface brightness
galaxiesinM101,andpinkdiamondsfortheNGFSdwarfsinMun˜ozetal.(2015).
tudes of Fornax UFD1. Corresponding absolute magni- gion of Fornax with Dark Energy Camera on CTIO 4m
tudes of this galaxy are derived to be M = −7.6±0.2 as part of the program for the Next Generation Fornax
V
mag and M =−8.9±0.2 mag. These results show that Survey(NGFS),discovering158newdwarfgalaxieswith
I
this new system is indeed a genuine UFD. We list basic M <−8.0 mag. The effective radiiandeffective surface
i
parameters of Fornax UFD1 in Table 1. brightness of these galaxies are r =0.1 to 2.8 kpc and
eff
µ = 22.0 to 28.0 mag arcsec−2. Thus the parameters
4. DISCUSSION i
of Fornax UFD1 are similar to the faint limit in their
4.1. Comparison with Previous Surveys of Fornax sample. However,none ofthe starsinFornaxUFD1-like
Dwarf Galaxies dwarf galaxies can be resolved at the pixel scale of the
Dark Energy Camera images.
Ferguson (1989) published the Fornax Cluster Cata-
log (FCC) based on the wide field photographic sur-
vey of Fornax galaxies. Following this, several stud- 4.2. Comparison with Other Galaxies
ies used CCD images to find new fainter galaxies in We compare scaling relations of Fornax UFD1 with
Fornax and investigate their properties (Kambas et al. those of other early-type stellar systems in Fig-
2000;Hilker et al.2003;Mieske et al.2007;Mun˜oz et al. ure 4(a) and (b) (see Figure 4 for details of
2015). Kambas et al. (2000) imaged a long strip region the kinds of other early-type stellar systems). We
from NGC 1399 to NGC 1291 (an area of 13.8 deg2) in use the data for effective radii, total magnitudes,
Fornax,finding a large number of low surface brightness and central surface brightness of other stellar sys-
dwarf galaxies with MB < −12.0 mag. These are unre- tems compiled in Jang & Lee (2014) (see the references
solved faint galaxies. therein). We updated the data, adding data for new
Later deeper surveys of the central region in Fornax UFD galaxies: the new UFDs in the Local Group
found a number of fainter dwarf galaxies. The detection from the Dark Energy Survey (DES) (Bechtol et al.
limits of the dwarf spheroidal galaxies (dSphs) in the 2015; Koposov et al. 2015; Drlica-Wagner et al. 2015;
survey of Hilker et al. (2003) and Mieske et al. (2007) Kim et al. 2015; Kim & Jerjen 2015; Laevens et al.
are V ≈ 23 mag (MV ≈ −8.5 mag), and µV(0) ≈ 27 2015), a new UFD in the Centaurus A group
magarcsec−2. Mieske et al.(2007)determineddistances (Crnojevi´c et al. 2016) and Virgo UFD1 (Jang & Lee
to early-type dwarf galaxies in Fornax using the surface 2014). Wealsoaddedthe dataforthe newFornaxdwarf
brightnessfluctuation(SBF)method,andconfirmedthat galaxies (Mun˜oz et al. 2015).
30 of the dEs with −16.6 < M < −10.1 mag are the InthefigureitisfoundthatFornaxUFD1islocatedin
V
members of Fornax. Fornax UFD1 is about one mag- the regionofthe LocalGroupUFDs andthat the prop-
nitude fainter that the detection limits of Hilker et al. erties of Fornax UFD1 are very close to those of Virgo
(2003) and Mieske et al. (2007). UFD1 (Jang & Lee 2014). These results show that For-
Recently Mun˜oz et al. (2015) covered the central re- nax UFD1 is indeed a genuine UFD. The centralsurface
6 Lee et al. 2016
TABLE 1
Basic Parametersof Fornax UFD1
Parameter Value References
R.A.(2000) 3h23m11.s374 1
Dec(2000) −37◦20′46′.′53 1
Type UFD 1
Distancemodulus,(m−M)0 31.37±0.15 1
Distance,d[Mpc] 19.0±1.3 1
Imagescale 91.1pcarcsec−1 1
Totalmagnitude VT =23.8±0.2 1
Colorateffective radius (V −I)0=0.95±0.05 1
Foregroundreddening AV =0.057,E(V −I)=0.026 2
Absolutemagnitude, MV −7.6±0.2 1
Metallicity [Fe/H]≈−2.4 1
SersicIndex(n), V 0.54±0.06 1
SersicIndex(n), I 0.49±0.07 1
Effectiveradius(reff),V 1′.′6±0′.′1(146±9pc) 1
Centralsurfacebrightness,V µV,0=25.46±0.06magarcsec−1 1
Centralsurfacebrightness,I µI,0=24.66±0.05magarcsec−1 1
References. —(1)Thisstudy;(2)Schlafly&Finkbeiner(2011).
brightness of Fornax UFD1 as well as Virgo UFD1 cor- UFD1aresimilartothoseofVirgoUFD1. FornaxUFD1
responds to the upper range for the Local Group UFDs. is the most distant one among the known UFDs that
This implies that UFDs are ubiquitous and that there have been confirmed by resolved stars. Fornax UFD1
may be many more UFDs with lower surface brightness and Virgo UFD1 are, respectively, only the first known
and fainter magnitudes in Fornax and Virgo. UFD in each of Fornax and Virgo, and it is expected
that many more will be discovered in the future. We
justopenedanwindowtotherealmoftheUFDsbeyond
the Local Group.
5. SUMMARY
Taking advantage of high-resolution deep images of a Theauthorsaregratefultoanonymousrefereeforuse-
halo field in NGC 1316 obtained as part of the CCHP, ful comments. Tom Richtler is thanked for providing
wediscoveredanewUFDinFornax,FornaxUFD1. The deep CTIO 4m images of NGC 1316. MGL and ISJ are
CMD of the resolved stars in this UFD shows that it is supportedbytheNationalResearchFoundationofKorea
indeed a genuine UFD with low metallicity ([Fe/H] ≈ (NRF)grantfundedbythe KoreanGovernment(MSIP)
−2.4)andthatitislikelytobelongtotheFornaxcluster. (No. 2012R1A4A1028713).
The size, luminosity, and surface brightness of Fornax Facilities: HST (ACS, WFC3/IR).
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