arXiv:1305.5301v1 [astro-ph.CO] 23 May 2013 k.TeSonDgtlSySre SS)ealdthe the enabled of (SDSS) region Survey this Sky Digital of Sloan mainly surveys The thanks photometric decade, sky. large last two the over to soared has galaxies and multi-color, high-quality, photometry. our mapping, to wide-field access while complete has observable, its one questions easily provided facilitates these it tackle perspective renders to outsider’s al. proximity order the et in its arguably Ibata study as is 2012; to system system al. satellite ideal et Andromeda Pawlowski cosmolog- The favored 2013). (e.g. hi- the but the paradigm by test 2013) ical can al. induced host et al. formation their Koposov et understand Vargas erarchical around distribution (e.g. 2012; to their formation Jablonka also important & galaxy Revaz only of 2009; the end not with faint are decade the These past systems galaxies. the Group faint Local in faint evident been numerous of more has discovery context the cosmological all a made in formation galaxy ing Bell ila .Burgett S. William AET N ASOEAII W UIOSADDSATANDRO DISTANT AND LUMINOUS TWO III: CASSIOPEIA AND I LACERTA rneo,N 84,USA 08544, NJ Princeton, USA 96822, HI Honolulu, USA 02138, MA Cambridge, Street, UK 3HJ, EH9 Edinburgh Hill, Blackford servatory, 48109 MI Arbor, Ann St., Church Germany Heidelberg, 69117 l’Universit´e, F-6700 de rue 11 France 7550, Strasbourg, UMR CNRS, Strasbourg, [email protected] ioa .Martin F. Nicolas h ubro nw nrmd aelt dwarf satellite Andromeda known of number The understand- for galaxies dwarf satellite of salience The rpittpstuigL using 2018 typeset 8, Preprint September version Draft 7 6 5 4 3 2 1 nttt o srnm,Uiest fHwi tManoa, Universi Princeton at Sciences, Hawaii Astrophysical of of Department University Astronomy, Garden for 60 Institute Astrophysics, for Center Harvard-Smithsonian 500 Ob Royal Edinburgh, Michigan, of University Astronomy, of for Institute University Astronomy, of D- Department 17, K¨onigstuhl Astronomie, f¨ur Max-Planck-Institut de Universit´e Strasbourg, de astronomique Observatoire ueeA Magnier A. Eugene 3 ejmnP .Laevens M. P. Benjamin , r ls oteGlci ln.Ti atrlmtto snwalvae yte3 the by alleviated now is limitation latter This plane. Galactic the to close hc ol edt h icvr fohrdsatAdoeasatellit Andromeda distant other of discovery headings: the Subject to lead could which aaisoeterlt icvr oterlreszs( sizes large their to discovery late their owe galaxies 26 p n 144 and kpc 7 itne f756 of distances n 10 and adiaigdt.Bt r uiossses( systems luminous are Both data. imaging band n asoeaIIAdoeaXXI(a I/n XI) nsta in XXXII), III/And (Cas XXXII III/Andromeda Cassiopeia and h erpr h icvr ftonwdafglxe,LcraI/And Lacerta galaxies, dwarf new two of discovery the report We . mag/arcsec 0 6 = . . 5 5 ◦ − +1 rmM1 a n a I r ieystlie fteAdoeagala Andromeda the of satellites likely are III Cas and I Lac M31. from 1. 1 . . 2 0 A 1 rmno 1456 or arcmin INTRODUCTION T − +6 , 6 2 E 4 ent .Chambers C. Kenneth , oi .Slater T. Colin , − +44 tl mltajv 5/2/11 v. emulateapj style X 2 p.Tebiheto eetLclGopmme icvre,th discoveries, member Group Local recent of brightest The kpc. 28 ,a ela otelc fasseai uvyo ein tlrera large at regions of survey systematic a of lack the to as well as ), oa ru aais niiul a aais niiul n XXXI And individual: galaxies: — I Lac individual: galaxies: — Group Local 6 aais niiul a I aais niiul n XXXII And individual: galaxies: — III Cas individual: galaxies: p n 772 and kpc efe .Morgan S. Jeffrey , AAISFUDI H 3 THE IN FOUND GALAXIES 1 , 2 dur .Bernard J. Edouard , ± − +61 6 cfrCsII,adcneunl o ufc rgtes( brightness surface low consequently and III), Cas for pc 267 56 3 dadF Schlafly F. Edward , p,rsetvl,adcrepnigM1cnrcdsacso 2 of distances M31-centric corresponding and respectively, kpc, rf eso etme ,2018 8, September version Draft 6 6 lu .Hodapp W. Klaus , alA Price A. Paul , ABSTRACT ty, 0 - M 4 V net .N Ferguson N. M. Annette , π rmteMlyWyds,msl ot fM1 and M31, of of cone south a mostly to away disk, limited well Way is stay PAndAS Milky that regions the surveyed from only has SDSS yet ttos n I I XIIaealeape of about examples least all at are of XXVIII distances & projected 30 VII at lim- galaxies VI, coverage dwarf their And highlights also itations. system satellite M31 M n aa ste r oae nteLcraadCas- and Lacerta the in located imag- are Pan-STARRS1 they As stacked in data. ing found galaxies ra- dwarf virial the dwarf beyond ( and M31 a Andromeda to of provide the out dius to of system potential satellite understanding the galaxy complete has it more 2013), much SDSS al. the than with et deeper and, (Metcalfe slightly limitations reaching photome- these data have a stacked not is final does 2010) that al. Sys- survey et try Response Kaiser (Pan-STARRS1; Rapid 1 and tem Telescope Survey Panoramic ihttlmgiue agn from 2011) ranging al. et magnitudes al. Richardson et total satel- 2008; Ibata 2009; al. with Andromeda 2006, et al. McConnachie new et 2007; (Martin unambiguous popu- out galaxies dwarf galaxy, 16 stellar lite Andromeda the of the of discovery of of study distances halo the to the to within hand, dedicated lations other the Pan-Andromeda survey The region on a 2011). is, the al. (PAndAS) et of Survey XXIX; Bell Archaeological And 2011; parts X; al. XXVIII, (And of et And 8 Slater coverage Release Data IX, in systematic M31 complemented (And around more later was galaxy a which 2007), Andromeda by 2004, ma- the al. the et along of Zucker stripe axis dedicated a jor from initially systems, icvr fahnflo eaieybih ( bright relatively of handful a of discovery − ∼ A-TRS SURVEY PAN-STARRS1 h ra ucs fteesresi neln the unveiling in surveys these of success great The nti ae,w rsn h icvr ftetwo the of discovery the present we paper, this In r V ◦ h 2 crepnigto (corresponding − ≃ 7 rcMorganson Eric , 2 oae tpoetddsacso 20 of distances projected at located 12) 4 = onL Tonry L. John , 6 10 ihlsKaiser Nicholas , . 2 . − +0 0. 0 . . 4 5 oeaXX LcIAdXXXI) I/And (Lac XXXI romeda kdPan-STARRS1 cked rmno 912 or arcmin ∼ wr galaxies. dwarf e 5 p.Aoe thsealdthe enabled has it Alone, kpc. 150 ∼ 6 ∼ π ihr .Wainscoat J. Richard , 0 kpc). 300 2 EASTLIEDWARF SATELLITE MEDA asWle Rix Hans-Walter , 0 p 3-eti distances), M31-centric kpc 400 a-TRS survey, Pan-STARRS1 4 6 oga .Finkbeiner P. Douglas , ofPtrKudritzki Rolf-Peter , ywt heliocentric with xy s w e dwarf new two ese − +124 93 ∼ i rmM31, from dii cfrLcI; Lac for pc r 10 P1 ◦ and - M rmM1 The M31. from V µ M 75 0 i 2 − ≃ P1 V . rcF. Eric , — 3 ∼ ± ◦ - ∼ < 6 6 − . to 0 8 6 5 . , , 5) 2 N. F. Martin et al. siopeia constellations, but are also likely Andromeda satellites, we follow the dual-naming convention empha- sized in Martin et al. (2009) to avoid ambiguity. We therefore dub the first dwarf galaxy both Lacerta I and Andromeda XXXI (Lac I/And XXXI) and the sec- ond one both Cassiopeia III and Andromeda XXXII (Cas III/And XXXII). The data set used for the dis- covery is summarized in §2, while the derivation of the dwarf galaxies’ properties is described in §3, before we conclude in §4.

2. THE PAN-STARRS1 SURVEY The PS1 3π survey (Chambers et al., in prepara- tion) is a systematic imaging survey of the sky north of δ = −30◦ in five optical and near-infrared pho- tometric bands (gP1rP1iP1zP1yP1; Tonry et al. 2012). Conducted on a dedicated 1.8m telescope located on Haleakala, in Hawaii, the system relies on a 1.4-Gpixel, 3.3◦-field-of-view camera for rapid mapping of the sky. Any location in the survey is observed repeatedly for a planned four times per year per filter, conditions per- mitting, with exposure times of 43/40/45/30/30 sec- Figure 1. Map of dwarf galaxies located in the surroundings of onds in the gP1/rP1/iP1/zP1/yP1-bands, respectively M31 and M33, projected on the tangential plane centered on M31. (Metcalfe et al. 2013). The median seeing values in these Known satellites are shown as hollow circles while the two new bands are 1.27/1.16/1.11/1.06/1.01 arcseconds. Images discoveries, Lac I and Cas III, are represented by filled circles. The limits of the SDSS and PAndAS are represented by the thin- are automatically processed through the Image Process- line and the double-line polygons, respectively. The dotted lines ing Pipeline (Magnier 2006, 2007; Magnier et al. 2008) highlight lines of constant Galactic latitude at b = 0◦, −20◦, and to produce the stacked data catalogue. −40◦ from top to bottom. At the time of this paper, the region around An- dromeda had been observed for three consecutive sea- surements are affected at the 0.05 level in the r band, sons from May 2010, with up to twelve exposures per P1 and at the 0.1 level in the iP1 band. However, since filter. Chip gaps, poor observing conditions, and techni- the majority of point sources have a reliable photometry, cal problems, however, mean that a varying number of we have decided to nevertheless keep theses discrepant exposures are available for stacking at any given loca- sources in the catalogue. tion. For point-sources, this leads to 10σ-completeness Throughout, the distance to M31 is assumed to be limits8 of 22.1 in the r band, and 22.2 in the i band P1 P1 779+19 kpc and the uncertainties on this values are mod- for the surroundings of Lac I. Around Cas III, these lim- −18 eled by drawing directly from the posterior probabil- its are 22.0 and 21.9, respectively. Checking the spatial ity distribution function provided by Conn et al. (2012). completeness from the presence of 2MASS stars, bright All magnitudes are dereddened using the Schlegel et al. enough to be observed with PS1, yields an area coverage (1998) maps, with the extinction coefficients for the factor > 95 percent in either band. PS1 filters provided by Schlafly & Finkbeiner (2011): Flags were used to remove spurious detections that are A P 1 /E(B − V )=3.172, A P 1 /E(B − V )=2.271, and above the saturation limit, blended with other sources, g r A P 1 /E(B − V )=1.682. thought to be defect, saturated pixels, or are considered i significant cosmic rays or extended sources. Sources for 3. LACERTA I AND CASSIOPEIA III which either the point-spread function fit is bad, the mo- In a first pass at searching for unknown M31 satellites ments, the local sky, or the local sky variance could not ◦ be measured were also removed from the data set. Fi- within 30 from their host, a map was produced from nally, detections for which the peak lands on a diffraction the catalogue sources selected in the color-magnitude di- spike, a ghost, a glint, or near a chip edge were discarded agram (CMD) to loosely correspond to potential red gi- ant branch (RGB) stars at the distance of Andromeda as well, in addition to sources with signal-to-noise below < < 5. (0.0 ∼ (rP1−iP1)0 ∼ 0.8, iP1,0 < 20.8). This map showed The photometric calibration of the stacked PS1 data a handful of high-significance peaks, including the well- is still under development, with spurious offsets in the known M31 dwarf galaxies And I, II, III, V, VI, and photometry for subsets of detections. In the region of LGS3. The CMD of significant peaks which did not over- Lac I, a comparison of the stacked photometry with that lap with known satellites were inspected by eye, leading to the clear discovery of two compact stellar system lo- of the single exposure PS1 observations, calibrated at the h m s ◦ ′ ′′ 0.01-magnitude level by Schlafly et al. (2012), yields no cated at ICRS (α, δ) = (22 58 16.3 , +41 17 28 ) and h m s ◦ ′ ′′ significant offset in the iP1 band, whereas the rP1 band (α, δ) = (0 35 59.4 , +51 33 35 ) that have no coun- has ∼ 20 percent of sources offset by ∼ 0.2 magnitudes. terparts in NED or Simbad. We designate them as In the Cas III surroundings, a small fraction of mea- Lac I/And XXXI and Cas III/And XXXII and show the location of the two dwarf galaxies comparatively to the 8 These detection limits correspond to the median magnitude of M31 satellite dwarf galaxy system in Figure 1. local stars with photometric uncertainties close to 0.1 magnitudes. The top, left-hand panels of Figure 2 present the CMD Lacerta I and Cassiopeia III 3

Figure 2. Top, left-hand panels: CMD of the region within two half-light radii of Lac I (bottom-left) and that of a field region of the same coverage (top-left), along with the map of PS1 sources around Lac I (right). The dwarf galaxy’s CMD exhibits an overdensity of stars in the region expected for bright RGB stars at the distance of M31, matched by a spatial overdensity in the map. For the CMDs, the error-bars on the right-hand side of the panels represent the average photometric uncertainties and the red polygon delineate the selection box that was used to isolate candidate Lac I stars, shown as large dots in the spatial map (other sources are shown as small dots). The red ellipse overlaid on the map delineates the region within two half-light radii of the dwarf galaxy, according to the favored structural parameters determined in § 3.1. Top, right-hand panels: Similar plots for Cas III. Bottom panels: Similar plots for And I. The And I structural parameters are taken from McConnachie (2012). of stars within two half-light radii of Lac I (see below for Table 1 the estimation of its structural parameters); for compar- Derived properties of Lac I/And XXXI and Cas III/And XXXII ison, the panels also include the CMD of a field region with the same coverage, but offset by a degree towards Lac I Cas III α (ICRS) 22h58m16.3s 0h35m59.4s the south-east. Lac I produces a clear overdensity of stars δ (ICRS) +41◦17′28′′ +51◦33′35′′ within the RGB color-selection, typical of stars close to ℓ (◦) 101.1 120.5 the tip of the red giant branch (TRGB) at the distance b (◦) -16.7 -11.2 of M31. Isolating these stars yields the map shown to E(B − V )a 0.133 0.193 the right of the CMDs and reveals that they are also (m − M)0 24.40 ± 0.12 24.45 ± 0.14 +44 +61 clumped in a clear spatial overdensity, with a size of a Heliocentric distance (kpc) 756−28 772−56 +6 few arcminutes, typical of M31 satellite dwarf galaxies. M31-centric distance (kpc) 275 ± 7 144−4 The same is true for Cas III (top, right-hand panels), MV −11.7 ± 0.7 −12.3 ± 0.7 2 despite a more severe contamination from the prominent µ0 (mag/arcsec ) 25.8 ± 0.8 26.4 ± 0.8 Milky Way disk. Ellipticity 0.43 ± 0.07 0.50 ± 0.09 ◦ For comparison, the bottom panels of Figure 2 show Position angle (N to E; ) −59 ± 6 −90 ± 7 +0.4 +1.2 the same set of plots for the well-known Andromeda I rh (arcmin) 4.2−0.5 6.5−1.0 +124 dwarf galaxy, also present in the stacked PS1 data, albeit rh (pc) 912−93 1456 ± 267 in a region of deeper data. The CMDs of both And I and aFrom Schlegel et al. (1998). its background region are less populated owing to the ◦ lower Galactic foreground contamination (b = −16.7 stars when compared to the two discoveries. for Lac I and b = −11.2◦ for Cas III vs. b = −24.8◦ for And I), but otherwise displays a similar overdensity of 3.1. Structure 4 N. F. Martin et al.

Figure 3. Left-hand panels: The likelihood distribution functions for the structural parameters of Lac I, marginalized over other param- eters. The gray dash-dotted lines correspond to the maximum of the distributions, whose parameter values are taken as the favored model parameters. The right-most panel of this set displays the binned radial density profile of the dwarf galaxy built by assuming the favored centroid, ellipticity, and position angle (filled circles), compared to the favored exponential profile and background (full line). The dashed line corresponds to the favored estimation of the background. The uncertainties on counts are computed assuming Poisson uncertainties. Right-hand panels: Similar plots for Cas III.

The structural parameters of Lac I and Cas III are tively). The size of the two systems is on the high determined following the maximum likelihood technique side of the size distribution of M31 satellite dwarf galax- +0.4 +124 presented by Martin et al. (2008), and updated in Mar- ies: rh = 4.2−0.5 arcmin or 912−93 pc for Lac I and tin et al. (2013, in prep) to allow for a full Markov r = 6.5+1.2 arcmin or 1456 ± 267pc for Cas III (see Chain Monte Carlo estimate of the parameters, allow- h −1.0 below for their heliocentric distance estimates). ing for missing data. For systems that contain relatively The properties of Lac I and Cas III are summarized in few stars, a likelihood-based estimate of the structural Table 1 and the MCMC chains are available on demand parameters is far less biased and much more robust than to the corresponding author. estimates that rely on binning the data (Martin et al. 2008). In this instance, the algorithm explores the like- 3.2. Distance lihood of a family of exponential models with a constant background level to reproduce the distribution of PS1 The magnitude of the TRGB is a very good distance stars around Lac I or Cas III. The parameters of the indicator in the case of Andromeda dwarf galaxies (e.g., dwarf galaxy models are its centroid, its exponential half- Conn et al. 2011), especially in the absence of deep pho- tometric data. The tip’s location also has the benefit light radius (rh), the position angle of its major axis (θ), its ellipticity (ǫ = 1 − b/a, with a and b being, respec- of being relatively insensitive to the metallicity and age tively, the major and minor axis scale lengths of the sys- of stellar populations, as long as these are reasonably tem), and the number of member stars within the cho- old and metal-poor, a condition that is fulfilled for all sen CMD selection box (N ∗), which is tied to the back- known M31 dwarf spheroidal galaxies. We apply the like- ground level through the number of stars in the data lihood technique implemented by Slater et al. (2011) to set. Uncertainties on these parameters are determined determine the distance to And XXVIII, later confirmed from the one dimensional, marginalized likelihood distri- to be accurate from deeper observations of this system bution functions and the location at which the likelihood (Slater et al., in preparation). The TRGB of Lac I is −0.5 TRGB reaches e ≃ 0.61 of the peak value. For a Gaussian estimated at iP1,0 = 20.95 ± 0.07, and that of Cas III TRGB distribution, the corresponding parameter values bound at iP1,0 = 21.00±0.10. In both cases, the measurement the 1σ confidence interval. is in good agreement with the dwarf galaxy’s luminosity The likelihood distribution functions for these param- function, as shown in Figure 4. eters, constrained on the PS1 data of the region within ′ The TRGB magnitude has been estimated at 25 of the centroid of Lac I or Cas III, are presented in TRGB Mi,SDSS = −3.44 ± 0.10 by Bellazzini (2008). Its Figure 3 for the most relevant parameters. These are conversion onto the PS1 photometric system using the well constrained and, in particular, the high significance color equations of Tonry et al. (2012) yields M TRGB = of N ∗ in both cases leaves no ambiguity to the existence i,P1 −3.45 ± 0.10 and allows us to calculate the distance of the new dwarf galaxies. Their radial density profiles modulus of Lac I: (m − M)0 = 24.40 ± 0.12 . This are also displayed in the same figure and, for both dwarf distance modulus corresponds to a heliocentric distance galaxy, compares the data, binned following the most +44 likely centroid, ellipticity, and position angle, with the of 756−28 kpc and, when also accounting for the M31 distance uncertainty, an M31-centric distance of 275 ± exponential model of most likely half-light radius and 9 number of stars. In both cases, the very good agreement 7kpc . Given the dearth of isolated Local Group dwarf between the two is testimony to the quality of the fit. galaxies, it still appears that likely Lac I is an Andromeda The measured structure of Lac I and Cas III are rather 9 The low uncertainty on the M31-centric distance stems from typical of bright Andromeda satellite galaxies with mod- Lac I being close to the point of closest distance to M31 along the erate ellipticities (0.42 ± 0.07 and 0.50 ± 0.09, repec- line of sight, i.e. the geometric tangent point, irrespective of the moderate uncertainty on its heliocentric distance. Lacerta I and Cassiopeia III 5

Figure 4. Left-hand panels: CMD of stars within two half-light radii of Lac I, with Parsec isochrones of age 12.7 Gyr and metallicity [Fe/H] = −2.3 and −1.0 overlaid as a full blue line and a dashed red line, respectively. The binned luminosity function of the CMD stars is represented by a thin line in the right panel, while that of a local field region scaled to the same coverage is represented by a dotted line, and the resulting background-corrected luminosity function of Lac I is displayed with a thick line. The estimated location of the TRGB is indicated by the arrow, and its uncertainty is represented by the vertical error bar attached to it. Right-hand panels: Similar plots for Cas III. satellite despite its large M31-centric distance; its proper- ness of this method and the multitude of assumptions ties are similar to other remote satellites like And VI, VII, it relies on are made evident by the large uncertainties, XXVIII, or XXIX, which have now all been confirmed as measured as the dispersion of the magnitude derivations Andromeda satellites from radial velocity measurements from the four reference dwarf galaxies. Nevertheless, the (McConnachie 2012; Tollerud et al. 2013). CMDs of the two new dwarf galaxies apparently contain- In the case of Cas III, the distance modulus is (m − ing a similar number of stars to that of And I (Figure 2) M)0 = 24.45 ± 0.14, leading to a heliocentric distance acknowledges the fact that they have a total magnitude +61 +6 similar to that of And I (M = −11.7 ± 0.1). of 772−56 kpc, and an M31-centric distance of 144−4 kpc. V At this distance, Cas III is even more likely to be a mem- ber of the Andromeda satellite system. Of course, radial 4. FINAL REMARKS velocities are needed to add evidence that both Lac I and We report the discovery of two new dwarf galax- Cas III are truly bound to M31. ies, Lac I/And XXXI and Cas III/And XXXII, from 3.3. Total magnitude stacked PS1 imaging data. At a heliocentric distance +44 of 756−28 kpc, Lac I is located in the environs of To estimate the total magnitude of the two dwarf the Andromeda galaxy, at an M31-centric distance of galaxies, we first sum up the flux contribution of all 275 ± 7 kpc, making it a likely satellite of Andromeda. stars within their half-light radius that are in the in- Cas III is located closer to M31 in projection and, com- terval 20.7 < iP1,0 < 21.5 and also broadly fall in the bined with our measurement of its heliocentric distance RGB’s color range. The contaminating flux from fore- +61 (772−56 kpc), is well within the M31 virial radius with an ground sources is calculated in the same way from a +6 large local field region, later scaled to the area within M31-centric distance of only 144−4 kpc. The membership the half-light radius of Lac I and Cas III, and subtracted of both dwarf galaxies to the M31 satellite system shall from the flux of the dwarf galaxies. We correct for unob- be confirmed kinematically through our ongoing spectro- served stars by integrating a metal-poor ([Fe/H] = −1.7) scopic program of both Lac I and Cas III. The discov- and old (12.7 Gyr) Parsec luminosity function in the ery of Cas III is of particular interest as it is located on PS1 photometric system (Bressan et al. 2012). This the rotating disk of dwarf galaxies that includes about half of Andromeda’s dwarf galaxies (Conn et al. 2013; yields the corrected apparent magnitude: miP1,0 = 12.8 Ibata et al. 2013). and mr , = 13.3 for Lac I and mi , = 12.1 and P1 0 P1 0 +124 mrP1,0 = 12.6 for Cas III. Data are not deep enough Both dwarf galaxies are quite large (rh = 912−93 pc in gP1 to yield a reliable estimate of the apparent mag- and rh = 1456 ± 267 pc), yet this is not unexpected for nitude in that band, complicating the transformation to dwarf galaxies or their brightness. Brasseur et al. (2011) the widely used V -band magnitude. estimated that, for a total magnitude of MV = −12.0, In order to circumvent this difficulty, we rely on a dif- Andromeda dwarf galaxies have an average size of 775pc, ferential measurement with the rP1andiP1 magnitudes of with a lognormal dispersion of 0.23, implying that Lac I And I, II, III and V as measured from PS1 data, in the is barely above the average size and Cas III just slightly PS1 bandpasses. We repeat the above exercise for these more than one dispersion away. One also has to keep in four dwarf galaxies, account for their distance modulus mind that, of course, it is the largest systems that are the difference with those of Lac I and Cas III, and tie the most likely to have so far avoided detection, biasing new total magnitude of the two discoveries to that of these discoveries towards larger systems than were discovered well-known dwarf galaxies, yielding MV = −11.7 ± 0.7 from photographic surveys. for Lac I and MV = −12.3 ± 0.7 for Cas III. The crude- To a certain degree, the only unexpected property of 6 N. F. Martin et al.

knowledge support by the DFG through the SFB 881. CTS and EFB acknowledge support from NSF grant AST 1008342. The Pan-STARRS1 Surveys (PS1) have been made possible through contributions of the Institute for As- tronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society and its partic- ipating institutes, the Max Planck Institute for Astron- omy, Heidelberg and the Max Planck Institute for Ex- traterrestrial Physics, Garching, The Johns Hopkins Uni- versity, Durham University, the University of Edinburgh, Queen’s University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Figure 5. POSS II F (Red) image of And VI (left) and Lac I Central University of Taiwan, the Space Telescope Sci- (right). The images are 30 arcmin on the side, centered on the two dwarf galaxies, with north to the top and east to the left. And VI is ence Institute, the National Aeronautics and Space Ad- readily visible while Lac I appears as a faint amount of unresolved ministration under Grant No. NNX08AR22G issued light. More significant wispy features due to the foreground Milky through the Planetary Science Division of the NASA Sci- Way dust appear at the edge of the Lac I image. ence Mission Directorate, the National Science Founda- tion under Grant No. AST-1238877, and the University of Maryland. Lac I and Cas III are their total magnitude, MV = −11.7 ± 0.7 and −12.3 ± 0.7, respectively, which are an order of magnitude more luminous than any mem- REFERENCES ber of the cohort of Local Group dwarf galaxies discov- ered in the last decade. Although surprising at first, one must remember that, prior to PS1, the region of the Armandroff, T. E., Jacoby, G. H., & Davies, J. E. 1999, AJ, 118, two discoveries had only been systematically observed 1220 in the optical by photographic surveys. Regardless, one Bell, E. 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