Detection of an Optical Counterpart to the ALFALFA Ultra-compact High Velocity Cloud AGC 249525
William Janesh, Katherine L. Rhode, John J. Salzer, Steven Janowiecki, Elizabeth A. K. Adams, Martha P. Haynes, Riccardo Giovanelli, John M. Cannon
DDraft version November 13, 2018
Typeset using L A TEX twocolumn style in AASTeX61
DETECTION OF AN OPTICAL COUNTERPART TO THE ALFALFA ULTRA-COMPACT HIGH VELOCITYCLOUD AGC 249525
William Janesh, Katherine L. Rhode, John J. Salzer, Steven Janowiecki, Elizabeth A. K. Adams, Martha P. Haynes, Riccardo Giovanelli, and John M. Cannon Department of Astronomy, Indiana University, 727 E. Third Street, Bloomington, IN 47405, USA International Centre for Radio Astronomy Research, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia. Netherlands Institute for Radio Astronomy (ASTRON), Postbus 2, 7900 AA Dwingeloo, The Netherlands Center for Radiophysics and Space Research, Space Sciences Building, Cornell University, Ithaca, NY 14853, USA Department of Physics and Astronomy, Macalester College, 1600 Grand Avenue, Saint Paul, MN 55105, USA
ABSTRACTWe report on the detection at >
98% confidence of an optical counterpart to AGC 249525, an Ultra-Compact HighVelocity Cloud (UCHVC) discovered by the ALFALFA blind neutral hydrogen survey. UCHVCs are compact, isolatedH I clouds with properties consistent with their being nearby low-mass galaxies, but without identified counterparts inextant optical surveys. Analysis of the resolved stellar sources in deep g - and i -band imaging from the WIYN pODIcamera reveals a clustering of possible Red Giant Branch stars associated with AGC 249525 at a distance of 1.64 ± I synthesis map of AGC 249525 from Adams et al. (2016) showsthat the stellar overdensity is exactly coincident with the highest-density H I contour from that study. Combiningour optical photometry and the H I properties of this object yields an absolute magnitude of − . ≤ M V ≤ − .
5, astellar mass between 2 . ± . × M (cid:12) and 3 . ± . × M (cid:12) , and an H I to stellar mass ratio between 9 and 144.This object has stellar properties within the observed range of gas-poor Ultra-Faint Dwarfs in the Local Group, butis gas-dominated. Keywords: galaxies: dwarf, galaxies: photometry, galaxies: stellar content a r X i v : . [ a s t r o - ph . GA ] F e b Janesh et al. INTRODUCTIONThe Arecibo Legacy Fast ALFA survey (ALFALFA)is a blind neutral hydrogen (H I ) survey carried out withthe Arecibo radio telescope that covers ∼ × M (cid:12) of H I at the distance of the Virgo Cluster and < M (cid:12) within the Local Group at an angular resolution of ∼ (cid:48) and a spectral resolution of ∼ − (Giovanelliet al. 2005). Among these thousands of ALFALFAsources is a small population of objects first identifiedaccording to their common characteristics by Giovanelliet al. (2010) and dubbed Ultra-Compact High Veloc-ity Clouds (UCHVCs). The UCHVCs are very com-pact ( < (cid:48) diameter), isolated gas clouds that have H I properties (including narrow velocity widths) that sug-gest they may be nearby low-mass galaxies. The otherimportant characteristic of UCHVCs is that they haveno clear optical counterpart when their positions andredshifts are checked against optical surveys and cat-alogs like the Sloan Digital Sky Survey (SDSS; Eisen-stein et al. 2011) and sources in the NASA ExtragalacticDatabase (NED).Some have argued that compact high-velocity clouds(CHVCs, with ∼ (cid:48) diameters) detected in H I surveysare gas clouds in the Local Group with few or no stars,embedded within low-mass dark matter halos (Blitz etal. 1999; Braun & Burton 1999). The detection of asubstantial population of such objects would have im-plications for the “missing satellites problem”, the dis-crepancy between the numbers of low-mass halos pre-dicted by ΛCDM structure formation models comparedto the numbers detected in observational surveys (Kauff-mann et al. 1993; Klypin et al. 1999; Moore et al.1999). However Sternberg et al. (2002) demonstratedthat the structural properties of the CHVCs do notmatch the expected properties of the missing satellites inthe ΛCDM framework: if extragalactic, they are phys-ically too large. Giovanelli et al. (2010) brought re-newed attention to this topic by showing that UCHVCsdiscovered in ALFALFA, if they are indeed at distancesof ∼ do have sizes and masses consistent withtheir being baryonic material embedded within low-mass( (cid:46) M (cid:12) ) dark matter halos, or “minihalos”, that fitwithin the ΛCDM paradigm. Giovanelli et al. (2010)concluded that the UCHVCs are plausible minihalo can-didates, but exactly what they represent – e.g., star-less gas clouds, low-mass galaxies with stars that havebeen missed by past surveys, or some other poorly-understood HVC phenomenon – remains unclear. A preliminary list of 27 UCHVCs was presented inGiovanelli et al. (2010), and Adams et al. (2013) fol-lowed up with a more complete list of 59 objects chosenfrom the 40% ALFALFA catalog (Haynes et al. 2011).UCHVCs selected from ALFALFA have H I propertieslike those of Leo T (Irwin et al. 2007), which is thefaintest ( M V ∼ −
7) Local Group dwarf galaxy with ev-idence of recent star formation. At a distance of 1 Mpc,the UCHVCs would have H I masses of ∼ − M (cid:12) ,H I diameters of ∼ − ∼ − M (cid:12) .In order to derive the masses and sizes of these H I sources, and thus to better understand their nature, weneed to know their distances. One route for derivingdistances for the UCHVCs is to detect their stellar pop-ulations, construct a color-magnitude diagram (CMD),and then measure a distance from the Tip of the RedGiant Branch (TRGB) method. Accordingly, we arecarrying out a campaign to obtain deep optical imagingof UCHVCs with the WIYN 3.5-m telescope to look forstellar populations associated with these objects, and (ifpossible) derive their distances, masses, sizes, and otherproperties, and to study their star formation histories.The first UCHVC we observed with WIYN resultedin the discovery of Leo P, an extremely metal-poor gas-rich star-forming dwarf galaxy (Giovanelli et al. 2013;Rhode et al. 2013; Skillman et al. 2013). Located justoutside the Local Group at 1 . ± .
15 Mpc (McQuinnet al. 2015), Leo P has an H I mass of 8 . × M (cid:12) and an H I -to-stellar mass ratio of ∼
2. Its oxygen abun-dance is ∼
2% of the solar value, comparable to the abun-dances of I Zw 18, DDO 68, and the ALFALFA galaxyAGC 198691 (Hirschauer et al. 2016). Its total mag-nitude is M V = − . ± . . × M (cid:12) , making Leo P the lowest-mass galaxyknown that is currently forming stars.Since the discovery of Leo P, we have developed a sys-tematic observing strategy and procedure for searchingfor stellar counterparts to the UCHVCs. Details arepresented in Janesh et al. (2015), along with a tenta-tive detection of a counterpart to the ALFALFA sourceAGC 198606, nicknamed Friend of Leo T (Adams et al.2015) because it is close in position and velocity to dwarfgalaxy Leo T. We detected a stellar counterpart at a dis-tance of ∼
380 kpc, with M i ∼ − .
7, and an H I -to-stellarmass ratio of ∼ − The WIYN Observatory is a joint facility of the Universityof Wisconsin-Madison, Indiana University, the University of Mis-souri, and the National Optical Astronomy Observatory. etection of an Optical Counterpart to AGC 249525 I distribution and evidence of rotation infollow-up H I synthesis observations. OBSERVATIONS AND ANALYSISA16 presented results from Westerbork Radio Synthe-sis Telescope (WRST) observations of several UCHVCs.AGC 249525 shows a smooth H I morphology at im-proved (60 (cid:48)(cid:48) , 105 (cid:48)(cid:48) ) angular resolution and ordered mo-tion consistent with ∼
15 km s − rotation. AGC 249525,and the aforementioned AGC 198606, are among thehighest column density objects in the UCHVC sample,with peak N HI ∼ × atoms cm − at 105 (cid:48)(cid:48) reso-lution. A16 reported a mean H I angular diameter athalf-flux level of 8.5 (cid:48) for AGC 249525 and an H I mass of2 . × M (cid:12) , assuming a distance of 1 Mpc. A16 showedthat for a distance range of 0.4 − ∼ (cid:48) × (cid:48) field of viewand a pixel scale of 0 . (cid:48)(cid:48)
11 per pixel. Nine 300s exposureswere obtained in g and i filters. The raw images weretransferred to the ODI Portal, Pipeline, and Archive(ODI-PPA) at Indiana University and processed usingthe QuickReduce pipeline (Kotulla 2014) to remove theinstrumental signature. We then illumination-corrected,scaled, and stacked the images. The FWHM of the point spread function in the stacked images is 0 . (cid:48)(cid:48)
78 in g and0 . (cid:48)(cid:48)
73 in i .Sources were identified with the IRAF task DAOFIND and the source lists in g and i were matched. We per-formed photometry and calculated final, calibrated g and i magnitudes for all point sources in the matchedlist. Photometric calibration coefficients were calculatedbased on SDSS DR13 (SDSS Collaboration et al. 2016)standard stars in the images and Galactic extinction cor-rections were calculated using the relations in Schlafly& Finkbeiner (2011). The 5 σ detection limit is 25.5 in g and 24.5 in i .The full details of our analysis methods are given inJanesh et al. (2015). Briefly, we constructed a CMDfrom the g and i photometry and then applied a color-magnitude filter derived from Girardi et al. (2004)isochrones to the data, sampling a set of distances be-tween 0.3 and 2.5 Mpc. The CMD filter is intended to se-lect red giant branch (RGB) stars and horizontal branch(HB) stars from old, metal-poor stellar populations. Wethen smoothed the spatial distribution of the filteredstars by binning the positions into a 2-dimensional gridwith ∼ (cid:48)(cid:48) × (cid:48)(cid:48) bin sizes, and convolving the grid with aGaussian distribution with a 3 (cid:48) smoothing radius. Over-densities are regions in the grid that exceed the meangrid value by some number of standard deviations ( σ ).To quantify the significance of the overdensities, we cre-ated 25000 samples with the same number of points aswere found in the CMD filter, but with a uniform ran-dom distribution. We measured the peak σ value foreach of these samples. The distribution is well-fittedby a lognormal probability distribution function. Toidentify significant overdensities, we calculated the per-centage of the cumulative distribution function (CDF)of peak σ values below that of the overdensity detectedin the real data. We consider an overdensity significantif it has a peak σ value higher than 95% or more of thevalues in the simulated CDF. A POSSIBLE OPTICAL COUNTERPART TOAGC 249525Based on the analysis described above, we found sig-nificant stellar overdensities in the AGC 249525 imagesat a range of distances between 1.35 Mpc and 2.08 Mpc.In Figure 1, we show the results of the CMD filtering andsmoothing process for the highest-significance detectionat 1.64 Mpc ( m − M = 26 . ± . ± ≥ Janesh et al. D e c ( a r c m i n ) sky positions ( g − i ) i m-M = 26.07 (1.64 Mpc) D e c ( a r c m i n ) ( g − i ) i in circle 1 0 1 2 3 4 ( g − i ) Figure 1.
Results of the filtering and smoothing process for AGC 249525 at a filter distance of 1.64 Mpc. Top left: positionsof point sources relative to the corner of the field. Sources that fall outside the CMD filter are marked with small black dots;sources within the CMD filter are color-coded according to their ( g − i ) values. A magenta 3 (cid:48) -radius circle is centered on thedetection peak; a yellow circle of the same size marks a random location used to construct a reference CMD. Top right: CMDfor all point sources in the field; the CMD filter is shown in blue, and the sources selected by the filter are shown in red. Bottomleft: the smoothed stellar density in units of standard deviations above or below the mean; the 3 (cid:48) -radius circle (magenta) iscentered on the highest-signal pixel. Bottom right: CMDs for the stars inside the 3 (cid:48) -radius magenta circle (left) and the starsin the yellow reference circle (right); the latter provides a sampling of the foreground and background contamination present inthe detection CMD. for the detection at 1.64 Mpc. At this distance, 98.4% ofpeak overdensities in the random realizations are weakerthan the peak overdensity in the data, indicating thatthe overdensity is unlikely to be a random clustering ofsources.Figure 3 shows the i image overlaid with the H I con-tours from the WSRT data presented in A16. The lo-cation of the stellar overdensity, marked by a magentacircle, is coincident with the highest H I contour level,providing further evidence that the stellar overdensityis associated with the UCHVC.Another check on the validity of the overdensity ofRGB stars in the center of the AGC 249525 field is shown Figure 2.
Results of the significance testing described inSection 2 for the overdensity shown in Figure 1. The σ valuefor the overdensity is greater than 98.4% of peak sigma valuesin 25,000 random realizations. etection of an Optical Counterpart to AGC 249525 D e c ( J ) Figure 3.
The WIYN pODI i -band image ( ∼ (cid:48) × (cid:48) ; North-up, East-left) of AGC 249525 with H I contours from A16overlaid at [9, 15, 20, 30, 40] × atoms cm − (solid black lines). The WSRT beam is shown in the bottom-left corner. Alsomarked are stars selected by the CMD filter (red circles) and a 3 (cid:48) -radius circle at the location of the overdensity from Figure 1(magenta). A 75 (cid:48)(cid:48) -radius circle (blue) shows the aperture used to estimate optical properties in Section 3.1. The center of thestellar overdensity identified by the CMD filtering process coincides with the highest column density H I contour. in Figure 4. We divided the image into regions 4 . (cid:48) × . (cid:48) g − i ) (cid:39)
1, consistent with the colors of RGBstars in our CMD filter. When visualized in this way,the overdensity of potential RGB stars in the center ofthe image is readily apparent. We note that a modestoverdensity appears in the northwest corner of the im-age (top right of Figure 4). The peak σ value for this overdensity is greater than the peak value in only 2% ofthe random realizations, so it is not significant.3.1. Estimated H I and Optical Properties We can use the 1.64 Mpc distance to calculate theH I properties of AGC 249525. Combining the valuespresented in A16 with our distance yields an H I mass of Janesh et al.
Figure 4.
CMDs for 4 . (cid:48) × . (cid:48) g − i ) (cid:39) . ± . × M (cid:12) , an H I radius of 1 . ± . . ± . × M (cid:12) .To calculate the observed properties of the opticalcounterpart, we masked all objects in the images thatare obvious background galaxies or foreground stars.We then measured the magnitude in the masked imageswithin a circular aperture of radius 75 (cid:48)(cid:48) , centered on thelocation of the highest-density peak in Figure 1. The75 (cid:48)(cid:48) radius was chosen to minimize contributions fromsky background fluctuations while maximizing the num-ber of stars that fell within the CMD filter. This processyielded apparent magnitudes of g = 19 . ± .
10 and i = 18 . ± .
07 and a ( g − i ) color of 1 . ± .
12. At Measured at the 1 × atoms cm − level. a distance of 1.64 Mpc, this corresponds to an absolutemagnitude of M V = − .
1. Because these values includeall unmasked light inside the 75 (cid:48)(cid:48) radius aperture, in-cluding fluctuations in the sky background, they shouldbe considered upper limits.We also compute lower limit magnitudes by summingthe flux from only those sources selected by the CMDfilter that are inside the 75 (cid:48)(cid:48) -radius circle around thestellar overdensity peak. In this case we find apparentmagnitudes of g = 22 . ± .
02 and i = 20 . ± . g − i ) color of 1 . ± .
02. At a distanceof 1.64 Mpc, the “minimum” absolute magnitude ofAGC 249525 is M V = − .
5. Although extremely faintcompared to most dwarf galaxies, these absolute magni-tudes are consistent with the range of values for ultra- etection of an Optical Counterpart to AGC 249525 Table 1.
Properties of AGC249525Property ValueRA (J2000) 14 h m . s ◦ (cid:48) . (cid:48)(cid:48) . ± .
45 Mpc cz
47 km s − H I mass 3 . ± . × M (cid:12) H I radius 1 . ± . +6 − km s − Dynamical mass 1 . ± . × M (cid:12) Lower Limit Upper Limit i . ± .
02 18 . ± . M V -4.5 -7.1( g − i ) . ± .
02 1 . ± . L ∗ . ± . × L (cid:12) . ± . × L (cid:12) Stellar mass 2 . ± . × M (cid:12) . ± . × M (cid:12) M HI /M ∗
144 9 faint dwarfs (UFDs) in the Local Group and its environs(McConnachie 2012).We can use the distance to compute an H I -to-stellarmass ratio for this object. We begin by estimating thestellar mass-to-light ratio using the relations from Bellet al. (2003), finding for the redder, brighter magnitudelimit a value of ( M/L ) i = 3 .
79, and for the bluer, faintermagnitude limit a value of (
M/L ) i = 2 .
87. Using thesemass-to-light ratios we calculate a stellar luminosity of L ∗ = 9 . ± . × L (cid:12) and a stellar mass of M ∗ =3 . ± . × M (cid:12) for the bright limit, and a luminosityof L ∗ = 7 . ± . × L (cid:12) and mass of M ∗ = 2 . ± . × M (cid:12) for the faint limit. Using an H I mass of 3 . × M (cid:12) , we find H I -to-stellar mass ratios of M HI /M ∗ ∼ M HI /M ∗ ∼
144 for the faintlimit. The H I and optical properties are listed in Table1. A16 discuss the potential close neighbors of AGC 249525:Bootes I, Bootes II, and UGC 9128. Bootes I and BootesII are both located within 60 kpc of the Sun, and so arenot in close spatial proximity to AGC 249525. At thedistance we have estimated here, the most likely neigh-bor is the galaxy UGC 9128 at 2.27 Mpc (Tully et al.2013). UGC 9128 has a cz = 152 km s − (McConnachie2012) and is less than 10 ◦ away from AGC 249525,which has a cz = 48 km s − . AGC 249525: A GAS-RICH ULTRA-FAINTDWARF GALAXY?The possible detection of an optical counterpart forthe UCHVC AGC 249525 has interesting implications.Along with AGC198606 (Janesh et al. 2015), it wouldrepresent one of the most extreme galaxies in or near theLocal Group, with its sparse stellar population and large M HI /M ∗ ratio. While galaxies like these are difficult todetect, models have predicted that they should exist.High-resolution hydrodynamic cosmological simulationsfrom O˜norbe et al. (2015) indicate that isolated, low-mass dark matter halos result in galaxies with stellarmasses between 10 − M (cid:12) and large M gas /M ∗ ratiosand stellar masses between 10 − M (cid:12) , consistent withthe range of possible values for AGC 249525. Addition-ally, the N-body and semi-analytic models of Bovill &Ricotti (2011) predict that isolated UFDs should existin significant numbers in the Local Volume, with opti-cal properties similar to known UFDs, though most ofthese predicted UFDs have lost their gas via strippingor stellar feedback effects.The putative optical counterpart for AGC 249525 hasan absolute magnitude that places it in the middle ofthe range of the faint dwarf galaxies in or near the Lo-cal Group. UFDs like Segue 1 ( M V = − . M V = − . M V = − .
7) are atthe faintest end of the range of known nearby dwarfs(McConnachie 2012). Koposov et al. (2015) recentlydiscovered nine new UFDs in the Southern Hemispherewith M V = − . − M V = − .
3; Rhode et al. 2013; McQuinn et al. 2015)and Leo T ( M V = − .
0; Irwin et al. 2007) are signif-icantly brighter, though they are actively forming, orhave recently formed, stars. The possible optical coun-terpart to AGC 198606, which is gas-rich but withoutactive star formation, is closer to the UFDs in its lu-minosity, with M V ∼ − .
5. AGC 249525, while moreluminous than AGC 198606, is still substantially fainterthan Leo T and Leo P.Previous work has concluded that some UFDs couldbe the stellar populations left over after their progeni-tor dwarf galaxies were stripped of gas and stars by theMilky Way (Willman et al. 2006; Martin et al. 2007).In Janesh et al. (2015) we speculated that, with the dis-covery of gas-rich galaxies with extremely sparse stellarpopulations like AGC 198606 and now AGC 249525, adifferent formation scenario for UFDs is possible. Withless massive progenitors, less stripping would be nec-essary to reach the low stellar masses and faint totalmagnitudes observed in UFDs.Further observations are needed to confirm the opticalcounterpart to AGC 249525. Only the upper portion of
Janesh et al. the RGB is accessible in the current WIYN pODI data,whereas a detection of HB stars would provide a moredefinitive distance determination. Our derived distanceis similar to that of Leo P. McQuinn et al. (2015) usedHubble Space Telescope (HST) observations to measurea distance to Leo P of 1 . ± .
15 Mpc, based on acombination of TRGB stars, HB stars, and RR Lyraevariables. At 1.6 Mpc, HB stars would have M V ∼ . I gas, although at these magnitudessuch observations would be a challenge.We thank the anonymous referee for helpful com-ments. We thank the WIYN, NOAO, and ODI-PPA staff for their help at various stages of this project.W.F.J. and K.L.R. are supported by NSF grant AST-1615483. S.J. acknowledges support from the Aus-tralian Research Council’s Discovery Project fund-ing scheme (DP150101734). EAKA is supported byTOP1EW.14.105, which is financed by the NetherlandsOrganisation forScientific Research (NWO). The AL-FALFA team at Cornell is supported by NSF grantsAST-0607007 and AST-1107390 to R.G. and M.P.H.and by grants from the Brinson Foundation. J.M.C. issupported by NSF grant 1211683. This research madeuse of the NASA/IPAC Extragalactic Database (NED)which is operated by the Jet Propulsion Laboratory,California Institute of Technology, under contract withthe National Aeronautics and Space Administration.REFERENCES Adams, E. A. K., Giovanelli, R., & Haynes, M. P. 2013,ApJ, 768, 77Adams, E. A. K. 2014, Ph.D. Thesis,Adams, E. A. K., Faerman, Y., Janesh, W. F., et al. 2015,A&A, 573, LL3Adams, E. A. K., Oosterloo, T. A., Cannon, J. M.,Giovanelli, R., & Haynes, M. P. 2016, A&A, 596, A117Beccari, G., Bellazzini, M., Battaglia, G., et al. 2016, A&A,591, A56Bell, E. F., McIntosh, D. H., Katz, N., & Weinberg, M. D.2003, ApJS, 149, 289Bellazzini, M., Magrini, L., Mucciarelli, A., et al. 2015,ApJL, 800, L15Bellazzini, M., Beccari, G., Battaglia, G., et al. 2015, A&A,575, A126Blitz, L., Spergel, D. N., Teuben, P. J., Hartmann, D., &Burton, W. B. 1999, ApJ, 514, 818Bovill, M. S., & Ricotti, M. 2011, ApJ, 741, 18Braun, R., & Burton, W. B. 1999, A&A, 341, 437Dooley, G. A., Peter, A. H. G., Yang, T., et al. 2016,arXiv:1610.00708Eisenstein, D. J., Weinberg, D. H., Agol, E., et al. 2011,AJ, 142, 72Giovanelli, R., Haynes, M. P., Kent, B. R., et al. 2005, AJ,130, 2598Giovanelli, R., Haynes, M. P., Kent, B. R., & Adams,E. A. K. 2010, ApJL, 708, L22Giovanelli, R., Haynes, M. P., Adams, E. A. K., et al. 2013,AJ, 146, 15 Girardi, L., Grebel, E. K., Odenkirchen, M., & Chiosi, C.2004, A&A, 422, 205Haynes, M. P., Giovanelli, R., Martin, A. M., et al. 2011,AJ, 142, 170Hirschauer, A. S., Salzer, J. J., Skillman, E. D., et al. 2016,ApJ, 822, 108Irwin, M. J., Belokurov, V., Evans, N. W., et al. 2007,ApJL, 656, L13Janesh, W., Rhode, K. L., Salzer, J. J., et al. 2015, ApJ,811, 35Jones, M. G., Papastergis, E., Haynes, M. P., & Giovanelli,R. 2016, MNRAS, 457, 4393Kauffmann, G., White, S. D. M., & Guiderdoni, B. 1993,MNRAS, 264, 201Klypin, A., Kravtsov, A. V., Valenzuela, O., & Prada, F.1999, ApJ, 522, 82Koposov, S. E., Belokurov, V., Torrealba, G., & Evans,N. W. 2015, ApJ, 805, 130Kotulla, R. 2014, Astronomical Society of the PacificConference Series, 485, 375Martin, N. F., Ibata, R. A., Chapman, S. C., Irwin, M., &Lewis, G. F. 2007, MNRAS, 380, 281McConnachie, A. W. 2012, AJ, 144, 4McGaugh, S. S. 2012, AJ, 143, 40McQuinn, K. B. W., Skillman, E. D., Dolphin, A., et al.2015, ApJ, 812, 158Moore, B., Ghigna, S., Governato, F., et al. 1999, ApJL,524, L19O˜norbe, J., Boylan-Kolchin, M., Bullock, J. S., et al. 2015,arXiv:1502.02036 etection of an Optical Counterpart to AGC 2495259