J/ApJ/866/22 Spectroscopic analysis of Tuc III stream (Li+, 2018) ================================================================================ The first tidally disrupted ultra-faint dwarf galaxy: a spectroscopic analysis of the Tucana III stream. Li T.S., Simon J.D., Kuehn K., Pace A.B., Erkal D., Bechtol K., Yanny B., Drlica-Wagner A., Marshall J.L., Lidman C., Balbinot E., Carollo D., Jenkins S., Martinez-Vazquez C.E., Shipp N., Stringer K.M., Vivas A.K., Walker A.R., Wechsler R.H., Abdalla F.B., Allam S., Annis J., Avila S., Bertin E., Brooks D., Buckley-Geer E., Burke D.L., Carnero Rosell A., Carrasco Kind M., Carretero J., Cunha C.E., D'Andrea C.B., da Costa L.N., Davis C., De Vicente J., Doel P., Eifler T.F., Evrard A.E., Flaugher B., Frieman J., Garcia-Bellido J., Gaztanaga E., Gerdes D.W., Gruen D., Gruendl R.A., Gschwend J., Gutierrez G., Hartley W.G., Hollowood D.L., Honscheid K., James D.J., Krause E., Maia M.A.G., March M., Menanteau F., Miquel R., Plazas A.A., Sanchez E., Santiago B., Scarpine V., Schindler R., Schubnell M., Sevilla-Noarbe I., Smith M., Smith R.C., Soares-Santos M., Sobreira F., Suchyta E., Swanson M.E.C., Tarle G., Tucker D.L. =2018ApJ...866...22L ================================================================================ ADC_Keywords: Galaxies, spectra; Radial velocities; Equivalent widths; Abundances, [Fe/H]; Photometry; Optical Keywords: dark matter ; galaxies: dwarf ; galaxies: individual (Tucana III) ; Local Group ; stars: abundances Abstract: We present a spectroscopic study of the tidal tails and core of the Milky Way satellite Tucana III, collectively referred to as the Tucana III stream, using the 2dF+AAOmega spectrograph on the Anglo-Australian Telescope and the IMACS spectrograph on the Magellan Baade Telescope. In addition to recovering the brightest nine previously known member stars in the Tucana III core, we identify 22 members in the tidal tails. We observe strong evidence for a velocity gradient of 8.0+/-0.4km/s/deg over at least 3{deg} on the sky. Based on the continuity in velocity, we confirm that the Tucana III tails are real tidal extensions of Tucana III. The large velocity gradient of the stream implies that Tucana III is likely on a radial orbit. We successfully obtain metallicities for four members in the core and 12 members in the tails. We find that members close to the ends of the stream tend to be more metal-poor than members in the core, indicating a possible metallicity gradient between the center of the progenitor halo and its edge. The spread in metallicity suggests that the progenitor of the Tucana III stream is likely a dwarf galaxy rather than a star cluster. Furthermore, we find that with the precise photometry of the Dark Energy Survey data, there is a discernible color offset between metal-rich disk stars and metal-poor stream members. This metallicity-dependent color offers a more efficient method to recognize metal-poor targets and will increase the selection efficiency of stream members for future spectroscopic follow-up programs on stellar streams. Description: We observed candidate member stars in the Tuc III stream with the 2dF+AAOmega spectrograph on the 3.9m Anglo-Australian Telescope (AAT). This paper focuses on the spectra obtained with the red arm using the 1700D grating, which has a spectral resolution of R=10000 and a wavelength coverage of 8400-8810{AA}. Observations were conducted on 2016 June 30, July 9-13, and July 25-27 and 2017 August 22. In order to better probe the transition region (0.3{deg}<~r<~0.5{deg}) between the (presumably) bound core of Tuc III and the tidal tails, we also obtained additional spectroscopy of Tuc III with the IMACS spectrograph on the Magellan Baade telescope. We observed three slit masks on the nights of 2017 June 19 and 21. The spectrograph configuration provided a spectral resolution of R=11000 over the wavelength range ~7500-8800{AA}. File Summary: -------------------------------------------------------------------------------- FileName Lrecl Records Explanations -------------------------------------------------------------------------------- ReadMe 80 . This file table1.dat 124 609 Velocity and metallicity measurements of the observed stars table3.dat 78 51 Potential members in the Tuc III stream -------------------------------------------------------------------------------- See also: I/337 : Gaia DR1 (Gaia Collaboration, 2016) J/AJ/135/2177 : High proper motion stars in the DSS. IV. (Lepine, 2008) J/MNRAS/397/1748 : Spectroscopic study of Segue 2 (Belokurov+, 2009) J/ApJ/704/898 : Photometry of the Hercules dwarf galaxy (Sand+, 2009) J/ApJ/694/L144 : Spectroscopic study of Leo V dSph (Walker+, 2009) J/ApJ/710/1664 : BV photometry in Leo IV and Leo V dSphs (de Jong+, 2010) J/ApJ/756/121 : Variable stars in Her dwarf galaxy (Musella+, 2012) J/ApJ/756/79 : Opt. photom. in 3 Local Group dwarf galaxies (Sand+, 2012) J/MNRAS/434/1681 : Extremely metal-poor stars CaII triplet (Carrera+, 2013) J/ApJ/770/16 : Spectroscopic members of Segue 2 galaxy (Kirby+, 2013) J/AJ/145/102 : Sp. of bright M dwarfs in northern sky (Lepine+, 2013) J/A+A/570/A61 : PM-selected stars of Hercules galaxy (Fabrizio+, 2014) J/ApJ/808/95 : Spectroscopy of Reticulum II (Simon+, 2015) J/ApJ/819/1 : Wide-field photometry of the GC Palomar 5 (Ibata+, 2016) J/ApJ/838/44 : Abundances of the Tuc III brightest members (Hansen+, 2017) J/ApJ/842/120 : Large sp. survey of Palomar 5 stellar stream (Ibata+, 2017) J/ApJ/838/11 : Member stars in the MW satellite Tucana III (Simon+, 2017) J/ApJ/857/145 : MagLiteS Carina II and Carina III spectroscopy (Li+, 2018) Byte-by-byte Description of file: table1.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 1 I1 --- Set [0/3]? Subset (1) 3- 5 A3 --- --- [DES] 6- 24 A19 --- DES Identifier, Jhhmmss.ss-ddmmss.s (G1) 26- 34 F9.5 deg RAdeg Right Ascension (J2000) 36- 44 F9.5 deg DEdeg [-60.4/-58.8] Declination (J2000) 46- 51 F6.3 mag gmag [15.9/21.1] Dereddened g band PSF magnitude (G2) 53- 58 F6.3 mag rmag [15/20.5] Dereddened r band PSF magnitude (G2) 60- 62 A3 --- Cat Either DES DR1 or Y2Q catalog (G2) 64- 68 A5 --- Inst Telescope/instrument (G3) 70- 74 F5.1 --- SNR [4.1/237.3] Average spectra signal-to-noise ratio 76- 82 F7.2 km/s HRV [-326/430] Heliocentric Radial Velocity 84- 87 F4.2 km/s e_HRV [0.5/6.5] Uncertainty in HRV 89- 92 F4.2 0.1nm EW [1.2/8.4]? Equivalent Width, Angstroms 94- 97 F4.2 0.1nm e_EW [0.2/1.6]? Uncertainty in EW 99-103 F5.2 [-] [Fe/H] [-3/-2.2]? Metallicity 105-108 F4.2 [-] e_[Fe/H] [0.1/0.6]? Uncertainty in [Fe/H] 110-110 I1 --- Mm [0/1]? Subjective membership (1:member; 0:non-member) (2) 112-115 F4.2 % Prob ? Membership probability, objective mixture model (3) 117-124 A8 --- Comm Comment -------------------------------------------------------------------------------- Note (1): Subset of stars in catalog as follows: 1 = Tucana III Leading/West Tail with -140 < HRV < -70km/s; 2 = Tucana III Core with -140 < HRV < -70km/s. Also observed by Simon+ (2017, J/ApJ/838/11); 3 = Tucana III Trailing/East Tail with -140 < HRV < -70km/s; 0 = Other observed stars; HRV < -140 or HRV > -70km/s. Note (2): We identified a total of 31 members in the AAT and IMACS combined data set, nine of which are members of the Tuc III core and were previously confirmed in Simon+ (2017, J/ApJ/838/11). The other 22 members are in the Tuc III tails and are identified for the first time in this work. We note that the membership identification of Tuc III stream members is a subjective selection process using the following parameters: velocity, metallicity, color, magnitude, and the spectrum itself. See section 3.1 for further details. Note (3): Only stars observed with AAT has membership probability. -------------------------------------------------------------------------------- Byte-by-byte Description of file: table3.dat -------------------------------------------------------------------------------- Bytes Format Units Label Explanations -------------------------------------------------------------------------------- 1- 3 A3 --- --- [DES] 4- 22 A19 --- DES Identifier, Jhhmmss.ss-ddmmss.s (G1) 24- 32 F9.5 deg RAdeg Right Ascension, decimal degrees (J2000) 34- 42 F9.5 deg DEdeg [-60.3/-59] Declination, decimal degrees (J2000) 44- 49 F6.3 mag gmag [18.7/21.6] Dereddened g band PSF magnitude (G2) 51- 56 F6.3 mag rmag [18.3/21.3] Dereddened r band PSF magnitude (G2) 58- 60 A3 --- Cat Either DR1 or Y2Q catalog (G2) 62- 66 A5 --- Inst Telescope/instrument (G3) 68- 70 F3.1 --- SNR [1.6/6.7] Spectra Signal-to-noise ratio at 72- 78 F7.2 km/s HRV [-137.1/-71] Heliocentric Radial Velocity (1) -------------------------------------------------------------------------------- Note (1): Stars that are observed with AAT or IMACS and have measured RVs in the range of -140km/s < HRV < -70km/s, but the S/N of the spectra are too low to obtain robust velocity measurements, and therefore these stars are not included in the analysis. RVs should be used with caution. See details in Appendix C. -------------------------------------------------------------------------------- Global notes: Note (G1): The star IDs are computed based on the Right Ascension and Declination from either DES DR1 (Abbott+, 2018ApJS..239...18A) or Y2Q (Drlica-Wagner+, 2015ApJ...813..109D) catalog in the format of Jhhmmss.ss-ddmmss.s. Due to the small difference in the astrometry between the two catalogs, the star ID in DR1 catalog could be slightly different (in the last decimal point) from the Y2Q catalog; the latter one was used to produce the star IDs in the previous literatures for Tuc III (e.g., Simon+ 2017, J/ApJ/838/11 and Hansen+ 2017, J/ApJ/838/44). Note (G2): Quoted magnitudes represent the dereddened PSF magnitude in either DES DR1 (Abbott+, 2018ApJS..239...18A) or Y2Q catalog (Drlica-Wagner+, 2015ApJ...813..109D) as given by the 'Cat' column. Note (G3): Telescopes and instruments used for obtaining the spectra of the targets as follows: AAT = AAT/2df+AAOmega; IMACS = Magellan/IMACS. -------------------------------------------------------------------------------- History: From electronic version of the journal ================================================================================ (End) Prepared by [AAS], Emmanuelle Perret [CDS] 27-Sep-2019