Galactoseismology: Discovery of a cluster of receding, variable halo stars

0.6 MB·

by  Sukanya Chakrabarti

DRAFTVERSIONJANUARY14,2016
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
GALACTOSEISMOLOGY:DISCOVERYOFACLUSTEROFRECEDING,VARIABLEHALOSTARS
SUKANYACHAKRABARTI1,RODOLFOANGELONI2,KENNETHFREEMAN3,BENJAMINSARGENT1,JOSHUAD.SIMON4,PIOTR
KONORSKI5,WOLFGANGGIEREN6,BRANIMIRSESAR7,ANDREWLIPNICKY1 ,LEOBLITZ8,GIBORBASRI8,MASSIMOMARENGO
9,WILLIAMVACCA10,PURAGRAGUHATHAKURTA11,ALICEQUILLEN12,PHILIPCHANG13
DraftversionJanuary14,2016
ABSTRACT
Adynamicalcharacterizationofdarkmatterdominateddwarfgalaxiesfromtheirobservedeffectsongalac-
6 tic disks (i.e. Galactoseismology) has remained an elusive goal. Here, we present preliminary results from
1 spectroscopicobservationsofthreeclusteredCepheidcandidatesidentifiedfromK-bandlightcurvestowards
0 Norma. Theaverageheliocentricradialvelocityofthesestarsis∼156km/s,whichislargeanddistinctfrom
2 that of the Galaxy’s stellar disk. These objects at l ∼333◦ and b∼−1◦ are therefore halo stars; using the
3.6µmperiod-luminosityrelationofTypeICepheids,theyareat∼73kpc. OurongoingI-bandphotometry
n
indicatesvariabilityonthesametimescaleastheperioddeterminedfromtheK-bandlightcurve. Distances
a s
J determinedfromtheK-bandperiod-luminosityrelationandthe3.6µmperiod-luminosityrelationarecompa-
rable. Theobservedradialvelocityofthesestarsagreesroughlywithpredictionsfromdynamicalmodels. If
3
these stars are indeed members of the predicted dwarf galaxy that perturbed the outer HI disk of the Milky
1
Way,thiswouldmarkthefirstapplicationofGalactoseismology.

] Subjectheadings:galaxies: dwarf–galaxies: individual(MilkyWay),stars: Cepheids
A
G
1. INTRODUCTION Cepheid candidates identified from the analysis of Ks-band
h. The puzzlingly large perturbations in the outer HI disk of light curves from the VVV survey. The average heliocentric
p ourGalaxy(Levine,Blitz&Heiles06)maybeduetoanin- radialvelocityofthesesourcesis∼156km/s,whichislarge
- teractionwithadwarfgalaxy(Chakrabarti&Blitz09;hence- anddistinctfromtheGalacticstellardisk(Yannyetal. 2003;
o Bondetal. 2010).
forthCB09).Underlyingthesearchforthisculpritisthehope
r
WeaddresstherecentclaimsbyPietrukowiczetal. (2015;
t thatonecouldinprincipleinfercharacteristicsoftheGalac-
s henceforthP15)thatthesesourcesarecoolspottedstars,and
a tic potential and sub-structure from a dynamical analysis of
the broader claims that classification of variables from near-
[ observedperturbationsinthegasorstellardiskoftheMilky
IR light curves is problematic, and that there is no evidence
Way,i.e,Galactoseismology.
1 for a dwarf galaxy in this part of the sky. Given that the pe-
We recently presented evidence of an excess of variables
v riod of one of these sources has been confirmed by OGLE,
with periods greater than 3 days, and in particular clustered
1 Cepheid candidates at l ∼333◦, b∼−1◦ (Chakrabarti et al. themainfalsepositivearespottedstarsthatcanmimictheKs-
8 bandlightcurvesofCepheids. However,spottedstarswould
2015; henceforth C15) from the analysis of K-band light
3 s be part of the Galactic stellar disk, while the observed spec-
curves from the VVV survey (Saito et al. 2012). Using the
3 tra indicate that these objects are moving with a large (and
K-bandperiod-luminosityrelationofTypeICepheids(Mat-
0 s similar)radialvelocity,whichismuchlargerthanthatoftyp-
sunagaetal. 2013), andtheCardellietal. (1998)extinction
.
1 law, we derived an average distance of ∼ 90 kpc from the icaldiskstars. Moreover,thedistanceofthesesourcesasde-
0 Galactic Center, which is close to the current predicted dis- rivedfromtheperiod-luminosityrelationintheKs-bandandat
6 3.6µmarecomparable. OurownongoingI-bandphotometry
tanceoftheperturbingdwarfgalaxy.
1 indicates variability on roughly the same scale as the period
Withoutbothlightcurvesandspectroscopicobservations,it
: derived from the K-band light curves. The I-band Fourier
v isdifficulttounderstandthenatureofvariablestarsandtheir s
parametersofoneofthesesources(P15;Figure5inthatpa-
i dynamical relation to our Galaxy. Here, we present prelimi-
X per), while not lying in the main part of the distribution of
naryresultsfromH-bandspectroscopicobservationsofthree
Fourierparameters,showssomeoverlapwithSMCCepheids,
r
a aswouldbeexpectedforCepheidsinalow-metallicityenvi-
1School of Physics and Astronomy, Rochester Institute of ronment. Thus, the possibility that this source is a classical
Technology, 84 Lomb Memorial Drive, Rochester, NY 14623; Cepheidshouldnotbedismissed. Finally,the5−σ excessof
[email protected]
2GeminiObservatory,c/oAURA,Casilla603LaSerena,Chile variables in this part of the sky (C15) is difficult to explain
3ResearchSchoolofAstronomy&AstrophysicsANURSAAMount withoutinvokingagravitationallyboundstructure.
StromloObservatory The classification of variable stars using near-IR light
4CarnegieObservatories curves (Matsunaga et al. 2011; 2013; Angeloni et al. 2014)
5WarsawUniversityObservatory isindeedarelativelynewendeavor. Whilethetraditionalap-
6UniversidaddeConcepcion proach has been to classify variable stars in the optical or I-
7MPIA
band(Soszynskietal. 2011)andobtainafewepochsofnear-
8UCBerkeley,AstronomyDepartment
9IowaState IRphotometrytolowertheuncertaintyonthedistance(Feast
10NASAAmes etal. 2014),thechallengeofclassifyingsourcesthatarefaint
11UCSC,DepartmentofAstronomy in the optical bands motivates the development of templates
12UniversityofRochester,DepartmentofPhysicsandAstronomy inthenear-infrared(Angelonietal. 2014; Innoetal. 2015),
13UniversityofWisconsin-Milwaukee
2
As in C15, we derive the periods of the sources presented
inTable1usingtheLomb-Scarglealgorithmandretainonly
sources that have a significance level greater than 90-th per-
centile(Kleinetal.2014),andperiodsgreaterthanthreedays
and K >15 mag. We then perform a parametric bootstrap
s
assuming a Gaussian distribution of errors and at this stage
onlyretainsourceswherethemeanoftheperioddistribution
minusthestandarddeviationexceedsthreedays. Finally,we
compute the Fourier parameters of the sources and visually
inspectthelightcurves. ThefindingchartsandK-bandlight
s
curvesofthethreeCepheidcandidatesthatweobtainedspec-
troscopic observations of are shown in Figure 1. Our selec-
tionofCepheidcandidatesdiffersfromC15inonesignificant
way. InadditiontothecriteriausedbyC15,herewerequire
thatatleast90%oftheK-bandVVVphotometricobserva-
s
tionsareclassifiedas"stellar",i.e. haveflag=-1(Saitoetal.
2012); spuriousvariablesmaybeidentifiedifthisflagisnot
required.Thismaybethereasonwhytwoofthesourceslisted
in C15 (theP = 3.42d and the P= 4.19d) were misidentified
asvariables. Onlyoneofthesourcesinthispaperwasearlier
presentedinC15(theP=5.69dsource).
Our own ongoing I-band photometry confirms the vari-
ablityofthethreesourcesinthispaperonthesametemporal
scaleastheperioddeterminedfromtheK-bandlightcurves.
s
Thesourceswepresentherehavespectroscopicobservations,
aswellas I-bandphotometry(on average∼ 10epochs), K-
s
bandlightcurves,and3.6µmfluxes.ThereareotherCepheid
candidatesinthispartoftheskythatwewereunabletoobtain
spectroscopic observations of; the stars discussed in this pa-
FIG.1.—H-bandimageswithNorthupandEasttotheleft,withthehori- perdonotrepresentacompletesampleofCepheidcandidates
zontallinemarking30"oneachplot,andK-bandlightcurvesoftheCepheid inthispartofthesky.
candidatesofthe(top)P=5.69dsource,(middle)P=4.923dsource,and
Table1liststheRAandDECofthesourcesinJ2000, the
(bottom)P=4.9dsource
pulsationperiod,aswellasthedistancederivedfromtheK-
s
band period-luminosity relation (M13), the 3.6 µm period-
luminosityrelation(Scowcroftetal. 2011),themeanI-band
as well as other innovative approaches of multi-color classi- magnitude (and current number of epochs from our ongoing
ficationwithrelativelyfewepochs(Sesaretal. 2012). Light photometryontheSwopetelescopethatwillbepresentedin
curvesofCepheidsinthenear-IRarenearlysinusoidal(Mat- a forthcoming paper), the mean K-band magnitude, and the
s
sunaga et al. 2012), while light curves in the I-band tend to 3.6µmflux.The3.6µmphotometryfortheP=5.69dandP=
show more features (Soszynski et al. 2011), allowing easier 4.9dsourcesarefromtheDEEPGLIMPSEsurvey(Church-
classification. But even with I-band light curves, the clas- welletal. 2009;Benjaminetal. 2003),andthe3.6µmpho-
sification is not always accurate. Only five of the thirty-two tometryfortheP=4.923dsourceisfromWillisetal. (2015).
CepheidcandidatesidentifiedbySoszynskietal.(2011)were Table 1 also lists the extinction at K and 3.6 µm, and the
s
spectroscopicallyconfirmedbyFeastetal. (2014)(whilethe MJD(andphaseattimeoftheF2observation)aswellasour
others are not Type I Cepheids or else their identification as derivedvelocity,intheheliocentricframe.
classicalCepheidsisuncertain). Giventherelativepaucityof
data in the near-IR, it is reasonable to proceed initially with 2.2. Distances&FourierParameters
light curves (with a moderate number of epochs) and subse- We adopt the period-luminosity relations of classical
quentlyobtainspectroscopicdata,especiallyforsourcesclose Cepheids in the LMC (Matsunaga et al. 2011), with a LMC
totheGalacticplane. distance modulus of 18.5 mag (Freedman et al. 2001) and
Studying Galactic structure and sub-structure close to the interstellar extinction value of A = 0.02 mag for the LMC
planeisalsoanewfield.YoungCepheidvariables(orametal- direction(Caldwelletal. 1985). TKshisgivesthedistancemod-
rich component) are unexpected in models of the Galactic ulusµforaCepheidwithpulsationperiodP:
halo, especially in the outer halo. However, gas accretion
or infall of gas-rich dwarf galaxies may well produce an µ=K −A +3.284log(P)+2.383, (1)
yet undiscovered population of Type I Cepheids in the outer s Ks
Galaxy. Bullock & Johnston’s (2005) cosmologically moti- where AKs is the extinction in the Ks band, which is given
vated models of the stellar halo suggest that while most of in Table 1. We adopt the "no cut" 3.6 µm period-luminosity
thesatelliteaccretionoccurredearlyintheMilkyWay’spast, relationfromScowcroftetal. (2011)toderivedistancesfrom
theremayhavebeensomerecentaccretionevents. the3.6µmphotometry,whichgivesthedistancemodulus:
µ=m −A +3.15(logP−1)+5.83, (2)
2. CEPHEIDCANDIDATES 3.6 3.6
wherem istheobserved3.6µmfluxandA µmistheex-
2.1. IdentificationofCepheidCandidates 3.6 3.6
tinctionat3.6µm.Weusethe90thpercentileRGBextinction
3
TABLE1
DATAFORINDIVIDUALCEPHEIDCANDIDATESANDDERIVEDPARAMETERS
RA DEC P(day) DK−band(kpc) D3.6µm(kpc) <I> <Ks> 3.6µm AKs A3.6µm MJD(phase) vhelio(km/s)
245.330825 -52.042578 5.69 65±11 67±6 19.0±0.2(25) 15.1±0.02 14.55±0.07 0.76 0.48 57293.8(0.1) 162±31
244.968946 -50.176306 4.923 — 73±20 19.0±0.2(7) 15.6±0.039 15.53±0.11 1.71 1.08 57278(0.6) 151±39
243.140542 -54.022122 4.9 78±7 78±6 18.9±0.2(8) 15.3±0.02 14.89±0.08 0.45 0.28 57295(0.85) 154±113
<I>and<K>refertothemeanI-bandandK bandmagnitudesrespectivelyfromourongoingobservationsontheSwopetelescopeandthe
s s
VVVsurvey(Saitoetal.2012),respectively.3.6µmfluxesarefromtheDEEPGLIMPSEsurvey(Churchwelletal.2009;Benjaminetal.
2003)andfromWillisetal.2015.MJDisthemodifiedJuliandateatthetimeoftheF2spectroscopicobservationsandthepulsationphaseis
listedinparenthesis.HeliocentricvelocitiesarederivedfromH-bandF2spectrapresentedinthispaper.
relation for Type II Cepheids (Matsunaga et al. 2013), they
wouldlieat∼30kpcfromtheGalacticcenter.
Figure2depictstheFourierparameters(asdefinedinEqua-
tion3ofC15)oftheCepheidcandidatesover-plottedwiththe
Fourier parameters of classical Cepheids in the Milky Way
andtheLMCandSMCfromacompilationbyBhardwajetal.
(2015),aswellasTypeIandTypeIICepheids,andeclipsing
binariesfromMatsunagaetal. (2013). TheFourierparame-
ters of Cepheids in the near-IR cover a wider range than the
FourierparametersintheI-band,andthereisoverallagreee-
mentoftheFourierparametersoftheCepheidcandidateswith
known sources in the near-IR, which have nearly sinusoidal
shapes.OurongoingI-bandobservationsindicatethattheP=
5.69dandP=4.9dsourcesshowvariabilityonthesamescale
astheperioddeterminedfromK-bandlightcurves,butatthe
s
momentwecannotcomputeFourierparametersintheI-band
duetothesmallnumberofepochsandlargephotometricer-
rors. We also cannot yet confirm the level of variability for
theP=4.923dsourceintheI-band.
3. FOLLOW-UPSPECTROSCOPICOBSERVATIONS
Weperformednear-infraredspectroscopicfollow-upofthe
threeCepheidcandidateswithFlamingos-2(F2)(Eikenberry
FIG.2.—FourierparametersoftheCepheidcandidatesfromKs-bandlight
curves,overplottedwithFourierparametersofTypeICepheidsintheMilky et al. 2004) on Gemini-South under the DDT Program GS-
WayandLMCfromBhardwajetal.(2015),andTypeI,TypeII,andeclipsing 2015B-DD2. F2wasusedinthelong-slitmodewiththeR3K
binaries(EB)fromMatsunagaetal.(2012) grismandtheHfilter,thuscoveringawavelengthrangefrom
1.5 to 1.75 µm. We used the 2-pix (0.36 arcsec) slit for an
averagespectralresolutionR∼2900. Theeffectiveon-source
map from Nidever et al. (2012) to determine A whenever totalexposuretimewas4800secpertarget. Theseeingwas
Ks
available(fortheP=4.923dandP=4.9dsources),otherwise betterthan0.6arcsecfortheP=4.9dandP=4.923dsource,
we use the Bonifacio et al. (2000) correction to Schlegel et andbetterthan0.85arcsecfortheP=5.69dsource. Wealso
al. (1989) in conjunction with Cardelli et al. (1989) to get observedtwobrightB9VtoA0Vstars,selectedcloseintime
A (fortheP=5.69dsource). WethenusetheFlahertyetal. andairmasstoourtargetstodealwiththetelluriclines. The
Ks
(2007) extinction law towards Serpens to get A3.6µm from data reduction has been performed with the GEMINI IRAF
A forallthesources. Thesourcesofuncertaintyinthedis- Package (v1.13) following the standard procedures, and the
K,s
tance are due to the photometric error, the uncertainty in the telluric correction was done following Vacca et al. (2003).
extinction(takentobethestandarddeviationoftheextinction TheP=5.69dsourcewasalsoobservedontheFIREspectro-
values from Schlegel et al. 1998, Nidever et al. 2012 and graphonBaade/Magellan(Simcoeetal. 2013)withaneffec-
Schlaflyetal. 2011). Forsingle-epochmeasurementsofthe tiveon-sourceexposuretimeof1200sec,andatelluricstan-
3.6 µm flux, the (small) variability (Scowcroft et al. 2011) dardwasalsoobserved. Inaddition,weobtainedaspectrum
relative to the observed flux also contributes an uncertainty of the Gcc-a Cepheid on FIRE that was earlier observed by
thatiscomparabletothephotometricerror. Wecalculatethe Matsunagaetal. (2015). TheFIREGUIwasusedtoperform
uncertaintyonthedistancebyaddinginquadraturetheuncer- the data reduction. The P = 4.9d source was also observed
tainties due to these terms. The distance error derived from on WiFeS with an effective exposure time of 4000 sec. The
theK-bandfortheP=4.923dsourceisformallycomparable seeing was about 1.5 arcsec and the data were taken in the
s
tothedistanceitself. Thissourceistheonethatisclosestto red channel using the I7000 grating which gives a nominal
theplane,andevenintheK-band,theextinctionvaluesvary resolvingpowerof7000.
s
significantly. For this source, we only cite a distance from TheH-bandspectraofCepheids(andofotherFstars)show
the3.6µmrelation. Giventhatthe3.6µmfluxhastheleast strongerBrackettserieslinesandweakermetallines(Meyer
extinctionandtherebyprovidesthebestdeterminationofthe etal.1998;Rayneretal.2009)relativetocoolerstars.Inpar-
distance, this correspondence is reassuring. If instead these ticular, the spectra of M dwarfs are distinct from the spectra
sources were Type II Cepheids, using the period-luminosity ofCepheids, whilethespectraofcoolgiantshavesomecor-
4
Figure3(a)showstheH-bandspectraoftheP=5.69dsource
comparedwiththeGcc-aCepheid,withthehigherS/N lines
marked;inboththeF2andFIREspectrathe14-4(1.588µm)
and12-4(1.64µm)transitionsarevisible.ThehigherS/NF2
spectrumalsoshowsthe16-4(1.556µm)and15-4(1.57µm)
transitions. Figure3(b)showsthespectraoftheP=4.923d
and P = 4.9d source. The clearest lines for the P = 4.923d
sourcearethe14-4and10-4(1.734µm)transitions,whilefor
theP=4.9dsourcethe10-4transitionappearstobevisible.
DuetotherelativelylowS/N ofthesespectraandstrongtel-
luric lines that overlap with the Brackett lines (especially in
the longer halfof the H-band), not allthe Brackett lines can
beidentified. TodeterminetheeffectoftheS/N ontheradial
velocity, we added noise and degraded the spectrum of the
Gcc-aCepheidandfoundthatevenatS/N∼few,theinferred
velocityremainedunchanged.
3.1. RadialVelocities
We use a χ2 fitting routine in conjunction with cross-
correlation to determine the best-fit velocity and the uncer-
tainty in the velocity. Our procedure for determining radial
velocities is similar to Matsunaga et al. (2015; henceforth
M15) who obtained H-band spectroscopic observations of
classical Cepheids towards the Galactic center. They have
addressedtwoofthesignificantchallengesininterpretingH-
band spectra, namely the presence of telluric features in this
part of the spectrum, and the relative lack of near-IR radial
velocitytemplates. Toaddressthefirstissue,theyincorporate
the telluric features in the synthetic template spectra devel-
opedbyKurucz(1993)(bymultiplyingthetelluricspectrum
with the template). The model spectrum is shifted by a trial
redshift and compared with the target spectrum. Finally, the
targetspectrumisassignedtheredshiftthatminimizestheχ2
betweentheshiftedmodelspectrumandthetargetspectrum.
Cepheidpulsationscanaffecttheradialvelocitiesbytensof
km/s(Pejcha&Kochanek2012),andwecorrectforthepul-
sationusingradialvelocitytemplatesatthephasedetermined
from the K-band light curves. We adopt the radial velocity
templatesusedbyM15,namelytheinfraredphotometryand
radialvelocitiescompiledbyGroenewegen(2013). ForDelta
Cep,whichhasaperiodsimilartothestarsweconsiderhere,
the typical pulsation amplitude is ∼ 20 km/s (Barnes et al.
1987).
Forthesyntheticspectra,weusethefollowingparameters:
T =5000 K,log g=1.0 and Z =Z . To account for the
eff (cid:12)
telluric lines, the synthetic templates incorporate the telluric
linesfollowingearlierworkinM15. Wealsocomparetoob-
served templates from the Infrared Template Factory (IRTF;
Rayneretal. 2009),aswellasourownobservationofGcc-a.
We consider the errors in the spectrum and use a χ2 fitting
FIG.3.—(a)H-bandspectraoftheP=5.69dsourceonF2andonFIRE,
andtheGcc-a(theCepheidfromM15)spectrumonFIRE,(b)H-bandspectra routineinconjunctionwithcross-correlationtoderivetheun-
oftheP=4.923dandtheP=4.9dsourceonF2andtheGcc-aspectrumon certainties on the radial velocity (following earlier work in
FIRE,(c)WiFeSspectrumoftheP=4.9dsourceclosetothewavelength
Bhaleraoetal. 2012andtheimplementationintheIDLrou-
region of the expected Calcium triplet lines. The spectra are shown here
median-filtered. tineGETVEL.pro).
The heliocentric velocity that we determine for the three
starsis162±31km/s,151±39km/s,and154± 113km/s,
respondencewiththespectraofhotsupergiantstars.TheS/N after correcting for phase using known templates for the ra-
ofthespectraobtainedonGemini’sF2ofthesethreestarsare dial velocity of Cepheids (Groenewegen 2013; Barnes et al.
∼20,10,3respectively.Thus,whilethespectraarenotofsuf- 1987), as well as comparison with our own observation of
ficientS/Ntoidentifythespectraltypeofthestar,theyshould Gcc-a on FIRE. An alternate approach is to mask regions
be sufficient to measure the radial velocity. For the Gcc-a of high sky contamination and perform a cross-correlation,
Cepheid from Matsunaga et al. (2015), we obtained a simi- which yields values that are roughly comparable. The F2
larradialvelocityastheirlistedvaluefromcross-correlation. spectrumoftheP=4.9dsourceisquitelowS/N.TheWiFeS
5
observationoftheP=4.9dsourceproduced∼10countsper distinctfromtherotationofthestellardiskofourGalaxy,for
pixel,andthereappeartobeCalciumtripletlinesthatwould which |v |∼ few km/s (Bond et al. 2010). In this part of
r
correspondtothelinesat8498Åand8542Å(redshiftedby the sky, the velocity from the HI map is negative (Kalberla
∼200km/s)asshowninFigure3(c);however,theredshifted et al. 2005), and from models of the stellar disk (e.g. Be-
sanconetal. 2003)is∼−40km/s. Thus, weconcludethat,
line that would correspond to the 8662 Å line is not visible
whiletheidentificationofthesestarsasTypeICepheidsmay
(thelatterisinaforestofemissionlines). DuetothelowS/N
notbecertain,thesearehalostars,andnotpartoftheGalac-
inboththeF2andWiFeSobservationsfortheP=4.9dsource,
tic disk. Interestingly, the average radial velocity is roughly
ourradialvelocitymeasurementforthissourceisparticularly
consistentwiththepredictionsfromearlierdynamicalmodels
uncertain.
(CB09). Thisinteractionalsoproducesstructuresinthestel-
4. DISCUSSION&CONCLUSION lar disk reminiscent of the Monoceros Ring (Newberg et al.
2002),raisingthequestionifthestructuresinthegasdiskand
In this initial study, we were only able to procure spectro-
theMonocerosRingareconnected.
scopic observations of three sources. The excess of P>3d
variables in this part of the sky found by C15 is still present
whenweusethecriteriastatedinthispaper,andfuturework
should attempt to obtain radial velocities of other Cepheid SC is indebted to N. Matsunaga for many helpful discus-
candidates. Inaforthcomingpaper,wediscusstheexcessin sionsoninterpretingthespectra. WethankM.Richmond,N.
the K, J−K CMD when we extinction correct and subtract Morrell, and B. Madore for help with the I-band photome-
s s
outabackgroundfield, whichisconsistent withthelocation try, and D. Newman and R. Simcoe for help with the FIRE
ofred-clumpstarsat∼80kpc. pipeline. We also thank J. Kartaltelpe, V. Scowcroft, and L.
The average heliocentric radial velocity of the sources in Inno for helpful discussions. SC is supported by NSF grant
this paper is ∼ 156 km/s. This radial velocity is large and 1517488,BSbyNASA-ADAPgrantNNX15AF15G.
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