Detailed chemical composition and orbit of the new globular cluster FSR1758: Implications for the accretion of the Sequoia dwarf galaxy onto the Milky Way

TL;DR

This study provides detailed chemical, kinematic, and orbital data for the globular cluster FSR 1758, supporting its classification as a Milky Way halo cluster with a retrograde, eccentric orbit, and discussing implications for the accretion of the Sequoia dwarf galaxy.

Contribution

It offers the first high-resolution spectroscopic analysis of FSR 1758, confirming its metallicity, chemical abundance patterns, and orbital parameters, and discusses its origin and relation to galaxy accretion events.

Findings

Metallicity of [Fe/H] = -1.58 with small spread.

Cluster exhibits Na-O anti-correlations typical of Milky Way halo globulars.

Retrograde, highly eccentric orbit confirmed.

Abstract

We present detailed chemical abundances, radial velocities and orbital parameters for FSR 1758, a recently discovered star cluster in the direction of the Galactic Bulge. High resolution (R~42,000) spectra were obtained using the Magellan/Clay telescope instrumented with MIKE echelle spectrogragh, wavelength range 4900-8700 \AA. Cluster membership was determined using Gaia DR2 proper motions and confirmed with our radial velocity measurements. We find metallicity consistent with previous photometric estimates for this cluster, [Fe/H] = -1.58+-0.03 dex, with a small, 0.08 dex, spread. While other studies have suggested this massive object may be the result of a previous accretion event, our results are consistent with Milky Way Halo globular clusters with characteristic Na-O anti-correlations found for the metal-poor cluster members. The mean radial velocity of the cluster, +226.8+-1.6 km/s with a small velocity dispersion, 4.9+-1.2 km/s, is typical for globular clusters. We also confirm a retrograde Galactic orbit that appears to be highly eccentric.

Tables

Table 1. Table 1: ID, coordinates, proper motions and magnitudes of the observed stars. See text for details.

ID	RA(2000.0)	DEC(2000.0)	pmRA	pmDEC	G	BP	RP	J	H	K
	[hours]	[deg]	[mas/yr]	[mas/yr]	mag	mag	mag	mag	mag	mag
01	17:31:08.7	-39:49:42.2	-2.99056	2.35811	13.688	15.069	12.529	10.587	9.570	9.299
02	17:31:49.3	-39:56:16.5	-2.87837	2.64815	13.797	15.227	12.629	10.604	9.577	9.315
03	17:31:17.3	-39:50:30.5	-3.16035	2.51542	13.831	15.159	12.688	10.814	9.828	9.586
04	17:31:20.8	-39:45:48.1	-3.04149	2.58301	13.866	15.161	12.736	10.821	9.873	9.617
05	17:30:56.0	-39:50:38.5	-2.59973	2.61964	13.914	15.176	12.719	10.829	9.968	9.702
06	17:31:10.7	-39:50:30.6	-2.75174	2.51828	14.016	15.296	12.888	11.057	10.111	9.878
045	17:31:10.8	-39:47:02.8	-3.13759	2.31699	13.616	14.971	12.456	10.525	9.551	9.289
087	17:31:21.6	-39:50:12.2	-3.07022	2.52644	13.971	15.278	12.822	10.896	9.948	9.700
097	17:31:28.7	-39:54:06.2	-3.09229	2.51471	13.735	15.059	12.587	10.665	9.735	9.451

Table 2. Table 2: Parameters and abundances of the observed stars. The last rows give the mean abundances of the cluster and the relative error from the mean.

ID	$T_{\mathrm{eff}}$	log(g)	$v_t$	RVHELIO	[Fe/H]	[O/Fe]	[Na/Fe]	[Mg/Fe]	[Al/Fe]	[Ca/Fe]	[TiI/Fe]	[TiII/Fe]	[Cr/Fe]	[Ni/Fe]	[Ba/Fe]	[Eu/Fe]
	${}_{}{}^{o}K$		[km/s]	[km/s]	dex	dex	dex	dex	dex	dex	dex	dex	dex	dex	dex	dex
01	3880	0.00	1.83	229.6	-1.52	0.15	0.25	0.62	0.45	0.40	0.39	0.44	-0.18	-0.16	0.12	0.25
02	3880	0.00	1.91	223.8	-1.55	-0.06	0.61	0.74	0.77	0.42	0.34	-	-0.33	-0.05	0.07	0.28
03	4020	0.25	1.95	229.7	-1.62	0.29	0.05	0.69	0.41	0.37	0.33	0.42	-0.27	-0.04	-0.17	0.36
04	4010	0.28	1.88	226.2	-1.54	0.13	0.49	0.66	0.40	0.37	0.37	0.28	-0.22	-0.08	-0.13	0.19
05	4170	0.09	2.30	217.5	-1.77	0.27	0.01	0.74	-	0.17	0.08	0.38	-0.29	-0.14	0.08	0.19
06	4080	0.43	1.72	234.7	-1.57	0.32	-0.02	0.60	0.49	0.34	0.34	0.31	-0.18	-0.13	0.10	0.46
045	3870	0.15	1.93	228.5	-1.64	0.31	0.22	0.82	0.17	0.19	0.24	-	-0.31	-0.05	-0.05	0.39
087	4035	0.58	1.92	222.9	-1.52	0.25	0.24	0.55	0.46	0.34	0.39	0.25	-0.20	-0.03	-0.15	0.48
097	3950	0.10	1.85	228.1	-1.52	0.16	0.33	0.79	0.21	0.31	0.38	0.29	-0.23	-0.13	-0.05	0.24
Mean				226.8	-1.58	0.20	0.24	0.69	0.42	0.32	0.32	0.34	-0.25	-0.09	-0.02	0.32
Error				1.6	0.03	0.04	0.07	0.03	0.07	0.03	0.03	0.03	0.02	0.02	0.04	0.04

Table 3. Table 3: Estimated errors on abundances due to errors on atmospheric parameters and to spectral noise for star #097 (column 2 to 6). Column 7 gives the total error calculated as the root squared of the sum of the squared of columns 2 to 6. This total error must be compared with the observed dispersion (RMS) of the data with its error (column 8). The last column gives the significance of the difference between the total error and the observed dispersion, in units of σ 𝜎 \sigma . See text for more details.

El.	$\Delta$Teff=40 K	$\Delta$log(g)=0.15	$\Delta$vt=0.05 km/s	$\Delta$[Fe/H]=0.05	S/N	${\Delta }_{\mathrm{tot}}$	$RM{S}_{\mathrm{obs}}$	Significance ($\sigma$)
$\Delta$([O/Fe])	0.02	0.05	0.01	0.01	0.10	0.11	0.12$\pm$0.03	0.3
$\Delta$([Na/Fe])	0.02	0.01	0.00	0.00	0.05	0.05	0.21$\pm$0.05	3.2
$\Delta$([Mg/Fe])	0.01	0.01	0.01	0.00	0.07	0.07	0.09$\pm$0.02	1.0
$\Delta$([Al/Fe])	0.01	0.04	0.01	0.01	0.09	0.10	0.18$\pm$0.05	1.6
$\Delta$([Ca/Fe])	0.05	0.01	0.00	0.01	0.07	0.09	0.09$\pm$0.02	0.0
$\Delta$([TiI/Fe])	0.07	0.02	0.00	0.01	0.05	0.09	0.10$\pm$0.02	0.5
$\Delta$([Cr/Fe])	0.06	0.01	0.01	0.01	0.04	0.07	0.06$\pm$0.01	1.0
$\Delta$([Fe/H])	0.03	0.02	0.01	0.00	0.02	0.04	0.08$\pm$0.03	1.2
$\Delta$([Ni/Fe])	0.00	0.02	0.01	0.01	0.04	0.05	0.05$\pm$0.01	0.0
$\Delta$([Ba/Fe])	0.04	0.02	0.03	0.00	0.08	0.10	0.11$\pm$0.03	0.3
$\Delta$([Eu/Fe])	0.03	0.06	0.01	0.01	0.12	0.14	0.11$\pm$0.03	1.0

Equations

$$[Fe/H]=-1.58\pm 0.03$$

$${\sigma }_{[Fe/H]}=0.08\pm 0.02$$

Full text

optical: stars - open clusters and associations: general - stars: abundances

††journal: ApJ††facilities: Magellan-Clay:Mike††software: MOOG (Sneden, 1973)††thanks: Based on observations carried out at …………. under programs ………….

1 Introduction

The census of Galactic globular clusters (GCs) appears to be incomplete in the direction of the central regions of the Milky Way due to high interstellar absorption and source crowding ( e.g. Ivanov et al. 2005, 2017), as illustrated by several recent discoveries (e.g. Minniti et al. 2017a, b; Ryu et al. 2018; Borissova et al. 2018; Camargo 2018; Camargo & Minniti 2019; Palma et al. 2019). While most of the new GC candidates have low luminosity, one of the most unexpected discoveries is FSR 1758, that is a large metal-poor GC recently found in the direction of the Galactic bulge (Barba et al., 2019), centered at coordinates $RA=17:31:12$, and $DEC=-39:48:30$ (J2000), and Galactic coordinates $l=349.217$ deg, $b=-3.292$ deg. They use data from Gaia, the DECaPS optical survey (Schlafly et al., 2018) and the VVVX survey (Minniti et al., 2010, 2018) to estimate the GC parameters. In particular, Barba et al. (2019) determined a metallicity $[Fe/H]=-1.5\pm 0.3$ dex based on the optical and near-IR CMDs from Gaia and VVV, respectively. They also measure a distance $D=11.5\pm 1.0$ kpc, reddening $E(J-Ks)=0.20\pm 0.03$ mag, radii $Rc=10$ pc, and $Rt=150$ pc, total magnitude $M_{i}\sim-8.6\pm 1.0$ mag, and proper motions $\mu_{\alpha}=-2.95$ mas/yr, and $\mu_{\delta}=2.55$ mas/yr.

The determination of the fundamental parameters of this new GC is important because it has been recently proposed that FSR 1758 belongs to the past accretion of a dwarf galaxy (named Sequoia) by the Milky Way, along with other GCs (Myeong et al., 2019). These authors argue that the Sequoia event along with the so called Sausage event are the two most significant accretion events that occurred in the Milky Way.

Barba et al. (2019) argues that FSR 1758 may be part of an extended dwarf galaxy disrupted by the Milky Way, along with possibly Ton2 and NGC 6380, that are located within 1 deg of FSR 1758. However, the RVs of these clusters ($RV_{Ton2}=-182$ km/s and $RV_{NGC6380}=-4$ km/s) are very different from that of FSR 1758 ($RV_{FSR1758}=+227km/s$), making the association very unlikely, based on the Gaia RV measurement of Simpson (2019), who concludes that this cluster has a retrograde orbit.

On the other hand, based on the strongly retrograde orbit, Myeong et al. (2019) argues that it belongs to the Sequoia dwarf galaxy, accreted by the Milky Way along with the GCs WCen, NGC3201, NGC6101, NGC6535, NGC6388, and NGC6401. They argue that these clusters share common properties in the age-metallicity space for example.

As a caveat, according to Myeong et al. (2019), the Gaia-Enceladus event (Helmi et al., 2018) includes both the Sequoia and Sausage events. Much remains to be understood in terms of the past Milky Way accretion events, and globular clusters with well determined physical parameters, like is now FSR 1758, are key elements of evidence.

We have obtained high dispersion spectra for 9 stars that are confirmed members of the cluster. The deep photometry and accurate proper motions measured by Gaia and VVV are complemented by our high dispersion spectroscopy to fully characterize this GC, defining its parameters. The spectroscopy presented in this paper allows us to measure accurate radial velocities and detailed chemical abundances for the target stars. In particular, the Gaia proper motions in combination with our accurate radial velocities are employed to improve the cluster’s orbit.

In this paper we aim to answer the following questions: What is the detailed chemical composition of the new GC FSR 1758? Does it have a measurable dispersion in chemical elements, suggesting the presence of multiple populations? How massive is this cluster? Is this a real GC or the nucleus of an accreted dwarf? Does this cluster belong to Milky Way GCS or to an external dwarf that has been accreted? What is the cluster orbit? Is this a halo, bulge, or disk GC?

2 Observations and data reduction

Our data set consists of high-resolution spectra collected in 2018 October-November. The spectra come from single 300s exposures, obtained with the MIKE slit echelle spectrograph at the Magellan/Clay telescope. Weather conditions were excellent with a typical seeing of 0.7 arcsec at the zenith and a clear sky. We selected 9 isolated stars around the RGB tip. The MIKE spectra have a spectral coverage from 4900 to 8700 Å with a resolution of $\sim$42,000 and signal-to-noise ratio (S/N)$\sim$40 at 6300 Å. Data were reduced using MIKE dedicated pipeline (Kelson et al., 2000, 2003) 111 https://code.obs.carnegiescience.edu/mike, including bias subtraction, flat-field correction, and wavelength calibration, scattered-light and sky subtraction. Single orders were rectified and combined using IRAF 222IRAF is distributed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation. .

In order to select our targets we took advantage of the proper motions published in the Gaia DR2 (Gaia collaboration, 2018) since the cluster suffers of a severe foreground contamination. Proper motions of the cluster field stars are reported in Fig. 1, where cluster members are indicated with cyan symbols. As members we selected those stars within a radius of 0.7 mas/year around the center of the cluster in the proper motion space. The value of the radius was determined empirically assuming 1 mas/year as initial guess, and reducing it as much as possible in order to remove field contamination as shown from the CMD. We found that reducing the radius below 0.7 mas/year did not improve the CMD significantly. Among these members we selected our targets, indicated by red circles. The targets are RGB stars close to the tip. The mean proper motions of the members are:

pmRA=-2.7911$\pm$0.0097 mas/year

pmDEC=+2.6042$\pm$0.0090 mas/year

Gaia photometry was cross-correlated with 2MASS and we present multi-band CMDs of the cluster in Fig. 2. The cluster presents well populated RGB and HB and shows to be affected by differential reddening. The HB is not visible in the infrared bands since HB stars emit most of their flux in the blue and ultraviolet. Radial velocities were measured with the fxcor package in IRAF, using a synthetic spectrum as a template. The mean radial velocity we obtained is 226.8$\pm$1.6 km/s. There are no clear outliers in the radial velocity distribution. If we consider also that all our targets have the same proper motions (see Table 1) we conclude that they are all cluster members. We found a radial velocity dispersion of 4.9$\pm$1.2 km/s Table 1 lists the basic parameters of the 9 observed targets: ID, J2000.0 coordinates, proper motions and G, BP, RP magnitudes from Gaia Data release 2, and J, H, Ks magnitudes from 2MASS.

3 Abundance analysis

Figure  2 shows that the cluster is affected by differential reddening. For this reason an estimation of stellar parameters bases on photometry would require a differential reddening correction. Therefore as initial temperature, gravity and microturbulence for our stars we just assumed tipical values for RGB stars close to the tip, that is Teff=4000 K, log(g)=0.5 dex, and vt=1.90 km/s. Atmospheric models were calculated using ATLAS9 code (Kurucz, 1970), assuming our estimations of Teff, log(g), and vt, and the [Fe/H] value from Barba et al. (2019).

Then Teff, log(g), and vt were re-adjusted and new atmospheric models calculated in an interactive way in order to remove trends in excitation potential and reduced equivalent width (EQW) versus abundance for Teff and vt, respectively, and to satisfy the ionization equilibrium for log(g). 60-70 FeI lines and 6-7 FeII lines (depending on the S/N of the spectrum) were used for the latter purpose. The [Fe/H] value of the model was changed at each iteration according to the output of the abundance analysis. The Local Thermodynamic Equilibrium (LTE) program MOOG (Sneden, 1973) was used for the abundance analysis.

CaI, TiI, TiII, CrI, FeI, FeII, and NiI abundances were estimated using the EQW method. For this purpose we measured EQW using the automatic program DAOSPEC (Stetson et al., 2008) 333DAOSPEC is freely distributed by http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/en/community/STETSON/daospec/. OI, NaI, MgI, AlI, BaII, and EuII abundances were obtained using the spectro-synthesis method. For this purpose 5 synthetic spectra were generated for each line with 0.25 dex abundance step and compared with the observed spectrum. The line-list and the methodology we used are the same used in previous papers (e.g. Villanova et al. 2013), so we refer to those articles for a detailed discussion about this point. Here we just underline that we took hyperfine splitting into account for Ba as in our previous studies. This is particularly important because Ba lines are very strong even in metal-poor stars and hyperfine splitting help to remove the line-core saturation producing a change in the final abundance as estimated by the spectro-synthesis method up to 0.1 dex. Also Eu has odd-isotopes affected by hyperfine splitting, but their lines are weak and the line-core saturation is not at work. So hyperfine splitting corrections are negligible.

The abundances we obtained are reported in Tab. 2together with the mean values for the cluster and the error on the mean. For Ti we reported the mean values of TiI and TiII abundances. Na is an element affected by NLTE effets. For this reason we looked in the INSPEC 444version 1.0 (http://inspect.coolstars19.com/index.php?n=Main.HomePage) database for suitable NLTE corrections. We found the they are very small ($\sim$-0.05 dex) with no significant variation (less then 0.02 dex) in our temperature range. For this reason we decided not to apply them to our Na abundances.

As a cross check of our abundance analysis, we plot in Fig. 3 for each element the difference between the abundance of each star and the mean abundance of the cluster. We see that there is no evident trend of the abundances with temperature. This indicates that the methodology used to obtain chemical abundances is consistent over the entire temperature range. A detailed internal error analysis was performed using the method described in Marino et al. (2008). It gives us $\sigma$(Teff)=40 K, $\sigma$(log(g))=0.15, and $\sigma$(vt)=0.05 km/s. The error on [Fe/H] due to the S/N is 0.03 dex. Then we choose star #097 as representative of the sample, varied its Teff, log(g), [Fe/H], and vt according to the atmospheric errors we just obtained, and redetermined the abundances. Results are shown in Tab. 3, including the error due to the noise of the spectra. This error was estimated for elements whose abundance was obtained by EQWs, as the errors on the mean given by MOOG, and for elements whose abundance was obtained by spectrum-synthesis, as the error given by the fitting procedure. $\Delta_{\rm tot}$ is the squared sum of the individual errors. For each element we report the observed spread of the sample ($RMS_{\rm obs}$) with its error and in the final column the significance (in units of $\sigma$) calculated as the absolute value of the difference between $RMS_{\rm obs}$ and $\Delta_{\rm tot}$ divided by the error on $RMS_{\rm obs}$. This tells us if the observed dispersion $RMS_{\rm obs}$ is intrinsic or due to observational errors. Values larger than 3$\sigma$ imply an intrinsic dispersion in the species chemical abundance among the cluster stars.

4 Results

4.1 Iron-peak, $\alpha$ elements and heavy elements

The mean iron content we obtained is:

[TABLE]

with a dispersion of:

[TABLE]

Reported errors are errors on the mean. The measured iron dispersion in Tab. 3 well agrees with the dispersion due to measurement errors so we no evidence for an intrinsic Fe abundance spread. As far as other iron-peak elements are concerned, both Cr and Ni are syb-solar.

The $\alpha$ elements Ca and Ti are overabundant compared to the Sun. This is a feature common to almost all Galactic GC and Halo field stars as well as to very metal-poor stars ([Fe/H]$<$-1.5) in outer galaxies. Based on these elements we derive for the cluster a mean $\alpha$ element abundance of:

$[\alpha/Fe]=+0.32\pm 0.01$

As far as heavy elements are concerned Ba is slightly sub-solar, while Eu is super-solar.

Figure 4 shows the star by star (black filled circles) $\alpha$-element abundances of the cluster compared with a variety of galactic and extra-galactic objects. We have included values from GGCs (Carretta et al., 2009, 2010, 2014a, 2014b, 2015a, 2015b; Villanova et al., 2010, 2011, 2013; Munoz et al., 2013; San Roman et al., 2006, red filled squares), Disk and Halo stars (Fulbright, 2000; Reddy et al., 2003, 2006; Cayrel et al., 2004; Simmerer, 2004; Barklem et al., 2005; Francois et al., 2007; Johnson et al., 2012, 2014, gray filled squares), the Bulge (Gonzalez et al., 2011, purple stars), and extra-galactic objects such as Magellanic clouds (Pompeia et al., 2008; Johnson et al., 2006; Mucciarelli et al., 2008, 2009, blue filled squares), Draco, Sextans, Ursa Minor and Sagittarius dwarf galaxy and the ultra-faint dwarf spheroidals Boötes I and Hercules (Monaco et al., 2005; Sbordone et al., 2007; Shetrone et al., 2001; Ishigaki et al., 2014; Koch et al., 2008, green filled squares).

The $\alpha$ elements in FSR 1758 follows the same trend as Galactic GCs and are fully compatible with Halo and Thick Disk field stars while it falls in a region scarcely populated by extragalactic objects. So, according to its $\alpha$-element content, FSR 1758 is likely a genuine Galactic cluster.

The chemical abundances for the iron-peak elements Cr and Ni and for the heavy-elements Ba and Eu are reported in Fig. 5 and  6 respectively. Around FSR 1758 metallicity, Galactic and extragalactic environments share the same abundances. We underline the low Cr content of the cluster compared with the general trend, at the very lower boundary.

Finally for all $\alpha$, iron-peak and heavy elements Tab. 3 shows that the observed dispersion agrees well with the measurement errors so we can rule out any intrinsic abundance spread. We check also for possible correlations of these elements with lights elements such Na and Al, but we did not find evidence for significant trends.

4.2 Light elements

The light element Na has an observed spread that exceeds the observational uncertainties (see Tab. 3). O, Mg and Al instead show homogeneity, at least as far as observed dispersions are concerned.

In Fig. 7 we compare the O and Na abundances of the targets with different environments. A clear Na-O anticorrelation appears. The content of the two elements for the FSR 1758 first generation stars (the targets with [O/Fe]$\sim$0.3, [Na/Fe]$\sim$0.0) well matches the mean O and Na abundances of the Milky Way Halo and the mean O and Na content of the other GC first generation stars. On the other hand the cluster shows depletion in O and Na enhancement as far as the subsequent generation targets are concerned (the targets with [O/Fe]$<$0.3, [Na/Fe]$>$0.1). Fig. 8 shows the relations between all the light elements available from our study. Al shows the second largest spread among the elements (0.18 dex), however it is not significant according to our error analysis. There is also no apparent relation between Na and Mg or Al at odd with wha found in other GCs (i.e. M28, Villanova et al. 2017), or a relation between Mg and Al. Further studies with larger datasets are required to confirm or disproof this result.

5 The orbit

FSR 1758 is located at 11.5 kpc from the Sun, on the other side of the Galactic Bulge. In order to check if it belongs to the Bulge we calculated its orbit using GravPot16 555https://gravpot.utinam.cnrs.fr/ (Fernandez-Trincado, 2017). This is a web-code that allow to calculate orbits based on a Galactic gravitational potential driven by the Besançon Galaxy Model mass distribution. As input parameters for the cluster we assumed:

RAcluster=262.8100

DECcluster=-39.8150

Distance=11.5 kpc

pmRA=-2.7911 mas/year

pmDEC=+2.6042 mas/year

RVHelio=226.7 km/s

As far as the model is concerned we used:

R*⊙*= 8.2 kpc

U${}_{\odot},V{\odot},W{\odot}$=10.0,225.2,7.2 km/s

We integrated to orbit forward for 2 Gyrs. In Fig. 9 and Fig. 10 we report the projection of the orbit on the X,Y and X,Z Galactic planes. Fig. 9 shows that FSR 1758 orbit does not enter the Bulge region and the cluster minimum distance from the center is about 3.8 kpc while the maximum distance is 13.6 kpc. Fig. 10 on the other hand suggests an orbit with a maximum height of about 7 kpc. The eccentricity and the Z component of the orbital angular momentum (LZ) are 0.56 and 1.20 kpc$\times$km/s respectively.

We compare our orbit with that published by Simpson (2019) where the author use initial parameters very similar to ours:

RAcluster=262.8060

DECcluster=-39.8220

Distance=11.5 kpc

pmRA=-2.85 mas/year

pmDEC=+2.55 mas/year

RVHelio=227 km/s

He find a perincenter at 3.8${}^{+0.9}_{-0.9}$ kpc and an apocenter at 16${}^{+8}_{-5}$ kpc. Both parameters anicely gree with ours. Also the maximum height agrees with Simpson (2019) value beeing 7$\div$8 kpc. GravPot16 includes a Bar as far as the Bulge potential is concerned, while Simpson (2019) assumes a spherical potential for the Bulge. Since the cluster does not enter the Bulge region, the gravitational potential of the Bar does not affect its orbits significantly as the comparison of the two results shows.

We then support the Simpson (2019) classification of FSR 1758 as a Halo cluster based on its orbital parameters, since both its highly eccentric orbits and the maximum distance from the Galactic plane do not match typical Disk orbits that are more circular and confined within 1 Kpc from the plane. We confirm also that, due to the LZ value we find, the orbit is retrograde.

6 Discussion and Conclusions

Our target stars were carefully selected to be members of the cluster based on Gaia proper motions. All the observed target stars turned out to be RV members of the cluster, giving confidence to our selection procedure. Based on the available spectra for 9 objects, we measure a velocity dispersion $\sigma_{RV}=4.9\pm 1.2$ km/s. This velocity dispersion is relatively small, typical for a globular cluster (e.g. Pryor & Meylan 1993).

We also confirm the mean RV measurement of Simpson (2019), and therefore his conclusion that this cluster has a retrograde orbit. The orbital parameters we find agree nicely with theirs.

Simpson (2019) and Myeong et al. (2019) find that this GC has a strongly retrograde orbit with high eccentricity ($e\sim 0.6$). This is important because the orbit of Simpson (2019) favors the interpretation of Myeong et al. (2019) that this GC belongs to an early accretion event in the Milky Way. Myeong et al. (2019) studies the action space and the GCs analyzed by Vasiliev (2019), concluding that among the different substructures present, there was a clear signature of an individual event of accretion that was retrograde, corresponding to the Sequoia dwaf galaxy.

The orbit that we compute places this GC in a singular position with common properties with $\omega$ Centauri. It then appears that both clusters are the defining members of this Sequoia dwarf galaxy.

In summary, our main conclusions are:

• •

FSR1758 is a bonafide GC, and not the nucleus of a dwarf galaxy.

• •

FSR1758 shows the typical Na-O anticorrelation common to almost all the GCs in the Galaxy.

• •

Its velocity dispersion is small, typical for a globular cluster.

• •

The orbit is retrograde and highly eccentric.

• •

Based on the orbit, this cluster does not appear to belong to the Disk or Bulge components of the Milky Way, and is more consistent with that of a Galactic Halo building block.

• •

This GC probably belonged to the Sequoia dwarf galaxy that contained at least two large GCs ($\omega$Cen and FSR1758), and was accreted in the past by the Milky Way.

• •

We suggest that the nucleus of this dwarf galaxy was $\omega$Cen, and not FSR1758 that appears to have homogeneous composition typical for a normal GC.

• •

We predict that the Sequoia dwarf galaxy may have had more GCs.

SV gratefully acknowledges the support provided by Fondecyt reg. n. 1170518. D.G. gratefully acknowledges support from the Chilean BASAL Centro de Excelencia en Astrofisica y Tecnologias Afines (CATA) grant PFB-06/2007. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. DM is supported by the BASAL Center for Astrophysics and Associated Technologies (CATA) through grant AFB 170002, by the Programa Iniciativa Científica Milenio grant IC120009, awarded to the Millennium Institute of Astrophysics (MAS), and by Proyecto FONDECYT No. 1170121.