The orbit of FSR1758

The very retrograde orbit of the globular cluster FSR1758 revealed with Gaia Dr2

Jeffrey D. Simpson
School of Physics, UNSW, Sydney, NSW 2052, Australia
E-mail: jeffrey.simpson@unsw.edu.au
Accepted XXX. Received YYY; in original form ZZZ
Abstract

We report the first radial velocity measurements of the recently identified globular cluster FSR1758. From the four stars with radial velocities from the Gaia Radial Velocity Spectrograph reported in Gaia DR2, we find FSR1758 has a radial velocity of  . Combined with Gaia proper motions and photometric distance estimates, this shows that FSR1758 is on a very retrograde, radial orbit with an pericentre of  kpc, an apocentre of  kpc, and eccentricity of . Although it is currently at a Galactocentric distance of  kpc — at the edge of the bulge — it is an intruder from the halo. We investigate whether a reported ‘halo’ of stars around FSR1758 is related to the cluster, and find that most of these stars are likely foreground dwarf stars. We conclude that FSR1758 is not a dwarf galaxy, but rather a globular cluster.

keywords:
globular clusters: individual: FSR1758
pubyear: 2019pagerange: The very retrograde orbit of the globular cluster FSR1758 revealed with Gaia DR2The very retrograde orbit of the globular cluster FSR1758 revealed with Gaia DR2

1 Introduction

The second data release of the Gaia mission (Brown et al., 2018) has revolutionized our view of the Milky Way Galaxy. Among its many results, it has proven very useful for investigating purported stellar clusters, in many cases confirming their existence (e.g., Simpson et al., 2017b; Soubiran et al., 2018; Cantat-Gaudin et al., 2018), but in some cases showing they are not real physical associations (e.g., Kos et al., 2018). The precise proper motions are especially helpful in the Milky Way bulge, where imaging surveys are hampered by the large and differential reddening and extinction, resulting in clusters hidden from easy view. Its radial velocity measurements, though only available for the brighter cohort of stars, enable us to calculate the 3D motion of many clusters.

Recently Barbá et al. (2019, hereafter B19) used data from Gaia DR2 and the DECam Plane Survey (DECaPS; Schlafly et al., 2018) to present a physical characterization of the large, massive stellar grouping FSR1758. They found the stellar cluster to be curiously large (on the order of the size of  Cen), and claimed that it could potentially be the core of a dwarf galaxy based upon a halo of common proper motion stars in the surrounding region. As noted by B19, the key data missing were spectroscopic observations of the cluster. These data are crucial for deriving an orbit, confirming the photometric metallicity, and understanding the ‘halo’ of common proper motion stars.

In this Letter we report the radial velocities of four members of FSR1758 measured with the Gaia Radial Velocity Spectrograph (RVS; Cropper et al., 2018; Katz et al., 2018) (Sec. 2), which are used to calculate an orbit for FSR1758 (Sec. 3). We also examine the ‘halo’ of common proper motion stars around FSR1758 that led B19 to propose that FSR1758 may be a dwarf galaxy (Sec. 4). We conclude that these stars are foreground field stars and are not associated with the cluster.

2 Data

Figure 1: Position (a), proper motion (b), color-magnitude diagrams (c,d), radial velocity (e), and parallax (f) distributions of all stars in Gaia within 1 deg of FSR1758 and with proper motions within 1.2   of the value found by B19. The stars have been divided into ‘cluster’ (red stars for those with radial velocity, otherwise purple points) and ’field’ (black stars for those with radial velocity, otherwise green points) samples based upon their angular distance from the cluster. Filled stars identify the radial velocity members of FSR1758, and the unfilled stars are non-members with radial velocities. There are no stars with radial velocities in (d) due to the bright limit of DECaPS in this region. The cluster stars show an obvious grouping in proper motion space (panel b), and the colour-magnitude diagram morphology expected for an ancient, metal-poor population (panels c and d). In (f) the parallaxes of the stars with radial velocities are shown with the vertical positions randomly adjusted to aid the eye. The distribution of the parallaxes of cluster stars is offset from the distribution of the field stars, and the radial velocity members tend to have smaller parallaxes than the other stars with radial velocities.

The primary source of data for this work was Gaia DR2, combined with DECaPS. We searched for Gaia DR2 targets within 1.0 deg of FSR1758 () which returned a catalogue of over 1.5 million stars111The data and analysis code is available at https://doi.org/10.5281/zenodo.2550945.. These were positionally cross-matched to the DECaPS catalogue using topcat. The bright limit for the DECaPS photometry is , which meant that not all of the Gaia targets have DECaPS photometry.

Visually, the cluster is not obvious as an over-density on the sky, but the cluster has a distinct proper motion (Cantat-Gaudin et al., 2018; Barbá et al., 2019). Stars with proper motions within 1.2   of    were extracted from the catalogue. Those within 0.2 deg of the cluster position were selected as ‘cluster’ stars, and those outside that as a ‘field’ sample. This ‘cluster’ sample and the surrounding field are shown in Fig. 1 with their position on the sky, proper motion, colour-magnitude, radial velocity and parallax. The proper motions of the cluster sample are clearly clumped, and the CMD shows the red giant branch (RGB) and horizontal branch (HB) morphology of an ancient, metal-poor stellar population in both the Gaia and DECaPS filters. The parallaxes of the likely members of FSR1758 are indistinguishable from zero within the uncertainties (expected for an object at 11 kpc), but show a clear offset from the parallax distribution of the surrounding field stars (Fig. 1f).

Of the 1345 likely cluster stars within 0.2 deg, five stars had radial velocities () measured by the Gaia RVS: three of the five stars have , while the other two stars have and (respectively filled and unfilled red stars on Fig. 1). The three stars with high are all found at the tip of the giant branch of FSR1758 (Fig. 1c), and have smaller parallaxes than most of the other stars with radial velocities. We conclude that these three stars are members of FSR1758, and that the radial velocity of the cluster is  .

B19 estimated the tidal radius to be  deg, so we extended the search for possible members beyond the initial  deg sample. Within the surrounding 1 deg, there is one additional star with a large radial velocity (; black filled star on Fig. 1)), and it is, like the three stars above, compatible with the proper motion, parallax, and photometry of the cluster. The difference between 233  and 227  is consistent with the dispersion seen in other globular clusters (e.g.,  Cen has a velocity dispersion of ; Johnson & Pilachowski, 2010).

The Gaia RVS spectra were not made public in DR2, so we cannot estimate a metallicity for FSR1758. There do not appear to have been any serendipitous spectroscopic observations of the cluster. Unfortunately it is outside the footprints of RAVE (Kunder et al., 2017), GALAH (De Silva et al., 2015; Buder et al., 2018), APOGEE-2 (Zasowski et al., 2017) and other bulge spectroscopic surveys (e.g., Freeman et al., 2013; Zoccali et al., 2014; Howes et al., 2016).

3 Orbit

Figure 2: The previous 1.25 Gyr of the orbit of FSR1758 projected into Cartesian space centred on the Galactic Centre. The red line is the orbit from the nominal values of the phase space coordinates, and the blue lines show 100 orbits randomly sampling the error distributions of the input parameters. The black dot shows the observed position of FSR1758.
Figure 3: Comparison of the orbit of FSR1758 to other globular clusters. In panels (a) and (b) the shaded region indicates a distance of 3 kpc (i.e., the radius of the Milky Way bulge), and the straight lines show the one-to-one line. For panel (a) a cluster on this line would have a circular orbit, and in panel (b) a cluster on this line is being observed at apocentre. As well as FSR1758, a number of clusters of interest are highlighted. Djorgovski 1 and Terzan 10 were recently found by Ortolani et al. (2018) to be halo interlopers in the bulge. M54, M2, M22,  Cen have been found to be multi-metallicity clusters, and could therefore be stripped dwarf galaxy cores. NGC2419, NGC3201, IC4499 have large angular momentum orbits.

It is now possible to calculate the orbit of FSR1758. We used gala (version 0.3; Price-Whelan, 2017; Price-Whelan et al., 2018a), with the default potential MilkyWayPotential. This is a simple mass-model for the Milky Way consisting of a spherical nucleus and bulge, a Miyamoto-Nagai disk, and a spherical NFW dark matter halo. The parameters of this model are set to match the circular velocity profile and disk properties of Bovy (2015). We place the Sun at a Galactocentric distance of  kpc and a height above the plane of 25 pc (Bland-Hawthorn & Gerhard, 2016). The Sun’s velocity is taken to be (Schönrich, 2012). The position and velocity of FSR1758 were . Errors in the calculated orbital parameters were estimated by taking 1000 samples of the error distributions and finding the 16th and 84th percentiles of the given results.

Fig. 2 shows the previous 1.25 Gyr of its orbit (red line), as well as 100 other possible orbits over the same time interval created by sampling the error distributions (blue lines). Fig. 3 shows how the orbit of FSR17578 compares to the orbit of other known globular clusters, with input data from Vasiliev (2019). The orbits were calculated in the same manner as FSR1758 (i.e., using gala with the input values for the Milky Way and Sun as described above).

FSR1758 is found to have a radial, retrograde orbit, with a pericentre of  kpc, an apocentre of  kpc, and eccentricity of . FSR1758 is currently located at a Galactocentric distance of  kpc, placing it at the edge of the Milky Way bulge (clusters within  kpc of the Galactic centre tend to be classified as ‘bulge’ clusters; Barbuy et al., 2018). But we find that it is not a cluster that lives in the inner Galaxy like, e.g., Terzan 5. Instead it is a halo intruder into the inner Galaxy, like Djorgovski 1 and Terzan 10 (Ortolani et al., 2018). We have caught FSR1758 near the pericentre of its orbit.

4 Is FSR1758 a dwarf galaxy or globular cluster?

Figure 4: In B19, they found a halo of common proper motion stars within 2 deg of the cluster. We repeat their analysis, selecting stars along a locus formed by the RGB and HB in the Gaia filters within 2 deg of the cluster, with proper motions like the cluster, and with parallaxes  mas. As in Fig. 1 they are divided into the ‘cluster’ and ‘halo’ sample based upon their angular distance from the cluster. Considering the distributions of proper motion, colour and parallax, we conclude that the majority of this ‘halo’ are field stars that are coincidently share the proper motion of FSR1758.

One of the unresolved questions of B19 was whether FSR1758 is a dwarf galaxy or globular cluster. The orbital properties of FSR1758 do not distinguish it from other globular clusters, or proposed stripped dwarf galaxy cores (e.g., M54,  Cen). We find that FSR1758 has quite a retrograde orbit (i.e., large ) compared to most other clusters. Clusters with retrograde orbits are usually classified as being accreted by the Milky Way rather than forming in situ (e.g., Kruijssen et al., 2018). The large retrograde component of the inner halo of the Milky Way found in Gaia DR2 has been associated with a number of globular clusters, including  Cen (Helmi et al., 2018), but these have smaller than FSR1758, so we do not associated FSR1758 with Gaia-Enceladus. An intriguing future direction of research, beyond the scope of this work, is to look if any of the large number of stellar streams that are being found in Gaia and other surveys are related to FSR1758 (e.g., Shipp et al., 2018; Ibata et al., 2018; Malhan et al., 2018; Ibata et al., 2019).

Part of the reasoning for the claim in B19 that that FSR1758 could be a dwarf galaxy is a large ‘halo’ of common proper motion stars around the cluster, which they interpreted as possibly being tidal debris or that FSR1758 was the nucleus of a dwarf galaxy. In Fig. 4, we repeat their analysis: stars along a locus formed by the RGB and HB within 2 deg of the cluster, with parallax  mas, and proper motions within of the nominal cluster value. We divide this selection of stars into a ‘cluster’ and ’halo’ sample based upon angular distance from the cluster (cut at 0.2 deg).

We conclude that the majority of this halo of stars are not truly associated with FSR1758, but are instead mostly foreground field dwarf stars. There is a clear colour gap between the HB and RGB, especially obvious in the DECaPS photometry (Fig. 4d). Only 11 per cent (37/335) of the ‘cluster’ stars are found in the colour range , while for the ‘halo’ sample it is 59 per cent (134/226). This gap corresponds to the field dwarf sequence along the line of sight (Fig. 1c,d). FSR1758 shows an obvious blue horizontal branch, but almost none of the halo sample is found in this region of the CMD. The halo sample is not clumped in proper motion space like the cluster stars (Fig. 4b). Comparing the parallax distributions of the cluster and halo (Fig. 4f), the ‘halo’ does not peak at  mas like the cluster and is simply the low parallax selection of the broader field parallax distribution (Fig. 1f).

It is possible that a few of these common proper motion stars are extra-tidal stars, as these are known around other globular clusters (e.g., Simpson et al., 2017a), but the majority are unlikely to be related. We do not discount, with its retrograde orbit, the idea that FSR1758 could be the stripped core of a dwarf galaxy (e.g., like  Cen, M54) and it should be a high priority to observe this cluster to see if it has a metallicity spread.

5 Conclusion

We have identified four members of FSR1758 that have radial velocities measured by the Gaia RVS. Combined with the previously derived information about the cluster from B19, this allows us to calculate an orbit for the cluster which showed that it is a halo intruder into the inner Galaxy. We conclude that a possible large halo of common proper motion stars around the cluster are in fact likely to be less distant field stars.

We reiterate the words of B19 that “acid test for this cluster will be to obtain spectra for a number of members”. High resolution spectroscopic observations will be extremely useful, allowing us to know if the cluster has a metallicity spread. Given its highly retrograde orbit, a metallicity spread would indicate that FSR1758 is in fact a stripped dwarf galaxy core. With only four stars, it is not possible to make any meaningful comments about the radial velocity dispersion, and therefore the mass-to-light ratio of FSR1758. Where does it sit on the mass-metallicity relationship (Kirby et al., 2013)? If it is simply a globular cluster (i.e., no metallicity spread), then it is still an important object, representing the remnants of a dwarf galaxy system that has now been accreted by the Milky Way.

Acknowledgements

JDS acknowledges the support of the Australian Research Council through Discovery Project grant DP180101791.

The following software and programming languages made this research possible: astropy (v3.1; Price-Whelan et al., 2018b; The Astropy Collaboration, 2018), a community-developed core Python package for Astronomy; gala (v3.0; Price-Whelan, 2017; Price-Whelan et al., 2018a); pandas (v0.20.3; McKinney, 2010); seaborn (v0.8.1; Waskom et al., 2018); Tool for OPerations on Catalogues And Tables (topcat, v4.5; Taylor, 2005, 2006)

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.

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