Astronomical Science DOI: 10.18727/0722-6691/5076

MIKiS: the ESO-VLT Multi-Instrument Kinematic Survey of Galactic Globular Clusters

Francesco R. Ferraro 1, 2 black holes in clusters can finally Thus, despite its importance, our empiri- Alessio Mucciarelli 1, 2 be addressed, impacting our under- cal knowledge of internal Barbara Lanzoni 1, 2 standing of the formation and evolution- kinematics from the very centre (where Cristina Pallanca 1, 2 ary processes of globular clusters and the most interesting dynamical phenom- Livia Origlia 2 their interactions with the Galactic tidal ena occur) to the tidal radius (where the Emilio Lapenna 1, 2 field. effects of the Galactic tidal field are visible) Emanuele Dalessandro 2 is still critically inadequate. In particular, a Elena Valenti 3 detailed knowledge of the velocity disper- Giacomo Beccari 3 Galactic Globular Clusters are gas-free sion profile and the (possible) rotation Michele Bellazzini 2 stellar systems, made up of a few times curve is still missing in the majority of the Enrico Vesperini 4 105 of different masses (typically cases. This is essentially due to observa- 5 Anna Lisa Varri from ~ 0.1 to 0.8 M⊙). They are the tional difficulties. Antonio Sollima 2 most populous, and the oldest stellar systems in which stars can be individually In principle, velocity dispersion and observed. Moreover they are the sole ­rotation can be obtained from different 1 Dipartimento di Fisica e Astronomia, cosmic structures that, within the times- approaches. In practice, however, the Università degli Studi di Bologna, Italy cale of the age of the Universe, undergo standard methodology commonly used in 2 INAF–Osservatorio di Astrofisica e nearly all of the physical processes known extra-galactic astronomy (i.e., measuring ­Scienza dello Spazio di Bologna, Italy in stellar dynamics. Galactic globular radial velocities from Doppler shifts and 3 ESO clusters are true touchstones for astro- velocity dispersions from the line broad- 4 Department of Astronomy, Indiana physics. The study of their stellar popula- ening of integrated-light spectra) can ­University, Bloomington, USA tions is crucial to validating predictions ­suffer from severe shot-noise bias in 5 Institute for Astronomy, University of from stellar evolution theory, and they are globular clusters, because the Edinburgh, Royal Observatory, UK invaluable laboratories for multi-body acquired spectrum can be dominated by dynamics. Surprisingly, while remarkable the contribution of just a few bright stars progress has been made in the study (for example, Dubath et al., 1997). In the Globular clusters are collisional systems, of Galactic globular cluster stellar­ popu­ case of resolved stellar populations, a where stars of different masses orbit lations (see Carretta et al., 2009 and safer approach is to determine velocity and mutually interact. They are the best Piotto et al., 2015), the kinematical charac- dispersion and rotation from the veloci- “natural laboratories” in the Universe terisation of these systems is just in its ties of individual stars. By combining for studying multi-body dynamics and infancy and the modelling often relies on the line-of-sight information (measured their (reciprocal) effects on stellar evolu- a set of over-simplified assumptions. through resolved spectroscopy) with the tion. Although these objects have been two velocity components on the plane studied since the very beginning of In general, globular clusters are assumed of sky (from internal proper motions), a modern astrophysics, little is known to be quasi-relaxed, non-rotating stellar full 3D view of the velocity space of the observationally about their internal kine- systems, characterised by spherical system can be obtained. This begins to matics, thus preventing a complete ­symmetry and orbital isotropy, with struc- be feasible in Galactic globular clusters. understanding of their dynamical state, tural and kinematical properties (surface and of their formation and evolutionary brightness and velocity dispersion pro- Internal proper motions require high-­ history. We present the first results from files) that are well captured by a truncated precision photometry and astrometry on the Very Large Telescope (VLT) Multi- Maxwellian distribution function (for relatively long time baselines. These are Instrument Kinematic Survey of Galactic example, King, 1966). However, growing finally achievable thanks to multi-epoch globular clusters (MIKiS), which is spe- observational evidence demonstrates HST and Gaia observations. The former cifically designed to provide line-of-sight that although this scenario is correct to are providing the necessary information velocities of hundreds of individual a first approximation, it is largely over- on the centres of Galactic globular clus- stars over the entire radial extension simplified. Indeed, there is accumulating ters (even if the innermost regions of the of a selected sample of clusters. The evidence of significant deviations from densest systems might still be missed; survey allows the first kinematical sphericity (Chen & Chen, 2010), and from for example, see Bellini et al., 2014 and exploration of the innermost regions of a King density profile, a profile used to Watkins et al., 2015 for recent results). In high-density globular clusters. When fit the observed distribution of stars within the meantime, Gaia is providing the com- combined with proper motion measure- a globular cluster as a function of their plementary measures in the outskirts. ments, it will provide the full 3D view in distance from the centre (see Carballo- The line-of-sight kinematical information velocity-space for each system. Long- Bello et al., 2012; Lane et al., 2010). Sig- over the entire radial extension of Galactic running open issues, such as the accu- natures of systemic rotation and pressure globular clusters is still largely missing, rate shapes of the velocity dispersion anisotropy have been detected in several because it requires the collection of large profiles, the existence of systemic rota- clusters (for example, Watkins et al., 2015; samples of individual stellar spectra, both tion and orbital anisotropy (and thus Kamann et al., 2018; and references in highly crowded regions — the central the level of relaxation), and the contro- therein). density of the Milky Way globular clusters 5 –3 versial presence of intermediate-mass can reach up to 7 × 10 L⊙ (see

18 The Messenger 172 – June 2018 Harris, 1996 and 2010) — and over large cally sample the spatial distribution of size of 0.2 arcseconds each. We have sky areas (from 20–40-arcminute diameter stars (which is necessary to properly used the YJ grating covering the 1.00– and even more). This is conceptually easy construct the rotation curve) we 1.35 μm spectral range at a resolution but operationally challenging and requires planned a mosaic of between 4 and 9 R = 3400. This instrument setup is a lot of telescope time. To fill this gap in pointings in the 125-millarcsecond especially effective at simultaneously globular cluster studies and overcome mode (125 × 250 milliarcseconds/pixel, measuring a number of reference these difficulties, we recently conducted 8 × 8 arcseconds field of view). In the ­telluric lines in the spectra of giant a spectroscopic campaign based on densest clusters, an additional central stars for an accurate calibration of the a synergic use of three different VLT pointing was also planned in the , despite the relatively facilities. 50-milliarcsecond mode (50 × 100 milli- low spectral resolution. Typically, 7–8 arcseconds/pixel, 3 × 3 arcseconds pointings have been secured in each field of view). With the selected setup cluster. We selected red giant targets The survey (K grating, R = 4000), radial velocities with J < 14, with no stars brighter than can be properly measured from molec- J = 15 within 1 arcsecond from their The Multi-Instrument Kinematic Survey ular CO bandheads in the coolest centre. In order to minimise the effects of Galactic globular clusters (hereafter the giants (Teff < 5000 K) and from some of possible residual contamination MIKiS survey; Ferraro et al., 2018) has atomic lines in the warmer stars. Note from nearby stars and/or from the been specifically designed to provide the that, in the MIKiS approach, integral- unresolved stellar background, the velocity dispersion and rotation profiles field spectroscopy is used in a non- 1D spectra were extracted manually by of a representative sample of Milky Way conventional way; instead of using inte- visually inspecting each IFU and select- globular clusters over their entire radial grated-light spectra at various distances ing the brightest spaxel corresponding extension. With this aim, it takes advan- from the cluster centre, we extract the to each target star’s centroid. Normally, tage of the specific characteristics of spectrum of each distinguishable indi- one star was measured in each IFU three different instruments installed at the vidual star from the most exposed spa- though in a few cases two or more VLT, allowing multi-object spectroscopy tial pixel (spaxel) corresponding to the resolved stars were clearly distinguish- with the appropriate angular resolution, star centroid in the data cube. Typically able in a single KMOS IFU (see Figure 2), depending on the radial region surveyed. several dozens of giants have been and their spectra were extracted. The survey is based on two ESO Large observed down to K ~ 15 in each Programmes (193.D-0232 using the ­cluster with the planned mosaics (see 3. Wide-field multi-object spectroscopy K-band Multi-Object Spectrograph Figure 1). with FLAMES. This has been exploited [KMOS] and the Fibre Large Array Multi to sample the external cluster regions, Element Spectrograph [FLAMES], and 2. Seeing-limited observations with the i.e., out to several arcminutes. We used 197.D-0750 using SINFONI; Principal multi-integral-field spectrograph FLAMES in the combined GIRAFFE- Investigator Francesco Ferraro) and has KMOS MEDUSA mode, which consists of 132 been designed with the core scheme These observations have been per- described below. formed for an optimal coverage of the intermediate radial range (a scale Figure 1. Left panels: Comparison between a recon- 1. Diffraction-limited integral-field of tens of arcseconds). KMOS is a structed SINFONI image and an HST/ACS-HR C ­spectroscopy with adaptive-optics- spectrograph equipped with 24 deploy- image of the innermost 3 × 3 arcseconds of NGC 6388. Each SINFONI spaxel provides a spec- assisted SINFONI able integral-field units (IFUs) that can trum. The red crosses correspond to the stellar These observations are used to resolve be allocated within a 7.2-arcminute ­centroids identified from the HST image; 52 individual stars in the innermost few arcseconds diameter field of view. Each IFU covers RVs have been measured within 1.5 arcseconds of the highest-density clusters. We a ­projected area on the sky of about from the cluster centre. Right panels: The same in the case of NGC 2808, where 700 individual spectra adopted the SINFONI­ K-band grating to 2.8 × 2.8 arcseconds, sampled by an of resolved stars were extracted within 10 arcsec- achieve high Strehl ratios. To symmetri- array of 14 × 14 spaxels with an angular onds from the centre.

HST/ACS-HRC SINFONI

N

E

The Messenger 172 – June 2018 19 Astronomical Science Ferraro F. R. et al., MIKiS

Figure 2. The map in the upper panel shows the location of the targets observed with KMOS in NGC 6388. Lower panel: reconstructed images for 20 IFUs in a typical pointing.

deployable fibres that can be allocated within a 25-arcminute diameter field of view. The adopted HR 21 grating setup, with a resolving power R = 16 200 and spectral coverage from 8484 to 9001 Å, samples the prominent Ca II triplet lines, which are excellent features from which to measure radial velocities. The selected targets are essentially red giant branch stars brighter than I = 18.5. In order to avoid spurious contamination from other sources within the fibres, only isolated stars with no bright neighbours within 2 arcseconds from each target were selected. On average, 3–4 pointings were performed in each cluster.

This approach was first tested, as a proof of concept, on the Galactic globular clusters NGC 6388 (Lanzoni et al., 2013, ­Lapenna et al., 2015) and NGC 2808 (see Figures 1 and 2). Then, for the MIKiS ­survey we selected 30 massive globular clusters across the entire (Fig- ure 3), sampling both the bulge/disc stel- lar populations close to the Galactic plane (at z < 1.5 kpc) and the halo ones (at 1.5 < z < 13 kpc).

The targets properly map the parameter space most sensitive to the internal dynamical evolution of the cluster, cover- ing a wide range of central densities and concentration parametersa (2.5 < c < 0.5), and sampling all the stages of dynamical evolution, including both pre- and post- core-collapse globular clusters (with 2.5 a core relaxation time spanning almost 10 3 orders of magnitude). For a reliable determination of the line-of-sight velocity 2 dispersion profile the selected globular clusters are also populous enough (i.e.,

more luminous than MV = –6.5) to provide z 0 c 1.5 large samples of giant/sub-giant stars. To measure stars along a relevant portion

1 Figure 3. Distribution of the targets selected for the –10 MIKiS survey (large circles) showing the height on 0.5 the Galactic plane z vs. absolute integrated V-band magnitude MV (left) and the King concentration parameter c vs. core relaxation time t (right). The –7 –8 –9 789 c total Galactic globular cluster population is also M log t (years) V c ­plotted for reference (small dots).

20 The Messenger 172 – June 2018 Figure 4. Velocity dispersion profiles of NGC 6388 15 NGC 6388 NGC 2808 and NGC 2808 obtained from the radial velocities of individual stars (blue circles) measured following our multi-instrument approach: SINFONI+KMOS+- FLAMES. The number of individual spectra measured 20 with each instrument is labelled. The empty triangles­

) ) 10 in the left-hand panel show the velocity dispersion –1 –1 s s profile obtained from integrated-light spectroscopy m m (Lutzgendorf et al., 2011), which is probably affected (k (k ) ) by shot-noise bias. r r ( ( P P σ 10 σ 5 the innermost core regions of high-­ SINFONI KMOS FLAMES SINFONI KMOS FLAMES density systems, thus finally opening up (790) (96) (700) (52) (82) (276) the possibility­ of properly exploring the kine­matics of Galactic globular clusters 0 12 0 123 at sub-arcsecond scales (see Figures 1 log r (arcseconds) log r (arcseconds) and 4). For comparison, note that no proper motions have been determined in of the red giant branch with good signal- knowledge of the visible matter density the innermost 10 arcseconds of these to-noise in reasonable exposure times, distribution and the kinematic profiles dense globular clusters. For example, in the horizontal branch level is always can provide reliable estimates of the the case of NGC 6388, proper motions brighter than I = 16.5 (i.e., all the targets stellar densities, mass-to-light ratios, have been measured at only r > 20 arcsec- are located at distances < 16 kpc). More- and total cluster masses. onds (Watkins et al., 2015). over, we included only globular clusters 3. The exploration of the kinematics in with [Fe/H] > –1.8, showing metallic lines the proximity of the cluster tidal radius, Figure 5 summarises the results obtained deep enough to guarantee a few km s–1 which has major implications for the from the analysis of 6275 stars sampling accuracy in the radial velocities measure- understanding of the physical origin of the entire radial extension of 11 Galactic ments obtained from low-resolution recently claimed “extra-tidal structures”, globular clusters (Ferraro et al., 2018). This spectra. the interplay with the external tidal dataset allowed us to accurately deter- field, as well as the possible presence mine the systemic velocity and velocity As a first step, the MIKiS survey is of small dark matter halos or modifica- dispersion profile of each system, as well expected to provide the full characteri­ tions to the theory of gravity. as to investigate the presence of ordered sation of the line-of-sight internal kine- 4. A detailed characterisation of the kine- motions. It also provides the first kine- matics of each target cluster, from the matics of multiple populations with matical information for two poorly investi- innermost to the outermost regions. ­different light-element content, to gated clusters: NGC 1261 and NGC 6496. Once combined with measurements of ­provide crucial constraints to globular In the majority of the surveyed systems proper motion, the global project will cluster formation scenarios. we find evidence of rotation within a few ­provide the full 3D view of the velocity half-mass radii from the centre. These space of each system, obtained from results are in overall agreement with the hundreds individual stars, with crucial MIKiS first results predictions of recent theoretical studies, impact on many hot topics of globular suggesting that the detected signals cluster science. In particular, the MIKiS The results obtained so far clearly could be the relics of significant internal survey will provide: demonstrate the revolutionary potential rotation that was set at the epoch of the 1. The very first sub-arcsecond kinematic of the approach adopted in the MIKiS cluster’s formation. This evidence, com- exploration of Galactic globular cluster survey to study the kinematics of colli- bined with other recent results in the cores, thus allowing a systematic search sional systems. In this section, we give ­literature (see for example, Kamann et for signatures of systemic rotation an overview of the first set of results. We al., 2018), suggests that the vast majority 3 4 and intermediate-mass (10 –10 M⊙ ) find that the AO-corrected SINFONI of Galactic globular clusters (if not all of black holes, providing crucial new observations attained an angular resolu- them) display some level of internal rota- insights into the physics and formation tion comparable with that of the HST, thus tion. This might be the remnant signal of processes of both globular clusters allowing us to extract individual spectra a much larger amount of ordered motion and these elusive dark compact for 700 resolved stars within 10 arcsec- imprinted at birth (at the end of the initial objects. onds from the centre of NGC 2808 (see violent relaxation phase) which gradually 2. The determination of the mass distri- Figure 1), and even 52 individual star dissipated via two-body relaxation (see bution and the global amount of spectra within the central 2 arcseconds Tiongco et al., 2017). mass in dark remnants (white dwarfs, of the high-­density cluster NGC 6388 (see neutron stars, stellar mass black Lanzoni et al., 2013). This is indeed an Figure 6 shows the unprecedented rota- holes), since the kinematic profiles are unprecedented achievement; for the very tion curve derived for M5 (Lanzoni et al., sensitive to the whole mass enclosed first time, the radial velocities of hundreds 2018a). The velocity dispersion profile within stellar orbits. The simultaneous of individual stars have been measured in and the rotation curve in this cluster were

The Messenger 172 – June 2018 21 Astronomical Science Ferraro F. R. et al., MIKiS

Figure 5. Projected velocity dispersion profiles for 11 Galactic globular clusters observed in the MIKiS 4 survey (red filled circles). The solid lines correspond to the projected velocity dispersion profiles of the King models that best fit the observed density/­ 3 surface brightness distributions (from Ferraro et al., 2018). 2

1 NGC 288 NGC 6496 NGC 6171 determined from the radial velocity of more than 800 individual stars observed out to 700 arcseconds (~ 5 half-mass 5 radii) from the centre. The rotation curve 4 obtained is the cleanest and most coher- ent pattern ever observed in a globular 3 ) –1

cluster. The rotation axis has been s 2 NGC 3201 NGC 6723 NGC 1261

­measured in distinct concentric annuli at m (k

different distances from the cluster centre ) r ( and it turns out to be strikingly stable P (having a constant position angle of 145 σ 6 degrees with respect to the north-south direction at all surveyed radii). The well- 4 defined shape of the rotation curve is fully consistent with cylindrical rotation. The 2 NGC 1904 NGC 6254 NGC 5927 star density distribution shows a clear 10 flattening in the direction perpendicular to 10 1001000 the rotation axis, with the ellipticity (e) 8 increasing with increasing distance from the cluster centre, reaching a maximum 6 of e ~ 0.14 at r > 80 arcseconds. The peak 4 of the projected rotation velocity curve NGC 5272 NGC 362 (~ 3 km s−1) has been found at ~ 0.6 half- 2 mass radii, and its ratio with respect to 10 1001000 10 1001000 the central velocity dispersion is Vpeak /σ0 r (arcseconds) ~ 0.4. All of these results suggest that M5 is an oblate rotator that is seen almost edge on. Figure 6. The unprece- 4 dented rotation curve M5 obtained for M5 (­Lanzoni Figure 7 shows the results obtained in et al., 2018a). The blue another intriguing cluster: NGC 5986 2 circles mark the mean )

(Lanzoni et al., 2018b). The velocity dis- –1 stellar velocity as a s function of the projected persion profile (left panel) indicates a m

(k distance on either sides ­significant deviation from the King model 0 of the rotation axis (XR). R)

in the outer region (r > 100 arcseconds) (X The red line represents

t

ro the cylindrical rotation of the system. The clear-cut evidence V of systemic rotation is even more impres- –2 curve, which well repro- duces the observed sive. Indeed, the observed rotation curve profile. (right panel of Figure 7) is one of the best examples of solid-body rotation ever –4 found in a globular cluster. This is the first –400 –200 04200 00 time that these two kinematic features XR (arcseconds) have been jointly observed in a Galactic , and such co-existence in the physical interpretation of the struc- ubiquity of detections of even modest makes the case of NGC 5986 particularly ture and kinematics of the peripheries of signatures of rotation in globular clusters intriguing. In fact, these features may globular clusters and their possible role indicates that most of these systems­ result from the evolutionary interplay as new tools for near-field cosmology. were born with significant amounts of between the internal angular momentum ordered motion, thus pro­viding significant of the system and the effect of the tidal The first results of the MIKiS survey have constraints on globular cluster formation field of the host galaxy (Tiongco et al., highlighted the importance of an appro- models. On the other hand, the detection 2016, 2017). Such a discovery is particu- priate search for rotation signals over of intriguing features, such as those larly timely, in light of the growing interest the entire cluster extension. In fact, the observed in NGC 5986, sheds new light

22 The Messenger 172 – June 2018 4

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m

(k 0

R)

(X

t

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s 6 m (k ) r (

P – 4 σ –2 00 0 200 XR (arcseconds)

4 Figure 7. Left panel: Velocity dispersion profile of NGC 5986, showing a clear increasing trend in the outskirts. Right panel: Rotation curve of NGC 5986. The blue circles mark the mean stellar­ velocity as a function of the projected distance on either side of the rotation axis (XR). The solid line is the best least- squares fit to the observed points (from Lanzoni et 1.522.5 al., 2018b). log r (arcseconds)

on the complex interplay between globular­ Ferraro, F. R. et al. 2018, ApJ, 860, 50 Tiongco, M. A. et al. 2018, MNRAS, 475, L86 cluster internal evolution and the tidal Harris, W. E. 1996, AJ, 112, 1487 Watkins, L. L. et al. 2015, ApJ, 803, 29 Kamann, S. et al. 2018, MNRAS, 473, 5591 field of the Galaxy. King, I. R. 1966, AJ, 71, 64 Lane, R. R. et al. 2010, MNRAS, 406, 2732 Notes Lanzoni, B. et al. 2013, ApJ, 769, 107 References Lanzoni, B. et al. 2018a, ApJ, in press, a The shape of the profile is fixed by the concentration arXiv:1804.10509 parameter c, defined asc = log(rt /rc), where rc is Bellini, A. et al. 2014, ApJ, 797, 115 Lanzoni, B. et al. 2018b, ApJ, submitted the core radius of the model (roughly correspond- Carballo-Bello, J. A. et al. 2012, MNRAS, 419, 14 Lapenna, E. et al. 2015, ApJ, 798, 23 ing to the cluster-centric distance at which the Carretta, E. et al. 2009, A&A, 505, 117 Lutzgendorf, N. et al. 2011, A&A, 533, A36 ­projected density of stars drops to half of its central Chen, C. W. & Chen, W. P. 2010, ApJ, 721, 1790 Piotto, G. et al. 2015, AJ, 149, 91 value), and rc is the tidal radius (at which the stellar Dubath, P. et al. 1997, A&A, 324, 505 Tiongco, M. A. et al. 2016, MNRAS, 461, 402 density becomes zero). Tiongco, M. A. et al. 2017, MNRAS, 469, 683 )/ESO atacamaphoto.com G. Hüdepohl (

A drone’s eye view of the VLT.

The Messenger 172 – June 2018 23