The globular cluster NGC 7492 and the Sagittarius tidal stream: together but unmixed

Carballo‐Bello, J. A.; Corral-Santana, J. M.; Catelan, M.; Martínez–Delgado, David; Muñoz, Ricardo R.; Sollima, A.; Navarrete, C.; Duffau, S.; Côté, Patrick; Mora, Marcelo D.

doi:10.1093/mnras/stx3001

Cited by 7 publications

(8 citation statements)

References 38 publications

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“…The N-body simulation of Peñarrubia et al (2010) predicts the presence of the Sagittarius stream at a coincident distance in the same portion of the sky occupied by Whiting 1 and NGC 7492, although only the former has a compatible radial velocity. In a recent spectroscopic follow-up, Carballo-Bello et al (2017, 2018b confirmed this hypothesis by revealing the presence of peaks in the velocity distribution of their sample that cannot be associated with the Galactic field. From these studies, they identified the presence of the Sagittarius stream in the neighbourhood of these GCs although only Whiting 1 seems to be kinematically linked to this substructure.…”

Section: Discussionmentioning

confidence: 90%

“…We therefore conclude that the detected stellar populations are due to the remnants of dwarf galaxies within which these two GCs are embedded. In particular, these two GCs are known to be located along the path of the Sagittarius stream, and the presence of the stellar population of this remnant in the surroundings of these two GCs has been revealed by previous works (Carballo-Bello et al 2014, 2018b.…”

Section: E T H O Dmentioning

confidence: 86%

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A survey for dwarf galaxy remnants around 14 globular clusters in the outer halo

Sollima

Delgado

Muñoz

et al. 2018

Monthly Notices of the Royal Astronomical Society

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We report the results of a systematic photometric survey of the peripheral regions of a sample of 14 globular clusters in the outer halo of the Milky Way at distances d GC > 25 kpc from the Galactic Centre. The survey is aimed at searching for the remnants of the host satellite galaxies where these clusters could originally have been formed before being accreted on to the Galactic halo. The limiting surface brightness varies within our sample, but reaches μ V, lim = 30-32 mag arcsec −2. For only two globular clusters (NGC 7492 and Whiting 1; already suggested to be associated with the Sagittarius galaxy), we detect extended stellar populations that cannot be associated with either the clusters themselves or with the surrounding Galactic field population. We show that the lack of substructures around globular clusters at these Galactocentric distances is still compatible with the predictions of cosmological simulations whereby in the outer halo the Galactic globular cluster system is built up through hierarchical accretion at early epochs.

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Section: Discussionmentioning

confidence: 90%

Section: E T H O Dmentioning

confidence: 86%

A survey for dwarf galaxy remnants around 14 globular clusters in the outer halo

Sollima

Delgado

Muñoz

et al. 2018

Monthly Notices of the Royal Astronomical Society

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“…Moreover, globular clusters (GCs) are progressively gaining consideration as building blocks of yet unknown fractions of the Galactic stellar halo, which is also bolstered by their observed disruption in the tidal field of the MW via tidal tails in the Galactic halo (e.g., Odenkirchen et al 2001;Lee et al 2004;Lauchner et al 2006;Chun et al 2010;Jordi & Grebel 2010;Niederste-Ostholt et al 2010;Sollima et al 2011;Myeong et al 2017;Navarrete et al 2017;Carballo-Bello et al 2018;Kuzma et al 2018), the detection of extratidal stars with a likely GC origin based on, e.g., photometric filtering techniques or stellar abundances and velocities (e.g., Jordi & Grebel 2010;Kunder et al 2014;Anguiano et al 2015Anguiano et al , 2016Navin et al 2016; (Hollyhead et al 2017(Hollyhead et al , 2018 and down to cluster ages of ∼2 Gyr (Martocchia et al 2018a).…”

Section: Introductionmentioning

confidence: 99%

Purveyors of fine halos: Re-assessing globular cluster contributions to the Milky Way halo buildup with SDSS-IV

Koch

Grebel

Martell

2019

A&A

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There is ample evidence in the Milky Way for globular cluster (GC) disruption. Hence one may expect that also part of the Galactic halo field star population may once have formed in GCs. We seek to quantify the fraction of halo stars donated by GCs by searching for stars that bear the unique chemical fingerprints typical for a subset of GC stars often dubbed as "second-generation stars". These are stars showing light element abundance anomalies such as a pronounced CN-band strength accompanied by weak CH-bands. Based on this indicator, past studies have placed the fraction of halo stars with a GC origin between a few to up to 50%. Using low-resolution spectra from the most recent data release (DR14) of the latest extension of the Sloan Digital Sky Survey (SDSS-IV), we were able to identify 118 metal-poor (−1.8 ≤ [Fe/H] ≤ −1.3) CN-strong stars in a sample of 4470 halo giant stars out to ∼50 kpc. This increases the number of known halo stars with GC-like light-element abundances by a factor of two and results in an observed fraction of these stars of 2.6±0.2%. Using an updated formalism to account for the fraction of stars lost early on in the GCs' evolution we thus estimate the fraction of the Galactic halo that stems from disrupted clusters to be very low, at 11±1%. This number would represent the case that stars lost from GCs were entirely from the first generation and is thus merely an upper limit. Our conclusions are sensitive to our assumptions of the mass lost early on from the first generation formed in the GCs, the ratio of first-to-second generation stars, and other GC parameters. We carefully test the influence of varying these parameters on the final result and find that, under realistic scenarios, the above fraction depends on the main assumptions at less than 10 percentage points. We further recover a flat trend in this fraction with Galactocentric radius, with a marginal indication of a rise beyond 30 kpc that could reflect the ex-situ origin of the outer halo as is also seen in other stellar tracers.

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“…Other formation theories, where GCs formed in gasrich discs and major mergers of galaxies do not require the presence of a dark matter halo (Kravtsov & Gnedin 2005). In this scenario, the peculiarities could be explained by the GCs still being in the debris of a disrupted dwarf galaxy after a merger (Carballo-Bello et al 2018).…”

Section: Old and New Observables And Their Possible Dynamical Interpr...mentioning

confidence: 91%

“…Traditional expectations of Newtonian dynamics in the outskirts of GCs would suggest a decreasing velocity dispersion profile with increasing radius. Earlier and more recent empirical evidence suggests that, in some Galactic clusters, the velocity dispersion may be elevated and the surface density may be raised compared to this expectation (Drukier et al 1998;Lane et al 2011;Carballo-Bello et al 2018) which has led some to propose the inclusion of additional physics beyond Newtonian predictions. These explanations include modified theories of gravity (Hernandez & Jiménez 2012), where once a star passes below a threshold value in acceleration (and also the clusters orbital velocity around the galaxy is below the same threshold) the star can enter a modified dynamics regime (Milgrom 1983), which can lead to a flat velocity dispersion at large distances from the cluster centre.…”

Section: Old and New Observables And Their Possible Dynamical Interpr...mentioning

confidence: 99%

Spherical models of star clusters with potential escapers

Claydon

Gieles

Varri

et al. 2019

Monthly Notices of the Royal Astronomical Society

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An increasing number of observations of the outer regions of globular clusters (GCs) have shown a flattening of the velocity dispersion profile and an extended surface density profile. Formation scenarios of GCs can lead to different explanations of these peculiarities, therefore the dynamics of stars in the outskirts of GCs are an important tool in tracing back the evolutionary history and formation of star clusters. One possible explanation for these features is that GCs are embedded in dark matter halos. Alternatively, these features are the result of a population of energetically unbound stars that can be spatially trapped within the cluster, known as potential escapers (PEs). We present a prescription for the contribution of these energetically unbound members to a family of self-consistent, distribution function-based models, which, for brevity, we call the Spherical Potential Escapers Stitched (spes) models. We show that, when fitting to mock data of bound and unbound stars from an N -body model of a tidally-limited star cluster, the spes models correctly reproduce the density and velocity dispersion profiles up to the Jacobi radius, and they are able to recover the value of the Jacobi radius itself to within 20%. We also provide a comparison to the number density and velocity dispersion profiles of the Galactic cluster 47 Tucanae. Such a case offers a proof of concept that an appropriate modeling of PEs is essential to accurately interpret current and forthcoming Gaia data in the outskirts of GCs, and, in turn, to formulate meaningful present-day constraints for GC formation scenarios in the early universe.

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The globular cluster NGC 7492 and the Sagittarius tidal stream: together but unmixed

Cited by 7 publications

References 38 publications

A survey for dwarf galaxy remnants around 14 globular clusters in the outer halo

A survey for dwarf galaxy remnants around 14 globular clusters in the outer halo

Purveyors of fine halos: Re-assessing globular cluster contributions to the Milky Way halo buildup with SDSS-IV

Spherical models of star clusters with potential escapers

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