Radial Velocities of RR Lyrae Stars in and around NGC 6441

Abstract

Detailed elemental abundance patterns of metal-poor ([Fe/H]∼−1 dex) stars in the Galactic bulge indicate that a number of them are consistent with globular cluster (GC) stars and may be former members of dissolved GCs. This would indicate that a few per cent of the Galactic bulge was built up from destruction and/or evaporation of globular clusters. Here an attempt is made to identify such presumptive stripped stars originating from the massive, inner Galaxy globular cluster NGC 6441 using its rich RR Lyrae variable star (RRL) population. We present radial velocities of forty RRLs centered on the globular cluster NGC 6441. All of the 13 RRLs observed within the cluster tidal radius have velocities consistent with cluster membership, with an average radial velocity of 24 ± 5 km s −1 and a star-to-star scatter of 11 km s −1 . This includes two new RRLs that were previously not associated with the cluster. Eight RRLs with radial velocities consistent with cluster membership but up to three time the distance from the tidal radius are also reported. These potential extratidal RRLs also have exceptionally long periods, which is a curious characteristic of the NGC 6441 RRL population that hosts RRLs with periods longer than seen anywhere else in the Milky Way. As expected of stripped cluster stars, most are inline with the cluster's orbit. Therefore, either the tidal radius of NGC 6441 is underestimated and/or we are seeing dissolving cluster stars stemming from NGC 6441 that are building up the old spheroidal bulge.

A Chemical Composition Survey of the Iron-complex Globular Cluster NGC 6273 (M19)*

“…For the non-member stars, we found the average velocity and dispersion to be −29.36 km s -1 and σ=77.02 kms 1 . These values are in agreement with previous kinematic observations of similar off-axis bulge fields (e.g., Kunder et al 2012;Ness et al 2013a;Zoccali et al 2014 Alonso-García et al (2012). Note that the high reddening region on the eastern side of the cluster core correlates with the known position of an interstellar cloud (e.g., Harris et al 1976).…”

A Milky Way with a massive, centrally concentrated thick disc: new Galactic mass models for orbit computations

“…Moreover, this feature is not reported in all the observational studies (see, for example, Reid et al 2014). Secondly, there is growing evidence that the mass of any classical bulge, if present in the Milky Way, must be small (Shen et al 2010;Kunder et al 2012;Di Matteo et al 2014Kunder et al 2016). For these reasons, we prefer to present a second model, our Model II, which does not include any spherical central component, and which is still compatible with the rotation curve of the Galaxy, as given by Reid et al (2014).…”

“…The classical bulge appears to be very limited or not existent (Shen et al 2010;Kunder et al 2012;Di Matteo et al 2014Di Matteo 2016;Kunder et al 2016) and the α-enhanced thick disc appears to be as massive as the thin disc (Snaith et al 2014(Snaith et al , 2015. The latter was obtained (see Snaith et al 2014Snaith et al , 2015 by reconstructing the star formation history of the Milky Way disc by fitting a chemical evolution model to the age-[Si/Fe] relation found on solar vicinity data by Haywood et al (2013).…”

Bars and boxy/peanut bulges in thin and thick discs

“…Indeed, recent results suggest that the main component of the Milky Way bulge is a b/p (Ness et al 2012(Ness et al , 2013Wegg & Gerhard 2013); estimates place an upper limit on the mass of a possible classical bulge between ∼2-10% (Shen et al 2010;Kunder et al 2012Kunder et al , 2016Di Matteo et al 2014;Debattista et al 2017).…”