We present new H -band echelle spectra , obtained with the NIRSPEC spectrograph at Keck II , for the massive star cluster “ B ” in the nearby dwarf irregular galaxy NGC 1569 .
From spectral synthesis and equivalent width measurements we obtain abundances and abundance patterns .
We derive an Fe abundance of [ Fe/H ] = -0.63 \pm 0.08 , a super-solar [ \alpha /Fe ] abundance ratio of +0.31 \pm 0.09 , and an O abundance of [ O/H ] = -0.29 \pm 0.07 .
We also measure a low ^ { 12 } C / ^ { 13 } C \approx 5 \pm 1 isotopic ratio .
Using archival imaging from the Advanced Camera for Surveys on board HST , we construct a colour-magnitude diagram ( CMD ) for the cluster in which we identify about 60 red supergiant ( RSG ) stars , consistent with the strong RSG features seen in the H -band spectrum .
The mean effective temperature of these RSGs , derived from their observed colours and weighted by their estimated H -band luminosities , is 3790 K , in excellent agreement with our spectroscopic estimate of T _ { eff } = 3800 \pm 200 K. From the CMD we derive an age of 15–25 Myr , slightly older than previous estimates based on integrated broad-band colours .
We derive a radial velocity of \langle v _ { r } \rangle = -78 \pm 3 km/s and a velocity dispersion of 9.6 \pm 0.3 km/s .
In combination with an estimate of the half-light radius of 0 \aas@@fstack { \prime \prime } 20 \pm 0 \aas@@fstack { \prime \prime } 05 from the HST data , this leads to a dynamical mass of ( 4.4 \pm 1.1 ) \times 10 ^ { 5 } M _ { \odot } .
The dynamical mass agrees very well with the mass predicted by simple stellar population models for a cluster of this age and luminosity , assuming a normal stellar IMF .
The cluster core radius appears smaller at longer wavelengths , as has previously been found in other extragalactic young star clusters .