A Cluster of Receding Halo Stars Towards Norma

Mon, November 30, 2015

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We present results from follow-up spectroscopic observations of clustered Cepheid candidates identified from K-band light curves towards the Norma constellation (Chakrabarti et al. 2015), as well as others that we have found more recently. The average radial velocity of these stars is ~ 200 km/s, which is large and distinct from that of the Galaxy’s stellar disk. These objects at l ~ −27◦ and b ~ −1◦ are therefore halo stars; using the period- luminosity relation of Type I Cepheids, they are at ~ 90 kpc. While the spectra do not have sufficient S/N to independently determine the metallicity and spectral type of the stars, there is a clear correspondence between the observed Brackett series lines in these observations and in known Type I Cepheids. Distances determined from the K-band period-luminosity relation (Matsunaga et al. 2013) and the 3.6 μm period-luminosity relation (Scowcroft et al. 2011) agree closely. The extinction corrected J − Ks colors of these sources are comparable to known Type I Cepheids. The observed radial velocity of these stars agrees with predictions from dynamical models (Chakrabarti & Blitz 2009). If these stars are indeed members of the predicted dwarf galaxy that perturbed the outer HI disk of the Milky Way, this would represent the first application of Galactoseismology. Young Cepheid variables are unexpected in models of the Galactic halo, though star formation due to infall of gas-rich dwarf galaxies may well produce a small population of yet undiscovered Cepheids in the outer halo.