Anomalous Surface Brightness Fluctuations in NGC 4489

Abstract

Anomalously high K-band surface brightness fluctuations (SBF) have been reported in NGC 4489 by Pahre & Mould (1994), Jensen et al. (1996) and Jensen et al. (1998). However, these conclusions were uncertain because of relatively low signal-to-noise data. New high signal-to-noise data for NGC 4489 have been obtained at the NOAO/KPNO 2.1m and the ESO/La Silla 3.5m NTT telescopes. Adopting the I-band SBF distance modulus determined by Tonry et al. (1990) and the M_I versus (V-I) calibration of Tonry et al. (2000), a value of M_K =-6.18 +/-0.09 mag was derived. Relative to the average empirical M_K derived for giant ellipticals by Jensen et al. (1998) (M_K=-5.61 +/- 0.12 mag), the detection of an anomalous K-band SBF in NGC 4489 is confirmed at the two sigma level. Such anomalous fluctuations could be caused by an extended giant branch, consisting of either intermediate-age AGB stars above the tip of the first-ascent giant branch or high-metallicity first-ascent giants, or by lack of a full understanding of the K-band SBF calibration. This result raises questions about the accuracy of K-band SBF distance measurements for elliptical galaxies with unknown stellar composition and underscores the need for M_K measurements over a larger range of color and luminosity.