Infrared Surface Brightness Fluctuations in Virgo Cluster Elliptical Galaxies

We imaged seven Virgo Cluster elliptical galaxies and the bulge of M31 in the near-infrared K' band (2.1 micron). The amplitude of the spatial surface brightness fluctuations (SBFs) was measured for each galaxy. Using the I-band SBF distance measurements for the Virgo galaxies, and a distance of 0.77 Mpc for M31, we determined a mean absolute fluctuation magnitude of M^bar^_K'_ = -5.62 +/- 0.29. Our calibration is consistent with Luppino & Tonry's Local Group determination of M^bar^_K'_ = -5.61 +/- 0.09 for M31, and it is also consistent with, but somewhat fainter than, Pahre & Mould's value of M^bar^_K_ = -5.77 +/- 0.18 from observations of six Virgo elliptical galaxies. Our results also compare favorably with theoretical predictions based on simple stellar populations. The scatter in our measurements is significant and appears to result from both observational uncertainty and variations in stellar populations. Nevertheless, the consistency of the IR observations on average and the relative brightness of fluctuations at K' suggest that IR SBFs may be a useful tool for measuring galaxy distances out to ~8000 km s^-1^. We find that the key observational limitations in using IR SBFs are accurately measuring the point-spread function (PSF) and subtracting the sky background, both of which strongly influence the fluctuation amplitude measured. A relationship was found between the fluctuation amplitude and the signal-to-noise ratio (S/N) of the observation: fluctuation amplitudes are systematically larger in low- S/N measurements. We attribute this relationship to present-day limitations of infrared arrays, which have sensitivity and dark current variations on scales comparable to those of the surface brightness fluctuations. In low-S/N observations, variations due to imperfect dark current subtraction and flat-fielding contribute significantly to the power spectrum, resulting in a biased measurement of the fluctuation power. To measure accurate IR SBF distances, a S/N ~> 4 is required. Significant radial variations in fluctuation power were not conclusively observed in any of the elliptical galaxies for which our S/N was sufficiently high to permit radial SBF analysis.