Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...433..567p&link_type=abstract
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 433, no. 2, p. 567-576
Astronomy and Astrophysics
Astronomy
53
Astronomical Models, Brightness Distribution, Distance, Elliptical Galaxies, Infrared Astronomy, Variations, Virgo Galactic Cluster, Asymptotic Giant Branch Stars, Cameras, Color, M Stars, Magnesium, Mercury Cadmium Tellurides, Metallicity, Power Spectra, Red Shift, Reflecting Telescopes
Scientific paper
We observed NGC 3379 and nine Virgo elliptical galaxies at 2.16 micrometers using a NICMOS3 256 x 256 detector array. We have measured the surface brightness fluctuations in the K-band for all but one Virgo galaxy (an E/S0), thus extending the previous measurements in K by a factor of 3. We find for NGC 3379 that the apparent fluctuations magnitude (bar-m)K = 24.48 +/- 0.11, and we find no evidence at the 0.20 mag level for radial gradients in the apparent fluctuations magnitude. In the Virgo Cluster, we find that NGC 4365 has a similar fluctuation magnitude to the other galaxies in the sample, which is in contrast to previous I-band fluctuation observations which put it in the background W cloud. We also find an anomalous bright apparent fluctuation magnitude for the dwarf, low-surface brightness elliptical NGC 4489, which might be attributable to its lower signal-to-noise ratio (but is significant at the 3 sigma level) or to an unusual stellar population. The average fluctuation magnitude for the Virgo galaxies (excluding NGC 4489) is (bar-m)K = 25.16 +/- 0.18 with a 1 sigmarms dispersion of 0.25 mag. When we assume the distance moduli to these galaxies from I-band surface brightness fluctuation measurements, we find that for the sample of Virgo galaxies (excluding NGC 4365 and 4489), plus NGC 3379, M31, and M32 (the latter two from Luppino & Tonry 1993), the absolute fluctuation magnitude (bar-M)K = -5.74 +/- 0.18. We find an observed 1 sigmarms dispersion in the measurements of (bar-M)K of 0.20 magnitude, which is nearly that of our typical uncertainty of 0.18 mag in each measurement of (bar-M)K. The theoretical predictions of Worthey (1993) for (bar-M)K as a function of both (V-I) color and Mg2 index are shown to be consistent with the observations, differing only by +0.31 and -0.09 mag, respectively, from our mean measurement of (bar-M)K = -5.74 +/- 0.18. The simple stellar population models of Buzzoni (1993) are shown to be too faint in (bar-M)K by approximately 0.5 mag, which we suggest to be due to their exclusion of an empirically based M-giant population. We suggest that the origin of the dispersion in the measurement of (bar-M)K may be due to variations in metallicity; larger samples of early-type galaxies should be able to determine if there is such a relationship between metallicity and (bar-M)K. We present models of (bar-M)K based upon the addition to the stellar population of an anomalous asymptotic giant branch population contributing 10% of the light at 2 micrometers and show that this produces an absolute fluctuation magnitude 0.7 to 1.5 mag brighter. Such a population may be consistent with the observations of (bar-M)K in M32 by Luppino & Tonry (1993), but could only be present in NGC 4365 and 4489 of our sample, the two galaxies which are measured to have brighter (bar-m)K relative to (bar-m)I. Finally, we suggest that with the typically better seeing at the K band (compared with the optical), and with the brightness of the fluctuations at this wavelength, measurements of distances to greater redshifts are possible than with optical measurements of surface brightness fluctuations, although the problem of calibrating the relationship between (bar-M)K and (V-I) color or Mg2 index needs to be solved in order to reduce the uncertainties of such distance measurements.
Mould Jeremy R.
Pahre Michael A.
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