arXiv:astro-ph/0011276v1 14 Nov 2000 oprtv gemn ihteNtoa cec Foundatio Science National the with Inc., agreement Astronomy, in cooperative Research for Universities of th sociation by operated Observatories, Astronomical Optical tional rg empirical erage r bevtr,L il,Cie S rga N program ESO Chile; Silla, La Observatory, ern M qaeo h aaydsac.We rprycalibrated properly When distance. the to proportional the inversely vari- of the is square distance, fluctuations on these the depend of While not ance each does fluctuations. pixel produces in per image flux populations mean galaxy stellar a of of pixel statistics Poissonian The Introduction 1. 1 ta.(98 ( (1998) al. et 4489 NGC S.Mei in Fluctuations Brightness Surface Anomalous tla content stellar – words: Key luminosity. and u VI airto fTnye l 20) au of value a (2000), al. et Tonry of calibration (V–I) sus 2 lsdtrie yTnye l 19)adthe and (1990) al. et Tonry 3.5m mod- by distance Silla SBF determined ESO/La I-band obtained ulus the the been Adopting and telescopes. have 2.1m NTT NOAO/KPNO 4489 high the NGC New at for data. signal-to-noise data uncertain Jensen low were signal-to-noise and relatively conclusions (1996) of these al. However, because by et (1998). 4489 Jensen al. NGC (1994), et in Mould reported & been Pahre have (SBF) fluctuations Abstract. 2000; 18, February Received e ihukonselrcmoiinadudrcrsthe underscores for and need composition stellar unknown galax- accuracy elliptical with for the ies measurements cal- about distance SBF SBF questions K–band K-band of raises the of result understanding This full ibration. a of lack giants, by first-ascent or high-metallicity first- ei- or the of of branch tip consisting giant the ascent branch, above giant could at AGB extended fluctuations intermediate-age confirmed ther an anomalous is by Such caused 4489 level. be NGC sigma two in the SBF K-band anomalous ⋆⋆ 11.04.1 are: codes thesaurus later) Your hand by inserted be (will no. manuscript A&A ⋆ bevtieMd–y´nes 4ae .Bln 10 Toul 31400 Belin, Midi–Pyr´en´ees, E. Observatoire ave. 14 uoenSuhr bevtr,Karl-Schwarzschild-Stra Observatory, Southern European K iiigAtooe,Kt ekNtoa bevtr,Na- Observatory, National Peak Kitt Astronomer, Visiting ae nosrain efre tteErpa South- European the at performed observations on Based = 12 − ..Silva D.R. , 6 M . 18 K nmlul ihKbn ufc brightness surface K-band high Anomalously ± aais itne niiul G 4489 NGC individual: – distances : esrmnsoe agrrneo color of range larger a over measurements M M 0 K . 9mgwsdrvd eaiet h av- the to Relative derived. was mag 09 K = eie o in litcl yJensen by ellipticals giant for derived 2 − ⋆⋆ 5 . ..Quinn P.J. , 61 ± 0 . 2,tedtcino an of detection the 12), 2 o 62.N-0876. M I under As- e ver- s ,878Grhn,Germany Garching, 85748 2, sse n. ue rne [email protected] France; ouse, ae eecpsad70 ms km 7000 and telescopes based ainadicesn h akrudso noise. shot background contami- the galaxy increasing cor- background and and and nation detect cluster to globular difficult for more rect significantly much is it background making disadvantage the brighter - major studies one at ground-based has for galaxies al. K-band et the for (Jensen However, wavelengths should distances 1998). optical it at SBF than infrared determine distances al larger the to et in possible Mei that be 1999; 2000). suggest al. al. arguments et et These Mei 2000; al. Jensen al. et 1998; et Jensen Liu al. 1999; 1994; et Mould Jensen & 1996; (Pahre solar-metallicity for populations age with stellar constant almost K– is absolute SBF that back- band both suggest Finally studies and I–band. theoretical the and clusters in empirical than (globular larger K–band is sources galaxies) between ground external SBF contrast the and color enhances SBF intrinsically the also is which Moreover seeing seeing contrast. I–band K–band than facilities, better ground Sec- distance. at given the any in ond, at extreme SBF and larger more giants producing is K–band, red populations brightest stellar between underlying contrast the the all of Jensen First 1998; al. et 1999). Jensen al. 1996; & et al. K– Pahre et 1993; Jensen infrared Tonry 1994; SBF & the Mould (Luppino Recently reasons in 2000). several for attempted al. band 1999; et been 1997; al. Tonry have et al. Pahre 1999; measurements et 1998; al. Tonry al. et et 1997; Lauer Blakeslee al. 1997; al. et et Ajhar Thomsen 1996; Thomsen Sodemann 1995; & Thomsen & (Sodemann (HST) Telescope itcladS itne pt 00k s km 4000 el- to measure up to distances successfully S0 used and liptical being have SBF et I–band Blakeslee by given re- been (a has 1999). populations method al. stellar the func- old on luminosity review in intrinsic cent giants are the red SBF in typically the tion, stars to brightest most Schneider & the the (Tonry contribute then pixel that the each stars to in The galaxy normalized 1988). the fluctuations indicator. of the flux distance of mean the flux a as the defined as of are variance used (SBF) Fluctuations be Brightness then Surface can variance this ASTROPHYSICS − ASTRONOMY 1 oebr1,2018 15, November nteHbl Space Hubble the on AND − 1 rmground from ⋆ 2 S.Mei, D.R. Silva, P.J. Quinn: Anomalous SBF in NGC 4489

Fig. 1. NGC 3379 external source luminosity function derived from the NTT data. The continuous line shows the sum of the adopted globular cluster plus background galaxy luminosity functions. The dashed line shows the globular cluster luminosity function, while the dotted line shows the background galaxy luminosity function. In total, 38 sources were detected between 15′′ and 60′′. ≈ ≈

Initial K-band SBF studies suggested that the M K varied used here as a fiducial galaxy. NGC 4489 is a low luminos- very little from galaxy-to-galaxy (Pahre & Mould 1994; ity . Its position angle and ellipticity Jensen et al. 1996). However, these studies also argued vary in a significant way with radius. Its profile deviates that two Virgo ellipticals (NGC 4365 and NGC 4489) had from a De Vaucouleurs law, rising steeply towards the cen- significantly brighter than average M K values suggesting ter. It has a Mg2 index of 0.198, different from the other the presence of an extended giant branch. This conclusion, Virgo galaxies. however, was weakened by two problems: (1) no correc- tion was made for globular cluster and background galaxy 2.2. Observations contamination; and (2) the SBF measurements were based on relatively low signal-to-noise data. Jensen et al. (1998) Data for NGC 3379 and NGC 4489 were obtained dur- re-measured M K for NGC 4365, using I-band data to de- ing two different observing runs. The first dataset was termine a correction for globular cluster contamination, obtained at the Kitt Peak National Observatory 2.1m on and concluded that NGC 4365 was a member of the back- 1999 February 7. The imaging camera was ONIS (OSU- ground W cloud and the M K for this galaxy was consis- NOAO Infrared Imaging Spectrometer) with a InSb 512 x tent with other Virgo ellipticals. In short, NGC 4365 ap- 1024 array detector. The gain is 4 e−/ADU, the read out pears to have a normal giant branch. No additional study noise 9 ADU, the dark current 1.2 e−/s . The pixel ≈ ≈ of NGC 4489 has been published until now. scale was 0.34′′ and the field of view 6′ x 3′. The filter In this study, new observations with significantly higher that was used was a K short filter Ksh, centered at 2.16 signal-to-noise and a new SBF analysis for NGC 4489 are µm. The Ksh filter is centered at shorter wavelength than presented. The observations are described in 2, the SBF the standard K filter to reduce thermal background and at § ′ analysis in 3 . The results of this analysis are presented larger wavelength than the K filter (Wainscoat and Cowie in 4 and § 5. § § 1992). Pahre and Mould (1994) estimated the difference between this filter and the standard K-band is of the or- 2. Observations der of 0.02 magnitudes for elliptical galaxies. We do not scale our results to the standard K-band filter, meaning 2.1. NGC 3379 and NGC 4489 the calculated K-band SBF magnitudes are magnitudes in The galaxies NGC 3379 in the Leo cluster and NGC the Ksh filter. 4489 in Virgo were observed. NGC 3379 has very well– The seeing was 1′′ for NGC 3379 and 0.9′′ for NGC 4489. established near–IR photometry (Frogel et al. 1978; Silva The photometric zero-point and the extinction coefficient & Bothun 1998) and has been already deeply studied in were extracted from the observations of NICMOS red the I–band by Sodemann & Thomsen (1995). It has been standards (Persson et al. 1998). A photometric zero-point S.Mei, D.R. Silva, P.J. Quinn: Anomalous SBF in NGC 4489 3

Fig. 2. NGC 4489 external source luminosity function derived from the NTT data. The continuous line shows the sum of the adopted globular cluster plus background galaxy luminosity functions. The dashed line shows the globular cluster luminosity function, while the dotted line shows the background galaxy luminosity function. In total, 62 sources were detected between 2′′ and 60′′. ≈ ≈ m1 = 21.32 mag and an extinction coefficient of 0.1 mag pixels. The total exposure times were 1380 secs for NGC in the Ksh were determined. The sky brightness was on av- 3379 and 3780 secs for NGC 4489. 2 erage 12.7 mag arcsec− . NGC 3379 was observed during The two sets of data were similarly processed. Each galaxy a non-photometric period. image was background subtracted and then divided by a Each galaxy was observed in a sequence of background- normalized dome flat field. Dark current was subtracted galaxy-background sets. At each position, the integration by this operation. Bad pixels and cosmic rays were elimi- time was 10 x 6 secs. After each set, a small random dither nated by a sigma clipping algorithm while combining the was executed to facilitate removal of bad pixels. The total images using the IRAF task IMCOMBINE. Sub-pixel reg- exposure times were 360 secs for NGC 3379 and 1700 secs istration was not used to avoid the introduction of corre- for NGC 4489. lated noise between the pixels into the images. The second dataset was obtained using the 3.5 m New Technology Telescope (NTT) at the European Southern 3. Surface Brightness Fluctuations Analysis Observatory, La Silla, Chile, on 1999 March 26. The in- Both datasets were analyzed by the SBF extraction stan- strument used was SOFI (Son OF ISAAC) with a Hawaii dard technique used e.g. by Tonry and Schneider (1988). HgCdTe 1024 x 1024 detector array. The gain is 5.53 A smooth galaxy model was built by fitting the galaxy e−/ADU, the read out noise 2.1 ADU, the dark current isophotes and then subtracted from the image. Visible ex- < 0.1e−/s . The pixel scale was 0.145′′/pixel, with a field sh ternal sources were not included in the fitting procedure. of view 2.47′ x 2.47′. The filter was again a K filter. The External point sources were then identified using Sextrac- seeing was 0.75′′ for NGC 3379 and 0.85′′ for NGC 4489. A tor (Bertin & Arnouts 1996) and subtracted. The residual photometric zero-point m1 = 22.32 mag and an extinction image was smoothed on a scale ten times the width of the coefficient of 0.07 mag were determined from NICMOS PSF. This smoothed image was subtracted from the resid- standards with solar-type colors (Persson et al. 1998). The 2 ual image to correct residual sky-subtraction errors. That sky brightness was on average 12.6 mag arcsec− . Both image was divided by the square root of the galaxy model galaxies were observed under photometric conditions. to make the amplitude of the SBF fluctuations constant NGC 3379 was observed in a sequence of galaxy- across the entire image. Finally, a point spread function background sets. At each position, the integration time (PSF) profile was determined from the bright stars in the 1 was 4 x 15 secs. NGC 4489 was observed in a sequence of image and normalized at 1 ADUs− . background-galaxy-background sets. At each position, the Various annuli were then analysed. The photometric com- integration time was 3 x 20 secs. After each set, a small pleteness function was calculated in each annulus by random dither was executed to facilitate removal of bad adding simulated point source images to the original, 4 S.Mei, D.R. Silva, P.J. Quinn: Anomalous SBF in NGC 4489

Fig. 3. NGC 3379 NTT power spectra. The NGC 3379 power spectra as a function of the wavenumber k are shown for different annuli. The fit of the power spectrum is given by the continuous line, the PSF power spectrum by the dotted line and the dashed line is the fitted constant white noise spectrum. See Table 1.

galaxy subtracted image. The photometric completeness point sources were masked up to a cut-off magnitude mcut, was calculated in intervals of 0.1 mag, as the ratio of the whose value for each annulus is given in Table 2, for KPNO simulated and the extracted point sources. The external observations, and in Table 1 and Table 3 for NTT obser- S.Mei, D.R. Silva, P.J. Quinn: Anomalous SBF in NGC 4489 5

Table 1. SBF measurements for various annuli of NGC 3379 from NTT observations

0 σP 0 1 Pes mK σ mcut Annulus P 0 P /P mK ′′ − − − ( ) (ADU s 1) (ADU s 1) (ADU s 1) (mag) (mag) (mag) 12 - 16 0.085 0.007 25 0.0076 24.86 0.1 19 16 - 21 0.096 0.005 20 0.0076 24.71 0.07 19 21 - 26 0.094 0.003 15 0.0076 24.69 0.04 19 26 - 30 0.100 0.002 13 0.0076 24.66 0.04 19 30 - 35 0.100 0.001 9 0.0076 24.66 0.03 19 35 - 39 0.094 0.002 8 0.0076 24.73 0.04 19 2 - 39 0.093 0.003 13.5 0.0076 24.70 0.03 19 Mean – – 24.72 0.06 –

vations. Next, the image power spectrum was calculated (1999a). For the background galaxies a power-law lumi- in each annulus and normalized to the number of non- nosity function was assumed: zero points in the annulus. The power spectrum was az- γm imuthally averaged. Nbg(m)= Nobg10 (4) The total image power spectrum is the sum of two compo- with γ =0.3 (Cowie et al. 1994). N and N were esti- nents: a constant power spectrum due to the white noise, ogc obg mated by the fitting these equations to the observed com- P and a power spectrum of the fluctuation and point 1 posite luminosity function determined from the external sources. Both components are convolved by the PSF in the sources extracted from the image. The actual m used spatial domain. In the Fourier domain this second compo- cut are given in Table 1, Table 2, and Table 3, while the ob- nent is given by a constant P multiplied by the power 0 served external source luminosity functions for NGC 3379 spectrum of the PSF: and NGC 4489 are shown respectively in Figure 1 and Figure 2, as derived from the NTT dataset. The fit has Egal = P0 EP SF + P1. (1) been made as from the sum of the globular cluster plus To calculate P0 and P1, a robust linear least square fit, background galaxies luminosity function: minimizing absolute deviation (Numerical Recipes, Press et al. 1992), was made to the power spectra of each an- nuli. Low (typically less than 4) wave number points were N(m)= Ngc(m)+ Nbg(m) (5) − − 2 (m mpeak) excluded from the fit since they are contaminated by the Nogc γm = e 2σ2 + Nobg 10 . galaxy subtraction and subsequent smoothing errors, plus √2πσ residual background variance contribution, as pointed out We keep all the parameters fixed in the fit, but Nogc e.g. by Jensen et al. (1999). and Nobg. We have assumed a galaxy distance modulus of P0 was then corrected for the contribution of point source 30.04 mag for NGC 3379 and of 31.15 mag for NGC 4489 fluctuations Pes, estimated from the equations: (see discussion in next section). Identified foreground stars 2 2 were not included in the fit. Pes = σgc + σbg (2) Pes was then calculated as the sum of: 2 as described by Blakeslee & Tonry (1995): σgc is the con- tribution to the fluctuations given by globular clusters, 2 σ2 = 1 N 100.8[m1 mpeak+0.4σ ln(10)] (6) σ2 is the contributions by background galaxies. It as as- gc 2 ogc − bg 2 mcut mpeak+0.8σ ln(10) sumed that the luminosity function of the globular clusters erfc[ − ] √2σ is given by: and −(m−m )2 Nogc peak Ngc(m)= e 2σ2 , (3) 2 Nobg 0.8(m mcut)+γ(mcut) √2πσ σ = 10 1− . (7) bg (0.8 γ)ln(10) − with σ = 1.35, M = 7.5 (Ferrarese et al. 2000) peakV where m is the zero magnitude which corresponds to a and (V–K) = 2.23 for NGC− 4489 and m = 22.70 1 peakB flux of 1 ADUs 1. and σ = 0.9 from Pritcher & Van den Bergh (1985) for − Finally, the apparent SBF magnitude was computed as: NGC 3379 and standard (V–K) color ( 3) from Kissler– Patig (2000). We have verified that exact≈ details in the mK = 2.5log(P0 Pes)+ m1 ǫextsec(z) (8) adopted globular cluster and background galaxy luminos- − − − ity functions have little effect on the final measurement of where ǫext is the extinction coefficient, and sec(z) the SBF amplitudes, as pointed out i.e. from Blakeslee et al. airmass for the observations. Color term and 6 S.Mei, D.R. Silva, P.J. Quinn: Anomalous SBF in NGC 4489 corrections are negligible (Liu et al. 2000). No galactic 4.2. NGC 4489 foreground reddening corrections were applied given that E(B–V) = 0.028 for NGC 4489 and E(B–V) = 0.028 The power spectra for NGC 4489 as derived from the NTT data are shown in Figure 4. The KPNO data produce for NGC 3379, implying AK = 0.01 for NGC 4489 and 0.009 for NGC 3379 (Schlegel et al. 1998). We measured similar results, albeit with lower signal-to-noise. External source detections from the NTT data were used to mask ǫext = 0.1 and m1 = 21.21 0.03 mag for the Kitt Peak ± the external sources from the KPNO dataset after appro- data and ǫext = 0.07 and m1 = 22.18 0.03 mag for the ± priate shifting and scaling. The results for each annulus for NTT data. The error on mK is given as the standard de- viation of the different annuli considered plus the entire the respective datasets are listed in Table 2 and Table 3. field. To this error, the errors due to the zero point magni- The standard deviation from each annulus calculated as the variance among different low wavelength cuts is listed. tude and ǫext calibration have been added in quadrature. To the fitting error the errors due to the photometric zero When a distance estimation from mK is made, the uncer- point calibration and to external source residual contribu- tainty of the SBF absolute magnitude calibration M K cal- culated from previous K-band SBF observations (Jensen tion subtraction are added in quadrature. et al. 1998) is also added in quadrature. The signal-to-noise ratio P0/P1 for NTT data in the four annuli permit us to assess the SBF absolute magnitude 4. Results for this galaxy with a percent error of approximately 10% (Mei et al. 2000). From the NTT data we derive a 4.1. NGC 3379 value of mK = 24.97 0.06 mag and from the KPNO data m = 24.98 0±.06 mag. The errors were calcu- The resultant NGC 3379 power spectra derived from the K lated by adding in quadrature± the standard deviation of NTT data are shown in Figure 3. The results for each an- the magnitudes derived in each annulus, divided by the nulus for NTT data are listed in Table 1. The standard number of considered values. Based on the most recent deviation from each annulus calculated as the variance m = 29.10 0.13 mag and (V I)=1.081 0.015 among different low wavelength cuts is listed. To the fit- I (Ferrarese et± al. 2000), and using the− M vs (V–I)± Tonry ting error, the errors due the photometric zero point cali- I calibration from Tonry et al. (2000), a distance modu- bration and to external source residual contribution sub- lus for this galaxy equal to (m - M )=31.15 0.11 traction were added in quadrature. The Kitt Peak data I I mag was adopted. K–band SBF absolute magnitudes± of for NGC 3379 were obtained under non-photometric con- NT T KPNO M = 6.18 0.13 mag and M = 6.17 0.13 ditions and are excluded from the final analysis. In both K − ± K − ± NGC 3379 datasets, here is a tendency to have fainter fluc- mag were determined. The average of these two values is M K = 6.18 0.09 mag. tuations in the center of the galaxy (Sodemann & Thom- − ± sen 1995). This result is in agreement with the Jensen et al. (1998) From NTT data a value of m = 24.72 0.06 mag was measurement of mK = 24.99 0.35 mag. It is not com- K ± derived. The error is given by adding in± quadrature the parable with previous measurements by Pahre & Mould standard deviation of the magnitudes derived in each an- (1994) and Jensen et al. (1996), because of the different nulus divided by the number of considered values. Pahre treatment of the external source contribution to the fluc- and Mould (1994) derived a value of mK = 24.48 0.1 mag tuations between this study and the previous studies. ± for NGC 3379 from the analysis of annuli between 4′′ The observed M K is inconsistent with the mean empiri- and 45 . They did not correct for contamination by≈ ex- cal value derived by Jensen et al. (1998) ( 5.61 0.12) ′′ − ± ternal≈ source contribution correction. This could explain at the two-sigma level, in the sense that NGC 4489 has why they reported slightly larger fluctuations. Based on brighter K-band surface brightness fluctuations than seen the Tonry et al. (1990) measurements of mI = 28.63 0.04 in bright, nearby, cluster ellipticals. This would appear mag and the most recent (V I)=1.223 0.015 mag± (Fer- to confirm the suggestion by Pahre & Mould (1994) and rarese et al. 2000) and the Tonry− et al. (2000)± calibration: Jensen et al. (1996) that NGC 4489 contains bright stellar component above the tip of the first ascent giant branch, M I = ( 1.74 0.08)+ (4.5 0.25) ((V I) 1.15), (9) − ± ± − − a so-called extended giant branch population. Those ini- a distance modulus of (mI - M I )=30.04 0.11 mag was tial studies argued that such a population could be sim- adopted. ± ilar to the K-band bright stellar population reported by Using the adopted I-band distance modulus, an absolute Freedman (1992) and Elston & Silva (1992) in M32 (see magnitude of M K = 5.32 0.13 mag was derived. This also Luppino & Tonry 1993 and Davidge 2000). The exact value is consistent with− measurements± for other ellipti- nature of such extended giant branches remains contro- cals presented in Jensen et al. (1998) sample and with versial, with most studies arguing for the presence of an nearly solar metallicity, 15 Gyr old models from Bruzual intermediate-age ( 2 Gyr) AGB. The most likely alter- & Charlot (2000), as analyzed by Mei et al. (1999), Mei native population currently∼ appears to be high-metallicity et al. (2000) and Liu et al. (2000) and from Worthey, as first ascent giant stars, brighter than normally observed in analyzed by Jensen et al. (1998). Galactic globular clusters (Guarnieri et al. 1997). S.Mei, D.R. Silva, P.J. Quinn: Anomalous SBF in NGC 4489 7

Fig. 4. NGC 4489 NTT power spectra. The NGC 4489 power spectra are shown as a function of the wavenumber k from NTT observation in various annuli. The fit of the power spectrum is given by the continuous line, the PSF power spectrum by the dotted line and the dashed line is the fitted constant white noise spectrum. See Table 3.

Table 2. SBF measurements for various annuli of NGC 4489 from KPNO observations

0 σP 0 1 Pes mK σ mcut Annulus P 0 P /P mK ′′ − − − ( ) (ADU s 1) (ADU s 1) (ADU s 1) (mag) (mag) (mag) 2 - 7 0.032 0.005 10 0.0009 24.85 0.15 19.9 7 - 12 0.026 0.004 4 0.0009 25.08 0.14 19.9 12 - 16 0.030 0.005 3 0.0009 24.92 0.13 19.9 2 - 16 0.026 0.001 3 0.0009 25.08 0.03 19.9 Mean – – 24.98 0.06 –

However, NGC 4489 is in fact significantly fainter and are no published empirical M K measures for ellipticals bluer (V–I = 1.081) than the mean elliptical in the Jensen for V–I 1, only theoretical predictions can be used to et al. calibration. This suggests that a comparison be- probe this∼ color range. Unfortunately, currently available tween the M K measured for NGC 4489 and the Jensen theoretical calculations of M K provide conflicting results. et al. mean value may not be strictly valid. Since there Models based on the latest Bruzual & Charlot models (e.g. 8 S.Mei, D.R. Silva, P.J. Quinn: Anomalous SBF in NGC 4489

Table 3. SBF measurements for various annuli of NGC 4489 from NTT observations

0 σP 0 1 Pes mK σ mcut Annulus P 0 P /P mK ′′ − − − ( ) (ADU s 1) (ADU s 1) (ADU s 1) (mag) (mag) (mag) 2 - 7 0.074 0.01 50 0.0022 24.96 0.18 19.9 7 - 12 0.068 0.007 40 0.0022 25.04 0.11 19.9 12 - 16 0.076 0.008 25 0.0022 24.91 0.13 19.9 2 - 16 0.070 0.002 25 0.0022 25.00 0.03 19.9 Mean – – 24.97 0.06 –

Liu et al. 2000; Mei et al. 2000), predict M K = 4.7 – 5.5 result of this paper concerns the issue of calibration. The for galaxies with V–I color similar to NGC 4489 for single- presently small sample of K–band measurements (around burst old populations. For these models, the near-IR SBF 20) would seem to suggest that M K varies very little from magnitudes become brighter as the galaxies becomes red- galaxy to galaxy. However, our result – a galaxy with der. On the other hand, independent models by Blakeslee, fluctuations different than the current average, illustrates Vazdekis, & Ahjar (2000) predict M K as bright as –6.5 the importance of an extended K–band calibration over a in this V–I color range, with near-IR SBF magnitudes be- larger range of luminosity and color. Only larger samples coming somewhat fainter as galaxies become redder. will allow us to determine whether or not NGC 4489 (or In the end, one thing is certain: NGC 4489 illustrates that for that matter M32) is an exceptional case, and to un- the Jensen et al. empirical mean M K is not universal. derstand the real potential of K–band SBF distance de- Whether or not this means that NGC 4489 has an unusual terminations. stellar population mixture cannot be answered without further empirical observations of ellipticals with similar Acknowledgements. S. Mei is grateful to P. Rosati and M. Ro- colors. maniello for the useful discussions and acknowledges support from the European Southern Observatory Studentship pro- gramme and Director General’s Discretionary Fund. 5. Summary and Conclusions New, high signal-to-noise K-band SBF measurements have References been obtained for NGC 3379 and NGC 4489. In summary: Ajhar E.A., Lauer, T.R., Tonry, J.L. et al., 1997, AJ, 114, 626 – K–band SBF M K values of 5.32 0.13 mag for NGC 3379 and 6.18 0.09 mag− for NGC± 4489 have been Bertin, E. & Arnouts, S. 1996, A&AS, 117, 393; derived. Although− ± the new apparent SBF magnitudes Blakeslee, J. P., Ajhar, E. A., Tonry, J. L., 1999, in Post- presented here are independent observations and are Hipparcos Cosmic Candles, eds. A. Heck & F. Caputo more accurate for NGC 4489 than previous determi- (Boston: Kluwer), 181 nations, the final K-band SBF absolute magnitudes Blakeslee J.P. & Tonry, J.L., 1995, ApJ, 442, 579; rely on the accuracy of the respective distance moduli Blakeslee J.P., Vazdekis, A., & Ajhar, E.A. 2000, MNRAS, determined in the I-band. in press. – Keeping in mind the caveat above about absolute mag- Cowie L.L. Gardner, J. P., Hu, E. M. et al., 1994, ApJ, nitudes, the measurement by Jensen et al. (1998) and 434, 114 Pahre & Mould (1994) of large K–band SBF in NGC Davidge, T.J. 2000, PASP, 112, 1177. 4489 has been confirmed. These fluctuations could be Elston, R. & Silva D.R., 1992, AJ, 104, 1360; the signature of an extended giant branch, but this Ferrarese L., Ford H. C., Huchra J. et al. 2000, ApJS, 128, cannot be confirmed until K-band SBF magnitudes are 431 determined for a larger sample of galaxies similar in Freedman, W.L. 1992, AJ, 104, 1349; color to NGC 4489. Frogel, J. A., Persson, S. E., Matthews, K. et al. 1978, ApJ, 220, 75 These results raise questions concerning the accuracy Gebhardt, K. & Kissler-Patig, M., 2000, AJ, 118, 1526 of K–band SBF distance measurements for elliptical galax- Guarnieri M.D., Renzini, A., Ortolani, S. 1997, ApJL, 477, ies. SBF measurements have been extended to this band 21; in the belief that one could determine with the same ac- Jensen, J.B., Luppino, G.A., Tonry, J.L., 1996, ApJ, 468, curacy K–band SBF distances to larger distances than 519 possible at optical wavelengths. This is possible only if Jensen, J.B., Tonry, J.L., Luppino, G.A., 1998, ApJ, 505, the contribution of external sources to the observed image 111 fluctuations can be correctly estimated and if the K–band Jensen, J.B., Tonry, J.L., Luppino, G.A., 1999, ApJ, 510, SBF absolute magnitude can be precisely calibrated. The 71 S.Mei, D.R. Silva, P.J. Quinn: Anomalous SBF in NGC 4489 9

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