This work continues our efforts to calibrate model surface brightness fluctuation luminosities for the study of unresolved stellar populations , through the comparison with data of Magellanic Cloud star clusters .
We present here the relation between absolute K _ { s } -band fluctuation magnitude and ( V - I ) integrated colour , using data from the 2MASS and DENIS surveys , and from the literature .
We compare the star cluster sample with the sample of early-type galaxies and spiral bulges studied by Liu et al .
( 50 ) .
We find that intermediate-age to old star clusters lie along a linear correlation with the same slope , within the errors , of that defined by the galaxies in the \bar { M } _ { K _ { s } } vs. ( V - I ) diagram .
While the calibration by Liu et al . was determined in the colour range 1.05 < ( V - I _ { c } ) _ { 0 } < 1.25 , ours holds in the interval -5 \ga \bar { M } _ { K _ { s } } \ga -9 , 0.3 \la ( V - I ) \la 1.25 .
This implies , according to Bruzual & Charlot ( 12 ) and Mouhcine & Lançon ( 56 ) models , that the star clusters and the latest star formation bursts in the galaxies and bulges constitute an age sequence .
At the same time , a slight offset between the galaxies and the star clusters [ the latter are \sim 0.7 mag fainter than the former at a given value of ( V - I ) ] , caused by the difference in metallicity of roughly a factor of two , confirms that the \bar { M } _ { K _ { s } } vs. ( V - I ) plane may contribute to break the age-metallicity degeneracy in intermediate-age and old stellar populations .
The confrontation between models and galaxy data also suggests that galaxies with K _ { s } fluctuation magnitudes that are brighter than predicted , given their ( V - I ) colour , might be explained in part by longer lifetimes of TP-AGB stars .
A preliminary comparison between the H 2MASS data of the Magellanic star clusters and the sample of 47 early-type galaxies and spiral bulges observed by Jensen et al .
( 42 ) through the F 160 W HST filter leads to the same basic conclusions : galaxies and star clusters lie along correlations with the same slope , and there is a slight offset between the star cluster sample and the galaxies , caused by their different metallicities .
Magellanic star clusters are single populations , while galaxies are composite stellar systems ; moreover , the objects analised live in different environments .
Therefore , our findings mean that the relationship between fluctuation magnitudes in the near-IR , and ( V - I ) might be a fairly robust tool for the study of stellar population ages and metallicities , could provide additional constraints on star formation histories , and aid in the calibration of near-IR SBFs for cosmological distance measurements .