Specific Frequency
The specific frequency ($S_N$) is a metric used by astronomers to compare the globular cluster populations of galaxies by normalizing their total counts to a standard galactic luminosity. It is formally defined as:
$$S_N \equiv N_t 10^{0.4(M_V + 15)}$$
where:
$N_t$: The total number of globular clusters in the galaxy and $M_V$: The absolute visual magnitude of the parent galaxy.
This formula effectively calculates how many globular clusters a galaxy would have if its absolute magnitude were adjusted to a standard value of $M_V = -15$.
Observations indicate that the specific frequency correlates strongly with a galaxy’s Hubble type:
Spiral and Irregular Galaxies: These “late-type” systems generally have low specific frequencies. For instance, $Sc$ and $Sb$ spirals typically have $S_N$ values ranging from $0.5$ to $1.2$.
Elliptical Galaxies: Earlier-type, more spheroidally dominant galaxies exhibit much higher specific frequencies. Giant $cD$ galaxies found at the centers of clusters can host tens of thousands of clusters, with $S_N$ values reaching as high as $15$.
Evolutionary Significance: This trend suggests that galaxies with more prominent spheroidal components were significantly more efficient at forming globular clusters during their early evolutionary stages. Consequently, the specific frequency serves as a vital probe for understanding the early formation history and environment of different galaxy types.