Institution(s): 1. University of Hong Kong
The central giant elliptical galaxies of cool-core clusters have been found to exhibit CO luminosities rivaling those of ultraluminous infrared galaxies. The standard CO-to-H2 conversion yields H2 gas masses of up to ~1011 M⊙ in these galaxies. In the central giant elliptical galaxy of the Perseus cluster, NGC 1275, the molecular gas is distributed in a filamentary network that extends to ~60 kpc from the center of the galaxy. Much of the CO emission, however, originates from a few radial filaments having H2 masses of ~108-9 M⊙ and lying within ~10 kpc of the center of NGC 1275. Detectable dust emission originates from the same region, and has an overall morphology similar to the CO-luminous filaments. Based on the H2 surface-mass density, the Kennicutt–Schmidt law predicts a star-formation rate of at least 0.1-10 M⊙/yr in each of these filaments. Based on the infrared dust luminosity, the Kennicutt relation predicts an overall star-formation rate of ~24 M⊙/yr. Yet, apart from one and perhaps two isolated giant HII regions, there is no evidence for vigorous star formation in the CO-luminous filaments. Instead, hundreds of blue star clusters have been found distributed all over NGC 1275, lying as far out as ~30 kpc from the center of the galaxy. We show that the vast majority of the blue star clusters projected against the central region of NGC 1275 are actually associated with giant HII regions in a foreground galaxy rather than with NGC 1275. The remainder preferentially avoid rather than coincide with the CO-luminous filaments, similar to those found elsewhere in the galaxy. We explore reasons for why the standard CO-to-H2 conversion and/or the empirical relationships between gas/dust emission and star-formation rate appears to break down in NGC 1275. The answer is probably rooted in the origin of the molecular gas, which in central cluster elliptical galaxies may be related to an X-ray cooling flow. Because (some of) the molecular gas and dust in massive high-z galaxies also may originate from cooling flows, caution must be taken when applying empirical relationships between their gas/dust emission and star-formation rates.