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Paper: Gas Properties of the Medusa Minor Merger---Comparing with ULIRGs
Volume: 249, The Central Kiloparsec of Starbursts and AGN: the La Palma Connection
Page: 626
Authors: Aalto, S.; Hüttemeister, S.; Walter, F.
Abstract: High-resolution observations of 12CO and 13CO (1 --> 0) in the Medusa (NGC 4194) minor merger show the 12CO / 13CO (1 --> 0) intensity ratio (r) increasing from normal values (5--10) in the outer parts of the galaxy to high (> 20) values in the central, extended starburst region. Ratios >20 are otherwise typical of more luminous mergers. The Medusa LFIR / LCO ratio rivals that of ultraluminous galaxies (ULIRGs), despite the comparatively modest luminosity, indicating an exceptionally high star formation efficiency (SFE). We present models of the high-pressure ISM in a ULIRG and the relatively low-pressure ISM of Medusa. We discuss how these models may explain large cal R1 --> 0 in both types of distributions. Since the HCN emission is faint towards Medusa, we suggest that the SFE is not primarily controlled by the mass fraction of dense (n >~ 104 cm-3) gas, but is probably strongly dependent on dynamics. The bright HCN emission towards ULIRGs is not necessarily evidence that the IR emission there is always powered by starbursts.
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