Galaxy interactions – poor starburst triggers

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Abstract
We report on a study of tidally triggered star formation in galaxies based on spectroscopic/photometric observations in the optical/near-IR of a magnitude limited sample of 59 systems of interacting and merging galaxies and a comparison sample of 38 normal isolated galaxies. From a statistical point of view the sample gives us a unique opportunity to trace the effects of tidally induced star formation. In contrast to results from previous investigations, our global UBV colours do not support a significant enhancement of starforming activity in the interacting/merging galaxies. We also show that, contrary to previous claims, there is no significantly increased scatter in the colours of Arp galaxies as compared to normal galaxies. We do find support for moderate (a factor of ∼2-3) increase in star formation in the very centres of the interacting galaxies of our sample, contributing marginally to the total luminosity. The interacting and in particular the merging galaxies are characterized by increased far infrared (hereafter FIR) luminosities and temperatures that weakly correlate with the central activity. The LFIR/LB ratio however, is remarkably similar in the two samples, indicating that true starbursts normally are not hiding in the central regions of the FIR luminous cases. The gas mass-to-luminosity ratio in optical-IR is practically independent of luminosity, lending further support to the paucity of true massive starburst galaxies triggered by interactions/mergers. We estimate the frequency of such cases to be of the order of ∼0.1% of the galaxies in an apparent magnitude limited sample. Our conclusion is that interacting and merging galaxies, from the global star formation aspect, generally do not differ dramatically from scaled up versions of normal, isolated galaxies. No drastic change with redshift is expected. One consequence is that galaxy formation probably continued over a long period of time and did not peak at a specific redshift. The effects of massive starbursts, like blowouts caused by superwinds and cosmic reionization caused by starburst populations would also be less important than what is normally assumed.