1. Star Formation Enhancement in Close Pairs Each galaxy pair consists of a primary galaxy and a companion - we examine the specific star- formation rate of the primary galaxy in terms of the properties of the interacting pair. Figure 4. plots the dependence of specific star formation rate (  m ) on projected pair separation. We distinguish between three morphologically defined sub-samples: late, `mixed’ and early type galaxies. Data points plot the mean specific star formation rate for a given projected separation bin. A two power law function is fitted to the un-binned data. Error bars show sample variance. For each morphological group there is an increase in the specific star formation rate for projected separations less than about 25 kpc. The effect is strongest for late-type galaxies. For late-type systems the mean specific star- formation rate decreases systematically with projected separation out to separations of about 300 kpc. Dependence on Perturber Investigated the dependence of tidal triggering on the nature of the perturbing galaxy - examine morphological type and the mass of the perturber. Examine the distribution of specific star formation rates for late-type galaxies that have a companion within 50 kpc as a function of morphological type and mass. Morphological Type: galaxy pairs split into three sub-sample: late-late, late-mixed and late-early pairs. – Kolmogorov-Smirnov (K-S) test indicates no significant differences between the SSFRs of these sub-samples. Mass: galaxy pairs split into three sub-samples according to z-band magnitude difference between primary and companion galaxy – K-S test indicates no significant difference between the SSFRs of these sub-samples. z-band cut-off means we only probe a maximum of a factor of 6 difference in mass. Bulge Stability ? Trend of decreasing specific star formation rate out to 300 kpc for late-type systems. Result may be due to bulge stability (Mihos & Hernquist 1996) => simulations show galaxies with weak bulge components (high concentration indices) are more susceptible to bar formation. Bars provide a mechanism for gas inflow to nucleus over prolonged period of time => enhanced star formation at larger separations. Systems with weak bulges (lower concentration indices), are stabilized against bar formation => star formation enhancement delayed until final stage of merger. Figure 4. Specific star formation rates (  m ) as a function of pair separation for late (circles), mixed (diamonds) and early (squares) galaxy types. Comparison with FIR Investigate biases due to Hα star formation rate estimator by comparison with the Far Infrared (FIR). Scanpi service used to obtain IRAS fluxes for the late-type systems in primary catalogue - biased towards galaxies with higher star formation rates. These are the systems in which we anticipate greatest problems due to aperture and extinction corrections. IRAS data low resolution - IRAS fluxes could be artificially enhanced by the companion galaxy for close pairs => plot two mass normalisations. - Normalise FIR SFR by mass of primary galaxy alone. - For galaxy pairs closer than 1.5 arcminutes – normalise FIR SFR by sum of the primary and companion galaxy mass. Good agreement between the FIR and Hα data both in terms of trend with projected pair separation and magnitude of specific star formation rate. For close pairs agreement improved by normalisation using both the mass of the primary and companion galaxy. Figure 5. Specific star formation rates for galaxies detected by IRAS; Hα (open circles) ; FIR (open diamonds); FIR normalised by mass of primary and companion galaxies if their separation is less than 1.5 arcminutes (solid squares). Nuclear Concentration of Light in Close Pairs Figure 6. plots the mean concentration index, C, against projected pair separation for the full volume limited sample. The concentration index peaks at separations of  75 kpc and declines rapidly for separations less than 50 kpc => galaxies with close companions have unusually centrally concentrated light profiles. Images of low concentration index systems at small separations indicate they are late-type galaxies with strong nuclear starbursts. Turnover of mean concentration index at 50-75 kpc could be indicating the characteristic scale for triggering of a nuclear starburst. Problem: Intense nuclear starburst may result in a galaxy being placed in the wrong morphological class. –Enhanced SSFR for early and mixed systems may result partly from the misclassification of late-type galaxies with strong nuclear starbursts. C < 0.35 Figure 6. Mean concentration index, C, as a function of pair separation for the full volume limited sample. 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