Galaxy-galaxy interactions are predicted to cause gas inflows leading to enhanced nuclear star formation. This prediction is borne out observationally, and is also supported by the gas-phase metallicity dilution in the inner regions of galaxies in close pairs. In this paper we test the further prediction that the gas inflows lead to enhanced accretion on to the central supermassive black hole, triggering activity in the nucleus. Based on a sample of 11 060 Sloan Digital Sky Survey galaxies with a close companion (r p < 80 h −1 70 kpc, V < 200 km s −1 ), we classify active galactic nuclei (AGN) based either on emission line ratios or on spectral classification as a quasar. The AGN fraction in the close pairs sample is compared to a control sample of 110 600 mass-and redshift-matched control galaxies with no nearby companion. We find a clear increase in the AGN fraction in close pairs of galaxies with projected separations < 40 h −1 70 kpc by up to a factor of 2.5 relative to the control sample [although the enhancement depends on the chosen signal-to-noise ratio (S/N) cut of the sample]. The increase in AGN fraction is strongest in equal-mass galaxy pairings, and weakest in the lower mass component of an unequal-mass pairing. The increased AGN fraction at small separations is accompanied by an enhancement in the number of 'composite' galaxies whose spectra are the result of photoionization by both AGN and stars. Our results indicate that AGN activity occurs (at least in some cases) well before final coalescence and concurrently with ongoing star formation. Finally, we find a marked increase at small projected separations of the fraction of pairs in which both galaxies harbour AGN. We demonstrate that the fraction of double AGN exceeds the expected random fraction, indicating that some pairs undergo correlated nuclear activity. We discuss some of the factors that have led to conflicting results in previous studies of AGN in close pairs. Taken together with complementary studies, we favour an interpretation where interactions trigger AGN, but are not the only cause of nuclear activity.

In order to investigate the effects of galaxy mergers throughout the interaction sequence, we present a study of 10,800 galaxies in close pairs and a smaller sample of 97 post-mergers identified in the Sloan Digital Sky Survey. We find that the average central star formation rate (SFR) enhancement (×3.5) and the fraction of starbursts (20 per cent) peak in the postmerger sample. The post-mergers also show a stronger deficit in gas phase metallicity than the closest pairs, being more metal-poor than their control by −0.09 dex. Combined with the observed trends in SFR and the timescales predicted in merger simulations, we estimate that the post-mergers in our sample have undergone coalescence within the last few hundred Myr. In contrast with the incidence of star-forming galaxies, the frequency of active galactic nuclei (AGN) peaks in the post-mergers, outnumbering AGN in the control sample by a factor of 3.75. Moreover, amongst the galaxies that host an AGN, the black hole accretion rates in the closest pairs and post-mergers are higher by a factor of ∼ 3 than AGN in the control sample. These results are consistent with a picture in which star formation is initiated early on in the encounter, with AGN activity peaking post-coalescence.

We present a sample of 1899 galaxies with a close companion taken from the Sloan Digital Sky Survey Data Release 7. The galaxy pairs are selected to have velocity differences Δv < 300 km s−1, projected separations (rp) < 80 h70−1 kpc, mass ratios between 0.1 and 10, and robust measurements of star formation rates and gas‐phase metallicities. We match the galaxies in total stellar mass, redshift and local density to a set of 10 control galaxies per pair galaxy. For each pair galaxy, we can therefore calculate the statistical change in star formation rate (SFR) and metallicity associated with the interaction process. Relative to the control sample, we find that galaxies in pairs show typical SFR enhancements that are, on average, 60 per cent higher than the control sample at rp < 30 h70−1 kpc. It is at these small separations that the strongest enhancements in SFR (by up to a factor of ∼10) are measured, although such starbursts are rare, even amongst the closest pairs. In addition, the pairs demonstrate more modest SFR enhancements of ∼30 per cent out to at least 80 h70−1 kpc (the widest separations in our sample). This is the first time that enhanced SFRs have been robustly detected out to such large projected separations. Galaxies in both major and minor mergers show significant SFR enhancements at all rp, although the strongest starbursts (with SFR enhancements of a factor of ∼10) appear to be found only in the major mergers. We also find evidence that SFR enhancements are synchronized in an interacting pair, such that a higher SFR in one galaxy is accompanied by an increased SFR in its companion. For the first time, we are also able to trace the metallicity changes in galaxy pairs as a function of projected separation. The metallicity is generally diluted in galaxy pairs by ∼0.02 dex, with an average metallicity decrement of −0.03 dex at the smallest separations, a trend that mirrors the SFR enhancements as a function of rp. The SFR and metallicity trends with projected separation are interpreted through a comparison with theoretical models. These simulations indicate that the peak in SFR enhancements at small separations is due to systems near the end of the merger process. The extended plateau in SFR enhancements out to at least 80 h70−1 kpc is dominated by galaxies that have made a pericentric passage and are now experiencing triggered star formation on their trajectory towards apogalacticon, or on a subsequent close approach.

We use pair and environmental classifications of ∼ 211 000 star-forming galaxies from the Sloan Digital Sky Survey, along with a suite of merger simulations, to investigate the enhancement of star formation as a function of separation in galaxy pairs. Using a new technique for distinguishing between the influence of nearby neighbours and larger scale environment, we find a clear enhancement in star formation out to projected separations of ∼ 150 kpc, beyond which there is no net enhancement. We find the strongest enhancements at the smallest separations (especially < 20 kpc), consistent with earlier work. Similar trends are seen in the simulations, which indicate that the strongest enhancements are produced in highly disturbed systems approaching final coalescence, whereas the more modest enhancements seen at wider separations are the result of starburst activity triggered at first pericentre passage, which persists as the galaxies move to larger separations. The absence of any net enhancement beyond 150 kpc provides reassurance that the detected enhancements are due to galaxy−galaxy interactions, rather than larger scale environmental effects or potential pair selection biases. A rough census indicates that 66 per cent of the enhanced star formation in our pair sample occurs at separations > 30 kpc. We conclude that significant interaction-induced star formation is not restricted to merger remnants or galaxies with close companions; instead, a larger population of wider separation pairs exhibit enhanced star formation due to recent close encounters.

The star formation rates (SFRs) and metallicities of a sample of 294 galaxies with visually classified, strong, large‐scale bars are compared to a control sample of unbarred disc galaxies selected from the Sloan Digital Sky Survey Data Release 4. The fibre (inner few kpc) metallicities of barred galaxies are uniformly higher (at a given mass) than the unbarred sample by ∼0.06 dex. However, the fibre SFRs of the visually classified barred galaxies are higher by about 60 per cent only in the galaxies with total stellar mass M★ > 1010 M⊙. The metal enhancement at M★ < 1010 M⊙ without an accompanying increase in the SFR may be due to a short‐lived phase of early bar‐triggered star formation in the past, compared to on‐going SFR enhancements in higher mass barred galaxies. There is no correlation between bar length or bar axial ratio with the enhancement of the SFR. In order to assess the relative importance of star formation triggered by bars and galaxy–galaxy interactions, SFRs are also determined for a sample of close galaxy pairs. Both mechanisms appear to be similarly effective at triggering central star formation for galaxies with M★ > 1010 M⊙. However, due to the much lower fraction of pairs than bars, bars account for ∼3.5 times more triggered central star formation than interactions.