We present a new morphological indicator designed for automated recognition of galaxies with faint asymmetric tidal features suggestive of an ongoing or past merger. We use the new indicator, together with preexisting diagnostics of galaxy structure to study the role of galaxy mergers in inducing (post-)starburst spectral signatures in local galaxies, and investigate whether (post-)starburst galaxies play a role in the the build up of the 'red sequence'. Our morphological and structural analysis of an evolutionary sample of 335 (post-)starburst galaxies in the SDSS DR7 with starburst ages 0 < t SB < 0.6 Gyr, shows that 45% of galaxies with young starbursts (t SB < 0.1 Gyr) show signatures of an ongoing or past merger. This fraction declines with starburst age, and we find a good agreement between automated and visual classifications. The majority of the oldest (post-)starburst galaxies in our sample (t SB ∼ 0.6 Gyr) have structural properties characteristic of early-type disks and are not as highly concentrated as the fully quenched galaxies commonly found on the 'red sequence' in the present day Universe. This suggests that, if (post-)starburst galaxies are a transition phase between active star-formation and quiescence, they do not attain the structure of presently quenched galaxies within the first 0.6 Gyr after the starburst.

Galaxy interactions are thought to be one of the main triggers of Active Galactic Nuclei (AGN), especially at high luminosities, where the accreted gas mass during the AGN lifetime is substantial. Evidence for a connection between mergers and AGN, however, remains mixed. Possible triggering mechanisms remain particularly poorly understood for luminous AGN, which are thought to require triggering by major mergers, rather than secular processes. We analyse the host galaxies of a sample of 20 optically and X-ray selected luminous AGN (log(L bol [erg/s]) > 45) at z ∼ 0.6 using HST WFC3 data in the F160W/H band. 15/20 sources have resolved host galaxies. We create a control sample of mock AGN by matching the AGN host galaxies to a control sample of non-AGN galaxies. Visual signs of disturbances are found in about 25% of sources in both the AGN hosts and control galaxies. Using both visual classification and quantitative morphology measures, we show that the levels of disturbance are not enhanced when compared to a matched control sample. We find no signs that major mergers play a dominant role in triggering AGN at high luminosities, suggesting that minor mergers and secular processes dominate AGN triggering up to the highest AGN luminosities. The upper limit on the enhanced fraction of major mergers is 20%. While major mergers might increase the incidence of (luminous AGN), they are not the prevalent triggering mechanism in the population of unobscured AGN.

Post-starburst galaxies can be identified via the presence of prominent Hydrogen Balmer absorption lines in their spectra. We present a comprehensive study of the origin of strong Balmer lines in a volume-limited sample of 189 galaxies with 0.01 < z < 0.05, log(M /M ) > 9.5 and projected axis ratio b/a > 0.32. We explore their structural properties, environments, emission lines and star formation histories, and compare them to control samples of star-forming and quiescent galaxies, and simulated galaxy mergers. Excluding contaminants, in which the strong Balmer lines are most likely caused by dust-star geometry, we find evidence for three different pathways through the post-starburst phase, with most events occurring in intermediate-density environments: (1) a significant disruptive event, such as a gas-rich major merger, causing a starburst and growth of a spheroidal component, followed by quenching of the star formation (70% of post-starburst galaxies at 9.5 < log(M /M ) < 10.5 and 60% at log(M /M ) > 10.5); (2) at 9.5 < log(M /M ) < 10.5, stochastic star formation in blue-sequence galaxies, causing a weak burst and subsequent return to the blue sequence (30%); (3) at log(M /M ) > 10.5, cyclic evolution of quiescent galaxies which gradually move towards the high-mass end of the red sequence through weak starbursts, possibly as a result of a merger with a smaller gas-rich companion (40%). Our analysis suggests that AGN are 'on' for 50% of the duration of the post-starburst phase, meaning that traditional samples of post-starburst galaxies with strict emission line cuts will be at least 50% incomplete due to the exclusion of narrow-line AGN.

We present optical integral field spectroscopy (IFS) observations of the Mice, a major merger between two massive ( 10 11 M ) gas-rich spirals NGC 4676A and B, observed between first passage and final coalescence. The spectra provide stellar and gas kinematics, ionised gas properties, and stellar population diagnostics, over the full optical extent of both galaxies with ∼1.6 kpc spatial resolution. The Mice galaxies provide a perfect case study that highlights the importance of IFS data for improving our understanding of local galaxies. The impact of first passage on the kinematics of the stars and gas has been significant, with strong bars most likely induced in both galaxies. The barred spiral NGC 4676B exhibits a strong twist in both its stellar and ionised gas disk. The edge-on disk galaxy NGC 4676A appears to be bulge free, with a strong bar causing its "boxy" light profile. On the other hand, the impact of the merger on the stellar populations has been minimal thus far. By combining the IFS data with archival multiwavelength observations we show that star formation induced by the recent close passage has not contributed significantly to the total star formation rate or stellar mass of the galaxies. Both galaxies show bicones of high ionisation gas extending along their minor axes. In NGC 4676A the high gas velocity dispersion and Seyfert-like line ratios at large scaleheight indicate a powerful outflow. Fast shocks (v s ∼ 350 km s −1 ) extend to ∼6.6 kpc above the disk plane. The measured ram pressure (P/k = 4.8 × 10 6 K cm −3 ) and mass outflow rate (∼8−20 M yr −1 ) are similar to superwinds from local ultra-luminous infrared galaxies, although NGC 4676A only has a moderate infrared luminosity of 3 × 10 10 L . Energy beyond what is provided by the mechanical energy of the starburst appears to be required to drive the outflow. Finally, we compare the observations to mock kinematic and stellar population maps extracted from a hydrodynamical merger simulation. The models show little enhancement in star formation during and following first passage, in agreement with the observations. We highlight areas where IFS data could help further constrain the models.

About 35 years ago a class of galaxies with unusual spectral characteristics was discovered in distant galaxy clusters [1, 2]. These objects, alternatively referred to as post-starburst, E+A or k+a galaxies, are now known to occur in all environments and at all redshifts [3][4][5][6][7], with many exhibiting compact morphologies and low-surface brightness features indicative of past galaxy mergers [3, 8]. They are commonly thought to represent galaxies that are transitioning from blue to red sequence, making them critical to our understanding of the origins of galaxy bimodality [9][10][11][12][13][14]. However, recent observational studies have questioned this simple interpretation [15, 18]. From observations alone, it is challenging to disentangle the different mechanisms that may lead to the post-starburst phase. Here we present examples of four different evolutionary pathways through the post-starburst phase found in the EAGLE cosmological simulation [19, 20]: blue→blue, blue→red, red→red and truncated. Each pathway is consistent with scenarios hypothesised for observational samples [2, 15, 18, 22]. The fact that post-starburst spectral characteristics can be attained via various evolutionary channels explains the diversity of properties of post-starburst galaxies found in observational studies, and lends support to the idea that slower quenching channels are dominant at low-redshift [23, 24] Most galaxies in the local Universe are either blue, gas-rich systems featuring a prominent disky component, where new stars are being formed from the interstellar gas reservoirs, or red, often spheroidal galaxies with little to no cold gas. The build up of the red population over cosmic time indicates that star-forming spirals are transformed into quiescent ellipticals. The paucity of intermediate "green valley" galaxies may imply that galax-ies preferentially undergo a rapid blue-to-red transformation, rather than gradually consuming their gas supply via steady star formation. Numerous scenarios have been proposed that could lead to such a rapid transformation of galaxies, with or without concurrent alteration of morphology from disk to spheroid; however, observations have found it difficult to corroborate or falsify these theories.Observations of post-starburst galaxies opened a new door into the study of galaxy evolution as their characteristics place them in a transition phase between the two main classes of the general galaxy population [9][10][11][12][13][14]. Idealised simulations of galaxy mergers confirm that a brief ( 600 Myr) post-starburst phase can follow a gas-rich merger, in which a powerful central starburst is succeeded by rapid quenching of star formation and significant morphological transformation [11, 25, 26]. However, a large fraction of local post-starburst galaxies contain considerable cold gas reservoirs that may return them to the blue sequence [16, 17], although the gas may not be in a favourable state for star formation [27]. Additionally, those in the densest environments may reflect stage...