Quantifying galaxy morphology

How do the different shapes of galaxies arise? Milena Pawlik describes work to identify the role of galaxy mergers and starbursts in galactic evolution.Publisher PDFPeer reviewe

The origin and fate of post-starburst galaxies : morphological and structural image analysis of local galaxies with recent episodes of enhanced star formation

Post-starburst galaxies are a rare class of objects with unusual spectroscopic characteristics. Previous studies have shown that the presence of strong Balmer absorption lines in the spectra of these galaxies are consistent with a recent bust of star formation; however, the cause of such events remains unclear. Their environment and disturbed morphology suggest that many of them are likely remnants of gas-rich mergers of galaxies with comparable masses and models of galaxy mergers support that claim; however, some studies disagree and the origin of these curious systems remains an open debate. Post-starburst galaxies are also often regarded as a plausible transition channel between the blue continuously star-forming and quiescent red galaxies, commonly observed in the local Universe. This is supported by models of merger- driven starbursts, which cause structural transformation of galaxies consistent with evolution towards the red population, and can ultimately lead to quenching of star formation in the merger remnant; however, observational evidence for this scenario remains elusive. In my study I aimed to place further constraints on the role of galaxy mergers in triggering starbursts in local galaxies and to investigate whether the post-starburst galaxies are indeed in transition between the star-forming and passive phases of galaxy evolution, by the analysis of the morphology and structure of galaxies with spectroscopic signatures of a recent starburst. An important difference between this work and many previous studies is the post-starburst sample selection. Traditionally, post-starburst galaxies are selected to have completely quenched their star-formation; here, I also considered those which show spectroscopic sign of residual star-formation, selected using a method introduced by Wild et al. (2007). I have also followed a different approach to quantifying the morphology of galaxies, by means of an automated method for detecting visual post-merger signatures in galaxies, introduced as a part of this work. My analysis suggests that major mergers play a significant role in inducing starbursts in local Universe, but their significance declines as a function of the stellar mass of the galaxies. I also found that post-starburst galaxies have different structural properties in the low- and high-mass regimes and concluded that they are likely to be transitioning between the blue and red galaxy populations through a multi-stage scenario where both major and minor mergers are at play.European Career Reintegration Grant Phiz-Ev (P.I. Vivienne Wild

The redshift evolution of major merger triggering of luminous AGN:a slight enhancement at z∼2

TH was supported by STFC grant ST/M503812/1 during the course of this work. VW, JM-A, MP and KR acknowledge support from the European Research Council Starting Grant SEDmorph (P.I. V. Wild).Active galactic nuclei (AGN), particularly the most luminous AGN, are commonly assumed to be triggered through major mergers, however observational evidence for this scenario is mixed. To investigate any influence of galaxy mergers on AGN triggering and luminosities through cosmic time, we present a sample of 106 luminous X-ray selected type 1 AGN from the COSMOS survey. These AGN occupy a large redshift range (0.5 < z < 2.2) and two orders of magnitude in X-ray luminosity (∼1043 - 1045 erg s-1). AGN hosts are carefully mass and redshift matched to 486 control galaxies. A novel technique for identifying and quantifying merger features in galaxies is developed, subtracting galfit galaxy models and quantifying the residuals. Comparison to visual classification confirms this measure reliably picks out disturbance features in galaxies. No enhancement of merger features with increasing AGN luminosity is found with this metric, or by visual inspection. We analyse the redshift evolution of AGN associated with galaxy mergers and find no merger enhancement in lower redshift bins. Contrarily, in the highest redshift bin (z∼2) AGN are ∼4 times more likely to be in galaxies exhibiting evidence of morphological disturbance compared to control galaxies, at 99% confidence level (∼2.4σ) from visual inspection. Since only ∼15% of these AGN are found to be in morphologically disturbed galaxies, it is implied that major mergers at high redshift make a noticeable but subdominant contribution to AGN fuelling. At low redshifts other processes dominate and mergers become a less significant triggering mechanism.PostprintPeer reviewe

The Mice at play in the CALIFA survey: A case study of a gas-rich major merger between first passage and coalescence

We present optical integral field spectroscopy (IFS) observations of the Mice, a major merger between two massive (>10^11Msol) gas-rich spirals NGC4676A 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. The Mice provide a perfect case study highlighting 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 likely induced in both galaxies. The barred spiral NGC4676B exhibits a strong twist in both its stellar and ionised gas disk. On the other hand, the impact of the merger on the stellar populations has been minimal thus far: star formation induced by the recent close passage has not contributed significantly to the global star formation rate or stellar mass of the galaxies. Both galaxies show bicones of high ionisation gas extending along their minor axes. In NGC4676A the high gas velocity dispersion and Seyfert-like line ratios at large scaleheight indicate a powerful outflow. Fast shocks extend to ~6.6kpc above the disk plane. The measured ram pressure and mass outflow rate (~8-20Msol/yr) are similar to superwinds from local ULIRGs, although NGC4676A has only a moderate infrared luminosity of 3x10^10Lsol. Energy beyond that provided by the mechanical energy of the starburst appears to be required to drive the outflow. We compare the observations to mock kinematic and stellar population maps from a 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.Comment: 23 pages, 13 figures, accepted to A&A. A version with a complete set of high resolution figures is available here: http://www-star.st-and.ac.uk/~vw8/resources/mice_v8_astroph.pd

Secreted Human Amyloid Precursor Protein Binds Semaphorin 3a and Prevents Semaphorin-Induced Growth Cone Collapse

The amyloid precursor protein (APP) is well known for giving rise to the amyloid-β peptide and for its role in Alzheimer's disease. Much less is known, however, on the physiological roles of APP in the development and plasticity of the central nervous system. We have used phage display of a peptide library to identify high-affinity ligands of purified recombinant human sAPPα695 (the soluble, secreted ectodomain from the main neuronal APP isoform). Two peptides thus selected exhibited significant homologies with the conserved extracellular domain of several members of the semaphorin (Sema) family of axon guidance proteins. We show that sAPPα695 binds both purified recombinant Sema3A and Sema3A secreted by transfected HEK293 cells. Interestingly, sAPPα695 inhibited the collapse of embryonic chicken (Gallus gallus domesticus) dorsal root ganglia growth cones promoted by Sema3A (Kd≤8·10−9 M). Two Sema3A-derived peptides homologous to the peptides isolated by phage display blocked sAPPα binding and its inhibitory action on Sema3A function. These two peptides are comprised within a domain previously shown to be involved in binding of Sema3A to its cellular receptor, suggesting a competitive mechanism by which sAPPα modulates the biological action of semaphorins