Complex trajectories of a simple pendulum

Accepted versio

An infrared study of local galaxy mergers

A. Carpineti, et al., “An infrared study of local galaxy mergers”, Astronomy & Astrophysics, Vol. 577, May 2015. This version of record is available online at: https://doi.org/10.1051/0004-6361/201425276 Reproduced with Permission from Astronomy and Astrophysics, © ESO 2016.We combine a large, homogeneous sample of $\sim$3000 local mergers with the Imperial IRAS Faint Source Redshift Catalogue (IIFSCz), to perform a blind far-infrared (FIR) study of the local merger population. The IRAS-detected mergers are mostly ($98\%$) spiral-spiral systems, residing in low density environments, a median FIR luminosity of $10^{11} L_\odot$ (which translates to a median star formation rate of around 15$M_\odot yr^{-1}$). The FIR luminosity -- and therefore the star formation rate -- shows little correlation with group richness and scales with the total stellar mass of the system, with little or no dependence on the merger mass ratio. In particular, minor mergers (mass ratios $Peer reviewedFinal Published versio

Galaxy interactions II: High density environments

With the aim to assess the role of dense environments in galaxy interactions, properties we present an analysis of close galaxy pairs in groups and clusters, obtained from the Sloan Digital Sky Survey Data Release 7 (SDSS-DR7). We identified pairs that reside in groups by cross-correlating the total galaxy pair catalogue with the SDSS-DR7 group catalogue from Zapata et al. (2009). We classify pair galaxies according to the intensity of interaction. We analysed the effect of high density environments on different classes of galaxy-galaxy interactions and we have also studied the impact of the group global environment on pair galaxies. We find that galaxy pairs are more concentrated towards the group centres with respect to the other group galaxy members, and disturbed pairs show a preference to contain the brightest galaxy in the groups. The color-magnitude relation exhibits significant differences between pair galaxies and the control sample, consisting in color tails with a clear excess of extremely blue and red galaxies for merging systems. In addition, pair galaxies show a significant excess of young stellar populations with respect to galaxies in the control sample; this finding suggests that, in dense environments, strong interactions produce an important effect in modifying galaxy properties. We find that the fraction of star forming galaxies decreases toward the group centre; however, galaxy pairs show a more efficient star formation activity than galaxies without a close companion. We have also found that pair galaxies prefer groups with low density global environments with respect to galaxies of the corresponding control sample. Blue, young stellar population galaxies prefer groups within low density global environments.Comment: 10 pages, 11 figures, accepted for publication in A&

General Aspects of PT-Symmetric and P-Self-Adjoint Quantum Theory in a Krein Space

In our previous work, we proposed a mathematical framework for PT-symmetric quantum theory, and in particular constructed a Krein space in which PT-symmetric operators would naturally act. In this work, we explore and discuss various general consequences and aspects of the theory defined in the Krein space, not only spectral property and PT symmetry breaking but also several issues, crucial for the theory to be physically acceptable, such as time evolution of state vectors, probability interpretation, uncertainty relation, classical-quantum correspondence, completeness, existence of a basis, and so on. In particular, we show that for a given real classical system we can always construct the corresponding PT-symmetric quantum system, which indicates that PT-symmetric theory in the Krein space is another quantization scheme rather than a generalization of the traditional Hermitian one in the Hilbert space. We propose a postulate for an operator to be a physical observable in the framework.Comment: 32 pages, no figures; explanation, discussion and references adde

MASSIV: Mass Assembly Survey with SINFONI in VVDS. V. The major merger rate of star-forming galaxies at 0.9 < z < 1.8 from IFS-based close pairs

We aim to measure the major merger rate of star-forming galaxies at 0.9 < z <1.8, using close pairs identified from integral field spectroscopy (IFS). We use the velocity field maps obtained with SINFONI/VLT on the MASSIV sample, selected from the star-forming population in the VVDS. We identify physical pairs of galaxies from the measurement of the relative velocity and the projected separation (r_p) of the galaxies in the pair. Using the well constrained selection function of the MASSIV sample we derive the gas-rich major merger fraction (luminosity ratio mu = L_2/L_1 >= 1/4), and, using merger time scales from cosmological simulations, the gas-rich major merger rate at a mean redshift up to z = 1.54. We find a high gas-rich major merger fraction of 20.8+15.2-6.8 %, 20.1+8.0-5.1 % and 22.0+13.7-7.3 % for close pairs with r_p <= 20h^-1 kpc in redshift ranges z = [0.94, 1.06], [1.2, 1.5) and [1.5, 1.8), respectively. This translates into a gas-rich major merger rate of 0.116+0.084-0.038 Gyr^-1, 0.147+0.058-0.037 Gyr^-1 and 0.127+0.079-0.042 Gyr^-1 at z = 1.03, 1.32 and 1.54, respectively. Combining our results with previous studies at z < 1, the gas-rich major merger rate evolves as (1+z)^n, with n = 3.95 +- 0.12, up to z = 1.5. From these results we infer that ~35% of the star-forming galaxies with stellar masses M = 10^10 - 10^10.5 M_Sun have undergone a major merger since z ~ 1.5. We develop a simple model which shows that, assuming that all gas-rich major mergers lead to early-type galaxies, the combined effect of gas-rich and dry mergers is able to explain most of the evolution in the number density of massive early-type galaxies since z ~ 1.5, with our measured gas-rich merger rate accounting for about two-thirds of this evolution.Comment: Published in Astronomy and Astrophysics, 24 pages, 30 figures, 2 tables. Appendix with the residual images from GALFIT added. Minor changes with respect to the initial versio

Galaxy Zoo: The properties of merging galaxies in the nearby Universe - Local environments, colours, masses, star formation rates and AGN activity

‘The definitive version is available at www3.interscience.wiley.com '. Copyright Royal Astronomical Society.Following the study of Darg et al., we explore the environments, optical colours, stellar masses, star formation and active galactic nucleus activity in a sample of 3003 pairs of merging galaxies drawn from the Sloan Digital Sky Survey using visual classifications from the Galaxy Zoo project. While Darg et al. found that the spiral-to-elliptical ratio in (major) mergers appeared higher than that of the global galaxy population, no significant differences are found between the environmental distributions of mergers and a randomly selected control sample. This makes the high occurrence of spirals in mergers unlikely to be an environmental effect and must therefore arise from differing time-scales of detectability for spirals and ellipticals. We find that merging galaxies have a wider spread in colour than the global galaxy population, with a significant blue tail resulting from intense star formation in spiral mergers. Galaxies classed as star-forming using their emission-line properties have average star formation rates approximately doubled by the merger process though star formation is negligibly enhanced in merging elliptical galaxies. We conclude that the internal properties of galaxies significantly affect the time-scales over which merging systems can be detected (as suggested by recent theoretical studies) which leads to spirals being 'over-observed' in mergers. We also suggest that the transition mass 3 × 10 M, noted by Kauffmann et al., below which ellipticals are rare could be linked to disc survival/destruction in mergers. © 2009 RAS.Peer reviewe

Galaxy interactions I: Major and minor mergers

We study galaxy pair samples selected from the Sloan Digital Sky Survey (SDSS-DR7) and we perform an analysis of minor and major mergers with the aim of investigating the dependence of galaxy properties on interactions. We build a galaxy pair catalog requiring rp < 25 kpc h-1 and Delta V < 350 km s-1 within redshift z<0.1. By visual inspection of SDSS images we removed false identifications and we classify the interactions into three categories: pairs undergoing merging, M; pairs with evident tidal features, T; and non disturbed, N. We also divide the pair sample into minor and major interactions according to the luminosity ratio of the galaxy members. We study star formation activity through colors and star formation rates. We find that 10% of the pairs are classified as M. These systems show an excess of young stellar populations as inferred from the Dn(4000) spectral index, colors, and star formation rates of the member galaxies, an effect which we argue, is directly related to the ongoing merging process. We find 30% of the pairs exhibiting tidal features (T pairs) with member galaxies showing evidence of old stellar populations. Regardless of the color distribution, we find a prominent blue peak in the strongest mergers, while pairs with tidal signs under a minor merger show a strong red peak. Therefore, our results show that galaxy interactions are important in driving the evolution of galaxy bimodality. By adding stellar masses and star formation rates of the two members of the pairs, we explore the global efficiency of star formation of the pairs as a whole. We find that, at a given total stellar mass, major mergers are significantly more efficient (a factor 2) in forming new stars, with respect to both minor mergers or a control sample of non-interacting galaxies.Comment: 10 pages, 14 figures, submitted to A&

Activity in galactic nuclei of cluster and field galaxies in the local universe

We study the environmental effects on the activity in galactic nuclei by comparing galaxies in clusters and in the field. Using a spectroscopic sample of galaxies in Abell clusters from the SDSS DR7, we investigate the dependence of nuclear activity on the physical parameters of clusters as well as the nearest neighbor galaxy. We also compare galaxy properties between AGN hosts and non-AGN galaxies. We find that the AGN fraction of early-type galaxies starts to decrease around one virial radius of clusters (r_{200,cl}) as decreasing clustercentric radius, while that of late types starts to decrease close to the cluster center (R~0.1-0.5r_{200,cl}). The AGN fractions of early-type cluster galaxies on average are found to be lower than those of early-type field galaxies by a factor ~3. However, the mean AGN fractions of late-type cluster galaxies are similar to those of late-type field galaxies. The AGN fraction of early-type BCGs lies between those of other early-type, cluster and field galaxies with similar luminosities. In the field, the AGN fraction strongly depends on the morphology of and the distance to the nearest neighbor galaxy. We find an anti-correlation between the AGN fraction and the velocity dispersion of clusters for all subsamples divided by morphology and luminosity of host galaxies. The AGN power indicated by L_[OIII]/M_BH is found to depend strongly on the mass of host galaxies rather than the clustercentric radius. The difference in physical parameters such as luminosity, (u-r) colors, SFRs, and (g-i) color gradients between AGN hosts and non-AGN galaxies is seen for both early and late types at all clustercentric radii, while the difference in structure parameters between the two is significant only for late types. These results support the idea that the nuclear activity is triggered through galaxy-galaxy interactions and mergers when gas supply for AGN is available.Comment: 15 pages, 13 figures. To appear in A&

Pattern Recognition in Astrophysics and the Anthropic Principle

The role of the Anthropic Principle in astrophysics and cosmology is examined in two principal parts. The first (minor) part takes a chiefly philosophical perspective and examines the manner in which human cognition features into discussions on cosmic origins. It is shown that the philosophical questions raised by the Anthropic Principle and ‘fine-tuning of life’ bear resemblances to problems within the philosophy of mind and we seek a common origin for this surprising parallel. A form of ‘epistemic structural realism’ is defended and used to critique the physicalist identity thesis. It is argued that equating ‘reality’ with mathematical structures, which is the basis of the identity thesis, leads to incoherent conclusions. Similar reasoning is used to critique infinite Multiverse theories. In the second (major) part, we gradually transition into mainstream astrophysics, first presenting a new line of research to explore counterfactual universes using semi-analytic models (SAMs) and offering a preliminary study wherein the cosmological constant is varied and the effects on ‘advanced civilisations’ are examined. The importance of galaxy mergers is highlighted and leads to their study. We first try solving the pattern-recognition problem of locating mergers using the Galaxy Zoo database and produce the largest homogenous merger catalogue to date. We examine their properties and compare them with the SAMs of the Millennium Simulation finding good general agreement. We develop the Galaxy Zoo approach with a new visual-interface design and double the size of the merger catalogue of SDSS mergers in the local Universe.</p

Tidal dwarf galaxies in the nearby Universe

We present a statistical observational study of the tidal dwarf (TD) population in the nearby Universe, by exploiting a large, homogeneous catalogue of galaxy mergers compiled from the SDSS. 95% of TD-producing mergers involve two spiral progenitors, while most remaining systems have at least one spiral progenitor. The fraction of TD-producing mergers where both parents are early-type galaxies is <2%, suggesting that TDs are unlikely to form in such mergers. The bulk of TD-producing systems inhabit a field environment and have mass ratios greater than 1:7 (the median value is 1:2.5). TDs forming at the tidal-tail tips are ~4 times more massive than those forming at the base of the tails. TDs have stellar masses that are less than 10% of the stellar masses of their parents and typically lie within 15 optical half-light radii of their parent galaxies. The TD population is typically bluer than the parents, with a median offset of ~0.3 mag in the (g-r) colour and the TD colours are not affected by the presence of AGN activity in their parents. An analysis of their star formation histories indicates that TDs contain both newly formed stars (with a median age of ~30 Myr) and old stars drawn from the parent disks, each component probably contributing roughly equally to their stellar mass. Thus, TDs are not formed purely through gas condensation in tidal tails but host a significant component of old stars from the parent disks. Finally, an analysis of the TD contribution to the local dwarf-to-massive galaxy ratio indicates that ~6% of dwarfs in nearby clusters may have a tidal origin, if TD production rates in nearby mergers are representative of those in the high-redshift Universe. Even if TD production rates at high redshift were several factors higher, it seems unlikely that the entire dwarf galaxy population today is a result of merger activity over the lifetime of the Universe