ALBERT

Description: With ALMA (Atacama Large Millimeter/submillimeter Array) making it possible to resolve giant molecular clouds (GMCs) in other galaxies, it is becoming necessary to quantify the observational bias on measured GMC properties. Using a hydrodynamical simulation of a barred spiral galaxy, we compared the physical properties of GMCs formed in position–position–position (PPP) space to the observational position–position–velocity (PPV) space. We assessed the effect of disc inclination: face-on (PPV face ) and edge-on (PPV edge ), and resolution: 1.5 pc versus 24 pc, on GMC properties and the further implications of using Larson's scaling relations for mass–radius and velocity dispersion–radius. The low-resolution PPV data are generated by simulating ALMA Cycle 3 observations using the casa package. Results show that the median properties do not differ strongly between PPP and PPV face under both resolutions, but PPV edge clouds deviate from these two. The differences become magnified when switching to the lower, but more realistic resolution. The discrepancy can lead to opposite results for the virial parameter's measure of gravitational binding, and therefore the dynamical state of the clouds. The power-law indices for the two Larson's scaling relations decrease going from PPP, PPV edge to PPV face and decrease from high to low resolutions. We conclude that the relations are not entirely driven by the underlying physical origin and therefore have to be used with caution when considering the environmental dependence, dynamical state, and the extragalactic CO-to-H 2 conversion factor of GMCs.

Description: We explore the regulation of star formation in star-forming galaxies through a suite of high-resolution isolated galaxy simulations. We use the smoothed particle hydrodynamics code gasoline , including photoelectric heating and metal cooling, which produces a multi-phase interstellar medium (ISM). We show that representative star formation and feedback sub-grid models naturally lead to a weak, sub-linear dependence between the amount of star formation and changes to star formation parameters. We incorporate these sub-grid models into an equilibrium pressure-driven regulation framework. We show that the sub-linear scaling arises as a consequence of the non-linear relationship between scaleheight and the effective pressure generated by stellar feedback. Thus, simulated star formation regulation is sensitive to how well vertical structure in the ISM is resolved. Full galaxy discs experience density waves which drive locally time-dependent star formation. We develop a simple time-dependent, pressure-driven model that reproduces the response extremely well.

Description: We use cosmological hydrodynamic simulations to consistently compare the assembly of dwarf galaxies in both dominated, cold dark matter (CDM) and self-interacting dark matter (SIDM) models. The SIDM model adopts a constant cross-section of 2 cm 2 g –1 , a relatively large value to maximize its effects. These are the first SIDM simulations that are combined with a description of stellar feedback that naturally drives potential fluctuations able to create dark matter (DM) cores. Remarkably, SIDM fails to significantly lower the central DM density within the central 500 pc at halo peak velocities V max  〈 30 km s –1 . This is due to the fact that the central regions of very low mass field haloes have relatively low central velocity dispersion and densities, leading to time-scales for SIDM collisions greater than a Hubble time. CDM haloes with V max  〈 30 km s –1  have inefficient star formation, and hence weak supernova feedback. At a fixed 2 cm 2 g –1 SIDM cross-section, the DM content of very low mass CDM and SIDM haloes differs by no more than a factor of 2 within 100–200 pc. At larger halo masses (~10 10 M ), the introduction of baryonic processes creates field dwarf galaxies with DM cores and central DM+baryon distributions that are effectively indistinguishable between CDM and SIDM. Both models are in broad agreement with observed Local Group field galaxies across the range of masses explored. To significantly differentiate SIDM from CDM at the scale of faint dwarf galaxies, a velocity-dependent cross-section that rapidly increases to values larger than 2 cm 2 g –1 for haloes with V max  〈 25–30 km s –1 needs to be introduced.

Description: No large group of recently extinct placental mammals remains as evolutionarily cryptic as the approximately 280 genera grouped as 'South American native ungulates'. To Charles Darwin, who first collected their remains, they included perhaps the 'strangest animal[s] ever discovered'. Today, much like 180 years ago, it is no clearer whether they had one origin or several, arose before or after the Cretaceous/Palaeogene transition 66.2 million years ago, or are more likely to belong with the elephants and sirenians of superorder Afrotheria than with the euungulates (cattle, horses, and allies) of superorder Laurasiatheria. Morphology-based analyses have proved unconvincing because convergences are pervasive among unrelated ungulate-like placentals. Approaches using ancient DNA have also been unsuccessful, probably because of rapid DNA degradation in semitropical and temperate deposits. Here we apply proteomic analysis to screen bone samples of the Late Quaternary South American native ungulate taxa Toxodon (Notoungulata) and Macrauchenia (Litopterna) for phylogenetically informative protein sequences. For each ungulate, we obtain approximately 90% direct sequence coverage of type I collagen alpha1- and alpha2-chains, representing approximately 900 of 1,140 amino-acid residues for each subunit. A phylogeny is estimated from an alignment of these fossil sequences with collagen (I) gene transcripts from available mammalian genomes or mass spectrometrically derived sequence data obtained for this study. The resulting consensus tree agrees well with recent higher-level mammalian phylogenies. Toxodon and Macrauchenia form a monophyletic group whose sister taxon is not Afrotheria or any of its constituent clades as recently claimed, but instead crown Perissodactyla (horses, tapirs, and rhinoceroses). These results are consistent with the origin of at least some South American native ungulates from 'condylarths', a paraphyletic assembly of archaic placentals. With ongoing improvements in instrumentation and analytical procedures, proteomics may produce a revolution in systematics such as that achieved by genomics, but with the possibility of reaching much further back in time.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Welker, Frido -- Collins, Matthew J -- Thomas, Jessica A -- Wadsley, Marc -- Brace, Selina -- Cappellini, Enrico -- Turvey, Samuel T -- Reguero, Marcelo -- Gelfo, Javier N -- Kramarz, Alejandro -- Burger, Joachim -- Thomas-Oates, Jane -- Ashford, David A -- Ashton, Peter D -- Rowsell, Keri -- Porter, Duncan M -- Kessler, Benedikt -- Fischer, Roman -- Baessmann, Carsten -- Kaspar, Stephanie -- Olsen, Jesper V -- Kiley, Patrick -- Elliott, James A -- Kelstrup, Christian D -- Mullin, Victoria -- Hofreiter, Michael -- Willerslev, Eske -- Hublin, Jean-Jacques -- Orlando, Ludovic -- Barnes, Ian -- MacPhee, Ross D E -- England -- Nature. 2015 Jun 4;522(7554):81-4. doi: 10.1038/nature14249. Epub 2015 Mar 18.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉1] BioArCh, University of York, York YO10 5DD, UK [2] Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, 04103 Leipzig, Germany. ; BioArCh, University of York, York YO10 5DD, UK. ; Department of Earth Sciences, Natural History Museum, London SW7 5BD, UK. ; Centre for GeoGenetics, Natural History Museum of Denmark, University of Copenhagen, Oster Voldgade 5-7, 1350 Copenhagen K, Denmark. ; Institute of Zoology, Zoological Society of London, London NW1 4RY, UK. ; CONICET- Division Paleontologia de Vertebrados, Museo de La Plata. Facultad de Ciencias Naturales y Museo de La Plata, Universidad Nacional de La Plata. Paseo del Bosque s/n, B1900FWA, La Plata, Argentina. ; Seccion Paleontologia de Vertebrados. Museo Argentino de Ciencias Naturales "Bernardino Rivadavia", 470 Angel Gallardo Av., C1405DJR, Buenos Aires, Argentina. ; Institute of Anthropology, Johannes Gutenberg-University, Anselm-Franz-von-Bentzel-Weg 7, D-55128 Mainz, Germany. ; Department of Chemistry, University of York, York YO10 5DD, UK. ; Bioscience Technology Facility, Department of Biology, University of York, York YO10 5DD, UK. ; Department of Biological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA. ; Target Discovery Institute, Nuffield Department of Medicine, University of Oxford, Roosevelt Drive, Oxford OX3 7FZ, UK. ; Applications Development, Bruker Daltonik GmbH, 28359 Bremen, Germany. ; Novo Nordisk Foundation Center for Protein Research, Faculty of Health Sciences, University of Copenhagen, Blegdamsvej 3b, 2200 Copenhagen, Denmark. ; Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, UK. ; Smurfit Institute of Genetics, Trinity College Dublin, Dublin 2, Ireland. ; 1] BioArCh, University of York, York YO10 5DD, UK [2] Institute for Biochemistry and Biology, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam OT Golm, Germany. ; Department of Human Evolution, Max Planck Institute for Evolutionary Anthropology, 04103 Leipzig, Germany. ; Department of Mammalogy, American Museum of Natural History, New York, New York 10024, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/25799987" target="_blank"〉PubMed〈/a〉

Description: We examine gas accretion and subsequent star formation in representative galaxies from the McMaster Unbiased Galaxy Simulations. Accreted gas is bimodal with a natural temperature division at 10 5 K, near the peak of the cooling curve. Cold-mode accretion dominates inflows at early times, creating a peak in total accretion at redshift z = 2–4 and declining exponentially below z  ~ 2. Hot-mode accretion peaks near z = 1–2 and declines gradually. Hot-mode exceeds cold-mode accretion at z  ~ 1.8 for all four galaxies rather than when the galaxy reaches a characteristic mass. Cold-mode accretion can fuel immediate star formation, while hot-mode accretion preferentially builds a large, hot gas reservoir in the halo. Late-time star formation relies on reservoir gas accreted 2–8 Gyr prior. Thus, the reservoir allows the star formation rate to surpass the current overall gas accretion rate. Stellar feedback cycles gas from the interstellar medium back into the hot reservoir. Stronger feedback results in more gas cycling, gas removal in a galactic outflow and less star formation overall, enabling simulations to match the observed star formation history. For lower mass galaxies in particular, strong feedback can delay the star formation peak to z = 1–2 from the accretion peak at z = 2–4.

Description: We present a cosmological hydrodynamical simulation of a representative volume of the Universe, as part of the Making Galaxies in a Cosmological Context (MaGICC) project. MaGICC uses a thermal implementation for supernova and early stellar feedback. This work tests the feedback model at lower resolution across a range of galaxy masses, morphologies and merger histories. The simulated sample compares well with observations of high-redshift galaxies ( z  ≥ 2) including the stellar mass–halo mass ( M * - M h ) relation, the galaxy stellar mass function (GSMF) at low masses ( M *  〈 5 10 10 M ) and the number density evolution of low-mass galaxies. The poor match of M * - M h and the GSMF at high masses ( M *  ≥ 5 10 10 M ) indicates that supernova feedback is insufficient to limit star formation in these haloes. At z  = 0, our model produces too many stars in massive galaxies and slightly underpredicts the stellar mass around L * mass galaxy. Altogether our results suggest that early stellar feedback, in conjunction with supernova feedback, plays a major role in regulating the properties of low-mass galaxies at high redshift.

Description: Massive gas-rich galaxy discs at z  ~ 1–3 host massive star-forming clumps with typical baryonic masses in the range 10 7 –10 8  M which can affect the orbital decay and concurrent growth of supermassive black hole (BH) pairs. Using a set of high-resolution simulations of isolated clumpy galaxies hosting a pair of unequal-mass BHs, we study the interaction between massive clumps and a BH pair at kiloparsec scales, during the early phase of the orbital decay. We find that both the interaction with massive clumps and the heating of the cold gas layer of the disc by BH feedback tend to delay significantly the orbital decay of the secondary, which in many cases is ejected and then hovers for a whole gigayear around a separation of 1–2 kpc. In the envelope, dynamical friction is weak and there is no contribution of disc torques: these lead to the fastest decay once the orbit of the secondary BH has circularized in the disc mid-plane. In runs with larger eccentricities the delay is stronger, although there are some exceptions. We also show that, even in discs with very sporadic transient clump formation, a strong spiral pattern affects the decay time-scale for BHs on eccentric orbits. We conclude that, contrary to previous belief, a gas-rich background is not necessarily conducive to a fast BH decay and binary formation, which prompts more extensive investigations aimed at calibrating event-rate forecasts for ongoing and future gravitational-wave searches, such as with Pulsar Timing Arrays and the future evolved Laser Interferometer Space Antenna.

Description: We explore the effect of galactic environment on properties of molecular clouds. Using clouds formed in a large-scale galactic disc simulation, we measure the observable properties from synthetic column density maps. We confirm that a significant fraction of unbound clouds forms naturally in a galactic disc environment and that a mixed population of bound and unbound clouds can match observed scaling relations and distributions for extragalactic molecular clouds. By dividing the clouds into inner and outer disc populations, we compare their distributions of properties and test whether there are statistically significant differences between them. We find that clouds in the outer disc have lower masses, sizes, and velocity dispersions as compared to those in the inner disc for reasonable choices of the inner/outer boundary. We attribute the differences to the strong impact of galactic shear on the disc stability at large galactocentric radii. In particular, our Toomre analysis of the disc shows a narrowing envelope of unstable masses as a function of radius, resulting in the formation of smaller, lower mass fragments in the outer disc. We also show that the star formation rate is affected by the environment of the parent cloud, and is particularly influenced by the underlying surface density profile of the gas throughout the disc. Our work highlights the strengths of using galaxy–scale simulations to understand the formation and evolution of cloud properties – and the star formation within them – in the context of their environment.

Description: As both simulations and observations reach the resolution of the star-forming molecular clouds, it becomes important to clarify if these two techniques are discussing the same objects in galaxies. We compare clouds formed in a high-resolution galaxy simulation identified as continuous structures within a contour, in the simulator's position–position–position (PPP) coordinate space and the observer's position–position–velocity space (PPV). Results indicate that the properties of the cloud populations are similar in both methods and up to 70 per cent of clouds have a single counterpart in the opposite data structure. Comparing individual clouds in a one-to-one match reveals a scatter in properties mostly within a factor of 2. However, the small variations in mass, radius and velocity dispersion produce significant differences in derived quantities such as the virial parameter. This makes it difficult to determine if a structure is truly gravitationally bound. The three cloud types originally found in the simulation in Fujimoto et al. are identified in both data sets, with around 80 per cent of the clouds retaining their type between identification methods. We also compared our results when using a peak decomposition method to identify clouds in both PPP and PPV space. The number of clouds increased with this technique, but the overall cloud properties remained similar. However, the more crowded environment lowered the ability to match clouds between techniques to 40 per cent. The three cloud types also became harder to separate, especially in the PPV data set. The method used for cloud identification therefore plays a critical role in determining cloud properties, but both PPP and PPV can potentially identify the same structures.

Description: We explore the chemical distribution of stars in a simulated galaxy. Using simulations of the same initial conditions but with two different feedback schemes (McMaster Unbiased Galaxy Simulations – MUGS – and Making Galaxies in a Cosmological Context – MaGICC), we examine the features of the age–metallicity relation (AMR), and the three-dimensional age–[Fe/H]–[O/Fe] distribution, both for the galaxy as a whole and decomposed into disc, bulge, halo and satellites. The MUGS simulation, which uses traditional supernova feedback, is replete with chemical substructure. This substructure is absent from the MaGICC simulation, which includes early feedback from stellar winds, a modified initial mass function and more efficient feedback. The reduced amount of substructure is due to the almost complete lack of satellites in MaGICC. We identify a significant separation between the bulge and disc AMRs, where the bulge is considerably more metal-rich with a smaller spread in metallicity at any given time than the disc. Our results suggest, however, that identifying the substructure in observations will require exquisite age resolution, of the order of 0.25 Gyr. Certain satellites show exotic features in the AMR, even forming a ‘sawtooth’ shape of increasing metallicity followed by sharp declines which correspond to pericentric passages. This fact, along with the large spread in stellar age at a given metallicity, compromises the use of metallicity as an age indicator, although alpha abundance provides a more robust clock at early times. This may also impact algorithms that are used to reconstruct star formation histories from resolved stellar populations, which frequently assume a monotonically increasing AMR.