Andrew Pontzen

Astropy: A community Python package for astronomy

We present the first public version (v0.2) of the open-source and community-developed Python package, Astropy. This package provides core astronomy-related functionality to the community, including support for domain-specific file formats such as flexible image transport system (FITS) files, Virtual Observatory (VO) tables, and common ASCII table formats, unit and physical quantity conversions, physical constants specific to astronomy, celestial coordinate and time transformations, world coordinate system (WCS) support, generalized containers for representing gridded as well as tabular data, and a framework for cosmological transformations and conversions. Significant functionality is under active development, such as a model fitting framework, VO client and server tools, and aperture and point spread function (PSF) photometry tools. The core development team is actively making additions and enhancements to the current code base, and we encourage anyone interested to participate in the development of future Astropy versions.

How supernova feedback turns dark matter cusps into cores

We propose and successfully test against new cosmological simulations a novel analytical description of the physical processes associated with the origin of cored dark matter density profiles. In the simulations, the potential in the central kiloparsec changes on sub-dynamical time-scales over the redshift interval 4 > z > 2, as repeated, energetic feedback generates large underdense bubbles of expanding gas from centrally concentrated bursts of star formation. The model demonstrates how fluctuations in the central potential irreversibly transfer energy into collisionless particles, thus generating a dark matter core. A supply of gas undergoing collapse and rapid expansion is therefore the essential ingredient. The framework, based on a novel impulsive approximation, breaks with the reliance on adiabatic approximations which are inappropriate in the rapidly changing limit. It shows that both outflows and galactic fountains can give rise to cusp flattening, even when only a few per cent of the baryons form stars. Dwarf galaxies maintain their core to the present time. The model suggests that constant density dark matter cores will be generated in systems of a wide mass range if central starbursts or active galactic nucleus phases are sufficiently frequent and energetic.

Cusp-core transformations in dwarf galaxies: observational predictions

The presence of a dark matter core in the central kiloparsec of many dwarf galaxies has been a long standing problem in galaxy formation theories based on the standard cold dark matter paradigm. Recent simulations, based on Smooth Particle Hydrodynamics and rather strong feedback recipes have shown that it was indeed possible to form extended dark matter cores using baryonic processes related to a more realistic treatment of the interstellar medium. Using adaptive mesh renement, together with a new, stronger supernovae feedback scheme that we have recently implemented in the RAMSES code, we show that it is also possible to form a prominent dark matter core within the well-controlled framework of an isolated, initially cuspy, 10 billion solar masses dark matter halo. Although our numerical experiment is idealized, it allows a clean and unambiguous identication of the dark matter core formation process. Our dark matter inner prole is well tted by a pseudo-isothermal prole with a core radius of 800 pc. The core formation mechanism is consistent with the one proposed recently by Pontzen & Governato. We highlight two key observational predictions of all simulations that nd cusp-core transformations: (i) a bursty star formation history (SFH) with peak to trough ratio of 5 to 10 and a duty cycle comparable to the local dynamical time; and (ii) a stellar distribution that is hot with v= 1. We compare the observational properties of our model galaxy with recent measurements of the isolated dwarf WLM. We show that the spatial and kinematical distribution of stars and HI gas are in striking agreement with observations, supporting the fundamental role played by stellar feedback in shaping both the stellar and dark matter distribution.

BARYONS MATTER: WHY LUMINOUS SATELLITE GALAXIES HAVE REDUCED CENTRAL MASSES

Using high-resolution cosmological hydrodynamical simulations of Milky Way-massed disk galaxies, we demonstrate that supernovae feedback and tidal stripping lower the central masses of bright (–15 < MV < –8) satellite galaxies. These simulations resolve high-density regions, comparable to giant molecular clouds, where stars form. This resolution allows us to adopt a prescription for H2 formation and destruction that ties star formation to the presence of shielded, molecular gas. Before infall, supernova feedback from the clumpy, bursty star formation captured by this physically motivated model leads to reduced dark matter (DM) densities and shallower inner density profiles in the massive satellite progenitors (M vir ≥ 109 M ☉, M * ≥ 107 M ☉) compared with DM-only simulations. The progenitors of the lower mass satellites are unable to maintain bursty star formation histories, due to both heating at reionization and gas loss from initial star-forming events, preserving the steep inner density profile predicted by DM-only simulations. After infall, gas stripping from satellites reduces the total central masses of satellites simulated with DM+baryons relative to DM-only satellites. Additionally, enhanced tidal stripping after infall due to the baryonic disk acts to further reduce the central DM densities of the luminous satellites. Satellites that enter with cored DM halos are particularly vulnerable to the tidal effects of the disk, exacerbating the discrepancy in the central masses predicted by baryon+DM and DM-only simulations. We show that DM-only simulations, which neglect the highly non-adiabatic evolution of baryons described in this work, produce denser satellites with larger central velocities. We provide a simple correction to the central DM mass predicted for satellites by DM-only simulations. We conclude that DM-only simulations should be used with great caution when interpreting kinematic observations of the Milky Ways dwarf satellites.

Damped Lyman α systems in galaxy formation simulations

We investigate the population of z = 3 damped Lyman alpha systems (DLAs) in a recent series of high resolution galaxy formation simulations. The simulations are of interest because they form at z = 0 some of the most realistic disk galaxies to date. No free parameters are available in our study: the simulation parameters have been fixed by physical and z = 0 observational constraints, and thus our work provides a genuine consistency test. The precise role of DLAs in galaxy formation remains in debate, but they provide a number of strong constraints on the nature of our simulated bound systems at z = 3 because of their coupled information on neutral H I densities, kinematics, metallicity and estimates of star f ormation activity. Our results, without any parameter-tuning, closely match the observed incidence rate and column density distributions of DLAs. Our simulations are the first to reproduce the distribution of metallicities (with a median of ZDLA ≃ Z⊙/20) without invoking observationally unsupported mechanisms such as significant dust biasin g. This is especially encouraging given that these simulations have previously been shown to have a realistic 0 < z < 2 stellar mass-metallicity relation. Additionally, we see a strong p ositive correlation between sightline metallicity and low-ion velocity width, the normalization and slope of which comes close to matching recent observational results. However, we somewhat underestimate the number of observed high velocity width systems; the severity of this disagreement is comparable to other recent DLA-focused studies. DLAs in our simulations are predominantly associated with dark matter haloes with virial masses in the range 10 9 < Mvir/M⊙ < 10 11 . We are able to probe DLAs at high resolution, irrespective of their masses, by using a range of simulation s of differing volumes. The fully constrained feedback prescription in use causes the majority of DLA haloes to form stars at a very low rate, accounting for the low metallicities. It i s also responsible for the massmetallicity relation which appears essential for reproduc ing the velocity-metallicity correlation. By z = 0 the majority of the z = 3 neutral gas forming the DLAs has been converted into stars, in agreement with rough physical expectations.

Hierarchical formation of bulgeless galaxies: why outflows have low angular momentum

Using high resolution, fully cosmological smoothed particle hydrodynamical simulations of dwarf galaxies in a Lambda cold dark matter Universe, we show how high redshift gas outflows can modify the baryon angular momentum distribution and allow pure disc galaxies to form. We outline how galactic outflows preferentially remove low angular momentum material due a combination of (a) star formation peaking at high redshift in shallow dark matter potentials, an epoch when accreted gas has relatively low angular momentum, (b) the existence of an extended reservoir of high angular momentum gas in the outer disc to provide material for prolonged SF at later times and (c) the tendency for outflows to follow the path of least resistance which is perpendicular to the disc. We also show that outflows are enhanced during mergers, thus expelling much of the gas which has lost its angular momentum during these events, and preventing the formation of ‘classical’, merger driven bulges in low-mass systems. Stars formed prior to such mergers form a diffuse, extended stellar halo component similar to those detected in nearby dwarfs.

Cold dark matter heats up

A principal discovery in modern cosmology is that standard model particles comprise only 5 per cent of the mass-energy budget of the Universe. In the ΛCDM paradigm, the remaining 95 per cent consists of dark energy (Λ) and cold dark matter. ΛCDM is being challenged by its apparent inability to explain the low-density ‘cores’ of dark matter measured at the centre of galaxies, where centrally concentrated high-density ‘cusps’ were predicted. But before drawing conclusions, it is necessary to include the effect of gas and stars, historically seen as passive components of galaxies. We now understand that these can inject heat energy into the cold dark matter through a coupling based on rapid gravitational potential fluctuations, explaining the observed low central densities.

Dynamical friction in constant density cores: a failure of the Chandrasekhar formula

Using analytic calculations and N-body simulations we show that in constant density (harmonic) cores, sinking satellites undergo an initial phase of very rapid (super-Chandrasekhar) dynamical friction, after which they experience no dynamical friction at all. For density profiles with a central power law profile, ρ∝r−α, the infalling satellite heats the background and causes α to decrease. For α < 0.5 initially, the satellite generates a small central constant density core and stalls as in the α= 0 case. We discuss some astrophysical applications of our results to decaying satellite orbits, galactic bars and mergers of supermassive black hole binaries. In a companion paper we show that a central constant density core can provide a natural solution to the timing problem for Fornaxs globular clusters.

MAGICC haloes: confronting simulations with observations of the circumgalactic medium at z = 0

We explore the circumgalactic medium (CGM) of two simulated star-forming galaxies with luminosities L ≈ 0.1 and 1 L⋆ generated using the smooth particle hydrodynamic code gasoline. These simulations are part of the Making Galaxies In a Cosmological Context (magicc) program in which the stellar feedback is tuned to match the stellar mass–halo mass relationship. For comparison, each galaxy was also simulated using a ‘lower feedback’ (LF) model which has strength comparable to other implementations in the literature. The ‘magicc feedback’ (MF) model has a higher incidence of massive stars and an approximately two times higher energy input per supernova. Apart from the low-mass halo using LF, each galaxy exhibits a metal-enriched CGM that extends to approximately the virial radius. A significant fraction of this gas has been heated in supernova explosions in the disc and subsequently ejected into the CGM where it is predicted to give rise to substantial O vi absorption. The simulations do not yet address the question of what happens to the O vi when the galaxies stop forming stars. Our models also predict a reservoir of cool H i clouds that show strong Lyα absorption to several hundred kpc. Comparing these models to recent surveys with the Hubble Space Telescope, we find that only the MF models have sufficient O vi and H i gas in the CGM to reproduce the observed distributions. In separate analyses, these same MF models also show better agreement with other galaxy observables (e.g. rotation curves, surface brightness profiles and H i gas distribution). We infer that the CGM is the dominant reservoir of baryons for galaxy haloes.

IN-N-OUT: The GAS CYCLE from DWARFS to SPIRAL GALAXIES

We examine the scalings of galactic outflows with halo mass across a suite of twenty high-resolution cosmological zoom galaxy simulations covering halo masses from 10^9.5 - 10^12 M_sun. These simulations self-consistently generate outflows from the available supernova energy in a manner that successfully reproduces key galaxy observables including the stellar mass-halo mass, Tully-Fisher, and mass-metallicity relations. We quantify the importance of ejective feedback to setting the stellar mass relative to the efficiency of gas accretion and star formation. Ejective feedback is increasingly important as galaxy mass decreases; we find an effective mass loading factor that scales as v_circ^(-2.2), with an amplitude and shape that is invariant with redshift. These scalings are consistent with analytic models for energy-driven wind, based solely on the halo potential. Recycling is common: about half the outflow mass across all galaxy masses is later re-accreted. The recycling timescale is typically about 1 Gyr, virtually independent of halo mass. Recycled material is re-accreted farther out in the disk and with typically about 2-3 times more angular momentum. These results elucidate and quantify how the baryon cycle plausibly regulates star formation and alters the angular momentum distribution of disk material across the halo mass range where most of cosmic star formation occurs.