arXiv subjects

D. Potter

Publications and source records attributed to D. Potter.

15 recordsLinked to original sources

arXiv2026

Euclid. Populating a dark universe with galaxies using SciPIC

High-fidelity galaxy mocks are crucial for validating analysis pipelines and for cosmological inference. In this context, the Science Pipeline at PIC (SciPIC) is a pipeline specifically designed for the fast generation of synthetic galaxy catalogues from the halo properties identified in cosmological simulations. SciPIC delivers galaxy catalogues that aim to reproduce the observed luminosity function and clustering above a given flux detection limit over a wide redshift range. In this work, we introduce SciPICal, an automated pipeline that calibrates the parameters that set the main mock galaxy properties, namely number density, luminosities, colours, and positions. The pipeline is applied to the Euclid Flagship 2 Wide and Deep halo catalogues, specifically built to support the \textit{Euclid} wide and deep surveys. Compared to the recently released Flagship 2 Wide mock, our calibrated version improves the clustering predictions by approximately 50\% based on chi-squared values. Furthermore, we produce the Euclid Deep mock catalogue, which reaches up to $z = 10$ by populating a light-cone and a complementary snapshot at $z = 0$. We validate these catalogues using measurements from spectroscopic and photometric galaxy surveys, as well as with results from a hydrodynamical simulation. The obtained good agreement (within $15\%$ for most of the samples) in the clustering predictions across the different galaxy samples considered, validates our calibration strategy and demonstrates the strong predictive power of the generated mocks. This pipeline will allow us to improve the methodology applied in assigning the galaxy properties and ensures that the galaxy mocks remain up-to-date by incorporating constraints from upcoming observational data in the calibration procedure.

astro-ph.CO↗

arXiv2024

Euclid preparation. Simulations and nonlinearities beyond $\Lambda$CDM. 1. Numerical methods and validation

To constrain models beyond $\Lambda$CDM, the development of the Euclid analysis pipeline requires simulations that capture the nonlinear phenomenology of such models. We present an overview of numerical methods and $N$-body simulation codes developed to study the nonlinear regime of structure formation in alternative dark energy and modified gravity theories. We review a variety of numerical techniques and approximations employed in cosmological $N$-body simulations to model the complex phenomenology of scenarios beyond $\Lambda$CDM. This includes discussions on solving nonlinear field equations, accounting for fifth forces, and implementing screening mechanisms. Furthermore, we conduct a code comparison exercise to assess the reliability and convergence of different simulation codes across a range of models. Our analysis demonstrates a high degree of agreement among the outputs of different simulation codes, providing confidence in current numerical methods for modelling cosmic structure formation beyond $\Lambda$CDM. We highlight recent advances made in simulating the nonlinear scales of structure formation, which are essential for leveraging the full scientific potential of the forthcoming observational data from the Euclid mission.

astro-ph.CO↗

arXiv2024

Euclid. V. The Flagship galaxy mock catalogue: a comprehensive simulation for the Euclid mission

We present the Flagship galaxy mock, a simulated catalogue of billions of galaxies designed to support the scientific exploitation of the Euclid mission. Euclid is a medium-class mission of the European Space Agency optimised to determine the properties of dark matter and dark energy on the largest scales of the Universe. It probes structure formation over more than 10 billion years primarily from the combination of weak gravitational lensing and galaxy clustering data. The breath of Euclid's data will also foster a wide variety of scientific analyses. The Flagship simulation was developed to provide a realistic approximation to the galaxies that will be observed by Euclid and used in its scientific analyses. We ran a state-of-the-art N-body simulation with four trillion particles, producing a lightcone on the fly. From the dark matter particles, we produced a catalogue of 16 billion haloes in one octant of the sky in the lightcone up to redshift z=3. We then populated these haloes with mock galaxies using a halo occupation distribution and abundance matching approach, calibrating the free parameters of the galaxy mock against observed correlations and other basic galaxy properties. Modelled galaxy properties include luminosity and flux in several bands, redshifts, positions and velocities, spectral energy distributions, shapes and sizes, stellar masses, star formation rates, metallicities, emission line fluxes, and lensing properties. We selected a final sample of 3.4 billion galaxies with a magnitude cut of H_E<26, where we are complete. We have performed a comprehensive set of validation tests to check the similarity to observational data and theoretical models. In particular, our catalogue is able to closely reproduce the main characteristics of the weak lensing and galaxy clustering samples to be used in the mission's main cosmological analysis. (abridged)

astro-ph.CO↗

arXiv2022

Euclid: Modelling massive neutrinos in cosmology -- a code comparison

The measurement of the absolute neutrino mass scale from cosmological large-scale clustering data is one of the key science goals of the Euclid mission. Such a measurement relies on precise modelling of the impact of neutrinos on structure formation, which can be studied with $N$-body simulations. Here we present the results from a major code comparison effort to establish the maturity and reliability of numerical methods for treating massive neutrinos. The comparison includes eleven full $N$-body implementations (not all of them independent), two $N$-body schemes with approximate time integration, and four additional codes that directly predict or emulate the matter power spectrum. Using a common set of initial data we quantify the relative agreement on the nonlinear power spectrum of cold dark matter and baryons and, for the $N$-body codes, also the relative agreement on the bispectrum, halo mass function, and halo bias. We find that the different numerical implementations produce fully consistent results. We can therefore be confident that we can model the impact of massive neutrinos at the sub-percent level in the most common summary statistics. We also provide a code validation pipeline for future reference.

astro-ph.CO↗

arXiv2020

Euclid preparation: IX. EuclidEmulator2 -- Power spectrum emulation with massive neutrinos and self-consistent dark energy perturbations

We present a new, updated version of the EuclidEmulator (called EuclidEmulator2), a fast and accurate predictor for the nonlinear correction of the matter power spectrum. Percent-level accurate emulation is now supported in the eight-dimensional parameter space of $w_0w_a$CDM$+\sum m_\nu$models between redshift $z=0$ and $z=3$ for spatial scales within the range 0.01 $h$/Mpc $\leq k \leq$ 10 $h$/Mpc. In order to achieve this level of accuracy, we have had to improve the quality of the underlying N-body simulations used as training data: (1) we use self-consistent linear evolution of non-dark matter species such as massive neutrinos, photons, dark energy and the metric field, (2) we perform the simulations in the so-called N-body gauge, which allows one to interpret the results in the framework of general relativity, (3) we run over 250 high-resolution simulations with $3000^3$ particles in boxes of 1 (Gpc/$h$)${}^3$ volumes based on paired-and-fixed initial conditions and (4) we provide a resolution correction that can be applied to emulated results as a post-processing step in order to drastically reduce systematic biases on small scales due to residual resolution effects in the simulations. We find that the inclusion of the dynamical dark energy parameter $w_a$ significantly increases the complexity and expense of creating the emulator. The high fidelity of EuclidEmulator2 is tested in various comparisons against N-body simulations as well as alternative fast predictors like Halofit, HMCode and CosmicEmu. A blind test is successfully performed against the Euclid Flagship v2.0 simulation. Nonlinear correction factors emulated with EuclidEmulator2 are accurate at the level of 1% or better for 0.01 $h$/Mpc $\leq k \leq$ 10 $h$/Mpc and $z\leq3$ compared to high-resolution dark matter only simulations. EuclidEmulator2 is publicly available at https://github.com/miknab/EuclidEmulator2 .

astro-ph.CO↗

arXiv2009

Dark Matter Direct Detection with Non-Maxwellian Velocity Structure

The velocity distribution function of dark matter particles is expected to show significant departures from a Maxwell-Boltzmann distribution. This can have profound effects on the predicted dark matter - nucleon scattering rates in direct detection experiments, especially for dark matter models in which the scattering is sensitive to the high velocity tail of the distribution, such as inelastic dark matter (iDM) or light (few GeV) dark matter (LDM), and for experiments that require high energy recoil events, such as many directionally sensitive experiments. Here we determine the velocity distribution functions from two of the highest resolution numerical simulations of Galactic dark matter structure (Via Lactea II and GHALO), and study the effects for these scenarios. For directional detection, we find that the observed departures from Maxwell-Boltzmann increase the contrast of the signal and change the typical direction of incoming DM particles. For iDM, the expected signals at direct detection experiments are changed dramatically: the annual modulation can be enhanced by more than a factor two, and the relative rates of DAMA compared to CDMS can change by an order of magnitude, while those compared to CRESST can change by a factor of two. The spectrum of the signal can also change dramatically, with many features arising due to substructure. For LDM the spectral effects are smaller, but changes do arise that improve the compatibility with existing experiments. We find that the phase of the modulation can depend upon energy, which would help discriminate against background should it be found.

astro-ph.GA↗

arXiv2008

Fossil remnants of reionization in the halo of the Milky Way

Our recently completed one billion particle Via Lactea II simulation of a Milky Way-sized dark matter halo resolves over 50,000 gravitationally bound clumps orbiting today within the virialized region of the main host. About 2,300 of these subhalos have one or more "progenitors" above 1e6 Msun at redshift 11, i.e. massive enough for their gas to have cooled via excitation of H2 and fragmented prior to the epoch of cosmic reionization. We count 4,500 such progenitors: if these were able to convert a fraction of their gas content into very metal-poor stars with a Salpeter initial mass function (IMF), they would be shining today with a visual magnitude M_V=6.7 per solar mass in stars. Assuming a universal baryon fraction, we show that mean star formation efficiencies as low as 0.1% in progenitors below 1e8 Msun would overproduce the abundance of the faint Galatic dwarf spheroidals observed by the Sloan Digital Sky Survey. Star formation at first light must have occurred either with an IMF lacking stars below 0.9 Msun, or was intrinsically very inefficient in small dark matter halos. If the latter, our results may be viewed as another hint of a minimum scale in galaxy formation.

astro-ph↗

arXiv2008

Clumps and streams in the local dark matter distribution

In cold dark matter cosmological models, structures form and grow by merging of smaller units. Numerical simulations have shown that such merging is incomplete; the inner cores of halos survive and orbit as "subhalos" within their hosts. Here we report a simulation that resolves such substructure even in the very inner regions of the Galactic halo. We find hundreds of very concentrated dark matter clumps surviving near the solar circle, as well as numerous cold streams. The simulation reveals the fractal nature of dark matter clustering: Isolated halos and subhalos contain the same relative amount of substructure and both have cuspy inner density profiles. The inner mass and phase-space densities of subhalos match those of recently discovered faint, dark matter-dominated dwarf satellite galaxies and the overall amount of substructure can explain the anomalous flux ratios seen in strong gravitational lenses. Subhalos boost gamma-ray production from dark matter annihilation, by factors of 4-15, relative to smooth galactic models. Local cosmic ray production is also enhanced, typically by a factor 1.4, but by more than a factor of ten in one percent of locations lying sufficiently close to a large subhalo. These estimates assume that gravitational effects of baryons on dark matter substructure are small.

astro-ph↗

arXiv2006

Ultracold Rb-OH collisions and prospects for sympathetic cooling

We have computed ab inito cross sections for cold collisions of Rb atoms with OH radicals. We predict collision rate constants of order 10^{-11} cm^3/s at temperatures in the range 10-100 mK at which molecules have already been produced experimentally. However, we also find that in these collisions the molecules have a strong propensity for changing their internal state, which could make sympathetic cooling of OH in a Rb buffer gas problematic in magnetostatic or electrostatic traps.

physics.atom-ph↗

arXiv2005

Testing Theoretical Evolutionary Models with AB Dor C and the Initial Mass Function

We assess the constraints on the evolutionary models of young low-mass objects that are provided by the measurements of the companion AB Dor C by Close and coworkers and by a new comparison of model-derived IMFs of star-forming regions to the well-calibrated IMF of the solar neighborhood. After performing an independent analysis of Close's imaging and spectroscopic data for AB Dor C, we find that AB Dor C is not detected at a significant level (SN 1.2) in the SDI images when one narrow-band image is subtracted from another, but that it does appear in the individual SDI frames as well as the images at JHK. Using the age of 75-150 Myr for AB Dor from Luhman, Stauffer, & Mamajek, the luminosity predicted by the models of Chabrier & Baraffe is consistent with the value that we estimate. We measure a spectral type of M6+/-1 from the K-band spectrum of AB Dor C, which is earlier than the value of M8+/-1 from Close and is consistent with the model predictions when a dwarf temperature scale is adopted. In a test of these models at much younger ages, we show that the low-mass IMFs that they produce for star-forming regions are similar to the IMF of the solar neighborhood. If the masses of the low-mass stars and brown dwarfs in these IMFs of star-forming regions were underestimated by a factor of two as suggested by Close, then the IMF characterizing the current generation of Galactic star formation would have to be radically different from the IMF of the solar neighborhood.

astro-ph↗

arXiv2004

High Resolution Mid - Infrared Imaging of the AGB Star RV Boo with the Steward Observatory Adaptive Optics System

We present high resolution (~0.1"), very high Strehl ratio (0.97+-0.03) mid-infrared (IR) adaptive optics (AO) images of the AGB star RV Boo utilizing the MMT adaptive secondary AO system. RV Boo was observed at a number of wavelengths over two epochs (9.8 um in May 2003, 8.8, 9.8 and 11.7 um in February 2004) and appeared slightly extended at all wavelengths. While the extension is very slight at 8.8 and 11.7 um data, the extension is somewhat more pronounced at 9.8 um. With such high Strehls we can achieve super-resolutions of 0.1" by deconvolving RV Boo with a point-spread function (PSF) derived from an unresolved star. We tentatively resolve RV Boo into a 0.16" FWHM extension at a position angle of 120 degrees. At a distance of 390(+250)(-100) pc, this corresponds to a FWHM of 60(+40)(-15) AU. We measure a total flux at 9.8 um of 145+-24 Jy for the disk and star. Based on a dust thermal emission model for the observed IR spectral energy distribution and the 9.8 um AO image, we derive a disk dust mass of 1.6x10^-6 Msun and an inclination of 30 to 45 degrees from edge-on. We discuss whether the dust disk observed around RV Boo is an example of the early stages in the formation of asymmetric structure in planetary nebula.

astro-ph↗

arXiv2002

Hokupa'a-Gemini Discovery of Two Ultracool Companions to the Young Star HD 130948

We report the discovery of two faint ultracool companions to the nearby (d~17.9 pc) young G2V star HD 130948 (HR 5534, HIP 72567) using the Hokupa'a adaptive optics instrument mounted on the Gemini North 8-meter telescope. Both objects have the same common proper motion as the primary star as seen over a 7 month baseline and have near-IR photometric colors that are consistent with an early-L classification. Near-IR spectra taken with the NIRSPEC AO instrument on the Keck II telescope reveal K I lines, FeH, and water bandheads. Based on these spectra, we determine that both objects have spectral type dL2 with an uncertainty of 2 spectral subclasses. The position of the new companions on the H-R diagram in comparison with theoretical models is consistent with the young age of the primary star (<0.8 Gyr) estimated on the basis of X-ray activity, lithium abundance and fast rotation. HD 130948 B and C likely constitute a pair of young contracting brown dwarfs with an orbital period of about 10 years, and will yield dynamical masses for L dwarfs in the near future.

astro-ph↗

arXiv2001

Discovery of a 0.15" Binary Brown Dwarf 2MASSJ 1426316+155701 With Gemini/Hokupa'a Adaptive Optics

Use of the highly sensitive Hokupa'a curvature wavefront sensor has allowed for the first time direct adaptive optics (AO) guiding on brown dwarfs and VLM stars (SpT=M7-L2). An initial survey of 9 such objects discovered one 0.15" binary (2MASSJ 1426316+155701). The companion is about half as bright as the primary (Delta K = 0.61+/-0.05$, Delta H = 0.70+/-0.05) and has even redder colors H-K=0.59+/-0.14 than the primary. The blended spectrum of the binary has been previously determined to be M9.0. We modeled a blend of an M8.5 template and a L1-L3 template reproducing a M9.0 spectrum in the case of Delta K = 0.61+/-0.05,Delta H = 0.70\pm0.05$. These spectral types also match the observed H-K colors of each star. Based the previously observed low space motion and $H_α$ activity we assign an age of $0.8^{+6.7}_{-0.3} Gyr$. Utilizing this age range and the latest DUSTY models of the Lyon group we assign a photometric distance of $18.8^{+1.44}_{-1.02} pc$ and masses of $M_{A}=0.074^{+0.005}_{-0.011} M_\odot$ and $M_{B}=0.066^{+0.006}_{-0.015} M_\odot$. We therefore estimate a system separation of $2.92_{+0.22}^{-0.16}AU$ and a period of $13.3{+3.18}^{-1.51} yr$ respectively. Hence, 2M1426 is among the smallest separation brown dwarf binaries resolved to date.

astro-ph↗

arXiv2001

Imaging Polarimetric Observations of a New Circumstellar Disk System

Few circumstellar disks have been directly observed. Here we use sensitive differential polarimetric techniques to overcome atmospheric speckle noise in order to image the circumstellar material around HD 169142. The detected envelope or disk is considerably smaller than expectations based on the measured strength of the far-IR excess from this system.

astro-ph↗

arXiv2000

The Peak Brightness and Spatial Distribution of AGB Stars Near the Nucleus of M32

The bright stellar content near the center of the Local Group elliptical galaxy M32 is investigated with 0.12 arcsec FWHM H and K images obtained with the Gemini Mauna Kea telescope. Stars with K = 15.5, which are likely evolving near the tip of the asymptotic giant branch (AGB), are resolved to within 2 arcsec of the nucleus, and it is concluded that the peak stellar brightness near the center of M32 is similar to that in the outer regions of the galaxy. Moreover, the projected density of bright AGB stars follows the visible light profile to within 2 arcsec of the nucleus, indicating that the brightest stars are well mixed throughout the galaxy. Thus, there is no evidence for an age gradient, and the radial variations in spectroscopic indices and ultraviolet colors that have been detected previously must be due to metallicity and/or some other parameter. We suggest that either the bright AGB stars formed as part of a highly uniform and coherent galaxy-wide episode of star formation, or they originated in a separate system that merged with M32.

astro-ph↗