1901.07571
Near-field cosmology with the lowest-mass galaxies
Weisz, Boylan-Kolchin
The premise of "near-field cosmology" is that the study of nearby low-mass galaxies on a star-by-star basis has implications that extend far beyond the local Universe and include the nature of DM, lives and deaths of the first stars, cosmic deionization, and galaxy formation across cosmic time. In response to the Astro2020 call for science white papers, provide a high-level synopsis of several major questions in near-field cosmology, the potential for transformative progress in the next decade and beyond, and mechanisms for achieving these goals. The recommendations include: (i) obtaining kinematic information from large samples of individual LoS and transverse velocities of stars and from spatially resolved gas observations in nearby dwarf galaxies to directly inform the understanding of DM interactions in the deeply nonlinear regime; and (ii) an intensive program of optical imaging and UV and optical spectroscopy of resolved stars in low-mass galaxies, including low-metallicity massive stars and HII regions (which are central to understanding faint galaxies at all earlier epochs). Realizing these aims requires a large aperture (>~9m) UV/optical space telescope, a two-fold community investment in ground-based optical spectroscopy through both ELTs and a dedicated wide-field spectroscopic survey on a 10m class telescope, and radio facilities that can measure high-precision, spatially resolved rotation curves of low-mass galaxies. Finally, suggest that a servicing mission to extend the life of HST beyond 2025 should be seriously explored.
1901.07578
Using surface brightness fluctuations to study nearby satellite galaxy systems: the complete satellite system of M101
Carlsten, et al
Use surface brightness fluctuation (SBF) measurements to constrain the distance to low surface brightness (LSB) dwarfs in the vicinity of M101. Recent work has discovered many LSB candidate satellite companions of M101. However, without accurate distances, it is problematic to identify these dwarfs as physical satellites of M101. Use CFHTLS data to measure the SBF signal for 43 candidate dwarfs. The data is deep enough that can constrain 29 of these to be unassociated background galaxies by their lack of SBF. Measure high S/N SBF signals for 2 of the candidate dwarfs, which are consist with being at the distance of M101. The remaining candidates are too LSB and/or small for their distances to be constrained. Still, by comparison with LG dwarfs, argue that the M101 satellite system is likely now complete down to stellar masses of ~5e5 Msun. Also provide a new SBF distance for the nearby dwarf UGC 8882, which suggests that it is significantly outside of the viral radius of M101 and is thus not a physical satellite. By constraining the distances to a majority of the candidates using only archival data, the work demonstrate the usefulness of SBF for nearby LSB galaxies and for studying the satellite systems of nearby massive galaxies.
1901.07726
On model selection in cosmology
Kerscher, Weller
Review some of the common methods for model selection: the goodness of fit, the likelihood ratio test, Bayesian model selection using Bayes factors, and the classical as well as the Bayesian information theoretic approaches. Illustrate these different approaches by comparing models for the expansion history of the Universe. In the discussion, highlight the premises and objectives entering these different approaches to model selection and finally recommend the information theoretic approach.
1901.07801
Nightingale, Massey, et al
Investigate how strong gravitational lensing can test contemporary models of massive elliptical (ME) galaxy formation, by combining a traditional decomposition of their visible stellar distribution with a lensing analysis of their mass distribution. As a proof of concept, study a sample of 3 ME lenses, observing that all are composed of two distinct baryonic structures, a 'red' central bulge surrounded by an extended envelope of stellar material. Whilst these two components look photometrically similar, their distinct lensing effects permit a clean decomposition of their mass structure. This allows inference of two key pieces of information about each lens galaxy: (i) the stellar mass distribution (without invoking stellar populations models) and (ii) the inner DM halo mass. Argue that these two measurements are crucial to testing models of ME formation, as the stellar mass profile provides a diagnostic of baryonic accretion and feedback whilst the DM mass places each galaxy in the context of LCDM LSS formation. Also detect large rotational offsets between the two stellar components and a lopsidedness in their outer mass distributions, which hold further information on the evolution of each ME. Finally, discuss how this approach can be extended to galaxies of all Hubble types and what implication the results have for studies of SL.
1901.08175
Reconstructing the weak lensing magnification distribution of Type Ia Supernovae
Zhai, Wang
WL of SN Ia is a systematic uncertainty in the use of Sneetch Ia as standard candles as well as an independent cosmological probe, if the corresponding magnification distribution can be extracted from data. Study the peak brightness distribution of SNe Ia in the Pantheon sample, and find that the high z sub-sample shows distinct WL signature compared to the low z subsample: a long tail at the bright end due to high magnifications and a shift of the peak brightness toward the faint end, consistent with findings from earlier work. Developed a technique to reconstruct the WL magnification distribution of SNe Ia, p(mu), from the measured SN Ia flux distribution, and applied it to the Pantheon sample. Find that p(mu) can be reconstructed at a significance better than 2 sigma fo the subsample of SNe Ia at z>0.7 (124 SNe Ia), and at a lower significance for the SNe Ia at z>0.9 (49 SNe Ia), due to the small number of SNe Ia at high redshifts. The large number of z>1 SNe Ia from future surveys will enable the use of p(nu) reconstructed from SNe Ia as an independent cosmological probe.
1901.08586
Galaxy shape measurement synergies between LSST and Euclid
Schuhmann, Heymans, Zuntz
Demonstrate that a joint analysis of LSST-like ground-based imaging with Euclid-like space-based imaging leads to increased precision and accuracy in galaxy shape measurements. At galaxy magnitudes of i~24.5, a combined survey analysis increases the effective galaxy number density for cosmic shear studies by ~50% in comparison to an analysis of each survey alone. Using a realistic distribution of galaxy sizes, ellipticities and magnitudes down to i=25.2, simulate LSST-like and Euclid-like images of over one million isolated galaxies. Compare the precision and accuracy of the recovered galaxy ellipticities for four different analyses: LSST-only, Euclid-only, a simultaneous joint-pixel analysis of the two surveys, and a simple catalogue-level survey combination. In the faint and small-galaxy regimes, where neither survey excels alone, find a ~20% increase in the precision of galaxy shape measurement when a joint-pixel analysis is adopted, compared to a catalogue-level combination. As the statistical power of cosmic shear is dominated by intrinsic ellipticity noise, however, this improvement in shape measurement noise only leads to a ~5% improvement in the effective number density of galaxies for lensing studies. View this as the minimum improvement that should be expected from a joint-pixel analysis over a less accurate catalogue-level combination, as the former will also improve the capability of LSST to de-blend close objects.
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