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Upcoming cosmological intensity mapping surveys will open new windows on the Universe, but they must first overcome a number of significant systematic effects, including polarization leakage. We present a formalism that uses scan strategy information to model the effect of different instrumental systematics on the recovered cosmological intensity signal for `single-dish' (autocorrelation) surveys. This modelling classifies different systematics according to their spin symmetry, making it particularly relevant for dealing with polarization leakage. We show how to use this formalism to calculate the expected contamination from different systematics as a function of the scanning strategy. Most importantly, we show how systematics can be disentangled from the intensity signal based on their spin properties via map-making. We illustrate this, using a set of toy models, for some simple instrumental systematics, demonstrating the ability to significantly reduce the contamination to the observed intensity signal. Crucially, unlike existing foreground removal techniques, this approach works for signals that are non-smooth in frequency, e.g. polarized foregrounds. These map-making approaches are simple to apply and represent an orthogonal and complementary approach to existing techniques for removing systematics from upcoming 21cm intensity mapping surveys., Comment: 19 pages, 14 Figures, 2 Tables, published in MNRAS

Model-independent constraints on modified gravity models hitherto exist mainly on linear scales. A recently developed formalism presented a consistent parameterisation that is valid on all scales. Using this approach, we perform model-independent modified gravity $N$-body simulations on all cosmological scales with a time-dependent $\mu$. We present convergence tests of our simulations, and we examine how well existing fitting functions reproduce the non-linear matter power spectrum of the simulations. We find that although there is a significant variation in the accuracy of all of the fitting functions over the parameter space of our simulations, the ReACT framework delivers the most consistent performance for the matter power spectrum. We comment on how this might be improved to the level required for future surveys such as Euclid and the Vera Rubin Telescope (LSST). We also show how to compute weak-lensing observables consistently from the simulated matter power spectra in our approach, and show that ReACT also performs best when fitting the weak-lensing observables. This paves the way for a full model-independent test of modified gravity using all of the data from such upcoming surveys., Comment: 32 pages, 14 Figures; Comments welcome

We derive ages, metallicities, and individual element abundances of early- and late-type galaxies (ETGs and LTGs) out to 1.5 R$_e$. We study a large sample of 1900 galaxies spanning $8.6 - 11.3 \log M/M_{\odot}$ in stellar mass, through key absorption features in stacked spectra from the SDSS-IV/MaNGA survey. We use mock galaxy spectra with extended star formation histories to validate our method for LTGs and use corrections to convert the derived ages into luminosity- and mass-weighted quantities. We find flat age and negative metallicity gradients for ETGs and negative age and negative metallicity gradients for LTGs. Age gradients in LTGs steepen with increasing galaxy mass, from $-0.05\pm0.11~\log$ Gyr/R$_e$ for the lowest mass galaxies to $-0.82\pm0.08~\log$ Gyr/R$_e$ for the highest mass ones. This strong gradient-mass relation has a slope of $-0.70\pm0.18$. Comparing local age and metallicity gradients with the velocity dispersion $\sigma$ within galaxies against the global relation with $\sigma$ shows that internal processes regulate metallicity in ETGs but not age, and vice versa for LTGs. We further find that metallicity gradients with respect to local $\sigma$ show a much stronger dependence on galaxy mass than radial metallicity gradients. Both galaxy types display flat [C/Fe] and [Mg/Fe], and negative [Na/Fe] gradients, whereas only LTGs display gradients in [Ca/Fe] and [Ti/Fe]. ETGs have increasingly steep [Na/Fe] gradients with local $\sigma$ reaching $6.50\pm0.78$ dex/$\log$ km/s for the highest masses. [Na/Fe] ratios are correlated with metallicity for both galaxy types across the entire mass range in our sample, providing support for metallicity dependent supernova yields., Comment: 21 pages, 21 figures, 4 tables + Appendix

Instrumental systematics need to be controlled to high precision for upcoming Cosmic Microwave Background (CMB) experiments. The level of contamination caused by these systematics is often linked to the scan strategy, and scan strategies for satellite experiments can significantly mitigate these systematics. However, no detailed study has been performed for ground-based experiments. Here we show that under the assumption of constant elevation scans (CESs), the ability of the scan strategy to mitigate these systematics is strongly limited, irrespective of the detailed structure of the scan strategy. We calculate typical values and maps of the quantities coupling the scan to the systematics, and show how these quantities vary with the choice of observing elevations. These values and maps can be used to calculate and forecast the magnitude of different instrumental systematics without requiring detailed scan strategy simulations. As a reference point, we show that inclusion of even a single boresight rotation angle significantly improves over sky rotation alone for mitigating these systematics. A standard metric for evaluating cross-linking is related to one of the parameters studied in this work, so a corollary of our work is that the cross-linking will suffer from the same CES limitations and therefore upcoming CMB surveys will unavoidably have poorly cross-linked regions if they use CESs, regardless of detailed scheduling choices. Our results are also relevant for non-CMB surveys that perform constant elevation scans and may have scan-coupled systematics, such as intensity mapping surveys., Comment: Narrative adjusted to make scope of paper and results clearer; version published in the Open Journal of Astrophysics

We present the first detailed case study using quadratic estimators (QE) to diagnose and remove systematics present in observed Cosmic Microwave Background (CMB) maps. In this work we focus on the temperature to polarization leakage. We use an iterative QE analysis to remove systematics, in analogy to de-lensing, recovering the primordial B-mode signal and the systematic maps. We introduce a new Gaussian filtering scheme crucial to stable convergence of the iterative cleaning procedure and validate with comparisons to semi-analytical forecasts. We study the limitations of this method by examining its performance both on idealized simulations and on more realistic, non-ideal simulations, where we assume varying de-lensing efficiencies. Finally, we quantify the systematic cleaning efficiency by presenting a likelihood analysis on the tensor to scalar ratio, $r$, and demonstrate that the blind cleaning results in an un-biased measurement of $r$, reducing the systematic induced B-mode power by nearly two orders of magnitude., Comment: 30 pages, 12 figures, prepared for submission to JCAP

We study a phenomenological class of models where dark matter converts to dark radiation in the low redshift epoch. This class of models, dubbed DMDR, characterizes the evolution of comoving dark matter density with two extra parameters, and may be able to help alleviate the observed discrepancies between early- and late-time probes of the universe. We investigate how the conversion affects key cosmological observables such as the CMB temperature and matter power spectra. Combining 3x2pt data from Year 1 of the Dark Energy Survey, {\it Planck}-2018 CMB temperature and polarization data, supernovae (SN) Type Ia data from Pantheon, and baryon acoustic oscillation (BAO) data from BOSS DR12, MGS and 6dFGS, we place new constraints on the amount of dark matter that has converted to dark radiation and the rate of this conversion. The fraction of the dark matter that has converted since the beginning of the universe in units of the current amount of dark matter, $\zeta$, is constrained at 68\% confidence level to be $<0.32$ for DES-Y1 3x2pt data, $<0.030$ for CMB+SN+BAO data, and $<0.037$ for the combined dataset. The probability that the DES and CMB+SN+BAO datasets are concordant increases from 4\% for the $\Lambda$CDM model to 8\% (less tension) for DMDR. The tension in $S_8 = \sigma_8 \sqrt{\Omega_{\rm m}/0.3}$ between DES-Y1 3x2pt and CMB+SN+BAO is slightly reduced from $2.3\sigma$ to $1.9\sigma$. We find no reduction in the Hubble tension when the combined data is compared to distance-ladder measurements in the DMDR model. The maximum-posterior goodness-of-fit statistics of DMDR and $\Lambda$CDM model are comparable, indicating no preference for the DMDR cosmology over $\Lambda$CDM., Comment: 23 pages, 11 figures. DES extensions group. Accepted by PRD

The CMB $B$-mode polarisation signal -- both the primordial gravitational wave signature and the signal sourced by lensing -- is subject to many contaminants from systematic effects. Of particular concern are systematics that result in mixing of signals of different ``spin'', particularly leakage from the much larger spin-0 intensity signal to the spin-2 polarisation signal. We present a general formalism, which can be applied to arbitrary focal plane setups, that characterises signals in terms of their spin. We provide general expressions to describe how spin-coupled signals observed by the detectors manifest at map-level, in the harmonic domain, and in the power spectra, focusing on the polarisation spectra -- the signals of interest for upcoming CMB surveys. We demonstrate the presence of a previously unidentified cross-term between the systematic and the intrinsic sky signal in the power spectrum, which in some cases can be the dominant source of contamination. The formalism is not restricted to intensity to polarisation leakage but provides a complete elucidation of all leakage including polarisation mixing, and applies to both full and partial (masked) sky surveys, thus covering space-based, balloon-borne, and ground-based experiments. Using a pair-differenced setup, we demonstrate the formalism by using it to completely characterise the effects of differential gain and pointing systematics, incorporating both intensity leakage and polarisation mixing. We validate our results with full time ordered data simulations. Finally, we show in an Appendix that an extension of simple binning map-making to include additional spin information is capable of removing spin-coupled systematics during the map-making process., Comment: Minimal revisions - some additional references added, typos fixed, etc. 31 pages, 8 figures, 1 table. Accepted by MNRAS

We conduct a quantitative analysis of the star formation history (SFH) of the Milky Way's bulge by exploiting the constraining power of its stellar [Fe/H] and [Mg/Fe] distribution functions. Using APOGEE data, we confirm the previously-established bimodal [Mg/Fe]--[Fe/H] distribution within 3 kpc of the inner Galaxy. Compared to that in the solar vicinity, the high-$\alpha$ population in the bulge peaks at a lower [Fe/H]. To fit these observations, we use a simple but flexible star formation framework, which assumes two distinct stages of gas accretion and star formation, and systematically evaluate a wide multi-dimensional parameter space. We find that the data favor a three-phase SFH that consists of an initial starburst, followed by a rapid star formation quenching episode and a lengthy, quiescent secular evolution phase. The metal-poor, high-$\alpha$ bulge stars ([Fe/H]<0.0 and [Mg/Fe]>0.15) are formed rapidly (<2 Gyr) during the early starburst. The density gap between the high- and low-$\alpha$ sequences is due to the quenching process. The metal-rich, low-$\alpha$ population ([Fe/H]>0.0 and [Mg/Fe]<0.15) then accumulates gradually through inefficient star formation during the secular phase. This is qualitatively consistent with the early SFH of the inner disk. Given this scenario, a notable fraction of young stars (age<5 Gyr) is expected to persist in the bulge. Combined with extragalactic observations, these results suggest that a rapid star formation quenching process is responsible for bimodal distributions in both the Milky Way's stellar populations and in the general galaxy population and thus plays a critical role in galaxy evolution., Comment: 16 pages, 12 figures. MNRAS in press

We extend our previous work on the age-chemical abundance structure of the Galactic outer disc to the inner disc (4 < r < 8 kpc) based on the SDSS/APOGEE survey. Different from the outer disc, the inner disc stars exhibit a clear bimodal distribution in the [Mg/Fe]-[Fe/H] plane. While a number of scenarios have been proposed in the literature, it remains challenging to recover this bimodal distribution with theoretical models. To this end, we present a chemical evolution model embedding a complex multi-phase inner disc formation scenario that matches the observed bimodal [Mg/Fe]-[Fe/H] distribution. In this scenario, the formation of the inner disc is dominated by two main starburst episodes 6 Gyr apart with secular, low-level star formation activity in between. In our model, the first starburst occurs at early cosmic times (t~1 Gyr) and the second one 6 Gyr later at a cosmic time of t~7 Gyr. Both these starburst episodes are associated with gas accretion events in our model, and are quenched rapidly. The first starburst leads to the formation of the high-$\alpha$ sequence, and the second starburst leads to the formation of the metal-poor low-$\alpha$ sequence. The metal-rich low-$\alpha$ stars, instead, form during the secular evolution phase between the two bursts. Our model shows that the $\alpha$-dichotomy originates from the rapid suppression of star formation after the first starburst. The two starburst episodes are likely to be responsible for the formation of the geometric thick disc (z >1 kpc), with the old inner thick disc and the young outer thick disc forming during the first and the second starbursts, respectively., Comment: 16 pages, 10 figures. MNRAS in press

We report the stellar atmospheric parameters for 7503 spectra contained in the first release of the MaNGA stellar library (MaStar) in SDSS DR15. The first release of MaStar contains 8646 spectra measured from 3321 unique stars, each covering the wavelength range 3622 \AA\ to 10354 \AA\ with a resolving power of $R \sim$ 1800. In this work, we first determined the basic stellar parameters: effective temperature ($\rm T_{eff}$), surface gravity ($\log g$), and metallicity ($\rm[Fe/H]$), which best fit the data using an empirical interpolator based on the Medium-resolution Isaac Newton Telescope library of empirical spectra (MILES), as implemented by the University of Lyon Spectroscopic analysis Software (Koleva et al. 2008, ULySS) package. While we analyzed all 8646 spectra from the first release of MaStar, since MaStar has a wider parameter-space coverage than MILES, not all of these fits are robust. In addition, not all parameter regions covered by MILES yield robust results, likely due to the non-uniform coverage of the parameter space by MILES. We tested the robustness of the method using the MILES spectra itself and identified a proxy based on the local density of the training set. With this proxy, we identified 7503 MaStar spectra with robust fitting results. They cover the range from 3179K to 20,517K in effective temperature ($\rm T_{eff}$), from 0.40 to 5.0 in surface gravity ($\log g$), and from $-$2.49 to $+$0.73 in metallicity ($\rm[Fe/H]$)., Comment: 14 pages, 13 figures

We investigate the environmental dependence of the stellar populations of galaxies in SDSS DR7. Echoing earlier works, we find that satellites are both more metal-rich (<0.1 dex) and older (<2 Gyr) than centrals of the same stellar mass. However, after separating star-forming, green valley and passive galaxies, we find that the true environmental dependence of both stellar metallicity (<0.03 dex) and age (<0.5 Gyr) is in fact much weaker. We show that the strong environmental effects found when galaxies are not differentiated result from a combination of selection effects brought about by the environmental dependence of the quenched fraction of galaxies, and thus we strongly advocate for the separation of star-forming, green valley and passive galaxies when the environmental dependence of galaxy properties are investigated. We also study further environmental trends separately for both central and satellite galaxies. We find that star-forming galaxies show no environmental effects, neither for centrals nor for satellites. In contrast, the stellar metallicities of passive and green valley satellites increase weakly (< 0.05 dex and < 0.08 dex, respectively) with increasing halo mass, increasing local overdensity and decreasing projected distance from their central; this effect is interpreted in terms of moderate environmental starvation (`strangulation') contributing to the quenching of satellite galaxies. Finally, we find a unique feature in the stellar mass--stellar metallicity relation for passive centrals, where galaxies in more massive haloes have larger stellar mass (~0.1 dex) at constant stellar metallicity; this effect is interpreted in terms of dry merging of passive central galaxies and/or progenitor bias., Comment: 22 pages, 18 figures, accepted for publication in MNRAS

The cosmological phenomenology of gravity is typically studied in two limits: relativistic perturbation theory (on large scales) and Newtonian gravity (required for smaller, non-linear, scales). Traditional approaches to model-independent modified gravity are based on perturbation theory, so do not apply on non-linear scales. Future surveys such as Euclid will produce significant data on both linear and non-linear scales, so a new approach is required to constrain model-independent modified gravity by simultaneously using all of the data from these surveys. We use the higher order equations from the post-Friedmann approach to derive a single set of "simple 1PF" (first post-Friedmann) equations that apply in both the small scale and large scale limits, and we examine the required conditions for there to be no intermediate regime, meaning that these simple equations are valid on all scales. We demonstrate how the simple 1PF equations derived here can be used as a model-independent framework for modified gravity that applies on all cosmological scales, and we present an algorithm for determining which modified gravity theories are subsumed under this approach. This modified gravity framework provides a rigorous approach to phenomenological N-body simulations, and paves the way to consistently using all of the data from upcoming surveys to constrain modified gravity in a model-independent fashion., Comment: Minor updates to match version published in PRD, primarily the addition of section V.C.6. Chosen as editor's suggestion

We perform the first test of dark matter (DM) stress-energy evolution through cosmic history, using cosmic microwave background measurements supplemented with baryon acoustic oscillation data and the Hubble Space Telescope key project data. We constrain the DM equation of state (EoS) in 8 redshift bins, and its sound speed and (shear) viscosity in 9 redshift bins, finding no convincing evidence for non-$\Lambda$CDM values in any of the redshift bins. Despite this enlarged parameter space, the sound speed and viscosity are constrained relatively well at late times (due to the inclusion of CMB lensing), whereas the EoS is most strongly constrained around recombination. These results constrain for the first time the level of "coldness" required of DM across various cosmological epochs at both the background and perturbative levels. We show that simultaneously allowing time dependence for both the EoS and sound speed parameters shifts the posterior of the DM abundance before recombination to a higher value, while keeping the present day DM abundance similar to the $\Lambda$CDM value. This shifts the posterior for the present day Hubble constant compared to $\Lambda$CDM, suggesting that DM with time-dependent parameters is well-suited to explore possible solutions to persistent tensions within the $\Lambda$CDM model. We perform a detailed comparison with our previous study involving a vanishing sound speed and viscosity using the same datasets in order to explain the physical mechanism behind these shifts., Comment: 20 pages, 10 figures, 5 tables; updated to match version published in PRD