2006.12507
The age of the Milky Way inner stellar spheroid from RR Lyrae population synthesis
Savino, et al
The central kiloparsecs of the Milky Way are known to host an old, spheroidal stellar population, whose spatial and kinematical properties set it apart from the boxy/peanut structure that constitutes most of the central stellar mass. The nature of this spheroidal population, whether a small classical bulge, the innermost stellar halo or a population of disk stars with large initial velocity dispersion, remains unclear. This structure is also a promising candidate to host some of the oldest stars in the Galaxy. Here we address the topic of the inner stellar spheroid age, using spectroscopic and photometric metallicities for a sample of 935 RR Lyrae stars that are constituents of this component. By means of stellar population synthesis, we derive an age-metallicity relation for RR Lyrae populations. We infer, for the RR Lyrae stars in the bulge spheroid, an extremely ancient age of $13.41 \pm 0.54$ Gyr and conclude they were among the first stars to form in what is now the Milky Way galaxy. Our age estimate for the central spheroid shows remarkable agreement with the age profile that has been inferred for the Milky Way stellar halo, suggesting a connection between the two structures. However, we find mild evidence for a transition in the halo properties at $r_{\rm GC} \sim 5$~kpc. We also investigate formation scenarios for metal-rich RR Lyrae stars, such as binarity and helium variations, and whether they can provide alternative explanations for the properties of our sample. We conclude that, within our framework, the only viable alternative is to have younger, slightly helium-rich, RR Lyrae stars, a hypothesis that would open intriguing questions for the formation of the inner stellar spheroid.
The age of the Milky Way inner stellar spheroid from RR Lyrae population synthesis
Savino, et al
The central kiloparsecs of the Milky Way are known to host an old, spheroidal stellar population, whose spatial and kinematical properties set it apart from the boxy/peanut structure that constitutes most of the central stellar mass. The nature of this spheroidal population, whether a small classical bulge, the innermost stellar halo or a population of disk stars with large initial velocity dispersion, remains unclear. This structure is also a promising candidate to host some of the oldest stars in the Galaxy. Here we address the topic of the inner stellar spheroid age, using spectroscopic and photometric metallicities for a sample of 935 RR Lyrae stars that are constituents of this component. By means of stellar population synthesis, we derive an age-metallicity relation for RR Lyrae populations. We infer, for the RR Lyrae stars in the bulge spheroid, an extremely ancient age of $13.41 \pm 0.54$ Gyr and conclude they were among the first stars to form in what is now the Milky Way galaxy. Our age estimate for the central spheroid shows remarkable agreement with the age profile that has been inferred for the Milky Way stellar halo, suggesting a connection between the two structures. However, we find mild evidence for a transition in the halo properties at $r_{\rm GC} \sim 5$~kpc. We also investigate formation scenarios for metal-rich RR Lyrae stars, such as binarity and helium variations, and whether they can provide alternative explanations for the properties of our sample. We conclude that, within our framework, the only viable alternative is to have younger, slightly helium-rich, RR Lyrae stars, a hypothesis that would open intriguing questions for the formation of the inner stellar spheroid.
2006.12538
Optimising LSST observing strategy for weak lensing systematics
Almoubayyed, Mandelbaum, et al
The LSST survey will provide unprecedented statistical power for measurements of dark energy. Consequently, controlling systematic uncertainties is becoming more important than ever. The LSST observing strategy will affect the statistical uncertainty and systematics control for many science cases; here, we focus on weak lensing systematics. The fact that the LSST observing strategy involves hundreds of visits to the same sky area provides new opportunities for systematics mitigation. We explore these opportunities by testing how different dithering strategies (pointing offsets and rotational angle of the camera in different exposures) affect additive weak lensing shear systematics on a baseline operational simulation, using the $\rho-$statistics formalism. Some dithering strategies improve systematics control at the end of the survey by a factor of up to $\sim 3-4$ better than others. We find that a random translational dithering strategy, applied with random rotational dithering at every filter change, is the most effective of those strategies tested in this work at averaging down systematics. Adopting this dithering algorithm, we explore the effect of varying the area of the survey footprint, exposure time, number of exposures in a visit, and exposure to the Galactic plane. We find that any change that increases the average number of exposures (in filters relevant to weak lensing) reduces the additive shear systematics. Some ways to achieve this increase may not be favorable for the weak lensing statistical constraining power or for other probes, and we explore the relative trade-offs between these options given constraints on the overall survey parameters.
2006.12566
Astronomy Paper seminar participation guide & Reading
Cooke, et al
Welcome to the wonderful world of scientific inquiry! On this journey you'll be reading many$\times 10^N$ papers in your discipline. Therefore, efficiency in digesting and relaying this information is paramount. In this guide, we'll review how you can participate in your local astronomy seminars. Participation takes many forms, from contributing a recently discovered article to the discussion of a published paper. In this guide, we'll begin by providing some suggested introductory activities for beginner scientists. Then we discuss how to locate papers and assimilate their results. Finally we conclude with a discussion on paper presentation and note storage. This guide is intended for an undergraduate and graduate student audience, and we encourage faculty to read and distribute this guide to students.
2006.12574
A dynamo amplifies the magnetic field of a Milky-Way-like galaxy
Ntormousi, et al
The magnetic fields of spiral galaxies are so strong that they cannot be primordial. Their typical values are over one billion times higher than any value predicted for the early Universe. Explaining this immense growth and incorporating it in galaxy evolution theories is one of the long-standing challenges in astrophysics. So far, the most successful theory for the sustained growth of the galactic magnetic field is the alpha-omega dynamo. This theory predicts a characteristic dipolar or quadrupolar morphology for the galactic magnetic field, which has been observed in external galaxies. However, so far, there has been no direct demonstration of a mean-field dynamo operating in direct, multi-physics simulations of spiral galaxies. We do so in this work. We employ numerical models of isolated, star-forming spiral galaxies that include a magnetized gaseous disk, a dark matter halo, stars, and stellar feedback. Naturally, the resulting magnetic field has a complex morphology that includes a strong random component. Using a smoothing of the magnetic field on small scales, we are able to separate the mean from the turbulent component and analyze them individually. We find that a mean-field dynamo naturally occurs as a result of the dynamical evolution of the galaxy and amplifies the magnetic field by an order of magnitude over half a Gyr. Despite the highly dynamical nature of these models, the morphology of the mean component of the field is identical to analytical predictions. This result underlines the importance of the mean-field dynamo in galactic evolution. Moreover, by demonstrating the natural growth of the magnetic field in a complex galactic environment, it brings us a step closer to understanding the cosmic origin of magnetic fields.
2006.12751
A redifinition of the halo boundary leads to a simple yet accurate halo model of large scale structure
Garcia, Rozo, Becker, More, et al
We present a model for the halo--mass correlation function that explicitly incorporates halo exclusion. We assume that halos trace mass in a way that can be described using a single scale-independent bias parameter. However, our model exhibits scale dependent biasing due to the impact of halo-exclusion, the use of a ``soft'' (i.e. not infinitely sharp) halo boundary, and differences in the one halo term contributions to $\xi_{\rm hm}$ and $\xi_{\rm mm}$. These features naturally lead us to a redefinition of the halo boundary that lies at the ``by eye'' transition radius from the one--halo to the two--halo term in the halo--mass correlation function. When adopting our proposed definition, our model succeeds in describing the halo--mass correlation function with $\approx 2\%$ residuals over the radial range $0.1\ h^{-1}{\rm Mpc} < r < 80\ h^{-1}{\rm Mpc}$, and for halo masses in the range $10^{13}\ h^{-1}{\rm M}_{\odot} < M < 10^{15}\ h^{-1}{\rm M}_{\odot}$. Our proposed halo boundary is related to the splashback radius by a roughly constant multiplicative factor. Taking the 87-percentile as reference we find $r_{\rm t}/R_{\rm sp} \approx 1.3$. Surprisingly, our proposed definition results in halo abundances that are well described by the Press-Schechter mass function with $\delta_{\rm sc}=1.449\pm 0.004$. The clustering bias parameter is offset from the standard background-split prediction by $\approx 10\%-15\%$. This level of agreement is comparable to that achieved with more standard halo definitions.
2006.13219
Interpreting LIGO/Virgo "Mass-Gap" events as lensed neutron star-black hole binaries
Broadhurst, et al
Gravitational lensing allows the detection of binary black holes (BBH) at cosmological distances with chirp masses that appear to be enhanced by $1+z$ in the range $1<z<4$, in good agreement with the reported BBH masses. We propose this effect also accounts for the puzzling "mass gap" events (MG) newly reported by LIGO/Virgo, as distant, lensed NSBH events with $1<z<4$. The fitted mass of the neutron star member becomes $(1+z)\times 1.4M_\odot$, and is therefore misclassified as a low mass black hole. In this way, we derive a redshift of $z\simeq 3.5$ and $z\simeq 1.0$ for two newly reported "mass asymmetric" events GW190412 \& GW190814, by interpreting them as lensed NSBH events, comprising a stellar mass black hole and neutron star. Over the past year an additional 31 BBH events and 5 MG events have been reported with high probability ($>95\%$), from which we infer a factor $\simeq 5$ higher intrinsic rate of NSBH events than BBH events, reflecting a higher proportion of neutron stars formed by early star formation. We predict a distinctive locus for lensed NSBH events in the observed binary mass plane, spanning $1<z<4$ with a narrow mass ratio, $q \simeq 0.2$, that can be readily tested when the waveform data are unlocked. All such events may show disrupted NS emission and are worthy of prompt follow-up as the high lensing magnification means EM detections are not prohibitive despite the high redshifts that we predict. Such lensed NSBH events provide an exciting prospect of directly charting the history of coalescing binaries via the cosmological redshift of their waveforms, determined relative to the characteristic mass of the neutron star member.
2006.13236
The Keck baryonic structure survey: using foreground/background galaxy pairs to trace the structure and kinematics of circumgalactic neutral's hydrogen at $z \sim 2$
Chen, et al
We present new measurements of the spatial distribution and kinematics of neutral hydrogen in the circumgalactic and intergalactic medium surrounding star-forming galaxies at z ~ 2. Using the spectra of ~ 3000 galaxies with redshifts <z> +/- 0.4 from the Keck Baryonic Structure Survey (KBSS), we assemble a sample of more than 200,000 distinct foreground-background pairs with projected angular separations of 3 - 500 arcsec and spectroscopic redshifts, with <$z_{fg}$> = 2.23 and <$z_{bg}$> = 2.57. The ensemble of sightlines and foreground galaxies is used to construct a 2D map of the mean excess Ly$\alpha$ optical depth relative to the intergalactic mean as a function of projected galactocentric distance (20 < $D_{tran}$/pkpc < 4000) and line-of-sight velocity. We provide information on the line-of-sight kinematics of H I gas as a function of projected distance $D_{tran}$. We compare the map with cosmological zoom-in simulation, finding qualitative agreement between them. A simple two-component (accretion, outflow) analytical model generally reproduces the observed line-of-sight kinematics and projected spatial distribution of H I. The best-fitting model suggests that galaxy-scale outflows with initial velocity $v_{out}$ ~ 600 km/s dominate the kinematics of circumgalactic H I out to $D_{tran}$ ~ 50 kpc, while H I at $D_{tran}$ > 100 kpc is dominated by infall with characteristic $v_{in}$ < $v_c$, where $v_c$ is the circular velocity of the host halo ($M_h$ ~ $10^{12} M_\odot$). Over the impact parameter range 80 < $D_{tran}$/pkpc < 200, the H I line-of-sight velocity range reaches a minimum, with a corresponding flattening in the rest-frame Ly$\alpha$ equivalent width. These observations can be naturally explained as the transition between outflow-dominated and accretion-dominated flows. Beyond $D_{tran}$ ~ 300 kpc, the line of sight kinematics are dominated by Hubble expansion.
2006.13685
(Un)conscious bias in the astronomical profession: universal recommendations to improve fairness, inclusiveness, and representation
Aloisi, Reid
(Un)conscious bias affects every aspect of the astronomical profession, from scientific activities (e.g., invitations to join collaborations, proposal selections, grant allocations, publication review processes, and invitations to attend and speak at conferences) to activities more strictly related to career advancement (e.g., reference letters, fellowships, hiring, promotion, and tenure). For many, (un)conscious bias is still the main hurdle to achieving excellence, as the most diverse talents encounter bigger challenges and difficulties to reach the same milestones than their more privileged colleagues. Over the past few years, the Space Telescope Science Institute (STScI) has constructed tools to raise awareness of (un)conscious bias and has designed guidelines and goals to increase diversity representation and outcome in its scientific activities, including career-related matters and STScI sponsored fellowships, conferences, workshops, and colloquia. STScI has also addressed (un)conscious bias in the peer-review process by anonymizing submission and evaluation of Hubble Space Telescope (and soon to be James Webb Space Telescope) observing proposals. In this white paper we present a plan to standardize these methods with the expectation that these universal recommendations will truly increase diversity, inclusiveness and fairness in Astronomy if applied consistently throughout all the scientific activities of the Astronomical community.
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