If the dark matter (DM) were composed of axions, then structure formation in the Universe would be suppressed below the axion Jeans scale. Using an analytic model for the halo mass function of a ...mixed DM model with axions and cold dark matter, combined with the abundance-matching technique, we construct the UV-luminosity function. Axions suppress high-z galaxy formation and the UV-luminosity function is truncated at a faintest limiting magnitude. From the UV-luminosity function, we predict the reionization history of the universe and find that axion DM causes reionization to occur at lower redshift. We search for evidence of axions using the Hubble Ultra Deep Field UV-luminosity function in the redshift range z = 6–10, and the optical depth to reionization, τ, as measured from cosmic microwave background polarization. All probes we consider consistently exclude m
a
≲ 10−23 eV from contributing more than half of the DM, with our strongest constraint ruling this model out at more than 8σ significance. In conservative models of reionization a dominant component of DM with m
a
= 10−22 eV is in 3σ tension with the measured value of τ, putting pressure on an axion solution to the cusp-core problem. Tension is reduced to 2σ for the axion contributing only half of the DM. A future measurement of the UV-luminosity function in the range z = 10–13 by JWST would provide further evidence for or against m
a
= 10−22 eV. Probing still higher masses of m
a
= 10−21 eV will be possible using future measurements of the kinetic Sunyaev–Zel'dovich effect by Advanced ACTPol to constrain the time and duration of reionization.
We report new constraints on the local escape speed of our Galaxy. Our analysis is based on a sample of high-velocity stars from the RAVE survey and two previously published data sets. We use ...cosmological simulations of disc galaxy formation to motivate our assumptions on the shape of the velocity distribution, allowing for a significantly more precise measurement of the escape velocity compared to previous studies. We find that the escape velocity lies within the range 498 < vesc < 608 km s−1 (90 per cent confidence), with a median likelihood of 544 km s−1. The fact that v2esc is significantly greater than 2v2circ (where vcirc= 220 km s−1 is the local circular velocity) implies that there must be a significant amount of mass exterior to the solar circle, that is, this convincingly demonstrates the presence of a dark halo in the Galaxy. We use our constraints on vesc to determine the mass of the Milky Way halo for three halo profiles. For example, an adiabatically contracted NFW halo model results in a virial mass of 1.42+1.14−0.54× 1012 M⊙ and virial radius of (90 per cent confidence). For this model the circular velocity at the virial radius is 142+31−21 km s−1. Although our halo masses are model dependent, we find that they are in good agreement with each other.
Abstract
Establishing the spatial extents and the nature of the outer stellar populations of dwarf galaxies is necessary for the determination of their total masses, current dynamical states, and ...past evolution. We here describe our investigation of the outer stellar content of the Boötes I ultra-faint dwarf galaxy, a satellite of the the Milky Way. We identify candidate member blue horizontal branch and blue straggler stars of Boötes I, both tracers of the underlying ancient stellar population, using a combination of multiband Pan-STARRS photometry and Gaia astrometry. We find a total of twenty-four candidate blue horizontal branch member stars with apparent magnitudes and proper motions consistent with membership of Boötes I, nine of which reside at projected distances beyond the nominal King profile tidal radius derived from earlier fits to photometry. We also identify four blue straggler stars of appropriate apparent magnitude to be at the distance of Boötes I, but all four are too faint to have high-quality astrometry from Gaia. The outer blue horizontal branch stars that we have identified confirm that the spatial distribution of the stellar population of Boötes I is quite extended. The morphology on the sky of these outer envelope candidate member stars is evocative of tidal interactions, a possibility that we explore further with simple dynamical models.
We investigate aluminum abundance variations in the stellar populations of globular clusters using both literature measurements of sodium and aluminum and APOGEE measurements of nitrogen and aluminum ...abundances. For the latter, we show that the Payne is the most suitable of the five available abundance pipelines for our purposes. Our combined sample of 42 globular clusters spans approximately 2 dex in Fe/H and 1.5 dex in . We find no fewer than five globular clusters with significant internal variations in nitrogen and/or sodium with little to no corresponding variation in aluminum, and that the minimum present-day cluster mass for aluminum enrichment in metal-rich systems is . We demonstrate that the slopes of the Al/Fe versus Na/Fe and Al/Fe versus N/Fe relations for stars without field-like abundances are approximately log-linearly dependent on both the metallicity and the stellar mass of the globular clusters. In contrast, the relationship between Na/Fe and N/Fe shows no evidence of such dependencies. This suggests that there were (at least) two classes of non-supernova chemical polluters that were common in the early universe, and that their relative contributions within globular clusters somehow scaled with the metallicity and mass of globular clusters. The first of these classes is predominantly responsible for the CNO and NeNa abundance variations, and likewise the second for the MgAl abundance variations. Particularly striking examples of this dichotomy include 47 Tuc and M4. As an auxiliary finding, we argue that abundance variations among Terzan 5 stars are consistent with it being a normal globular cluster.
Abstract
Insights from JWST observations suggest that active galactic nuclei feedback evolved from a short-lived, high-redshift phase in which radiatively cooled turbulence and/or momentum-conserving ...outflows stimulated vigorous early star formation (“positive” feedback), to late, energy-conserving outflows that depleted halo gas reservoirs and quenched star formation. The transition between these two regimes occurred at
z
∼ 6, independently of galaxy mass, for simple assumptions about the outflows and star formation process. Observational predictions provide circumstantial evidence for the prevalence of massive black holes at the highest redshifts hitherto observed, and we discuss their origins.
Abstract
This compilation is the fourth data release from the
R
-Process Alliance (RPA) search for
r
-process-enhanced stars and the second release based on “snapshot” high-resolution (
R
∼ 30,000) ...spectra collected with the du Pont 2.5 m Telescope. In this data release, we propose a new delineation between the
r
-I and
r
-II stellar classes at
, instead of the empirically chosen
level previously in use, based on statistical tests of the complete set of RPA data released to date. We also statistically justify the minimum level of Eu/Fe for definition of the
r
-I stars, Eu/Fe > +0.3. Redefining the separation between
r
-I and
r
-II stars will aid in the analysis of the possible progenitors of these two classes of stars and determine whether these signatures arise from separate astrophysical sources at all. Applying this redefinition to previous RPA data, the number of identified
r
-II and
r
-I stars changes to 51 and 121, respectively, from the initial set of data releases published thus far. In this data release, we identify 21 new
r
-II, 111 new
r
-I (plus 3 re-identified), and 7 new (plus 1 re-identified) limited-
r
stars out of a total of 232 target stars, resulting in a total sample of 72 new
r
-II stars, 232 new
r
-I stars, and 42 new limited-
r
stars identified by the RPA to date.
Abstract
We use the second data releases of the European Space AgencyGaia astrometric survey and the high-resolution Galactic Archaeology with HERMES (GALAH) spectroscopic survey to analyse the ...structure of our Galaxy’s disc components. With GALAH, we separate the α-rich and α-poor discs (with respect to Fe), which are superposed in both position and velocity space, and examine their distributions in action space. We study the distribution of stars in the zV$\mathrm{ z}$ phase plane, for both Vϕ and VR, and recover the remarkable ‘phase spiral’ discovered by Gaia. We identify the anticipated quadrupole signature in zV$\mathrm{ z}$ of a tilted velocity ellipsoid for stars above and below the Galactic plane. By connecting our work with earlier studies, we show that the phase spiral is likely to extend well beyond the narrow solar neighbourhood cylinder in which it was found. The phase spiral is a signature of corrugated waves that propagate through the disc, and the associated non-equilibrium phase mixing. The radially asymmetric distribution of stars involved in the phase spiral reveals that the corrugation, which is mostly confined to the α-poor disc, grows in z-amplitude with increasing radius. We present new simulations of tidal disturbance of the Galactic disc by the Sagittarius (Sgr) dwarf. The effect on the zV$\mathrm{ z}$ phase plane lasts ${\gtrsim } 2\, \mathrm{Gyr}$, but a subsequent disc crossing wipes out the coherent structure. We find that the phase spiral was excited ${\lesssim } 0.5\, \mathrm{Gyr}$ ago by an object like Sgr with total mass ∼3 × 1010 M⊙ (stripped down from ∼5 × 1010 M⊙ when it first entered the halo) passing through the plane.
We present chemical abundance measurements of two metal-poor red giant stars in the ultra-faint dwarf galaxy Bootes I, based on Magellan/MIKE high-resolution spectra. For Boo-980, with Fe/H = -3.1, ...we present the first elemental abundance measurements, while Boo-127, with Fe/H = -2.0, shows abundances in good agreement with previous measurements. Light and iron-peak element abundance ratios in the two Bootes I stars, as well as those of most other Bootes I members, collected from the literature, closely resemble those of regular metal-poor halo stars. Neutron-capture element abundances Sr and Ba are systematically lower than the main halo trend and also show a significant abundance spread. Overall, this is similar to what has been found for other ultra-faint dwarf galaxies. We apply corrections to the carbon abundances (commensurate with stellar evolutionary status) of the entire sample and find 21% of stars to be carbon-enhanced metal-poor (CEMP) stars, compared to 13% without using the carbon correction. We reassess the metallicity distribution functions for the CEMP stars and non-CEMP stars, and confirm earlier claims that CEMP stars might belong to a different, earlier population. Applying a set of abundance criteria to test to what extent Bootes I could be a surviving first galaxy suggests that it is one of the earliest assembled systems that perhaps received gas from accretion from other clouds in the system, or from swallowing a first galaxy or building block type object. This resulted in the two stellar populations observable today.
Chemical separation of disc components using RAVE Wojno, Jennifer; Kordopatis, Georges; Steinmetz, Matthias ...
Monthly notices of the Royal Astronomical Society,
10/2016, Volume:
461, Issue:
4
Journal Article
Peer reviewed
Open access
We present evidence from the RAdial Velocity Experiment (RAVE) survey of chemically separated, kinematically distinct disc components in the solar neighbourhood. We apply probabilistic chemical ...selection criteria to separate our sample into α-low (‘thin disc’) and α-high (‘thick disc’) sequences. Using newly derived distances, which will be utilized in the upcoming RAVE DR5, we explore the kinematic trends as a function of metallicity for each of the disc components. For our α-low disc, we find a negative trend in the mean rotational velocity (V
ϕ) as a function of iron abundance (Fe/H). We measure a positive gradient ∂V
ϕ/∂Fe/H for the α-high disc, consistent with results from high-resolution surveys. We also find differences between the α-low and α-high discs in all three components of velocity dispersion. We discuss the implications of an α-low, metal-rich population originating from the inner Galaxy, where the orbits of these stars have been significantly altered by radial mixing mechanisms in order to bring them into the solar neighbourhood. The probabilistic separation we propose can be extended to other data sets for which the accuracy in α/Fe is not sufficient to disentangle the chemical disc components a priori. For such data sets which will also have significant overlap with Gaia DR1, we can therefore make full use of the improved parallax and proper motion data as it becomes available to investigate kinematic trends in these chemical disc components.
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