ABSTRACT
Planck data provide precise constraints on cosmological parameters when assuming the base ΛCDM model, including a 0.17 per cent measurement of the age of the Universe, $t_0=13.797 \pm ...0.023\, {\rm Gyr}$. However, the persistence of the ‘Hubble tension’ calls the base ΛCDM model’s completeness into question and has spurred interest in models such as early dark energy (EDE) that modify the assumed expansion history of the Universe. We investigate the effect of EDE on the redshift–time relation z↔t and find that it differs from the base ΛCDM model by at least ${\approx } 4{{\ \rm per\ cent}}$ at all t and z. As long as EDE remains observationally viable, any inferred t ← z or z ← t quoted to a higher level of precision do not reflect the current status of our understanding of cosmology. This uncertainty has important astrophysical implications: the reionization epoch – 10 > z > 6 – corresponds to disjoint lookback time periods in the base ΛCDM and EDE models, and the EDE value of t0 = 13.25 ± 0.17 Gyr is in tension with published ages of some stars, star clusters, and ultrafaint dwarf galaxies. However, most published stellar ages do not include an uncertainty in accuracy (due to, e.g. uncertain distances and stellar physics) that is estimated to be $\sim 7\!-\!10{{\ \rm per\ cent}}$, potentially reconciling stellar ages with $t_{0,\rm EDE}$. We discuss how the big data era for stars is providing extremely precise ages ($\lt 1{{\ \rm per\ cent}}$) and how improved distances and treatment of stellar physics such as convection could result in ages accurate to $4\!-\!5{{\ \rm per\ cent}}$, comparable to the current accuracy of t↔z. Such precise and accurate stellar ages can provide detailed insight into the high-redshift Universe independent of a cosmological model.
With the release of Gaia DR2, it is now possible to measure the proper motions (PMs) of the lowest-mass, ultrafaint satellite galaxies in the Milky Way's (MW) halo for the first time. Many of these ...faint satellites are posited to have been accreted as satellites of the Magellanic Clouds (MCs). Using their six-dimensional phase-space information, we calculate the orbital histories of 13 ultrafaint satellites and five classical dwarf spheroidals in a combined MW+LMC+SMC potential to determine which galaxies are dynamically associated with the MCs. These 18 galaxies are separated into four classes: (i) long-term Magellanic satellites that have been bound to the MCs for at least the last two consecutive orbits around the MCs (Carina 2, Carina 3, Horologium 1, Hydrus 1); (ii) Magellanic satellites that were recently captured by the MCs < 1 Gyr ago (Reticulum 2, Phoenix 2); (iii) MW satellites that have interacted with the MCs (Sculptor 1, Tucana 3, Segue 1); and (iv) MW satellites (Aquarius 2, Canes Venatici 2, Crater 2, Draco 1, Draco 2, Hydra 2, Carina, Fornax, Ursa Minor). Results are reported for a range of MW and LMC masses. Contrary to previous work, we find no dynamical association between Carina, Fornax, and the MCs. Finally, we determine that the addition of the SMC's gravitational potential affects the longevity of satellites as members of the Magellanic system (long-term versus recently captured), but it does not change the total number of Magellanic satellites.
Abstract
We measure the ionizing photon production efficiency (
ξ
ion
) of low-mass galaxies (10
7.8
–10
9.8
M
⊙
) at 1.4 <
z
< 2.7 to better understand the contribution of dwarf galaxies to the ...ionizing background and reionization. We target galaxies that are magnified by strong-lensing galaxy clusters and use Keck/MOSFIRE to measure nebular emission-line fluxes and Hubble Space Telescope to measure the rest-UV and rest-optical photometry. We present two methods of stacking. First, we take the average of the log of H
α
-to-UV luminosity ratios (
L
H
α
/
L
UV
) of galaxies to determine the standard log(
ξ
ion
). Second, we take the logarithm of the total
L
H
α
over the total
L
UV
. We prefer the latter, as it provides the total ionizing UV luminosity density of galaxies when multiplied by the nonionizing UV luminosity density. log(
ξ
ion
) calculated from the second method is ∼0.2 dex higher than the first method. We do not find any strong dependence between log(
ξ
ion
) and stellar mass, far-UV magnitude (
M
UV
), or UV spectral slope (
β
). We report a value of log(
ξ
ion
) ∼ 25.47 ± 0.09 for our UV-complete sample (
) and ∼25.37 ± 0.11 for our mass-complete sample (7.8 < log(
M
*
) < 9.8). These values are consistent with measurements of more massive, more luminous galaxies in other high-redshift studies that use the same stacking technique. Our log(
ξ
ion
) is 0.2–0.3 dex higher than low-redshift galaxies of similar mass, indicating an evolution in the stellar properties, possibly due to metallicity or age. We also find a correlation between log(
ξ
ion
) and the equivalent widths of H
α
and O
iii
λ
5007 fluxes, confirming that these equivalent widths can be used to estimate
ξ
ion
.
We use data from Gaia's second data release (DR2) to constrain the initial-final mass relation (IFMR) for field stars with initial masses 0.9 min/M 8. Precise parallaxes have revealed unprecedented ...substructure in the white dwarf (WD) cooling sequence on the color-magnitude diagram (CMD). Some of this substructure stems from the diversity of WD atmospheric compositions, but the CMD remains bimodal even when only spectroscopically confirmed DA WDs are considered. We develop a generative model to predict the CMD for DA WDs as a function of the initial mass function, stellar age distribution, and a flexibly parameterized IFMR. We then fit the CMD of 1100 bright DA WDs within 100 pc, for which atmospheric composition and completeness are well understood. The resulting best-fit IFMR flattens at 3.5 min/M 5.5, producing a secondary peak in the WD mass distribution at mWD ∼ 0.8 M . Our IFMR is broadly consistent with weaker constraints obtained from binaries and star clusters in previous work but represents the clearest observational evidence obtained to date of theoretically predicted nonlinearity in the IFMR. A visibly bimodal CMD is only predicted for mixed-age stellar populations: in single-age clusters, more massive WDs reach the bottom of the cooling sequence before the first lower-mass WDs appear. This may explain why bimodal cooling sequences have thus far evaded detection in cluster CMDs.
Abstract
We develop a data-driven spectral model for identifying and characterizing spatially unresolved multiple-star systems and apply it to APOGEE DR13 spectra of main-sequence stars. Binaries and ...triples are identified as targets whose spectra can be significantly better fit by a superposition of two or three model spectra, drawn from the same isochrone, than any single-star model. From an initial sample of ∼20 000 main-sequence targets, we identify ∼2500 binaries in which both the primary and secondary stars contribute detectably to the spectrum, simultaneously fitting for the velocities and stellar parameters of both components. We additionally identify and fit ∼200 triple systems, as well as ∼700 velocity-variable systems in which the secondary does not contribute detectably to the spectrum. Our model simplifies the process of simultaneously fitting single- or multi-epoch spectra with composite models and does not depend on a velocity offset between the two components of a binary, making it sensitive to traditionally undetectable systems with periods of hundreds or thousands of years. In agreement with conventional expectations, almost all the spectrally identified binaries with measured parallaxes fall above the main sequence in the colour–magnitude diagram. We find excellent agreement between spectrally and dynamically inferred mass ratios for the ∼600 binaries in which a dynamical mass ratio can be measured from multi-epoch radial velocities. We obtain full orbital solutions for 64 systems, including 14 close binaries within hierarchical triples. We make available catalogues of stellar parameters, abundances, mass ratios, and orbital parameters.
A Sun-like star orbiting a black hole El-Badry, Kareem; Rix, Hans-Walter; Quataert, Eliot ...
Monthly notices of the Royal Astronomical Society,
01/2023, Letnik:
518, Številka:
1
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
We report discovery of a bright, nearby ($G = 13.8;\, \, d = 480\, \rm pc$) Sun-like star orbiting a dark object. We identified the system as a black hole candidate via its astrometric ...orbital solution from the Gaia mission. Radial velocities validated and refined the Gaia solution, and spectroscopy ruled out significant light contributions from another star. Joint modelling of radial velocities and astrometry constrains the companion mass of $M_2 = 9.62\pm 0.18\, \mathrm{M}_{\odot }$. The spectroscopic orbit alone sets a minimum companion mass of $M_2\gt 5\, \mathrm{M}_{\odot }$; if the companion were a $5\, \mathrm{M}_{\odot }$ star, it would be 500 times more luminous than the entire system. These constraints are insensitive to the mass of the luminous star, which appears as a slowly rotating G dwarf ($T_{\rm eff}=5850\, \rm K$, log g = 4.5, $M=0.93\, \mathrm{M}_{\odot }$), with near-solar metallicity ($\rm Fe/H = -0.2$) and an unremarkable abundance pattern. We find no plausible astrophysical scenario that can explain the orbit and does not involve a black hole. The orbital period, Porb = 185.6 d, is longer than that of any known stellar-mass black hole binary. The system’s modest eccentricity (e = 0.45), high metallicity, and thin-disc Galactic orbit suggest that it was born in the Milky Way disc with at most a weak natal kick. How the system formed is uncertain. Common envelope evolution can only produce the system’s wide orbit under extreme and likely unphysical assumptions. Formation models involving triples or dynamical assembly in an open cluster may be more promising. This is the nearest known black hole by a factor of 3, and its discovery suggests the existence of a sizable population of dormant black holes in binaries. Future Gaia releases will likely facilitate the discovery of dozens more.
Where are the most ancient stars in the Milky Way? El-Badry, Kareem; Bland-Hawthorn, Joss; Wetzel, Andrew ...
Monthly Notices of the Royal Astronomical Society,
10/2018, Letnik:
480, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The oldest stars in the Milky Way (MW) bear imprints of the Galaxy's early assembly history. We use FIRE cosmological zoom-in simulations of three MW-mass disc galaxies to study the spatial ...distribution, chemistry, and kinematics of the oldest surviving stars (
≳ 5) in MW-like galaxies. We predict the oldest stars to be less centrally concentrated at
= 0 than stars formed at later times as a result of two processes. First, the majority of the oldest stars are not formed
but are accreted during hierarchical assembly. These
stars are deposited on dispersion-supported, halo-like orbits but dominate over old stars formed
in the solar neighbourhood, and in some simulations, even in the galactic centre. Secondly, old stars formed
are driven outwards by bursty star formation and energetic feedback processes that create a time-varying gravitational potential at
≳ 2, similar to the process that creates dark matter cores and expands stellar orbits in bursty dwarf galaxies. The total fraction of stars that are ancient is more than an order of magnitude higher for sight lines
from the bulge and inner halo than for inward-looking sight lines. Although the task of identifying specific stars as ancient remains challenging, we anticipate that million-star spectral surveys and photometric surveys targeting metal-poor stars already include hundreds of stars formed before
= 5. We predict most of these targets to have higher metallicity (-3 < Fe/H < -2) than the most extreme metal-poor stars.
ABSTRACT In the Local Group, nearly all of the dwarf galaxies ( ) that are satellites within kpc (the virial radius) of the Milky Way (MW) and Andromeda (M31) have quiescent star formation and ...little-to-no cold gas. This contrasts strongly with comparatively isolated dwarf galaxies, which are almost all actively star-forming and gas-rich. This near dichotomy implies a rapid transformation of satellite dwarf galaxies after falling into the halos of the MW or M31. We combine the observed quiescent fractions for satellites of the MW and M31 with the infall times of satellites from the Exploring the Local Volume in Simulations (ELVIS) suite of cosmological zoom-in simulations to determine the typical timescales over which environmental processes within the MW/M31 halos remove gas and quench star formation in low-mass satellite galaxies. The quenching timescales for satellites with are short, Gyr, and quenching is more rapid at lower . These satellite quenching timescales can be 1-2 Gyr longer if one includes the time that satellites were environmentally preprocessed by low-mass groups prior to MW/M31 infall. We compare with quenching timescales for more massive satellites from previous works to synthesize the nature of satellite galaxy quenching across the observable range of . The satellite quenching timescale increases rapidly with satellite , peaking at Gyr for , and the timescale rapidly decreases at higher to Gyr at . Overall, galaxies with , similar to the Magellanic Clouds, exhibit the longest quenching timescales, regardless of environmental or internal mechanisms.
We search for signatures of reionization in the star formation histories (SFHs) of 38 Local Group dwarf galaxies (10 super(4) < M sub(sstarf) < 10 super(9) M sub(middot in circle)). The SFHs are ...derived from color-magnitude diagrams using archival Hubble Space Telescope/Wide Field Planetary Camera 2 imaging. Only five quenched galaxies (And V, And VI, And XIII, Leo IV, and Hercules) are consistent with forming the bulk of their stars before reionization, when full uncertainties are considered. Observations of 13 of the predicted "true fossils" identified by Bovill & Ricotti show that only two (Hercules and Leo IV) indicate star formation quenched by reionization. However, both are within the virial radius of the Milky Way and evidence of tidal disturbance complicates this interpretation. We argue that the late-time gas capture scenario posited by Ricotti for the low mass, gas-rich, and star-forming fossil candidate Leo T is observationally indistinguishable from simple gas retention. Given the ambiguity between environmental effects and reionization, the best reionization fossil candidates are quenched low mass field galaxies (e.g., K.K.R 25).
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