88 research outputs found
The total stellar halo mass of the Milky Way
We measure the total stellar halo luminosity using red giant branch (RGB) stars selected from Gaia data release 2. Using slices in magnitude, colour, and location on the sky, we decompose RGB stars belonging to the disc and halo by fitting two-dimensional Gaussians to the Galactic proper motion distributions. The number counts of RGB stars are converted to total stellar halo luminosity using a suite of isochrones weighted by age and metallicity, and by applying a volume correction based on the stellar halo density profile. Our method is tested and calibrated using Galaxia and N-body models. We find a total luminosity (out to 100 kpc) of Lhalo = 7.9 ± 2.0 Ă 108 L excluding Sgr, and Lhalo = 9.4 ± 2.4 Ă 108 L including Sgr. These values are appropriate for our adopted stellar halo density profile and metallicity distribution, but additional systematics related to these assumptions are quantified and discussed. Assuming a stellar mass-to-light ratio appropriate for a Kroupa initial mass function (M/L = 1.5), we estimate a stellar halo mass of M halo = 1.4 ± 0.4 Ă 109 M. This mass is larger than previous estimates in the literature, but is in good agreement with the emerging picture that the (inner) stellar halo is dominated by one massive dwarf progenitor. Finally, we argue that the combination of a âŒ109 M mass and an average metallicity of [Fe/H]âŒâ1.5 for the Galactic halo points to an ancient (âŒ10 Gyr) merger event
The Imprint of Cosmic Reionization on the Luminosity Function of Galaxies
The (re)ionization of hydrogen in the early universe has a profound effect on the formation of the first galaxies: by raising the gas temperature and pressure, it prevents gas from cooling into small halos, thus affecting the abundance of present-day small galaxies. Using the Galform semi-analytic model of galaxy formation, we show that two key aspects of the reionization processâwhen reionization takes place and the characteristic scale below which it suppresses galaxy formationâare imprinted in the luminosity function of dwarf galaxies. We focus on the luminosity function of satellites of galaxies like the Milky Way and the LMC, which is easier to measure than the luminosity function of the dwarf population as a whole. Our results show that the details of these two characteristic properties of reionization determine the shape of the luminosity distribution of satellites in a unique way, and are largely independent of the other details of the galaxy formation model. Our models generically predict a bimodality in the distribution of satellites as a function of luminosity: a population of faint satellites and population of bright satellites separated by a "valley" forged by reionization. We show that this bimodal distribution is present at high statistical significance in the combined satellite luminosity function of the Milky Way and M31. We make predictions for the expected number of satellites around LMC-mass dwarfs where the bimodality may also be measurable in future observational programs. Our preferred model predicts a total of 26 ± 10 (68% confidence) satellites brighter than M V = 0 in LMC-mass systems
Can we really pick and choose? Benchmarking various selections of Gaia Enceladus/Sausage stars in observations with simulations
Large spectroscopic surveys plus Gaia astrometry have shown us that the inner stellar halo of the Galaxy is dominated by the debris of Gaia Enceladus/Sausage (GES). With the richness of data at hand, there are a myriad of ways these accreted stars have been selected. We investigate these GES selections and their effects on the inferred progenitor properties using data constructed from APOGEE and Gaia. We explore selections made in eccentricity, energy-angular momentum (E-Lz), radial action-angular momentum (Jr-Lz), action diamond, and [Mg/Mn]-[Al/Fe] in the observations, selecting between 144 and 1279 GES stars with varying contamination from in-situ and other accreted stars. We also use the Auriga cosmological hydrodynamic simulations to benchmark the different GES dynamical selections. Applying the same observational GES cuts to nine Auriga galaxies with a GES, we find that the Jr-Lz method is best for sample purity and the eccentricity method for completeness. Given the average metallicity of GES (â1.28 < [Fe/H] < â1.18), we use the z = 0 massâmetallicity relationship to find an average of âŒ4 Ă 108âMâ. We adopt a similar procedure and derive for the GES-like systems in Auriga and find that the eccentricity method overestimates the true by âŒ2.6 Ă while E-Lz underestimates by âŒ0.7 Ă. Lastly, we estimate the total mass of GES to be using the relationship between the metallicity gradient and the GES-to-in-situ energy ratio. In the end, we cannot just âpick and chooseâ how we select GES stars, and instead should be motivated by the science question
Can we really pick and choose? Benchmarking various selections of Gaia Enceladus/Sausage stars in observations with simulations
Large spectroscopic surveys plus Gaia astrometry have shown us that the inner
stellar halo of the Galaxy is dominated by the debris of Gaia Enceladus/Sausage
(GES). With the richness of data at hand, there are a myriad of ways these
accreted stars have been selected. We investigate these GES selections and
their effects on the inferred progenitor properties using data constructed from
APOGEE and Gaia. We explore selections made in eccentricity, energy-angular
momentum (E-Lz), radial action-angular momentum (Jr-Lz), action diamond, and
[Mg/Mn]-[Al/Fe] in the observations, selecting between 144 and 1,279 GES stars
with varying contamination from in-situ and other accreted stars. We also use
the Auriga cosmological hydrodynamic simulations to benchmark the different GES
dynamical selections. Applying the same observational GES cuts to nine Auriga
galaxies with a GES, we find that the Jr-Lz method is best for sample purity
and the eccentricity method for completeness. Given the average metallicity of
GES (-1.28 < [Fe/H] < -1.18), we use the mass-metallicity relationship to
find an average of . We
adopt a similar procedure and derive for the GES-like systems
in Auriga and find that the eccentricity method overestimates the true by while E-Lz underestimates by .
Lastly, we estimate the total mass of GES to be using the relationship between the metallicity gradient and
the GES-to-in-situ energy ratio. In the end, we cannot just `pick and choose'
how we select GES stars, and instead should be motivated by the science
question.Comment: 20 pages, 14 figures, submitted to MNRA
The effects of dynamical substructure on Milky Way mass estimates from the high-velocity tail of the local stellar halo
We investigate the impact of dynamical streams and substructure on estimates of the local escape speed and total mass of Milky-Way-mass galaxies from modelling the high-velocity tail of local halo stars. We use a suite of high-resolution magnetohydrodynamical cosmological zoom-in simulations that resolve phase space substructure in local volumes around solar-like positions. We show that phase space structure varies significantly between positions in individual galaxies and across the suite. Substructure populates the high-velocity tail unevenly and leads to discrepancies in the mass estimates. We show that a combination of streams, sample noise, and truncation of the high-velocity tail below the escape speed leads to a distribution of mass estimates with a median that falls below the true value by âŒ20 per cent âŒ20 per cent â , and a spread of a factor of 2 across the suite. Correcting for these biases, we derive a revised value for the Milky Way mass presented in Deason et al. of 1.29 +0.37 â0.47 Ă 10 12 M â 1.29â0.47+0.37Ă1012Mâ â
HALO7D III: Chemical Abundances of Milky Way Halo Stars from Medium Resolution Spectra
The Halo Assembly in Lambda Cold Dark Matter: Observations in 7 Dimensions
(HALO7D) survey measures the kinematics and chemical properties of stars in the
Milky Way (MW) stellar halo to learn about the formation of our Galaxy. HALO7D
consists of Keck II/DEIMOS spectroscopy and Hubble Space Telescope-measured
proper motions of MW halo main sequence turn-off (MSTO) stars in the four
CANDELS fields. HALO7D consists of deep pencil beams, making it complementary
to other contemporary wide-field surveys. We present the [Fe/H] and [alpha/Fe]
abundances for 113 HALO7D stars in the Galactocentric radial range of
kpc. Using the full 7D chemodynamical data (3D positions, 3D
velocities, and abundances) of HALO7D, we measure the velocity anisotropy,
, of the halo velocity ellipsoid for each field and for different
metallicity-binned subsamples. We find that two of the four fields have stars
on very radial orbits while the remaining two have stars on more isotropic
orbits. Separating the stars into high, mid, and low [Fe/H] bins at dex
and dex for each field separately, we find differences in the
anisotropies between the fields and between the bins; some fields appear
dominated by radial orbits in all bins while other fields show variation
between the [Fe/H] bins. These chemodynamical differences are evidence that the
HALO7D fields have different fractional contributions from the progenitors that
built up the MW stellar halo. Our results highlight the additional information
that is available on smaller spatial scales when compared to results from a
spherical average of the stellar halo.Comment: 32 pages, 15 figure
The nature of H I absorbers in gamma-ray burst afterglows: clues from hydrodynamic simulations
In recent work, we have shown that it is possible to link quantitatively many aspects of damped Lyman α (DLA) absorbers in the spectra of quasars to high-resolution simulations of galaxy formation. Using runs from the same series of hydrodynamic numerical studies, we consider the expected properties of intrinsic Lyman α absorbers seen in the spectra of high-redshift (z > 2) gamma-ray burst afterglows (GRBâDLAs). If GRBs are associated with the death of massive stars, their afterglows provide insights into otherwise unprobed regions of protogalactic objects, but detailed physical interpretations are currently embryonic.
We find that median impact parameters (measured from the potential minimum) are approximately 1 kpc for GRBs compared with 4 kpc for quasi-stellar objectâDLA (QSOâDLA). However, an equally important difference is that GRBâDLAs are predominantly associated with haloes of mass 10^(10) < M_(vir)/M_â < 10^(12) , an order of magnitude larger than the hosts of QSOâDLAs. Accordingly, there are differences in the stellar properties of hosts. For instance, mean star formation rates are higher: <M(overdot)_â
â 10 M_â yr^(-1) for GRBâDLAs compared with <M(overdot)_â
â 1 M_â yr^(-1) for QSOâDLAs.
Our simulations accurately predict the form of the GRBâDLA H I column density distribution, producing quantitative agreement for N_(H I) > 10^(19) cm^(â2) , but they somewhat underpredict the incidence of low column densities N_(H I_ < 10^(19) cm^(â2) . This is reflected in our estimate of the ionizing photon escape fraction, f_(esc) â 1 per cent, which is lower than the observational GRB-derived escape fraction (2 per cent). Line-of-sight neutral gas metallicities predicted by our simulations (10^(â2) < Z/Z_â < 1) are consistent with the modest observational constraints. Because of large internal dispersions in gas metallicities, this agreement is not significantly compromised by imposing a cut-off on the metallicity of stars able to launch GRBs (Z_â
< Z_â/3) , confounding claims that the observed metallicity of GRBâDLAs poses a severe challenge to current GRB models
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