The upper bound on the lowest mass halo Jethwa, P; Erkal, D; Belokurov, V
Monthly notices of the Royal Astronomical Society,
01/2018, Volume:
473, Issue:
2
Journal Article
Peer reviewed
Open access
Abstract
We explore the connection between galaxies and dark matter haloes in the Milky Way (MW) and quantify the implications on properties of the dark matter particle and the phenomenology of ...low-mass galaxy formation. This is done through a probabilistic comparison of the luminosity function of MW dwarf satellite galaxies to models based on two suites of zoom-in simulations. One suite is dark-matter-only, while the other includes a disc component, therefore we can quantify the effect of the MW's baryonic disc on our results. We apply numerous stellar-mass–halo-mass (SMHM) relations allowing for multiple complexities: scatter, a characteristic break scale, and subhaloes which host no galaxy. In contrast to previous works, we push the model/data comparison to the faintest dwarfs by modelling observational incompleteness, allowing us to draw three new conclusions. First, we constrain the SMHM relation for 102 < M*/ M⊙ < 108 galaxies, allowing us to bound the peak halo mass of the faintest MW satellite to Mvir > 2.4 × 108 M⊙ (1σ). Secondly, by translating to a warm dark matter (WDM) cosmology, we bound the thermal relic mass mWDM > 2.9 keV at 95 per cent confidence, on a par with recent constraints from the Lyman-α forest. Lastly, we find that the observed number of ultra-faint MW dwarfs is in tension with the theoretical prediction that reionization prevents galaxy formation in almost all 108 M⊙ haloes. This can be tested with the next generation of deep imaging surveys. To this end, we predict the likely number of detectable satellite galaxies in the Subaru/Hyper Suprime-Cam survey and the Large Synoptic Survey Telescope. Confronting these predictions with future observations will be amongst our strongest tests of WDM and the effect reionization on low-mass systems.
A Magellanic origin of the DES dwarfs Jethwa, P; Erkal, D; Belokurov, V
Monthly notices of the Royal Astronomical Society,
09/2016, Volume:
461, Issue:
2
Journal Article
Peer reviewed
Open access
We establish the connection between the Magellanic Clouds (MCs) and the dwarf galaxy candidates discovered in the Dark Energy Survey (DES) by building a dynamical model of the MC satellite ...populations, based on an extensive suite of tailor-made numerical simulations. Our model takes into account the response of the Galaxy to the MCs infall, the dynamical friction experienced by the MCs and the disruption of the MC satellites by their hosts. The simulation suite samples over the uncertainties in the MC's proper motions, the masses of the MW and the Clouds themselves, and allows for flexibility in the intrinsic volume density distribution of the MC satellites. As a result, we can accurately reproduce the DES satellites’ observed positions and kinematics. Assuming that Milky Way (MW) dwarfs follow the distribution of sub-haloes in Λ cold dark matter, we further demonstrate that, of 14 observed satellites, the MW halo contributes fewer than 4(8) of these with 68(95) per cent confidence and that 7(12) DES dwarfs have probabilities greater than 0.7(0.5) of belonging to the Large Magellanic Cloud (LMC). Marginalizing over the entire suite, we constrain the number of Magellanic satellites in the range −7 < MV
< −1 which exceed the DES surface brightness threshold at ∼70, and the mass of the LMC around 1011 M⊙. The data also strongly support a first-infall scenario for the LMC. Finally, we give predictions for the line-of-sight velocities and the proper motions of the satellites discovered in the vicinity of the LMC.
Co-formation of the disc and the stellar halo Belokurov, V; Erkal, D; Evans, N W ...
Monthly notices of the Royal Astronomical Society,
07/2018, Volume:
478, Issue:
1
Journal Article
Peer reviewed
Open access
ABSTRACT
Using a large sample of main sequence stars with 7D measurements supplied by Gaia and SDSS, we study the kinematic properties of the local (within ∼10 kpc from the Sun) stellar halo. We ...demonstrate that the halo’s velocity ellipsoid evolves strongly with metallicity. At the low-Fe/H end, the orbital anisotropy (the amount of motion in the radial direction compared with the tangential one) is mildly radial, with 0.2 <β< 0.4. For stars with Fe/H > −1.7, however, we measure extreme values of β∼ 0.9. Across the metallicity range considered, namely−3 < Fe/H < −1, the stellar halo’s spin is minimal, at the level of $20\lt \bar{v}_{\theta }(\mathrm{kms}^{-1}) \lt 30$. Using a suite of cosmological zoom-in simulations of halo formation, we deduce that the observed acute anisotropy is inconsistent with the continuous accretion of dwarf satellites. Instead, we argue, the stellar debris in the inner halo was deposited in a major accretion event by a satellite with Mvir > 1010M⊙ around the epoch of the Galactic disc formation, between 8 and 11 Gyr ago. The radical halo anisotropy is the result of the dramatic radialization of the massive progenitor’s orbit, amplified by the action of the growing disc.
In a companion paper by Koposov et al., RR Lyrae from Gaia Data Release 2 are used to demonstrate that stars in the Orphan stream have velocity vectors significantly misaligned with the stream track, ...suggesting that it has received a large gravitational perturbation from a satellite of the Milky Way. We argue that such a mismatch cannot arise due to any realistic static Milky Way potential and then explore the perturbative effects of the Large Magellanic Cloud (LMC). We find that the LMC can produce precisely, the observed motion-track mismatch and we therefore use the Orphan stream to measure the mass of the Cloud. We simultaneously fit the Milky Way and LMC potentials and infer that a total LMC mass of |$1.38^{+0.27}_{-0.24} \times 10^{11}\, \rm {M_\odot}$| is required to bend the Orphan stream, showing for the first time that the LMC has a large and measurable effect on structures orbiting the Milky Way. This has far-reaching consequences for any technique which assumes that tracers are orbiting a static Milky Way. Furthermore, we measure the Milky Way mass within 50 kpc to be |$3.80^{+0.14}_{-0.11}\times 10^{11} \, \mathrm{M}_\odot$|. Finally, we use these results to predict that, due to the reflex motion of the Milky Way in response to the LMC, the outskirts of the Milky Way’s stellar halo should exhibit a bulk, upwards motion.
ABSTRACT
The GD-1 stream is one of the longest and coldest stellar streams discovered to date, and one of the best objects for constraining the dark matter properties of the Milky Way. Using data ...from Gaia DR2, we study the proper motions (PMs), distance, morphology, and density of the stream to uncover small-scale perturbations. The PM cleaned data show a clear distance gradient across the stream, ranging from 7 to 12 kpc. However, unlike earlier studies that found a continuous gradient, we uncover a distance minimum at φ1 ≈ −40 deg, after which the distance increases again. We can reliably trace the stream between −85 < φ1 <15 deg, showing an even further extent to GD-1 beyond the earlier extension of Price-Whelan and Bonaca. We constrain the stream track and density using a Boolean matched filter approach and find three large under densities and significant residuals in the stream track lining up with these gaps. The gaps are located at φ1 = −36, −20, and −3 deg, with the gap at −3 deg being surrounded by a clear sinusoidal wiggle. We argue that this wiggle is due to a perturbation since it has the wrong orientation to come from a progenitor. We compute a total initial stellar mass of the stream segment of 1.58 ± 0.07 × 104 M⊙. With the extended view of the spur in this work, we argue that the spur may be unrelated to the adjacent gap in the stream. Finally, we show that an interaction with the Sagittarius dwarf can create features similar to the spur.
Abstract
We explore the possibility that the observed population of Galactic
hypervelocity stars (HVSs) originate as runaway stars from the Large Magellanic Cloud (LMC). Pairing a binary evolution ...code with an N-body simulation of the interaction of the LMC with the Milky Way, we predict the spatial distribution and kinematics of an LMC runaway population. We find that runaway stars from the LMC can contribute Galactic HVSs at a rate of 3 × 10−6 yr−1. This is composed of stars at different points of stellar evolution, ranging from the main sequence to those at the tip of the asymptotic giant branch. We find that the known B-type HVSs have kinematics that are consistent with an LMC origin. There is an additional population of hypervelocity white dwarfs whose progenitors were massive runaway stars. Runaways that are even more massive will themselves go supernova, producing a remnant whose velocity will be modulated by a supernova kick. This latter scenario has some exotic consequences, such as pulsars and supernovae far from star-forming regions, and a small rate of microlensing from compact sources around the halo of the LMC.
Abstract
We use astrometry, broad-band photometry, and variability information from the Data Release 2 of ESA’s Gaia mission (GDR2) to identify members of the Orphan Stream (OS) across the whole sky. ...The stream is traced above and below the celestial equator and in both Galactic hemispheres, thus increasing its visible length to ∼210° equivalent to ∼150 kpc in physical extent. Taking advantage of the large number of RR Lyrae stars in the OS, we extract accurate distances and proper motions across the entire stretch of the tidal debris studied. As delineated by the GDR2 RR Lyrae, the stream exhibits two prominent twists in its shape on the sky which are accompanied by changes in the tangential motion. We complement the RR Lyrae maps with those created using GDR2 Red Giants and the DECam Legacy Survey Main Sequence Turn-Off stars. The behaviour of the OS track on the sky is consistent across all three tracers employed. We detect a strong non-zero motion in the across-stream direction for a substantial portion of the stream. Such a misalignment between the debris track and the streaming velocity cannot be reproduced in a static gravitational potential and signals an interaction with a massive perturber.
Abstract
We exploit the first Gaia data release to study the properties of the Galactic stellar halo as traced by RR Lyrae. We demonstrate that it is possible to select a pure sample of RR Lyrae ...using only photometric information available in the Gaia+2MASS catalogue. The final sample contains about 21 600 RR Lyrae covering an unprecedented fraction ( ∼ 60 per cent) of the volume of the Galactic inner halo (R < 28 kpc). We study the morphology of the stellar halo by analysing the RR Lyrae distribution with parametric and non-parametric techniques. Taking advantage of the uniform all-sky coverage, we test halo models more sophisticated than usually considered in the literature, such as those with varying flattening, tilts and/or offset of the halo with respect to the Galactic disc. A consistent picture emerges: the inner halo is well reproduced by a smooth distribution of stars settled on triaxial density ellipsoids. The shortest axis is perpendicular to the Milky Way's disc, while the longest axis forms an angle of ∼70° with the axis connecting the Sun and the Galactic Centre. The elongation along the major axis is mild (p = 1.27), and the vertical flattening is shown to evolve from a squashed state with q ≈ 0.57 in the centre to a more spherical q ≈ 0.75 at the outer edge of our data set. Within the radial range probed, the density profile of the stellar halo is well approximated by a single power law with exponent α = −2.96. We do not find evidence of tilt or offset of the halo with respect to the Galaxy's disc.
ABSTRACT
Stars slingshotted by the supermassive black hole at the Galactic Centre escape from the Milky Way so quickly that their trajectories are almost straight lines. Previous works have shown how ...these ‘hypervelocity stars’ (stars moving faster than the local Galactic escape speed) are subsequently deflected by the gravitational field of the Milky Way and the Large Magellanic Cloud (LMC), but have neglected to account for the reflex motion of the Milky Way in response to the fly-by of the LMC. A consequence of this motion is that the hypervelocity stars we see in the outskirts of the Milky Way today were ejected from where the Milky Way centre was hundreds of millions of years ago. This change in perspective causes large apparent deflections of several degrees in the trajectories of the hypervelocity stars. We quantify these deflections by simulating the ejection of hypervelocity stars from an isolated Milky Way (with a spherical or flattened dark matter halo), from a fixed-in-place Milky Way with a passing LMC, and from a Milky Way that responds to the passage of the LMC, finding that LMC passage causes larger deflections than can be caused by a flattened Galactic dark matter halo in ΛCDM. The $10\, \mu \mathrm{as}\, \mathrm{yr}^{-1}$ proper motion precision necessary to measure these deflections will be possible with the combination of Gaia with the proposed GaiaNIR successor mission, and these measurements will directly probe the shape of the Milky Way, the mass of the LMC, and the dance of these two galaxies.
Abstract
We report the discovery of Pegasus IV, an ultra-faint dwarf galaxy found in archival data from the Dark Energy Camera processed by the DECam Local Volume Exploration Survey. Pegasus IV is a ...compact, ultra-faint stellar system (
r
1
/
2
=
41
−
6
+
8
pc;
M
V
= −4.25 ± 0.2 mag) located at a heliocentric distance of
90
−
6
+
4
kpc
. Based on spectra of seven nonvariable member stars observed with Magellan/IMACS, we confidently resolve Pegasus IV’s velocity dispersion, measuring
σ
v
=
3.3
−
1.1
+
1.7
km s
−1
(after excluding three velocity outliers); this implies a mass-to-light ratio of
M
1
/
2
/
L
V
,
1
/
2
=
167
−
99
+
224
M
⊙
/
L
⊙
for the system. From the five stars with the highest signal-to-noise spectra, we also measure a systemic metallicity of Fe/H =
−
2.63
−
0.30
+
0.26
dex, making Pegasus IV one of the most metal-poor ultra-faint dwarfs. We tentatively resolve a nonzero metallicity dispersion for the system. These measurements provide strong evidence that Pegasus IV is a dark-matter-dominated dwarf galaxy, rather than a star cluster. We measure Pegasus IV’s proper motion using data from Gaia Early Data Release 3, finding (
μ
α
*
,
μ
δ
) = (0.33 ± 0.07, −0.21 ± 0.08) mas yr
−1
. When combined with our measured systemic velocity, this proper motion suggests that Pegasus IV is on an elliptical, retrograde orbit, and is currently near its orbital apocenter. Lastly, we identify three potential RR Lyrae variable stars within Pegasus IV, including one candidate member located more than 10 half-light radii away from the system’s centroid. The discovery of yet another ultra-faint dwarf galaxy strongly suggests that the census of Milky Way satellites is still incomplete, even within 100 kpc.
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