66 research outputs found
The ALFALFA HI velocity width function
We make the most precise determination to date of the number density of extragalactic 21-cm radio sources as a function of their spectral line widths – the H I velocity width function (H I WF) – based on 21 827 sources from the final 7000deg2 data release of the Arecibo Legacy Fast ALFA (ALFALFA) survey. The number density of sources as a function of their neutral hydrogen masses – the H I mass function (H I MF) – has previously been reported to have a significantly different low-mass slope and ‘knee mass’ in the two sky regions surveyed during ALFALFA. In contrast with this, we find that the shape of the H I WF in the same two sky regions is remarkably similar, consistent with being identical within the confidence intervals implied by the data (but the overall normalization differs). The spatial uniformity of the H I WF implies that it is likely a stable tracer of the mass function of dark matter haloes, in spite of the environmental processes to which the measured variation in the H I MF are attributed, at least for galaxies containing enough neutral hydrogen to be detected. This insensitivity of the H I WF to galaxy formation and evolution can be exploited to turn it into a powerful constraint on cosmological models as future surveys yield increasingly precise measurements. We also report on the possible influence of a previously overlooked systematic error affecting the H I WF, which may plausibly see its low-velocity slope steepen by ∼40 per cent in analyses of future, deeper surveys. Finally, we provide an updated estimate of the ALFALFA completeness limit
The "Building Blocks" of Stellar Halos
The stellar halos of galaxies encode their accretion histories. In
particular, the median metallicity of a halo is determined primarily by the
mass of the most massive accreted object. We use hydrodynamical cosmological
simulations from the APOSTLE project to study the connection between the
stellar mass, the metallicity distribution, and the stellar age distribution of
a halo and the identity of its most massive progenitor. We find that the
stellar populations in an accreted halo typically resemble the old stellar
populations in a present-day dwarf galaxy with a stellar mass
dex greater than that of the stellar halo. This suggest that had they not been
accreted, the primary progenitors of stellar halos would have evolved to
resemble typical nearby dwarf irregulars.Comment: 7 pages, 3 figures, published in the proceedings of "On the Origin
(and Evolution) of Baryonic Galaxy Halos", Puerto Ayora, Ecuador, March 13-17
2017, Eds. Duncan A. Forbes and Ericson D. Lope
The many reasons that the rotation curves of low-mass galaxies can fail as tracers of their matter distributions
It is routinely assumed that galaxy rotation curves are equal to their circular velocity curves (modulo some corrections) such that they are good dynamical mass tracers. We take a visualization-driven approach to exploring the limits of the validity of this assumption for a sample of 33 low-mass galaxies (60<vmax/kms−1<120 ) from the APOSTLE suite of cosmological hydrodynamical simulations. Only three of these have rotation curves nearly equal to their circular velocity curves at z = 0, the rest are undergoing a wide variety of dynamical perturbations. We use our visualizations to guide an assessment of how many galaxies are likely to be strongly perturbed by processes in several categories: mergers/interactions (affecting 6/33 galaxies), bulk radial gas inflows (19/33), vertical gas outflows (15/33), distortions driven by a non-spherical DM halo (17/33), warps (8/33), and winds due to motion through the intergalactic medium (5/33). Most galaxies fall into more than one of these categories; only 5/33 are not in any of them. The sum of these effects leads to an underestimation of the low-velocity slope of the baryonic Tully–Fisher relation (α ∼ 3.1 instead of α ∼ 3.9, where Mbar ∝ vα) that is difficult to avoid, and could plausibly be the source of a significant portion of the observed diversity in low-mass galaxy rotation curve shapes
The north-south asymmetry of the ALFALFA HI velocity width function
The number density of extragalactic 21-cm radio sources as a function of
their spectral line-widths -- the HI width function (HIWF) -- is a sensitive
tracer of the dark matter halo mass function (HMF). The cold dark
matter model predicts that the HMF should be identical everywhere provided it
is sampled in sufficiently large volumes, implying that the same should be true
of the HIWF. The ALFALFA 21-cm survey measured the HIWF in northern and
southern Galactic fields and found a systematically higher number density in
the north. At face value, this is in tension with theoretical predictions. We
use the Sibelius-DARK N-body simulation and the semi-analytical galaxy
formation model GALFORM to create a mock ALFALFA survey. We find that the
offset in number density has two origins: the sensitivity of the survey is
different in the two fields, which has not been correctly accounted for in
previous measurements; and the algorithm used for
completeness corrections does not fully account for biases arising from spatial
clustering in the galaxy distribution. The latter is primarily driven by a
foreground overdensity in the northern field within , but
more distant structure also plays a role. We provide updated measurements of
the ALFALFA HIWF (and HIMF) correcting for the variations in survey
sensitivity. Only when systematic effects such as these are understood and
corrected for can cosmological models be tested against the HIWF.Comment: MNRAS accepte
The north–south asymmetry of the ALFALFA H I velocity width function
The number density of extragalactic 21-cm radio sources as a function of their spectral line widths – the H I width function
(H I WF) – is a sensitive tracer of the dark matter halo mass function (HMF). The Lambda cold dark matter model predicts that
the HMF should be identical everywhere provided it is sampled in sufficiently large volumes, implying that the same should be
true of the H I WF. The Arecibo Legacy Fast ALFA (ALFALFA) 21-cm survey measured the H I WF in northern and southern
Galactic fields and found a systematically higher number density in the north. At face value, this is in tension with theoretical
predictions. We use the Sibelius-DARK N-body simulation and the semi-analytical galaxy formation model GALFORM to create
a mock ALFALFA survey. We find that the offset in number density has two origins: the sensitivity of the survey is different in the
two fields, which has not been correctly accounted for in previous measurements; and the 1/Veff algorithm used for completeness
corrections does not fully account for biases arising from spatial clustering in the galaxy distribution. The latter is primarily
driven by a foreground overdensity in the northern field within 30 Mpc , but more distant structure also plays a role. We provide
updated measurements of the ALFALFA H I WF (and H I mass function) correcting for the variations in survey sensitivity. Only
when systematic effects such as these are understood and corrected for can cosmological models be tested against the H I WF
Constraining quenching timescales in galaxy clusters by forward-modelling stellar ages and quiescent fractions in projected phase space
We forward-model mass-weighted stellar ages (MWAs) and quiescent fractions in
projected phase space (PPS), using data from the Sloan Digital Sky Survey, to
jointly constrain an infall quenching model for galaxies in
galaxy clusters at . We
find the average deviation in MWA from the MWA- relation depends on
position in PPS, with a maximum difference between the inner cluster and
infalling interloper galaxies of Gyr. Our model employs infall
information from N-body simulations and stochastic star-formation histories
from the UniverseMachine model. We find total quenching times of
Gyr and Gyr after first
pericentre, for and
galaxies, respectively. By using
MWAs, we break the degeneracy in time of quenching onset and timescale of star
formation rate (SFR) decline. We find that time of quenching onset relative to
pericentre is Gyr and
Gyr for our lower and higher stellar
mass bins, respectively, and exponential SFR suppression timescales are
Gyr and Gyr for our
lower and higher stellar mass bins, respectively. Stochastic star formation
histories remove the need for rapid infall quenching to maintain the bimodality
in the SFR of cluster galaxies; the depth of the green valley prefers quenching
onsets close to first pericentre and a longer quenching envelope, in slight
tension with the MWA-driven results. Taken together these results suggest that
quenching begins close to, or just after pericentre, but the timescale for
quenching to be fully complete is much longer and therefore ram-pressure
stripping is not complete on first pericentric passage.Comment: 21 pages, 13 figures, submitted to MNRA
The low abundance and insignificance of dark discs in simulated Milky Way galaxies
We investigate the presence and importance of dark matter discs in a sample of 24 simulated Milky Way galaxies in the apostle project, part of the eagle programme of hydrodynamic simulations in ΛCDM cosmology. It has been suggested that a dark disc in the Milky Way may boost the dark matter density and modify the velocity modulus relative to a smooth halo at the position of the Sun, with ramifications for direct detection experiments. From a kinematic decomposition of the dark matter and a real space analysis of all 24 haloes, we find that only one of the simulated Milky Way analogues has a detectable dark disc component. This unique event was caused by a merger at late time with an LMC-mass satellite at very low grazing angle. Considering that even this rare scenario only enhances the dark matter density at the solar radius by 35 per cent and affects the high-energy tail of the dark matter velocity distribution by less than 1 per cent, we conclude that the presence of a dark disc in the Milky Way is unlikely, and is very unlikely to have a significant effect on direct detection experiments
- …