اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدVelocity Dispersion Profile of the Milky Way Halo
134
0
0.0
(
0
)
تأليف
Warren R. Brown
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
We present a spectroscopic sample of 910 distant halo stars from the Hypervelocity Star survey from which we derive the velocity dispersion profile of the Milky Way halo. The sample is a mix of 74% evolved horizontal branch stars and 26% blue stragglers. We estimate distances to the stars using observed colors, metallicities, and stellar evolution tracks. Our sample contains twice as many objects with R>50 kpc as previous surveys. We compute the velocity dispersion profile in two ways: with a parametric method based on a Milky Way potential model, and with a non-parametric method based on the caustic technique originally developed to measure galaxy cluster mass profiles. The resulting velocity dispersion profiles are remarkably consistent with those found by two independent surveys based on other stellar populations: the Milky Way halo exhibits a mean decline in radial velocity dispersion of -0.38+-0.12 km/s/kpc over 15<R<75 kpc. This measurement is a useful basis for calculating the total mass and mass distribution of the Milky Way halo.
قيم البحث
اقرأ أيضاً
We have compiled a new sample of 240 halo objects with accurate distance and radial velocity measurements, including globular clusters, satellite galaxies, field blue horizontal branch stars and red giant stars from the Spaghetti survey. The new data
lead to a significant increase in the number of known objects for Galactocentric radii beyond 50 kpc, which allows a reliable determination of the radial velocity dispersion profile out to very large distances. The radial velocity dispersion shows an almost constant value of 120 km/s out to 30 kpc and then continuously declines down to 50 km/s at about 120 kpc. This fall-off puts important constraints on the density profile and total mass of the dark matter halo of the Milky Way. For a constant velocity anisotropy, the isothermal profile is ruled out, while both a dark halo following a truncated flat model of mass $1.2^{+1.8}_{-0.5}times 10^{12}$ M_sun and an NFW profile of mass $0.8^{+1.2}_{-0.2}times 10^{12}$ M_sun and c=18 are consistent with the data. The significant increase in the number of tracers combined with the large extent of the region probed by these has allowed a more precise determination of the Milky Way mass in comparison to previous works. We also show how different assumptions for the velocity anisotropy affect the performance of the mass models.
We explore the fundamental relations governing the radial and vertical velocity dispersions of stars in the Milky Way, from combined studies of complementary surveys including GALAH, LAMOST, APOGEE, the NASA $Kepler$ and K2 missions, and $Gaia$ DR2.
We find that different stellar samples, even though they target different tracer populations and employ a variety of age estimation techniques, follow the same set of fundamental relations. We provide the clearest evidence to date that, in addition to the well-known dependence on stellar age, the velocity dispersions of stars depend on orbital angular momentum $L_z$, metallicity and height above the plane $|z|$, and are well described by a multiplicatively separable functional form. The dispersions have a power-law dependence on age with exponents of 0.441$pm 0.007$ and 0.251$pm 0.006$ for $sigma_z$ and $sigma_R$ respectively, and the power law is valid even for the oldest stars. For the solar neighborhood stars, the apparent break in the power law for older stars, as seen in previous studies, is due to the anti-correlation of $L_z$ with age. The dispersions decrease with increasing $L_z$ until we reach the Suns orbital angular momentum, after which $sigma_z$ increases (implying flaring in the outer disc) while $sigma_R$ flattens. The dispersions increase with decreasing metallicity, suggesting that the dispersions increase with birth radius. The dispersions also increase linearly with $|z|$. The same set of relations that work in the solar neighborhood also work for stars between $3<R/{rm kpc}<20$. Finally, the high-[$alpha$/Fe] stars follow the same relations as the low-[$alpha$/Fe] stars.
We report a detection of 3.5 keV line in the Milky Way in 5 regions offset from the Galactic Center by distances from 10 to 35 degrees. We build an angular profile of this line and compare it with profiles of several astrophysical lines detected in t
he same observations. We compare our results with other detections and bounds previously obtained using observations of the Milky Way.
We analyse the structure of the local stellar halo of the Milky Way using $sim$ 60000 stars with full phase space coordinates extracted from the SDSS--{it Gaia} catalogue. We display stars in action space as a function of metallicity in a realistic a
xisymmetric potential for the Milky Way Galaxy. The metal-rich population is more distended towards high radial action $J_R$ as compared to azimuthal or vertical action, $J_phi$ or $J_z$. It has a mild prograde rotation $(langle v_phi rangle approx 25$ km s$^{-1}$), is radially anisotropic and highly flattened with axis ratio $q approx 0.6 - 0.7$. The metal-poor population is more evenly distributed in all three actions. It has larger prograde rotation $(langle v_phi rangle approx 50$ km s$^{-1}$), a mild radial anisotropy and a roundish morphology ($qapprox 0.9$). We identify two further components of the halo in action space. There is a high energy, retrograde component that is only present in the metal-rich stars. This is suggestive of an origin in a retrograde encounter, possibly the one that created the stripped dwarf galaxy nucleus, $omega$Centauri. Also visible as a distinct entity in action space is a resonant component, which is flattened and prograde. It extends over a range of metallicities down to [Fe/H] $approx -3$. It has a net outward radial velocity $langle v_R rangle approx 12$ km s$^{-1}$ within the Solar circle at $|z| <3.5$ kpc. The existence of resonant stars at such extremely low metallicities has not been seen before.
The circumgalactic region of the Milky Way contains a large amount of gaseous mass in the warm-hot phase. The presence of this warm-hot halo observed through $z=0$ X-ray absorption lines is generally agreed upon, but its density, path-length, and mas
s is a matter of debate. Here I discuss in detail why different investigations led to different results. The presence of an extended (over 100 kpc) and massive (over ten billion solar masses) warm-hot gaseous halo is supported by observations of other galaxies as well. I briefly discuss the assumption of constant density and end with outlining future prospects.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
