اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدMilky Way Disk-Halo Transition in HI: Properties of the Cloud Population
147
0
0.0
(
0
)
تأليف
H. Alyson Ford
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
Using 21cm HI observations from the Parkes Radio Telescopes Galactic All-Sky Survey, we measure 255 HI clouds in the lower Galactic halo that are located near the tangent points at 16.9 < l < 35.3 degrees and |b| < 20 degrees. The clouds have a median mass of 700 Msun and a median distance from the Galactic plane of 660 pc. This first Galactic quadrant (QI) region is symmetric to a region of the fourth quadrant (QIV) studied previously using the same data set and measurement criteria. The properties of the individual clouds in the two quadrants are quite similar suggesting that they belong to the same population, and both populations have a line of sight cloud-cloud velocity dispersion of sigma_cc ~ 16 km/s. However, there are three times as many disk-halo clouds at the QI tangent points and their scale height, at h=800 pc, is twice as large as in QIV. Thus the observed line of sight random cloud motions are not connected to the cloud scale height or its variation around the Galaxy. The surface density of clouds is nearly constant over the QI tangent point region but is peaked near R~4 kpc in QIV. We ascribe all of these differences to the coincidental location of the QI region at the tip of the Milky Ways bar, where it merges with a major spiral arm. The QIV tangent point region, in contrast, covers only a segment of a minor spiral arm. The disk-halo HI cloud population is thus likely tied to and driven by large-scale star formation processes, possibly through the mechanism of supershells and feedback.
قيم البحث
اقرأ أيضاً
Using data from the Galactic All-Sky Survey, we have compared the properties and distribution of HI clouds in the disk-halo transition at the tangent points in mirror-symmetric regions of the first quadrant (QI) and fourth quadrant (QIV) of the Milky
Way. Individual clouds are found to have identical properties in the two quadrants. However, there are 3 times as many clouds in QI as in QIV, their scale height is twice as large, and their radial distribution is more uniform. We attribute these major asymmetries to the formation of the clouds in the spiral arms of the Galaxy, and suggest that the clouds are related to star formation in the form of gas that has been lifted from the disk by superbubbles and stellar feedback, and fragments of shells that are falling back to the plane.
In the $Gaia$ era stellar kinematics are extensively used to study Galactic halo stellar populations, to search for halo structures, and to characterize the interface between the halo and hot disc populations. We use distribution function-based model
s of modern datasets with 6D phase space data to qualitatively describe a variety of kinematic spaces commonly used in the study of the Galactic halo. Furthermore, we quantitatively assess how well each kinematic space can separate radially anisotropic from isotropic halo populations. We find that scaled action space (the ``action diamond) is superior to other commonly used kinematic spaces at this task. We present a new, easy to implement selection criterion for members of the radially-anisotropic $Gaia$-Enceladus merger remnant, which we find achieves a sample purity of 82 per cent in our models with respect to contamination from the more isotropic halo. We compare this criterion to literature criteria, finding that it produces the highest purity in the resulting samples, at the expense of a modest reduction in completeness. We also show that selection biases that underlie nearly all contemporary spectroscopic datasets can noticeably impact the $E-L_{z}$ distribution of samples in a manner that may be confused for real substructure. We conclude by providing recommendations for how authors should use stellar kinematics in the future to study the Galactic stellar halo.
We employ numerical simulations and simple analytical estimates to argue that dark matter substructures orbiting in the inner regions of the Galaxy can be efficiently destroyed by disk shocking, a dynamical process known to affect globular star clust
ers. We carry out a set of fiducial high-resolution collisionless simulations in which we adiabatically grow a disk, allowing us to examine the impact of the disk on the substructure abundance. We also track the orbits of dark matter satellites in the high-resolution Aquarius simulations and analytically estimate the cumulative halo and disk shocking effect. Our calculations indicate that the presence of a disk with only 10% of the total Milky Way mass can significantly alter the mass function of substructures in the inner parts of halos. This has important implications especially for the relatively small number of satellites seen within ~30 kpc of the Milky Way center, where disk shocking is expected to reduce the substructure abundance by a factor of ~2 at 10^9 M$_{odot}$ and ~3 at 10^7 M$_{odot}$. The most massive subhalos with 10^10 M$_{odot}$ survive even in the presence of the disk. This suggests that there is no inner missing satellite problem, and calls into question whether these substructures can produce transient features in disks, like multi-armed spiral patterns. Also, the depletion of dark matter substructures through shocking on the baryonic structures of the disk and central bulge may aggravate the problem to fully account for the observed flux anomalies in gravitational lens systems, and significantly reduces the dark matter annihilation signal expected from nearby substructures in the inner halo.
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.
We present deep ACS images of 3 fields in the edge-on disk galaxy NGC 891, which extend from the plane of the disk to 12 kpc, and out to 25 kpc along the major axis. The photometry of individual stars reaches 2.5 magnitudes below the tip of the RGB.
We use the astrophotometric catalogue to probe the stellar content and metallicity distribution across the thick disk and spheroid of NGC 891. The CMDs of thick disk and spheroid population are dominated by old RGB stars with a wide range of metallicities, from a metal-poor tail at [Fe/H] ~ -2.4 dex, up to about half-solar metallicity. The peak of the MDF of the thick disk is at -0.9 dex. The inner parts of the thick disk, within 14 kpc along the major axis show no vertical colour/metallicity gradient. In the outer parts, a mild vertical gradient of Delta(V-I)/Delta|Z| = 0.1 +/- 0.05 kpc^-1 is detected. This gradient is however accounted for by the mixing with the metal poor halo stars. No metallicity gradient along the major axis is present for thick disk stars, but strong variations of about 0.35 dex around the mean of [Fe/H] = -1.13 dex are found. The properties of the asymmetric MDFs of the thick disk stars show no significant changes in both the radial and the vertical directions. The stellar populations at solar cylinder-like distances show strikingly different properties from those of the Galaxy, suggesting that the accretion histories of both galaxies have been different. The spheroid population shows remarkably uniform stellar population properties. The median metallicity of the halo stellar population shows a shallow gradient from about -1.15 dex in the inner parts to -1.27 dex at 24 kpc distance from the centre. Similar to the thick disk stars, large variations around the mean relation are present.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
