اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدAnisotropy studies around the galactic center
46
0
0.0
(
0
)
اسأل ChatGPT حول البحث
ﻻ يوجد ملخص باللغة العربية
We present the first results for anisotropy searches around the galactic center at EeV energies using data from the Pierre Auger Observatory. Our analysis, based on a substantially larger data set, do not support previous claim of anisotropy found in this region by the AGASA and Sugar experiment. Furthermore we place un upper limit on a possible point like source located at the galactic center which exclude several scenarios predicting neutron sources in this location.
قيم البحث
اقرأ أيضاً
We present new proper motion measurements and simultaneous orbital solutions for three newly identified (S0-16, S0-19, and S0-20) and four previously known (S0-1, S0-2, S0-4, and S0-5) stars at the Galactic Center. This analysis pinpoints the Galaxys
central dark mass to within +-1 milli-arcsec and, for the first time from orbital dynamics, limits its proper motion to 1.5+-0.5 mas/y, which is consistent with our derivation of the position of Sgr A* in the infrared reference frame (+-10 mas). The estimated central dark mass from orbital motions is 3.7 (+-0.2) x 10^6 (Ro/8kpc)^3 Mo; this is a more direct measure of mass than those obtained from velocity dispersion measurements, which are as much as a factor of two smaller. The smallest closest approach is achieved by S0-16, which confines the mass to within a radius of a mere 45 AU and increases the inferred dark mass density by four orders of magnitude compared to earlier analyses based on velocity and acceleration vectors, making the Milky Way the strongest existing case by far for a supermassive black hole at the center of any normal type galaxy. The stellar orbital properties suggest that the distributions of eccentricities and angular momentum vector and apoapse directions are consistent with those of an isotropic system. Therefore many of the mechanisms proposed for the formation of young stars in the vicinity of a supermassive black hole, such as formation from a pre-existing disk, are unlikely solutions for the Sgr A* cluster stars. Unfortunately, all existing alternative theories are also somewhat problematic. Understanding the apparent youth of stars in the Sgr A* cluster, as well as the more distant He I emission line stars, has now become one of the major outstanding issues in the study of the Galactic Center.
We detected a compact ionized gas associated physically with IRS13E3, an Intermediate Mass Black Hole (IMBH) candidate in the Galactic Center, in the continuum emission at 232 GHz and H30$alpha$ recombination line using ALMA Cy.5 observation (2017.1.
00503.S, P.I. M.Tsuboi). The continuum emission image shows that IRS13E3 is surrounded by an oval-like structure. The angular size is $0.093pm0.006times 0.061pm0.004$ ( $1.14times10^{16}$ cm $times 0.74times10^{16}$ cm). The structure is also identified in the H30$alpha$ recombination line. This is seen as an inclined linear feature in the position-velocity diagram, which is usually a defining characteristic of a rotating gas ring around a large mass. The gas ring has a rotating velocity of $V_mathrm{rot}simeq230$ km s$^{-1}$ and an orbit radius of $rsimeq6times10^{15}$ cm. From these orbit parameters, the enclosed mass is estimated to be $M_{mathrm{IMBH}}simeq2.4times10^4$ $M_odot$. The mass is within the astrometric upper limit mass of the object adjacent to Sgr A$^{ast}$. Considering IRS13E3 has an X-ray counterpart, the large enclosed mass would be supporting evidence that IRS13E3 is an IMBH. Even if a dense cluster corresponds to IRS13E3, the cluster would collapse into an IMBH within $tau<10^7$ years due to the very high mass density of $rho gtrsim8times10^{11} M_odot pc^{-3}$. Because the orbital period is estimated to be as short as $T=2pi r/V_mathrm{rot}sim 50-100$ yr, the morphology of the observed ionized gas ring is expected to be changed in the next several decades. The mean electron temperature and density of the ionized gas are $bar{T}_{mathrm e}=6800pm700$ K and $bar{n}_{mathrm e}=6times10^5$ cm$^{-3}$, respectively. Then the mass of the ionized gas is estimated to be $M_{mathrm{gas}}=4times10^{-4} M_odot$.
We analyze deep near-IR adaptive optics imaging as well as new proper motion data of the nuclear star cluster of the Milky Way. The surface density distribution of faint stars peaks within 0.2 of the black hole candidate SgrA*. The radial density dis
tribution of this stellar cusp follows a power law of exponent 1.3-1.4. The K-band luminosity function of the overall nuclear stellar cluster (within 9 of SgrA*) resembles that of the large scale, Galactic bulge, but shows an excess of stars at K<14. We find that most of the massive early type stars at distances 1-10 from SgrA* are located in two rotating and geometrically thin disks. These disks are inclined at large angles and counter-rotate with respect to each other. Their stellar content is essentially the same, indicating that they formed at the same time. The star closest to SgrA* in 2002, S2, exhibits a 3.8 micron excess. We propose that the mid-IR emission either comes from the accretion flow around the black hole itself, or from dust in the accretion flow that is heated by the ultra-violet emission of S2.
We present 1-resolution ALMA observations of the circumnuclear disk (CND) and the environment around SgrA*. The images unveil the presence of small spatial scale CO (J=3-2) molecular cloudlets within the central pc of the Milky Way, moving at high sp
eeds, up to 300 km/s along the line-of-sight. The CO-emitting structures show intricate morphologies: extended and filamentary at high negative-velocities (v_LSR < -150 km/s), more localized and clumpy at extreme positive-velocities (v_LSR > +200 km/s). Based on the pencil-beam CO absorption spectrum toward SgrA* synchrotron emission, we also present evidence for a diffuse gas component producing absorption features at more extreme negative-velocities (v_LSR < -200 km/s). The CND shows a clumpy spatial distribution. Its motion requires a bundle of non-uniformly rotating streams of slightly different inclinations. The inferred gas density peaks are lower than the local Roche limit. This supports that CND molecular cores are transient. We apply the two standard orbit models, spirals vs. ellipses, invoked to explain the kinematics of the ionized gas streamers around SgrA*. The location and velocities of the CO cloudlets are inconsistent with the spiral model, and only two of them are consistent with the Keplerian ellipse model. Most cloudlets, however, show similar velocities that are incompatible with the motions of the ionized streamers or with gas bounded to the central gravity. We speculate that they are leftovers of more massive, tidally disrupted, clouds that fall into the cavity, or that they originate from instabilities in the inner rim of the CND and infall from there. Molecular cloudlets, all together with a mass of several 10 M_Sun, exist around SgrA*. Most of them must be short-lived: photoevaporated by the intense stellar radiation field, blown away by winds from massive stars, or disrupted by strong gravitational shears.
We present the results of 16 years of monitoring stellar orbits around the massive black hole in center of the Milky Way using high resolution NIR techniques. This work refines our previous analysis mainly by greatly improving the definition of the c
oordinate system, which reaches a long-term astrometric accuracy of 300 microarcsecond, and by investigating in detail the individual systematic error contributions. The combination of a long time baseline and the excellent astrometric accuracy of adaptive optics data allow us to determine orbits of 28 stars, including the star S2, which has completed a full revolution since our monitoring began. Our main results are: all stellar orbits are fit extremely well by a single point mass potential to within the astrometric uncertainties, which are now 6 times better than in previous studies. The central object mass is (4.31 +- 0.06|stat +- 0.36|R0) * 10^6 M_sun where the fractional statistical error of 1.5 percent is nearly independent from R0 and the main uncertainty is due to the uncertainty in R0. Our current best estimate for the distance to the Galactic Center is R0 = 8.33 +- 0.35 kpc. The dominant errors in this value is systematic. The mass scales with distance as (3.95 +- 0.06) * 10^6 M_sun * (R0/8kpc)^2.19. The orientations of orbital angular momenta for stars in the central arcsecond are random. We identify six of the stars with orbital solutions as late type stars, and six early-type stars as members of the clockwise rotating disk system, as was previously proposed. We constrain the extended dark mass enclosed between the pericenter and apocenter of S2 at less than 0.066, at the 99% confidence level, of the mass of Sgr A*. This is two orders of magnitudes larger than what one would expect from other theoretical and observational estimates.
سجل دخول لتتمكن من نشر تعليقات
التعليقات
جاري جلب التعليقات
سجل دخول لتتمكن من متابعة معايير البحث التي قمت باختيارها
