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Initially defined by the IAU in 1958, the galactic coordinate system was thereafter in 1984 transformed from the B1950.0 FK4-based system to the J2000.0 FK5-based system. In 1994, the IAU recommended that the dynamical reference system FK5 be replaced by the ICRS, which is a kinematical non-rotating system defined by a set of remote radio sources. However the definition of the galactic coordinate system was not updated. We consider that the present galactic coordinates may be problematic due to the unrigorous transformation method from the FK4 to the FK5, and due to the non-inertiality of the FK5 system with respect to the ICRS. This has led to some confusions in applications of the galactic coordinates. We tried to find the transformation matrix in the framework of the ICRS after carefully investigating the definition of the galactic coordinate system and transformation procedures, however we could not find a satisfactory galactic coordinate system that is connected steadily to the ICRS. To avoid unnecessary misunderstandings, we suggest to re-consider the definition of the galactic coordinate system which should be directly connected with the ICRS for high precise observation at micro-arcsecond level.
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The definition of the Galactic coordinate system was announced by the IAU Sub-Commission 33b on behalf of the IAU in 1958. For more than 50 years the definition of the Galactic coordinate system has remained unchanged from this IAU1958 version. On the basis of deep and all-sky catalogs, the position of the Galactic plane can be revised and updated definitions of the Galactic coordinate systems can be proposed. We re-determine the position of the Galactic plane based on modern large catalogs, such as the Two Micron All-Sky Survey (2MASS) and the SPECFIND v2.0. This paper also aims to propose a possible definition of the optimal Galactic coordinate system by adopting the ICRS position of the Sgr A* at the Galactic center. The near-infrared 2MASS point source catalog and the SPECFIND v2.0 catalog of radio continuum spectra are used to determine the mean position of the Galactic plane on the celestial sphere. By fitting the data to an ideal Galactic equator, the parameters defining the Galactic coordinate system are obtained. We find that the obliquity of the Galactic equator on the ICRS principal plane is about $0.4^circ$ (2MASS) and $0.6^circ$ (SPECFIND v2.0) larger than the J2000.0 value, which is widely used in coordinate transformations between the equatorial $(alpha, delta)$ and the Galactic $(ell, b)$. Depending on the adopted parameters, data, and methods, the largest difference between the resulting Galactic coordinate systems is several arcminutes. We derive revised transformation matrices and parameters describing the orientation of the Galactic coordinate systems in the ICRS at the 1 milli-arcsecond level to match the precision of modern observations. For practical applications, we propose that a revised definition of the Galactic coordinate system should be required in the near future.
A number of elite thinkers in Europe during the 16th and 17th centuries pursued an agenda which historian Paolo Rossi calls the quest for a universal language, a quest which was deeply interwoven with the emergence of the scientific method. From a modern perspective, one of the many surprising aspects of these efforts is that they relied on a diverse array of memorization techniques as foundational elements. In the case of Leibnizs universal calculus, the ultimate vision was to create a pictorial language that could be learned by anyone in a matter of weeks and which would contain within it a symbolic representation of all domains of contemporary thought, ranging from the natural sciences, to theology, to law. In this brief article, I explore why this agenda might have been appealing to thinkers of this era by examining ancient and modern memory feats. As a thought experiment, I suggest that a society built entirely upon memorization might be less limited than we might otherwise imagine, and furthermore, that cultural norms discouraging the use of written language might have had implications for the development of scientific methodology. Viewed in this light, the efforts of Leibniz and others seem significantly less surprising. I close with some general observations about cross-cultural origins of scientific thought.
We present a kinematic analysis of a sample of 23,908 G- and K-type dwarfs in the Galactic disk. Based on the $alpha$-abundance ratio, [$alpha$/Fe], we separated our sample into low-$alpha$ thin-disk and high-$alpha$ thick-disk stars. We find a $V_{rm phi}$ gradient of $-$28.2 km s$^{-1}$ dex$^{-1}$ over [Fe/H] for the thin disk, and an almost flat trend of the velocity dispersions of $V_{rm R}$, $V_{rm phi}$, and $V_{rm Z}$ components with [Fe/H]. The metal-poor (MP; [Fe/H] $<$ $-$0.3) thin-disk stars with low-$V_{rm phi}$ velocities have high eccentricities ($e$) and small perigalacticon distances ($r_{rm p}$), while the high-$V_{rm phi}$ MP thin-disk stars possess low $e$ and large $r_{rm p}$. Interestingly, half of the super metal-rich ([Fe/H] $>$ $+$0.1) stars in the thin disk exhibit low-$e$, solar-like orbits. Accounting for the inhomogeneous metallicity distribution of the thin-disk stars with various kinematics requires radial migration by churning $-$ it apparently strongly influences the current structure of the thin disk; we cannot rule out the importance of blurring for the high-$e$ stars. We derive a rotation velocity gradient of $+$36.9 km s$^{-1}$ dex$^{-1}$ for the thick disk, and decreasing trends of velocity dispersions with increasing [Fe/H]. The thick-disk population also has a broad distribution of eccentricity, and the number of high-$e$ stars increases with decreasing [Fe/H]. These kinematic behaviors could be the result of a violent mechanism, such as a gas-rich merger or the presence of giant turbulent clumps, early in the history of its formation. Dynamical heating by minor mergers and radial migration may also play roles in forming the current thick-disk structure.
If there is a null gradient field in 1+3 dimensional space-time, we can set up a kind of light-cone coordinate system in the space-time. In such coordinate system, the metric takes a simple form, which is much helpful for simplifying and solving the Einsteins field equation. This light-cone coordinate system has wonderful properties and has been widely used in astrophysics to calculate parameters. In this paper, we give a detailed discussion for the structure of space-time with light-cone coordinate system. We derive the conditions for existence of such coordinate system, and show how to construct the light-cone coordinate system from usual ones, then explain their geometrical and physical meanings by examples.
We perform an analysis of the diffuse low-frequency Galactic components in the Southern part of the Gould Belt system (130^circleq lleq 230^circ and -50^circleq bleq -10^circ). Strong ultra-violet (UV) flux coming from the Gould Belt super-association is responsible for bright diffuse foregrounds that we observe from our position inside the system and that can help us improve our knowledge of the Galactic emission. Free-free emission and anomalous microwave emission (AME) are the dominant components at low frequencies ( u < 40 GHz), while synchrotron emission is very smooth and faint. We separate diffuse free-free emission and AME from synchrotron emission and thermal dust emission by using Planck data, complemented by ancillary data, using the Correlated Component Analysis (CCA) component separation method and we compare with the results of cross-correlation of foreground templates with the frequency maps. We estimate the electron temperature T_e from H$alpha$ and free-free emission using two methods (temperature-temperature plot and cross-correlation) and we obtain T_e ranging from 3100 to 5200 K, for an effective fraction of absorbing dust along the line of sight of 30% (f_d=0.3). We estimate the frequency spectrum of the diffuse AME and we recover a peak frequency (in flux density units) of 25.5 pm 1.5 GHz. We verify the reliability of this result with realistic simulations that include the presence of biases in the spectral model for the AME and in the free-free template. By combining physical models for vibrational and rotational dust emission and adding the constraints from the thermal dust spectrum from Planck and IRAS we are able to get a good description of the frequency spectrum of the AME for plausible values of the local density and radiation field.
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