The Structure of Our Galaxy Derived from Observations of Neutral Hydrogen

Abstract

Although insight into the structure of our Galaxy can be sought in various ways, study of the radio emission from the neutral hydrogen component is particularly suitable. Neutral atomic hydrogen, HI, is the main observed constituent of the interstellar medium; its physical properties are closely related to the properties of other Galactic constituents, both stellar and interstellar. The interstellar medium is transparent enough to hydrogen emission at the 21-cm radio wavelength that investigation of the entire Galaxy is possible, with the exception of a few directions along the Galactic equator. This transparency allows investigation of regions of the Galaxy which are too distant to be studied optically. HI is particularly important for the information it provides on the form and overall mass in the outer Galaxy, where it is the only directly observable constituent. Interstellar neutral hydrogen is so abundant and is distributed in such a general fashion throughout the Galaxy that the 21-cm hyperfine transition line has been detected in emission in every direction in the sky at which a suitably equipped radio telescope has been pointed. No time variation of a neutral hydrogen line has been found.The 21-cm line of atomic hydrogen results from a hyperfine transition in the ground state of the atom. This state is characterized by two possible relative orientations of the spins of the electron and proton; when the spins are parallel the energy of the system is slightly greater than when they are antiparallel. The energy separation between the two hyperfine sublevels corresponds to a quantum of radiation with a natural frequency of 1420.406 MHz. The probability that this spin-flip transition from the F = 1 to the F = 0 state will occur spontaneously is given by the Einstein emission coefficient, A 10 = 2.85 X to-15 S-1. Thus a spontaneous electron spin flip would occur after some eleven million years. In fact, atomic encounter collisions largely determine the relative populations of the two energy levels. At the typical HI densities of about 0.4 em -3 which characterize much of interstellar space, encounters result in reorientation of the spins of a hydrogen atom about once every three or four hundred years. Effects of ambient interstellar radiation field on the spin states are less important than those of encounters, except in certain unusual interstellar environments such as found near recent supernovae or near regions of intense star formation.

Evolution over time of the Milky Way’s disc shape

“…Figure 10 compares the amplitude of the warp from different authors with our result for three age class. A good agreement is obtained with Burton (1988), López-Corredoira et al (2002) and Yusifov (2004) for stars with mean age equal to 2.5 Gyr in the distance range, 10.5 < R < 13.5 kpc. However, the warp slopes were determined using tracers in different Galactocentric distance ranges.…”

“…The asymmetry of the warp was long identified in HI gas (Burton 1988;Levine et al 2006;Kalberla & Dedes 2008) and even in stars (López-Corredoira et al 2002;Reylé et al 2009). But this is the first time that it is shown that the shape varies with the age of the tracers considered.…”

“…The amplitude of their warp is 2.1 × 10 −19 × R 5.25 . It means that the mid-plane is going up to 1.2 kpc at R = 14 kpc, their node angle is found to be Yusifov (2004) from pulsars (Y04: orange diamonds), Burton (1988) from HI gas (B88: orange plus signs), Levine et al (2006) model fitted on HI (L06: blue open triangles), Drimmel & Spergel (2001) from COBE/DIRBE data (DS01: black asterisks). Positive (resp.…”

“…The evidence comes from gas tracers such as HI (Henderson et al 1982;Burton & te Lintel Hekkert 1986;Burton 1988;Diplas & Savage 1991;Nakanishi & Sofue 2003;Levine et al 2006) or molecular clouds (Grabelsky et al 1987;Wouterloot et al 1990;May et al 1997). Recent analysis of the outer Galaxy either from 2MASS (López-Corredoira et al 2002;Momany et al 2006;Reylé et al 2009) or from SDSS (Hammersley & López-Corredoira 2011) has led to the conclusion that even the stars follow a warped structure.…”

Members of Centaurus OB1 and NGC 4755: new spectroscopic and astrometric studies

“…Table 4 shows the LSR radial velocities of these stars (Col. 4) and the minimal distance of these stars from the Sun, obtained with the galactic rotation model of the Galaxy (Fich et al 1989) (Col. 5). The weight of the individual distance determinations stems from the assumption of an uncertainty of ±10 km s −1 caused by noncircular motion (Burton 1988), from which we can infer that the value of the distance obtained in this way has an error of 1 kpc, approximately. This makes it possible for all IRAS sources studied by Walsh et al (1997) to share their location in the galaxy with the association.…”