Galaxies distance scale

Gravitahonal (micro) lensing is now recognised as a way of revealing otherwise hidden matter. It is universally used to estimate the distribution and quantity of dark matter on a variety of distance scales. The study of dark matter in the halo of our own Galaxy is not the least significant amongst these. 

Briefly, astronomers have two basic methods for estimating distance scales for spiral galaxies that are independent of the observation of special standard candles such as supernovae. 

Non-centrality of the Sun. The Curtis-Shapley debate (Curtis 1921, Shap-ley 1921) centered on this and on the distance scale of the galaxy. Curtis said small, sun-centered while Shapley said big, with the sun something like 20 kpc from the center, based on his distance scale for globular clusters, derived in turn from the apparent brightness of the RR Lyrae stars in them. Shapley was pretty much the winner on this one, and he has been compared with Copernicus for moving us away from the center. 

In order to compare numbers across the decades, it is essential to allow for changes in the best estimate of the cosmic distance scale or the Hubble constant (H). In most cases, the L that you deduce for a galaxy (etc.) from its brightness will be proportional to the square of its distance, d, or to H 2, and the mass you calculate, from some form of M = V2R/G, will be proportional to d or H 1. Thus M/L scales as d 1 or H, and a velocity dispersion for a cluster of galaxies plus its angular size on the sky that led to M/L = 1000 for H = 500 km/sec/Mpc now, with H - 70 km/sec/Mpc, corresponds to M/L = 140. When Schwarzschild produced his table, the community was just incorporating the first of the large drops in H, from about 500 to 250 km/sec/Mpc. 

Extension of the distance scale to, presumably more remote objects, hke galactic clusters, requires equally precarious assumptions. Like the brightest star in each galaxy, each cluster is assumed to harbour a dominant galaxy that shines as a standard candle. Finally the only criterion left to measure larger distances is Hubble s law and the redshift. 

One of the most important objectives of the HST is to determine more accurately the distance scale of the universe it can do this by observing galaxies at much greater distances with the same degree of detail as nearer galaxies are presently observed with ground-based telescopes. Ultraviolet observations are an important aspect of these studies, because the most luminous hot stars are useful distance indicators, and these are much brighter in the UV than in the visible. Also, for... 

Fig. 8.14. Surface densities of atomic and molecular hydrogen in the Galaxy as a function of Galactocentric distance the Sun is at 8.5 kpc. Beyond that distance, the deduced surface density depends on the assumed law of Galactic rotation KBH refers to Kulkarni, Blitz and Heiles (1982). Assuming their rotation curve, the total gas surface density falls by about a factor of 2 between 4.5 and 13 kpc, corresponding to an exponential fall-off with a scale length a l of about 12 kpc. After Dame (1993). Courtesy T.M. Dame.

Trends in emission lines from HII regions in Scd galaxies as a function of galactocentric distance interpreted by L. Searle as a consequence of large-scale abundance gradients. 

Observations of galaxy duster do not help in this respect since the available data stop our understanding of the DM density profile at distances r 10 kpc from their centres (see Fig.5.1). Down to these scales the NFW is still allowed and at smaller scales the inner slope remains quite uncertain even when the combined analysis of X-ray, gravitational lensing and galaxy dynamics data are taken into account (Dalai Keeton 2003). 

The sun is located near the edge of the arm O. It may be noted that this picture of the spiral structure in the vicinity of the sun is based on both optical and radio observations, whereas the large scale structure of the interstellar gas (Fig. 4) is based only on observations of the X21 cm hydrogen line and kinematic distances, which become highly uncertain towards the galactic center and anticenter. Kinematic distances are obtained from a model for the rotation of the Galaxy due to Schmidt. This model relates the circular orbital velocity of the gas to the distance from the galactic center (see Fig. 3). 

By contrast it may be helpful to re-examine the consensus model in which the universe is considered made up of galaxies which interact like gas molecules in a situation of zero pressure. By simple ideal-gas kinetic theory the kinetic energy, given by the product pV, and the temperature, T = pV/R must both be zero in such a sample. There is no further argument about this - the distance between a pair of such galaxies must remain constant, unless the measuring scale is shrinking, which means that space expands. 

That is, to observe waves from a system with a solar-mass worth of energy in nonsymmetric motion at the megaparsec scale (the distance to the nearest galaxy), astronomers need to measure strains on the order of 10 . This sensitivity is just now becoming possible for broadband detectors. 

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