NED-D Historical Distances

NED-D Historical Distances Introduction

A discovery by Miss Henrietta Leavitt (1908, 1912) led to the first standard candle method for estimating galaxy distances. Yet until recently when Leavitt's Law of Cepheids was recognized in papers by Freedman et al. (2009), Madore et al. (2009a, 2009b) and Marengo et al. (2010), the discoverer of the Cepheid period-luminosity relation remained a footnote. Previously, credit for the discovery had gone to Hertzsprung (1913) and independently Russell (1913), as they were first to calibrate Leavitt's method and first to publish distances based on it.

Improvements by Shapley (1918) led to a Cepheid-based distance to the Small Magellanic Cloud (SMC) galaxy that remained fiduciary for decades. Hubble's first Cepheid distances weren't published until 1925. They were by far the best however, based on observations with the world's largest telescope, the 100-inch Hooker, newly operating by 1919 on Mount Wilson, CA (see Hubble 1925a, 1925b).

Potentially, the first standard candle extragalactic distance could be credited to Very (1911) based on Novae. However, this estimate was un- calibrated and, giving under a thousandth today's distance to the Andromeda galaxy, unrealistic. The first calibrated Novae-based distance was made by Curtis (1917). It provided a quantum leap in extragalactic distances at 6 Megaparsecs (Mpc), albeit to no particular galaxy but rather "distant spirals" in general. Lundmark (1919) provided the first calibrated Novae distances to a specific galaxy, publishing two estimates for Andromeda. One year later, Curtis (1920) used Novae to publish two new distances for Andromeda, a low of 0.23 Mpc and a high of 1.2 Mpc. Curtis's distances averaged would place Andromeda at 0.7 Mpc, within 7% the modern value of 0.75 +/- 0.02 Mpc by Freedman et al. (2001) for NASA's Hubble Space Telescope (HST) Key Project (KP).

A third standard candle method used today, brightest stars, was first employed by Shapley (1917) to place Andromeda at 0.31 Mpc. Others followed, including Lindemann and Lindemann (1919) and Lundmark (1921, 1924, and 1925). It was however Hubble (1926) who first used a Cepheid-calibrated version of the brightest-stars method to provide 32 distances for as many individual galaxies.

The first standard-ruler measurement was made by Shapley (1922), using globular cluster radii. Used today in NASA's HST Advanced Camera for Surveys (ACS) Virgo Cluster Survey (VCS; see Jordan et al. 2005), and ACS Fornax Cluster Survey (FCS; see Masters et al. 2010), Shapley used the method in 1922 to place the Large Magellanic Cloud (LMC) at 35 kiloparsecs (kpc), within 27% of the latest multi-methods- based estimate of 48 kpc by Freedman & Madore (2010), believed precise to within 3%.

Two years before Hubble's 1926 paper proving galaxies existed, Lundmark (1924) used his distance of 0.2 Mpc from novae in Andromeda from 1919 to calibrate diameter distances to 44 galaxies. He legitimately discovered these "nebulae" existed as "island universes" at vast distances, out to his furthest, NGC 1700 at 42 Mpc. Extrapolating a single untried method, diameters however, based on one new novae distance did not convince others. Hubble succeeded by employing two independent methods both calibrated by Cepheids, one primary method based on brightest stars and one secondary method, based on galaxy apparent magnitudes to prove conclusively that the distances indicated were real.

Lundmark (1925) actually hit upon Hubble's Law years before Hubble, writing "more distant spirals have higher space-velocity." Again however, it was Hubble (1929) who used two methods to confirm this: Whether based on 24 individual galaxies, or on dividing those galaxies into nine groups, the apparent redshift versus distance relation, now legitimately called Hubble's Law, was the same.

Noteworthy also, the first ever extragalactic distance estimate was published by Nichol (1840), giving 0.6 Mpc for nearby spirals in general -- not bad compared to today's value for Andromeda at 0.75 Mpc, the HST KP value. Plus, an archaic Tully-Fisher relation, based on spiral rotational velocity and brightness, the most commonly used method today, was developed by Opik in 1922 to estimate Andromeda at 0.45 Mpc, within 40% of the modern value. Finally, a dozen improbable underestimates were published, including some based on "apparent" internal rotation and others based on Proper Motion, impossible then, barely possible now and with low precision, e.g. Katherine et al. (2010) and references therein. Such underestimates kept the great debate of 1920 on island universes between Curtis and Shapley, respectively pro and con, going until Hubble's 1926 paper, after which all such underestimates vanish from the record.