The image on the left is dominated visually by the spiral galaxy NGC 4319. But what makes this field-of-view truly compelling is that a distant quasar, Markarian 205, happens to be "peeking" through the lower arms of the foreground spiral, which is more than 10X closer to us than the quasar (mouse-over the image to display object labels or select version "G"). The spectrum of the quasar (also classified as a Seyfert 1 galaxy) is shown on the right in two forms: as a real CCD image of dispersed light captured from Mrk 205, and as a plot of relative intensity versus wavelength extracted from the calibrated spectrum data. The two galaxies have a curious scientific backstory, which is described below. 

I captured the deep-sky image and the spectrum using my own equipment. The image is 13.5' square, with a resolution of about 0.62"/pixel. A 30'-square version of the image is included with this post as frame "D". Wavelength in the spectrum plot corresponds directly with horizontal position in the spectrum image. I used a Shelyak Instruments LISA spectrograph, equipped with a monochrome CCD camera for greater sensitivity (native resolution about 3.6 Angstroms per pixel). I colourized the dispersive image for aesthetic display only, using a straightforward conversion between wavelength and RGB values. Four heavily-broadened Hydrogen Balmer emission lines are seen, along with two much narrower "forbidden" transitions due to doubly-ionized Oxygen atoms (the rest wavelengths of the Hα and Hß lines are shown for reference). The redshift of the quasar as estimated from these spectral lines is about 7.1% of the speed of light, or about 21 000 km/sec, in excellent agreement with professional measurements. Doppler broadening implies that the atoms that radiated the Balmer lines had typical speeds of around 1500 km/sec. According to Hubble's Law, the redshift of Mrk 205 implies that it is at a distance of about 1 billion light-years. NGC 4319 has a far more modest redshift of about 1400 km/sec and is only about 80 million light years away. The elliptical galaxy in the upper-right part of the image is NGC 4291, which is at a distance and redshift close to those of NGC 4319.

From the early 1970s some astronomers argued vociferously that Mrk 205 and NGC 4319 are connected by a tidal tail, which would imply that they lie at about the same distance, and that consequently the redshift of Mrk 205 would not be cosmological in origin. The image in this post has some features between the galaxies in the position of the purported tidal tail. A notable advocate for this viewpoint was Halton Arp, whose Atlas of Peculiar Galaxies is well-known to amateur astrophotographers (Arp, who died at the end of 2013, was the subject of a moving obituary by Dennis Overbye in the New York Times, and was profiled in a popular account of cosmology by Timothy Ferris). 

Increasingly elaborate accounts of "discordant" redshifts, in this and other conjectured groupings of low-redshift "normal" galaxies with quasars and other "active" galaxies, were claimed to cast doubt on Big Bang Cosmology. Proponents of this theory argued that the high-redshift galaxies in question had been ejected from their claimed low-redshift partners, so that the large redshifts were not related to the expansion of the universe. But they also had to rule out redshift due to motion through space of the ejected galaxies (since no large blueshifts were found, which would be just as likely for high-speed ejected objects); instead, they invoked some previously unknown and highly-speculative new physics to account for the redshifts.

In 1992 the Hubble Space Telescope found absorption lines with the redshift of NGC 4319 in the spectrum of Mrk 205, which is consistent with the light from the distant quasar having passed through the foreground spiral on its way to us, and in 2002 the Hubble Heritage Project produced a deep image of these galaxies. There is now overwhelming evidence for the Big Bang and for the nature of quasars, including the cosmological origin of their redshifts, while large-scale quasar surveys have catalogued more than 750,000 entries. Apparent overlaps between quasars and low-redshift galaxies are bound to occur as a matter of chance, and the so-called "redshift controversy" has largely been forgotten.

My equipment includes a PlaneWave CDK600 telescope, an FLI Kepler-4040 sCMOS camera for deep-sky images, and a Shelyak Instruments LISA spectrograph that is configured with an integrated Neon-Argon lamp for wavelength calibration, an Atik 460ex CCD camera for data acquisition, and a QHY 174M guide camera. The response of the spectrometer as a function of wavelength was estimated by taking the spectrum of a nearby A-type star, and comparing the result with a reference spectrum of the same spectral type. In addition to colourizing the integrated CCD image for display purposes, I enhanced its contrast and sharpness, and expanded the vertical scale by a factor of three, in order to make its features easier to see. However, the spectrum plot was obtained from the raw data using only standard image-reduction and calibration steps (a simple low-pass filter was used to reduce noise in the plot on very small wavelength scales).

About 30 hours of telescope time were used for this work, divided roughly equally between imaging and spectroscopy, with the data acquired between March 5 and June 6.