emission from the Sombrero Galaxy varies only 10–20%. Unidentified terahertz radiation In 2006, two groups published measurements of the terahertz radiation from the nucleus of the Sombrero Galaxy at a wavelength of . This terahertz radiation was found not to originate from the thermal emission from dust (which is commonly seen at infrared and submillimeter wavelengths), synchrotron radiation (which is commonly seen at radio wavelengths), bremsstrahlung emission from hot gas (which is uncommonly seen at millimeter wavelengths), or molecular gas (which commonly produces submillimeter spectral lines). The source of the terahertz radiation remains unidentified. Globular clusters The Sombrero Galaxy has a relatively large number of globular clusters, observational studies of which have produced population estimates in the range of 1,200 to 2,000. The ratio of globular clusters to the galaxy's total luminosity is high compared to the Milky Way and similar galaxies with small bulges, but comparable to other galaxies with large bulges. These results have often been used to demonstrate that the number of a galaxy's globular clusters is thought to be related to the size of its bulge. The surface density of the globular clusters generally follows the bulge's light profile, except near the galaxy's center. Distance and brightness At least two methods have been used to measure the distance to the Sombrero Galaxy. The first method relies on comparing the measured fluxes from the galaxy's planetary nebulae to the known luminosity of planetary nebulae in the Milky Way. This method gave the distance to the Sombrero Galaxy as . The second method is the surface brightness fluctuations method, which uses the grainy appearance of the galaxy's bulge to estimate the distance to it. Nearby galaxy bulges appear very grainy, while more distant bulges appear smooth. Early measurements using this technique gave distances of . Later, after some refinement of the technique, a distance of was measured. This was even further refined in 2003 to . The average distance measured through these two techniques is . The galaxy's absolute magnitude (in the blue) is estimated as −21.9 at (−21.8 at the average distance of above)—which, as stated above, makes it the brightest galaxy in a radius of around the Milky Way. A 2016 report used the Hubble Space Telescope to measure the distance to M104 based on the tip of the red-giant branch method, yielding 9.55 ± 0.13 ± 0.31 Mpc. Nearby galaxies and galaxy group information The