NGC 3675 from remote

In 2017, I received a high-quality dataset of the spiral galaxy NGC 3675 from Telescope Live, acquired with a telescope in the USA. The excellent observing conditions provided very clean and deep data, ideal for capturing the fine details of this remarkable object.

NGC 3675 is a spiral galaxy in the constellation Ursa Major, about 35–55 million light-years from Earth, and a member of the Ursa Major Cluster within the Virgo Supercluster. It was discovered by William Herschel on 14 January 1788 and is one of the brighter, more structured spirals in that part of the sky.

NGC 3675 is classified as type Sb or SA(s)b, meaning it is a spiral with a fairly prominent bulge and moderately wound arms, and no strong bar. Its apparent size is roughly 6 × 3 arcminutes for the main disk, with fainter outer regions extending to about 8–9 arcminutes. At a distance of around 35–55 million light-years, that translates into a physical diameter of roughly 90,000–100,000 light-years, so it is comparable in size to the Milky Way.

Visually, the galaxy is described as “very bright, considerably large, very much extended” with a “very suddenly much brighter middle and nucleus”. In images, it shows a bright central bulge surrounded by a fainter elliptical disk and patchy, filamentary spiral arms. Some sources call it a flocculent spiral, meaning its arms are not grand, continuous arcs but rather broken, woolly segments.

One of the most interesting features of NGC 3675 is its nucleus. It hosts a low-ionization nuclear emission-line region (LINER), a type of weak active galactic nucleus powered by a supermassive black hole. Hubble-based measurements of the central velocity dispersion imply a black hole mass of roughly 10–39 million solar masses. That is not a quasar-level monster, but it is enough to produce detectable emission lines and influence the innermost gas.

The galaxy is also classified as an H II/LINER type, meaning it has both star-formation-related emission and a low-level AGN signature. That combination makes it scientifically useful for studying how star formation and nuclear activity coexist in spiral galaxies.

NGC 3675 has a fairly complex structure. About 20% of its total mass is in the nuclear bulge, and the disk is inclined by roughly 67° to our line of sight. The galaxy features two ring structures, with diameters of about 1.62 and 2.42 arcminutes, and tightly wound spiral arms that form an inner pseudoring before continuing for roughly one revolution outside that ring. The outer arms are described as very patchy and filamentary, consistent with its flocculent classification.

Infrared images once suggested a strong bar, but later observations have not confirmed a clear bar structure, so it is usually treated as an unbarred or very weakly barred spiral.

One supernova has been observed in NGC 3675: SN 1984R, discovered by Kaoru Ikeya on 2 December 1984. Its type is listed as unknown in most summaries, with a peak magnitude around 13. The presence of a supernova confirms that the galaxy is still forming massive stars, even though its overall appearance is dominated by an older stellar population in the bulge and inner disk.

For amateur astronomers, NGC 3675 is a rewarding but somewhat under-imaged target. With a visual magnitude around 10.0–10.2, it is accessible to moderate telescopes under dark skies. Observers describe it as large and quite bright, with a bright center and a fainter elongated disk, and sometimes note two moderately bright stars near the southern end of the disk. Its flocculent arms are subtle, so larger apertures and good contrast help reveal the spiral structure.

Data calibration, regitration and the final processing were all performed with PixInsight. The result was as follows:

The image also reveals a remarkable number of distant quasars scattered throughout the background field. The most distant identified object is SDSS J112748.67+433217.2, with a measured redshift of z = 3.163. Its light has travelled for more than 11.5 billion years before reaching Earth, allowing us to observe this quasar as it appeared when the Universe was only around 2.1 billion years old.

The images were taken with the following equipment:

• Date: 2017
• Location: USA
• Telescope/Lens: Planwave CDK17
• Focal length [mm]: 2924
• Focal ratio: 6.8
• Mount: Paramount ME
• Camera: FLI Proline 16803
• Filter: L:R:G:B
• Exposure time [min]: 580:180:180:165
• Resolution: 0.64″/px