NGC 4490 is a barred spiral galaxy located in the northern constellation of Canes Venatici. It was discovered by William Herschel in 1788 and is catalogued as NGC 4490 in the New General Catalogue. Together with its smaller companion NGC 4485, it forms the interacting galaxy pair Arp 269, included in Halton Arp's Atlas of Peculiar Galaxies because of the remarkable distortions caused by their mutual gravitational interaction. The system lies at an estimated distance of approximately 25 to 30 million light-years from Earth.
Morphologically, NGC 4490 is generally classified as a late-type barred spiral galaxy of type Sd, although its structure has been significantly altered by the ongoing interaction with NGC 4485. The galaxy extends over about 50,000 light-years and exhibits an irregular appearance characterised by prominent dust lanes, numerous star-forming regions, ionised hydrogen (H II) regions, and a disturbed spiral pattern. The interaction has triggered intense star formation throughout the galaxy, making it considerably more active than a typical spiral galaxy of similar size.
Its companion, NGC 4485, is a much smaller irregular galaxy located just a few arcminutes away. The gravitational encounter between the two galaxies has produced a bridge of gas and stars connecting them, as well as large-scale tidal distortions that are visible at multiple wavelengths. Radio observations have revealed an extensive envelope of neutral hydrogen surrounding the system, extending far beyond the visible boundaries of either galaxy and providing evidence of the significant exchange of material between them.
NGC 4490 is sometimes referred to as the “Cocoon Galaxy”, a name inspired by its elongated shape and mottled appearance. Together, these galaxies offer a clear view of how galactic interactions redistribute gas and stimulate the birth of new stars.
Image Acquisition
All data for this image were acquired with the Takahashi TOA-150 telescope. The native image scale of 1.69"/pixel was later increased to 0.845"/pixel using 2× drizzle during integration.
The image combines broadband RGB data with narrowband Hα data. RGB exposures were acquired with individual sub-exposure times of 900 seconds, while the Hα data were collected using 1800-second sub-exposures in order to record the faint emission associated with the numerous star-forming regions present in NGC 4490.
The total integration time amounts to 30 hours.
Image Processing
The processing strategy for this image was based on removing the stars from the linear datasets using StarXTerminator, allowing the galaxy and the stellar component to be processed independently.
Several aspects of the processing workflow are worth highlighting.
Firstly, the use of 2× drizzle during integration made it possible to improve the effective image scale from 1.69"/pixel to 0.845"/pixel and to better sample the fine structures of the galaxy.
Secondly, for the colour calibration of the RGB data, I used the SpectrophotometricColorCalibration (SPCC) tool. After several tests, I selected the Sd Galaxy spectral profile as the white reference instead of the default Average Spiral Galaxy profile, as it provided the most satisfactory result for NGC 4490, which is generally classified as a late-type Sd galaxy.
The image also incorporates Hα data, which were blended into the galaxy following the methodology described by Vicent Peris. This approach enhances the numerous star-forming and H II regions while preserving the overall colour balance of the broadband image.
Once the galaxy processing was completed, the RGB stars, processed separately to preserve their natural colours, were seamlessly reinserted using the ScreenStars script developed by Mike Cranfield and Bill Blanshan.
In this image, north is up.
Click on the image for the full resolution version, or visit the Gallery section for complete exposure details.
Image processing: Pixinsight.
Observatory automation and remote operation: Talon6.
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