Copeland’s Septet

Copeland’s Septet (also known as Arp 320, VV 282, and Hickson Compact group 57) is a group of galaxies, located at a distance of around 430 million light years in the constellation of Leo. It was discovered in 1874 by Ralph Copeland.

Note that in the Hickson Compact group catalogue an 8th member has been included in the group and they are known as HCG57a to HCG57h. In the NGC catalogue they are known as NGC 3753, NGC 3746, NGC 3750, NGC 3754, NGC 3748, NGC 3751, and NGC 3745. The 8th member is not included in the NGC catalogue, since it was created decades before Hickson’s catalogue was compiled. The 8th member is included in the Catalogue of Principal Galaxies (PGC), and it is known as PGC 36010.

The spiral galaxy NGC 3753 is the largest member of the group. This is a LINER (Low Ionization Nuclear Emission-line Region) galaxy. These galaxies display emission lines in their spectra. The source of these emission lines is debatable, and astronomers have proposed two mechanisms. The first mechanism suggests that AGNs with supermassive holes are responsible for the emission in LINER galaxies. The second mechanism assumes that this emission comes from star forming regions. Another problem with these galaxies is how atoms are excited. Photoionization has been suggested as an excitation source, while shock waves that propagate through the gas have also been assumed.

The disk morphology of NGC 3753 suggests that in the past (i.e., 50 million years ago) it interacted gravitationally, with NGC 3754. The interaction has caused significant disturbances in the disk of NGC 3753 and in this case, galactic interaction suppressed star formation instead of triggering it.

In terms of the rest of the members of the group, NGC 3746 is a spiral barred galaxy, while NGC 3750 is an elliptical galaxy. NGC 3754 is also a spiral barred galaxy, NGC 3748 is a lenticular galaxy, which is also the case for NGC 3751. Finally, NGC 3745 is a lenticular galaxy, while PGC 36010 is a spiral barred galaxy.

Image: Composite image of Copeland’s Septet taken with the 4-metre Blanco Telescope at Cerro Tololo Inter-American Observatory, Chile. The image was created using broadband filters that are centred at 474 nm (G-band, blue), 644 nm (R-band, green), and 926 nm (Z-band, red). NGC 3753 is the biggest galaxy in the image towards the centre of the image. On the left of NGC 3753 is NGC 3754, while on its right is NGC 3750. Further on the right is NGC 3751. On the upper left of the image PGC 36010 is the galaxy closer to NGC 3753. Moving clockwise from the latter is NGC 3748, followed by NGC 3745, and then NGC 3746.

Image Credit: DESI Legacy Imaging Surveys/LBNL/DOE & KPNO/CTIO/NOIRLab/NSF/AURA

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