M106 (NGC 4258): The Spiral Galaxy Built from Hubble and Backyard Data

The M106 picture released on February 5, 2013 is a hybrid. Its center comes almost entirely from Hubble, but no single Hubble program ever covered the whole galaxy, so the outer disk and the gaps between camera fields were filled with frames shot by two amateur astrophotographers using telescopes in the New Mexico desert.

A galaxy with a busy core

M106, also cataloged as NGC 4258, is a spiral galaxy in the northern constellation Canes Venatici, roughly 23.5 million light-years from Earth. It belongs to the class called Seyfert galaxies, type II, which means its nucleus is far brighter than starlight alone can explain. The extra energy comes from a supermassive black hole at the center, fed by large amounts of gas thought to be falling inward.

The galaxy is best known for a feature its visible disk does not explain: a pair of so-called anomalous arms. In the 2013 image their visible-light component shows up as red, glowing hydrogen emission laid over the starlight of the disk. The Hubble portrait covers only the inner structure around the nucleus and the surrounding halo, which is where those arms are easiest to trace in visible light.

Pieces from three Hubble cameras

The image processor behind the portrait was Robert Gendler, working with image specialists at STScI in Baltimore, Maryland. He began with what Hubble already had. The Hubble Legacy Archive held more than 1,400 observations in and around M106 at the time of the release, from every instrument and including spectra as well as images. Today those data are available through the Mikulski Archive for Space Telescopes, MAST.

From that pool he built a mosaic of the galaxy's center. The bulk of it came from proposal 11570, led by A. Riess of STScI, with additional datasets from the Advanced Camera for Surveys and the Wide Field and Planetary Camera 2 taken for other programs. Data from Wide Field Camera 3 went in as well. Each camera has a different field shape and pixel scale, so the pieces had to be aligned and matched in brightness before any color was added.

Maps of the archive's footprints, the outlines of every camera field laid over the sky, show how patchy that coverage is. Rectangles from the Advanced Camera for Surveys and Wide Field Camera 3 overlap thickly on the nucleus, while the stepped, L-shaped outlines of WFPC2 fields land here and there along the disk. Large parts of the outer galaxy fall outside all of them.

Filters and assigned colors

Filter	What it records	Shown as
F435W	Blue light (B band), ACS	Blue
F555W / F606W	Visible light (V band), ACS and WFC3	Green
F814W	Near-infrared (I band), ACS and WFC3; also the detail layer	Red
F656N	Hydrogen-alpha, WFPC2	Red

The near-infrared F814W data did double duty. Besides supplying the red channel, they served as the luminance layer, the grayscale image that carries the fine detail while the other filters contribute mostly color. The hydrogen-alpha frames from the older WFPC2 camera were added in red to bring out the glowing gas, including the anomalous arms. How such color assignments are decided is covered in Are the Colors Real?

Filling the gaps from the ground

Where Hubble coverage thinned out or stopped, Gendler turned to his own observations and to those of fellow astrophotographer R. Jay GaBany. Their 12.5-inch and 20-inch telescopes are set up under the dark skies of remote New Mexico observing sites. In the outer spiral arms the ground data mostly provide color for structure that Hubble recorded, and only in the true holes, where no Hubble frame exists at all, does the ground image stand in by itself.

The approach is the reverse of the usual relationship between amateurs and space telescopes. Ground images have less resolution but cover the whole galaxy in one frame and can be exposed for many hours. Hubble frames have exquisite detail but are scattered patches. A careful merge keeps the detail where Hubble has it and the completeness that the ground data offer everywhere else.

Gendler is a physician by profession and had been photographing the sky for about two decades at the time. He started from his driveway in suburban Connecticut, later imaged remotely from New Mexico and Western Australia, and then turned to assembling hybrid images from several data sources, including the Hubble archive. His work has appeared on NASA's Astronomy Picture of the Day and in books and magazines.

Questions readers ask

What are the anomalous arms of M106?

They are an extra pair of arms in M106 that the ordinary spiral pattern does not account for, which is where the name comes from. In the 2013 Hubble image their visible-light part appears as red emission from glowing hydrogen, recorded through the hydrogen-alpha filter of WFPC2.

Is the M106 image entirely from Hubble?

No. The central region is almost all Hubble data from three cameras. The outer arms combine Hubble detail with color from two amateur telescopes in New Mexico, and in a few places with no Hubble coverage the ground-based frames fill the gap directly.

Where is M106 in the sky?

It lies in Canes Venatici, the Hunting Dogs, a small northern constellation, at a declination of about +47 degrees. That position keeps it high in the sky for observers at mid-northern latitudes. The galaxy is about 23.5 million light-years from Earth.

What is a Seyfert II galaxy?

It is a spiral galaxy with an unusually bright, active nucleus powered by a supermassive black hole that is taking in gas. In M106 large amounts of gas are thought to be falling toward the central black hole and fueling it. The type II label describes the nucleus as seen in its spectrum.