M101 (Pinwheel Galaxy) in emission lines and continuum light

M101 (also known as the Pinwheel Galaxy) is a spiral galaxy in the constellation Ursa Major, which lies in a distance of about 22 million light-years. It's a large galaxy with a diameter of about 170,000 light-years, and it is the largest member of the M101 group, which belongs (together with many others, including the Local Group) to the Virgo Supercluster.

M101 (Pinwheel Galaxy) in Hα and continuum light — Click on the image to load it at full resolution in a JavaScript viewer. Use the buttons to switch the color mapping.
In the OHS image, [OIII] is mapped to red, Hα is mapped to green, and [SII] is mapped to blue. All channels contain some continuum light to reduce the artifacts near stars. In the two other images, NIR (near infrared) is mapped to red, yellow is mapped to green, and blue is mapped to blue. In the version with Hα, that emission line is added to the red channel.
By toggling between Continuum and Hα+continuum, it can be seen that the HII regions (reddish) correlate with bluish regions. That's because the HII gas clouds typically contain many young (blue) stars, which are also responsible for the ionization. Furthermore, the HII regions also emit [OIII] and Hβ light which is collected by the blue filter.
OIII regions are shown in red in the OHS image. Only a few “smaller” ones do not emit Hα light. That can be visualized by toggling between Hα+continuum and OHS. The largest OIII region that can’t be seen in other wavelengths has an apparent diameter of about 16″ and a true diameter of about 500 pc (1,700 light-years).
The large emission nebula on the left side is NGC 5471. It can be seen at all wavelengths, but it is largest in [OIII], with an apparent diameter of 140″ and a true diameter of about 4,500 pc (15,000 light-years). Also see the next figure.

M101 (Pinwheel Galaxy) in continuum light — Click on the image to load it at full resolution in a JavaScript viewer. Use the buttons to switch the color mapping.
In the OHS image, [OIII] is mapped to red, Hα is mapped to green, and [SII] is mapped to blue. All channels contain some continuum light to reduce the artifacts near stars. In the two other images, NIR (near infrared) is mapped to red, yellow is mapped to green, and blue is mapped to blue. In the version with Hα, that emission line is added to the red channel.
By toggling between Continuum and Hα+continuum, it can be seen that the HII regions (reddish) correlate with bluish regions. That's because the HII gas clouds typically contain many young (blue) stars, which are also responsible for the ionization. Furthermore, the HII regions also emit [OIII] and Hβ light which is collected by the blue filter.
OIII regions are shown in red in the OHS image. Only a few “smaller” ones do not emit Hα light. That can be visualized by toggling between Hα+continuum and OHS. The largest OIII region that can’t be seen in other wavelengths has an apparent diameter of about 16″ and a true diameter of about 500 pc (1,700 light-years).
The large emission nebula on the left side is NGC 5471. It can be seen at all wavelengths, but it is largest in [OIII], with an apparent diameter of 140″ and a true diameter of about 4,500 pc (15,000 light-years). Also see the next figure.

M101 (Pinwheel Galaxy) in [OIII] — Emission line images (without continuum light) shown separately. Interestingly, the [OIII] emissions only occur in the outer regions. (Note that there are artifacts near stars / regions contaminated by too much continuum light.)

M101 (Pinwheel Galaxy) in H-alpha — Emission line images (without continuum light) shown separately. Interestingly, the [OIII] emissions only occur in the outer regions. (Note that there are artifacts near stars / regions contaminated by too much continuum light.)

Image data

FOV:

0.53° × 0.47° (full view)

Date:

2019-2025

Location:

Pulsnitz, Germany

Instrument:

400mm Newton at f=1520mm

Camera Sensor:

Panasonic MN34230

Orientation:

North is up (approximately)

Scale:

0.8 arcsec/pixel

Total exposure times:

Hα (3nm):	22.1 h
[OIII] (3nm):	34.5 h
[SII] (3nm):	51.3 h
NIR:	3.6 h
Yellow (540nm to 650nm):	6.3 h
Blue:	3.7 h

Image processing

All image processing steps are deterministic, i.e. there was no manual retouching or any other kind of non-reproducible adjustment. The software which was used can be downloaded here.

Image processing steps where:

Bias correction, dark current subtraction, flatfield correction
Alignment and brightness calibration using stars from reference image
Stacking with masking unlikely values and background correction
Denoising and deconvolution
Continuum subtraction (full or partial)
Dynamic range compression using non-linear high-pass filter
Color composition
Tonal curve correction

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