NGC 2403 and NGC 2404 in emission lines and continuum light

NGC 2403 (also known as Caldwell 7) is a spiral galaxy in the constellation Camelopardalis with many star-forming HII regions. One of these regions has its own NGC number: NGC 2404. The galaxy lies at a distance of 9.7 million light-years and is a member of the M81 Group, one of the nearest groups to the Local Group.

NGC 2403 (Caldwell 7) and NGC 2404 in H-alpha and continuum light — Click on the image to load it at full resolution in a JavaScript viewer. Use te 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. A few relatively large ones can't be seen in other wavelengths. That can be visualized by toggling between Hα+continuum and OHS. The largest OIII-only regions have an apparent diameter of about 30″ and a true diameter of about 430 pc (1,400 light-years).

NGC 2403 (Caldwell 7) and NGC 2404 in continuum light — Click on the image to load it at full resolution in a JavaScript viewer. Use te 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. A few relatively large ones can't be seen in other wavelengths. That can be visualized by toggling between Hα+continuum and OHS. The largest OIII-only regions have an apparent diameter of about 30″ and a true diameter of about 430 pc (1,400 light-years).

Image data

FOV:

0.62° × 0.44° (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):	22.2 h
[OIII] (3nm):	33.7 h
[SII] (3nm):	50.5 h
NIR:	3.5 h
Yellow (540nm to 650nm):	6.3 h
Blue:	3.1 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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