The Sombrero Galaxy in Virgo

Link to annotated image

M104 is located about 31 million light years away on the borders of Virgo and Corvus, and is a member of the Virgo Galaxy. It’s only about 1/3 the diameter of the Milky Way, but has a bright nucleus and an unusually large central bulge and halo. Astronomers initially thought that the halo was small and light, indicative of a spiral galaxy, but the Spitzer Space Telescope found that the dust ring around the Sombrero Galaxy is larger and more massive than previously thought, indicative of a giant elliptical galaxy. The galaxy also has a billion solar mass central black hole, one of the largest supermassive black holes in the nearby universe.

The annotated image shows that a lot is going on in this field. At least three dwarf satellite galaxies are shown. Sombrero UCD (ultra compact dwarf galaxy) 1 is an object intermediate between a globular cluster and a dwarf elliptical galaxy, and was first identified in 2009. See Mon. Not. R. Astron. Soc. 394, L97–L101 (2009). 17th magnitude NGC4594 DW1 is a dwarf elliptical, discussed in 804 Aston. J. 88(6), 808 (1983). And NGC4594 DGSAT 1 is a low surface brightness galaxy identified in a 2016 deep imaging search by amateurs, although I’m not sure that it wasn’t known before then. A&A 588, A89 (2016). On top of that, M104 has 1200-2000 globular clusters, and it turns out that dozens of them are visible in this image. That was quite a surprise to me, but apparently they’re as bright as magnitude 20, so they’re not out of reach of amateur imaging gear. I’ve marked a bright one at upper left of the galaxy, and Universe Today has an identification chart for many more. Plus the normal background LEDA (PGC) background galaxies, quasars, a couple distant clusters and a faint galaxy that hosted a supernova in 1997.

Exposure: Total exposure time about 10 hours, 259:14:13:13 x 2 minutes LRGB. All bin 1x1. Data collected in May and June 2020.
Light pollution: Bortle 7-8 (white zone, NELM about 4.5)
Seeing: FWHM of integrated luminance around 2.8 arcsecs
Image scale at capture: 0.6 arcsecs/pixel = f/5.7
Scale of presentation: 1.2 arcsecs/pixel (50% of full scale)

Equipment:
Scope: C11 (standard, not Edge) with Celestron 0.63 reducer
Mount: Paramount MX+, connected via ASCOM Telescope Driver 6.1 for TheSkyX, with MKS 5000 driver 6.0.0.0
Camera: SXVR-H694, connected via SX ASCOM driver 6.2.1.17140 (SX 1.2.2 also installed)
Filter wheel: Atik EFW2 with 7x1.25 carousel and Artemis 2.4.3.0 driver
Filters: Astrodon Type IIe LRGB
Rotator: Optec Pyxis 2", connected via Andy Galasso's 0.4 driver (Optec Pyxis Rotator AG)
Focuser: Rigel Systems GCUSB nStep motor with driver version 6.0.7 on stock Celestron focuser
OAG: Orion Thin OAG
Guide cam: Lodestar (first generation). 4 second exposures
Automation SW: Sequence Generator Pro 3.1.0.457
Guide SW: PHD 2.6.7, connected to guide cam via native SXV driver
ASCOM: ASCOM 6.3.0.2831
Platesolving: PlateSolve 2, failover to local Astrometry.net 0.19 server
Collimation: Metaguide 3, using ASI120MM connected via ZWO Direct Show driver 3.0.0.2

Processing Workflow by Workspace in PixInsight 1.8.8:

1. Calibration
Calibration with WeightedBatchPreProcessing with flats and bias, using Cosmetic Correction with a master dark
Blink to preview and reject a few frames
Weighting and registration with WBPP

2. Stack and Mure Denoise
Image Integration on each channel
Mure Denoise on each channel
RGB Combination for RGB frames
Dynamic Crop

3. Luminance Linear Processing
Dynamic Background Extraction
Deconvolution:
Dynamic PSF to create PSF image
Create luminance mask: copy main image, stretch with Screen Transfer Function, apply as mask
Deconvolution

4. Luminance Stretching
Histo Trans x 2
Curves Trans
TGV Denoise
Mild Localized Histogram Equalization to enhance dust lanes

5. RGB Linear Processing
Dynamic Background Extraction
Photometric Color Calibration, using Average Spiral Galaxy white reference

6. RGB Stretching
Histo Trans
Boost color saturation with Curves
Curves

7. Color Combination
LRGB Combination of luminance and RGB images to create a “Galaxy” image

8. Star Reduction
I followed Adam Block’s star reduction technique:
StarNet to create “Starless Image”
Extract two copies of luminance from main image, then apply MLT to one to create a rough star mask
Binarize to select only the stars
MorphTrans to enlarge stars
Convolution to blur star edges
Pixel Math: subtract luminance image from blurred star mask so that cores are excluded from mask, and on ly halos are represented in the mask = “Halo Mask”
Apply Halo Mask to Galaxy image, then run PixelMath to use Starless Image where halos otherwise would be

9. Photoshop: Background Subtraction
Use CloneStamp to remove halos from Starless Image, then subtract it from Galaxy image to remove remaining messy clumps in the background
Save as TIFF and move back into PI

10. Smooth Galaxy Halo
Create star mask with StarNet++ on luminance layer, then invert
Create smoothed image: on main image, run Multiscale Median Transform, unchecking layer 5, to smooth lumpiness in galaxy halo
Photoshop: layer smoothed image on top of main image, using star mask to protect stars and modifying mask to protect galaxy details
Save as TIFF and move back into PI

11. Final
Final Histogram Transformation
ICC Profile Transform to sRGB
Resample to 50% of scale
Save as JPG
ImageSolve
ImageAnnotation (using custom catalogs for galaxy clusters and quasars)