NGC 253 processed as a dim dwarf galaxy in colour

[sjastro]

I am quite pleased with the result.
The halo comes up particularly well in colour.

90m L 60m R, 60m G,60m B BRC-250 scope.
The processing involved subtracting the skyglow and applying 4 iterations of the pixel mapping routine.
Ordinarily sky subtracting for a bright object such as NGC 253 is not necessary but the halo is considerably fainter than the main body of the galaxy.

High resolution image here.
http://members.iinet.net.au/~sjastro...olour_halo.jpg

As a comparison the image was processed normally.
http://members.iinet.net.au/~sjastro/ngc253latest.jpg

Clear skies

Steven

[Rex]

Very Nice Steven. There is a dramatic difference between the two processing techniques, the halo shows up very well as you have said.

[Ross G]

Amazing work Steven.

Good to see a different view of this galaxy.

Great looking photo.

Ross.

[sjastro]

Thanks Rex and Ross.

Clear skies

Steven

[sjastro]

The attachment shows the star J004804.8-251749 which has a Vmag of 22.09.

This star is under study at ESO's VLT scope.
Astronomers think the star is being ejected out of the halo.

Regards

Steven

[Rex]

Wow Steven that is so cool!

[Shiraz (Ray)]

fine images and very interesting work Steven - thanks for posting.

[atalas]

Originally Posted by sjastro

The attachment shows the star J004804.8-251749 which has a Vmag of 22.09.

This star is under study at ESO's VLT scope.
Astronomers think the star is being ejected out of the halo.

Regards

Steven

Very interesting Steven...do they say what is ejecting the star out?very fine work.

[5ash (Philip)]

Fantastic image , so much fine detail and sense of depth. It really looks a dynamic picture , gives a sense of lots of relative movement going on. I really haven't seen an ngc253 with so much going on , can't wait to see your next post
Regards philip

[sjastro]

Originally Posted by 5ash

Fantastic image , so much fine detail and sense of depth. It really looks a dynamic picture , gives a sense of lots of relative movement going on. I really haven't seen an ngc253 with so much going on , can't wait to see your next post
Regards philip

Thanks Philip.

Originally Posted by atalas

Very interesting Steven...do they say what is ejecting the star out?very fine work.

Hi Louie,

If J004804.8-251749 is a runaway star as indicated by it's doppler redshift, astronomers however have struggled to explain the high velocity.

From the 2002 paper. Not sure what the status is today.

A bright early-type star in the halo of NGC 253: Runaway or in situ formation?[*]

F. Comerón 1 - A. E. Gómez 2 - J. Torra 3


1 - European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei Muenchen, Germany
2 - Observatoire de Paris-Meudon, GEPI, UMR 8111 du CNRS, 92195 Meudon Cedex, France
3 - Departament d'Astronomia i Meteorologia, Universitat de Barcelona, Av. Diagonal 647, 08028 Barcelona, Spain

Received 13 November 2002 / Accepted 19 December 2002

Abstract
We present observations of J004804.8-251749, a blue object in the direction of the halo of NGC 253 with (B-V) = -0.16 +\- 0.08, Vmag = 22.09 +\- 0.06 and a stellar spectrum showing the Balmer lines redshifted to a velocity of 279 +\- 15 km s-1. These data are consistent with a B5/B8 supergiant star with a mass M = 12 Mo and a maximum age t = 20 Myr in the halo of NGC 253. Based on its position, radial velocity, and inferred maximum age, we consider the possibility that J004804.8-251749 may be a runaway star ejected from the disk of NGC 253. We derive a lower limit of 172 km s-1 for the ejection velocity, or 148 km s-1 if the lower limit of the distance fork to NGC 253 is adopted. While being within the range of velocities accessible to runaway stars through the dynamical ejection mechanism, such high velocities are rather unlikely, especially since the inferred value is only a lower limit. This leads us to consider in situ formation as a possible alternative scenario, in which the formation of J004804.8-251749 would have been triggered by the interaction between the starburst-driven superwind stemming from the center of NGC 253 and a cloud of cold hydrogen in the halo. Such a formation mechanism would be consistent with our previous finding of an excess of blue point sources in the direction of the halo of NGC 253, which we interpreted as indicating the existence of superwind-triggered star formation.

Key words: stars: early-type - stars: formation - stars: kinematics - galaxies: starburst - galaxies: individual: NGC 253

Reference:- http://www.aanda.org/articles/aa/ful...300.right.html

Originally Posted by Shiraz

fine images and very interesting work Steven - thanks for posting.

Thanks for your comments Ray.

Wow Steven that is so cool!

Once again thanks Rex.

[David Fitz-Henr]

Great work steven; all nice images and interesting information about the runaway star. I like the extended galaxy halo though perhaps a little over-stretched giving halos around the stars as well, but a minor point.

[Geoff45 (Geoff)]

NGC 253 in a new light! It's never going to be the same again. I'll bet we see a lot more of the halo from now on. Nice to push the boundaries and get something out of the ordinary. Well done.
Geoff

[sjastro]

Thanks David and Geoff.

I have never seen the halo extend deeply in any other amateur or professional image in the visible spectrum.
The pros image the halo in radio, UV and X-ray wavelengths.
One needs to be certain the halo is the real deal rather than a processing artefact.

I've sent the image to the European Southern Observatory for comment.
Given their involvement in the Carina Dwarf galaxy thread, hopefully they will resolve the issue.

Regards

Steven

[multiweb (Marc)]

Very cool shot Steven and interesting project.

[Andre27 (Andre)]

Well done , so much detail.

[sjastro]

Originally Posted by Andre27

Well done , so much detail.

Thanks Andre.

Originally Posted by multiweb

Very cool shot Steven and interesting project.

It is a very interesting project Marc.
Hopefully it has the Pros interested as well.

Regards

Steven

[gvanhau (Geert)]

Hello Steven

What a big halo, never seen that before.

Are there any doccuments where I can read/study about the techniques involved to extract such info from (almost) ordinary images?

Geert

[sjastro]

Originally Posted by gvanhau

Hello Steven

What a big halo, never seen that before.

Are there any documents where I can read/study about the techniques involved to extract such info from (almost) ordinary images?

Geert

Hi Geert.

I'm intending to write up a procedure based on Pixinsight in the not too distant future. Any software package however that allows data manipulation using pixel maths will do.

Here is a general overview.

For skyglow subtraction process the luminance image normally and make sure there is no clipping in the black region.
To subtract the skyglow one must accurately measure the noise in the background.
The noise value is used in a conditional function.

For example suppose the background noise value is n.

The conditional function is IF(PV<n, 0, PV) where PV is the pixel value being tested for the condition.
If PV<n, then PV is replaced by 0.
If PV=>n then PV is unchanged.

The key to success is the accurate measurement of the background noise.

The background is now black and largely noiseless and allows aggressive stretching without loss of contrast as the background remains black.

I have found the mapping function:

PV(new)= PV(old)*exp(-0.2*PV(old))

where PV(new) is the PV after mapping and PV(old) the value before mapping, to be superior to linear and non linear stretching that are typically used, as both the low and high pixel range remain largely unaffected while stretching is performed in the mid range.

When this non linear stretch is used after the skyglow is subtracted, one can extract very faint details in the image.

Regards

Steven

[gvanhau (Geert)]

Thks Seven for the explanation

I see that the conditional expresion IF(PV<n, 0, PV) is a step function instead of a proportional or substaction function that is normally used to dim the background, so the distance between the new background and the data is heavily increased, making the further stretching more agressive on the non background areas.

Geert