APOD: The Stars of the Triangulum Galaxy (2019 Feb 23)

[Chris Peterson]

What we're seeing in the near-infrared filter is a good and interesting question. Is it hydrogen? Hm. Emission detected through the F814W is a combination of both continuum and multiple emission lines from Paschen jumps and S [III]. https://iopscience.iop.org/article/10.1 ... 8/1/26/pdf
So, hydrogen for sure, maybe S [III] too.

True, but through broadband filters these are all going to be very dim compared with the continuum, which is the source of the overwhelming majority of the energy.

[shaileshs]

I checked the "zoom" tool.. what a view at 2nd most zoomed level.. stars are next to eachother as if they are sand grains on a beach.. I wonder what's the smallest distance between any 2 stars in the densest region.. and, will there be darkness on any planet in one of those star's solar system ? They might be seeing multiple stars in their sky - all time. No ?

The stars themselves are unresolved, but the spaces between them are not. We see each point of a star as much, much larger than it actually is. That's what gives the "sand grains on a beach" appearance. In actuality, what we're seeing is a handful of sand grains sprinkled across a football field. Huge distances between the grains compared with their individual sizes.

We're looking here at the central bulge of a galaxy, where the stellar density is much higher than it is in the sparse section of the Milky Way where our own solar system resides. To a first order, the density in much of this image may be 100 times greater than what we have here around us. So yes, the sky will be brighter at night. But it will still get dark... imagine a sky similar to what we see during a full Moon.

Thank you Chris for responding. I can understand and imagine.

[shaileshs]

I checked the "zoom" tool.. what a view at 2nd most zoomed level.. stars are next to eachother as if they are sand grains on a beach.. I wonder what's the smallest distance between any 2 stars in the densest region.. and, will there be darkness on any planet in one of those star's solar system ? They might be seeing multiple stars in their sky - all time. No ?

Anyone can shed light (enlighten me) on my above query ? Thanks in advance!

I attempted to respond yesterday but my device's battery died just moments from hitting submit.

Since the rules of physics are universal star sizes, types, distribution, etc. would in general be the same over there as here in our galaxy shaileshs. Therefore the night skies on a planet like ours in the Triangulum galaxy would be much like ours. Stars only appear close together (except for close binary pairs which look like one star). Star systems are usually several or even a great number of light years apart.

Thank you for responding. I can understand and imagine.

[Ann]

By the way, I've got a 4 filter version of this mosaic put together here: https://geckzilla.prodibi.com/a/y8vmwyj ... q12BEbA%3D
Note it's meant to be zoomed into fully. It doesn't look very good zoomed out. The filters are
Red: F160W
Yellow: F814W
Cyan: F475W
Blue: F336W

With that data you could toss the F160W and map the others to RGB and get something approaching a true color image (not perfect, but reasonable).

Seeing such an image would be very interesting.

Ann

[Ann]

I find it most unlikely that the infrared filter has detected Hα. Instead, it must have detected dust.

The IR band here is barely out of the visible. It doesn't show cool dust, but rather, part of the direct output of stars. At that longer wavelength, however, dust is somewhat more transparent, so we see deeper into dusty regions. In the near IR, dust is less apparent than in the visible range (but in both cases it is absorbing light, not emitting it).

Click to view full size image

NGC 604. ESA/Hubble and NASA.

You are absolutely right that the 814 nm filter would not detect cool gas. The ESA/Hubble and NASA picture at right shows that there are indeed cool (orange-looking) stars inside NGC 604 that emit a lot of infrared light, so you are also right about the fact that the infrared filter used for the Hubble picture of M33 must have detected starlight from cool stars.

However, given that the infrared filter detected infrared light from cool stars inside NGC 604, I must assume that the blue filter used for the Hubble picture of M33 did not just detect OIII from the ionized nebula of NGC 604, but also blue starlight from the large numbers of hot stars inside this giant site of star formation.

Ann

[geckzilla]

By the way, I've got a 4 filter version of this mosaic put together here: https://geckzilla.prodibi.com/a/y8vmwyj ... q12BEbA%3D
Note it's meant to be zoomed into fully. It doesn't look very good zoomed out. The filters are
Red: F160W
Yellow: F814W
Cyan: F475W
Blue: F336W

With that data you could toss the F160W and map the others to RGB and get something approaching a true color image (not perfect, but reasonable).

Seeing such an image would be very interesting.

Ann

Yep, already did that one here: https://geckzilla.prodibi.com/a/y8vmwyj ... go44eqq67x

Honestly just find the four filter version a little more interesting. The F814W filter works differently enough from "red" that it doesn't really look all that much like RGB. The dust is just that much more transparent to it. I could probably mix it down and make it look a lot closer to RGB, though, and then force some saturation back into it just like a "natural" color image that people crave. It still won't have pink hydrogen clouds, though. F814W just isn't right for that.