Nightfall
July 22nd, 2021, 06:52 PM
We visual observers and imagers know that spiral galaxies make for some of the most beautiful deep sky objects to chase down and observe or image – especially if we are well-heeled enough to pay for using narrowband filters and the many hours of photon capture time it takes to make a scientifically meaningful image. Roughly 70% of spiral galaxies have central bar structures (though some of these are invisible except in the IR due to obscuring dust). Galaxy bars are as aesthetically delightful as they are astrophysically important. Why do they acquire those shapes, thicknesses, lengths, and configurations with respect to the spiral arms and the galaxy's central bulge? Whence comes the comely diversity with which they bless us with many nights of sheer delight either looking at them or reading the literature about them? As yummies in the celestial candy dish go, barred spirals are the chocolates with the cherries in the middle.
Yet even a cursory look raises questions – why are galactic bars even there? How did they get started in a beautiful Grand Design spiral that looks the textbook image of imperturbable serenity? (M51 is undergoing the precursor instabilities along the rim of its bulge right now that will turn it into a barred spiral in, say, a billion years, give or take a few.) Where does the immense energy come from that can redirect all those stars and gas masses in, say, the Scutum and Norma Clouds (Scutum is five times the mass of the starry specks we see as Aquila's tail feathers) and swerve that enormous mass roughly 90° to the right to flow down into the bulge region? Why, as we also readily see in most barred spiral images, does most of that redirected gas and dust keep right on going past the bulge till it reaches the spiral arm on the opposite side? What happens to it when it gets there? Why do the dark dust lanes on the inner edges of a spiral arm become dark dust lanes on the outer edge of a bar as the bar hoovers its way along the spiral arms? Why are the middle regions of bars so blandly diffuse and seemingly bereft of star-forming regions? There are a lot of photons streaming out of those bland bars, but what makes the photons if there are so few stars there?
Now step back for a larger view and ask, why do the outer regions of spiral arms beyond the bar-arm confluences have such spectacular star-forming regions when any undergrad fresh out of Astronomy 001 will tell you those regions are composed primarily of low-energy atomic hydrogen, while blazing new star clusters require large masses of gooey, dusty, brutally cold HII gas clouds? How does the vast mass of molecular hydrogen gas and metals-rich dust that typically inhabit the inner arms and bulges of spirals end up out there in the atomic hydrogen Galactic boondocks?
And when we think about it, with spirals gobbling up so much fuel to make stars for our viewing joy, who fills the gas tank? Where does fresh gas come from, how does it enter the galaxy, and how is it distributed once there? And since this question takes us into the wondrous land of the Big Picture, why do most barred spirals inhabit intergalactic filaments in the Cosmic Web, and why do so many of their polar axes of rotation align roughly perpendicular to the bulk flow of gas along those filaments? Why are most large spiral galaxy globular cluster populations distributed more or less spherically in the galaxy's halo while those galaxy's retinues of dwarf galaxies align along vast planes that align either planar with the disk or perpendicular to it?
There is, as any TV detective from 1950s TV will tell you, more to this than meets the eye. And we might now add: there is also more to this than the presence of the smoking gun on the table where the Nightfall (https://assa.saao.ac.za/sections/deep-sky/nightfall/)story begins.
The Astro Society of Southern Africa now offers a few answers and a LOT of pretty pictures. Google Drive downloadable version here. (https://drive.google.com/file/d/1Otj8YeYQjmz_bPXUVq1Xt7zFKz9rdP_P/view?usp=sharing) The ISSUU.com page-by-page view is here (https://issuu.com/douglasbullis/docs/special_report_11_why_do_galaxies_make_bars_300_), and also free to download.
And you don't have to pay a dime for it, look at pages of godawful ads you've already seen 20 times this month, and we don't sell your info or location data to any bozo with a smartphone.
Yet even a cursory look raises questions – why are galactic bars even there? How did they get started in a beautiful Grand Design spiral that looks the textbook image of imperturbable serenity? (M51 is undergoing the precursor instabilities along the rim of its bulge right now that will turn it into a barred spiral in, say, a billion years, give or take a few.) Where does the immense energy come from that can redirect all those stars and gas masses in, say, the Scutum and Norma Clouds (Scutum is five times the mass of the starry specks we see as Aquila's tail feathers) and swerve that enormous mass roughly 90° to the right to flow down into the bulge region? Why, as we also readily see in most barred spiral images, does most of that redirected gas and dust keep right on going past the bulge till it reaches the spiral arm on the opposite side? What happens to it when it gets there? Why do the dark dust lanes on the inner edges of a spiral arm become dark dust lanes on the outer edge of a bar as the bar hoovers its way along the spiral arms? Why are the middle regions of bars so blandly diffuse and seemingly bereft of star-forming regions? There are a lot of photons streaming out of those bland bars, but what makes the photons if there are so few stars there?
Now step back for a larger view and ask, why do the outer regions of spiral arms beyond the bar-arm confluences have such spectacular star-forming regions when any undergrad fresh out of Astronomy 001 will tell you those regions are composed primarily of low-energy atomic hydrogen, while blazing new star clusters require large masses of gooey, dusty, brutally cold HII gas clouds? How does the vast mass of molecular hydrogen gas and metals-rich dust that typically inhabit the inner arms and bulges of spirals end up out there in the atomic hydrogen Galactic boondocks?
And when we think about it, with spirals gobbling up so much fuel to make stars for our viewing joy, who fills the gas tank? Where does fresh gas come from, how does it enter the galaxy, and how is it distributed once there? And since this question takes us into the wondrous land of the Big Picture, why do most barred spirals inhabit intergalactic filaments in the Cosmic Web, and why do so many of their polar axes of rotation align roughly perpendicular to the bulk flow of gas along those filaments? Why are most large spiral galaxy globular cluster populations distributed more or less spherically in the galaxy's halo while those galaxy's retinues of dwarf galaxies align along vast planes that align either planar with the disk or perpendicular to it?
There is, as any TV detective from 1950s TV will tell you, more to this than meets the eye. And we might now add: there is also more to this than the presence of the smoking gun on the table where the Nightfall (https://assa.saao.ac.za/sections/deep-sky/nightfall/)story begins.
The Astro Society of Southern Africa now offers a few answers and a LOT of pretty pictures. Google Drive downloadable version here. (https://drive.google.com/file/d/1Otj8YeYQjmz_bPXUVq1Xt7zFKz9rdP_P/view?usp=sharing) The ISSUU.com page-by-page view is here (https://issuu.com/douglasbullis/docs/special_report_11_why_do_galaxies_make_bars_300_), and also free to download.
And you don't have to pay a dime for it, look at pages of godawful ads you've already seen 20 times this month, and we don't sell your info or location data to any bozo with a smartphone.