- 3507 - GALAXIES - how many are there? In a single image the “Hubble eXtreme Deep Field” has revealed 5,500 galaxies over an area that took up just 1 / 32,000,000 th of the sky. Scientists estimate that there are more than ten times as many galaxies out there than Hubble, even at its limits, is capable of seeing even in that little space. We see so much like looking through a straw.
--------------------- 3507 - GALAXIES - how many are there?
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- Only a century ago we thought our one galaxy was the entire universe. At that time we had not discovered another galaxy and the Milky Way was our entire universe. My how times have changed.
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- In a single image the “Hubble eXtreme Deep Field” has revealed 5,500 galaxies over an area that took up just 1 / 32,000,000 th of the sky. Scientists estimate that there are more than ten times as many galaxies out there than Hubble, even at its limits, is capable of seeing even in that little space. We see so much like looking through a straw.
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- There are some 2 trillion galaxies within the observable Universe. Each of those galaxies contains billions of stars. Around all the stars are billions of planets and around the planets billions of moons. There must be more life out there somewhere? We keep looking.
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- When you gaze up at the night sky, through the veil of stars and the plane of the Milky Way close by, you can’t help but feel small before the grand abyss of the Universe that lies beyond. Even though nearly all of the stars are invisible to our eyes, our observable Universe, extending tens of billions of light years in all directions and contains a fantastically large number of galaxies within it.
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- The exact number of galaxies out there has been a mystery, with estimates rising to 170 billion even to 2 trillion galaxies in our Universe.
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- Our deepest galaxy surveys can reveal objects tens of billions of light years away, but even with ideal technology, there will be a large distance gap between the farthest galaxy and the Big Bang.
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- Some of the Universe is obscured by intervening matter. And the more distant an object is, the fainter it appears; at some point, a source is far enough away that even observing for a century won’t collect enough photons to reveal such a galaxy.
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- The stars and galaxies we see today didn’t always exist, and the farther back we go, the closer to perfectly smooth the Universe gets, but there is a limit to the smoothness it could’ve achieved, otherwise we wouldn’t have any structure at all today.
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- The longer you stare at a single patch of sky, the more light you’ll collect and the more you’ll reveal about it. We first did this in the mid-1990s with the Hubble Space Telescope, pointing at a patch of sky that was known to have practically nothing in it, and simply sit on that spot, collect photons, and let the Universe reveal what was present.
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- The blank region of sky was the region chosen to be the observing location of the original “Hubble Deep Field image“. With no known stars or galaxies within it, in a region devoid of gas, dust, or known matter of any type, this was the ideal location to stare into the abyss of the empty Universe.
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- It was one of the riskiest astronomy strategies of all-time. If it failed, it would have been a waste of over a week of observing time on the newly-corrected Hubble Space Telescope, the most sought-after data collection observatory. But if it succeeded, it promised to reveal a glimpse of the Universe in a way we had never seen before.
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- We collected data for hundreds of orbits, across a multitude of different wavelengths, hoping to reveal galaxies that were fainter, more distant, and harder to see than any we had detected before. We hoped to learn what the ultra-distant Universe really looked like. And when that first image finally was processed and released, we got a view unlike any other.
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- The original Hubble Deep Field image, for the first time, revealed some of the faintest, most distant galaxies ever seen. Only with a multi-wavelength, long-exposure view of the ultra-distant Universe could we hope to reveal these never-before-seen objects.
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- Everywhere we looked, in all directions, there were galaxies. The most distant galaxies are caught up in the expansion of the Universe, causing distant galaxies to redshift past the point where our optical and near-infrared telescopes (like Hubble) could detect them.
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- Finite sizes and observing times meant that only the galaxies above a certain brightness threshold could be seen. And very small, low-mass galaxies, like “Segue 3” in our own backyard, would be far too faint and small to resolve.
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- The best attempt we ever made was the “Hubble eXtreme Deep Field” (XDF), which represented a composite image of ultraviolet, optical, and infrared data. By observing just a tiny patch of sky so small it would take 32 million of them to cover all the possible directions we could look, we accumulated a total of 23 days worth of data.
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- Stacking everything together into a single image revealed something never-before seen: a total of approximately 5,500 galaxies. This represented the highest density of galaxies ever observed through a narrow, pencil-like beam in space. Like looking through a straw.
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- You might think that we could estimate the number of galaxies in the Universe by taking the number we observed in this image and multiplying it by the number of such images it would take to cover the entire sky.
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------------------- 5500 multiplied by 32 million = 176 billion galaxies.
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- That’s a lower limit. Nowhere in that estimate do the too-faint, too-small, or too-close-to-another galaxies show up. Nowhere do the galaxies obscured by the neutral gas and dust appear, nor do the galaxies located beyond the redshift capabilities of Hubble.
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- Galaxies comparable to the present-day Milky Way are numerous, but younger galaxies that are Milky Way-like are inherently smaller, bluer, more chaotic, and richer in gas in general than the galaxies we see today.
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- There were more galaxies in the early Universe than there are today. But, they’re smaller, less massive, and are destined to merge together into the old spirals and ellipticals that dominate the Universe we inhabit at present.
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- As of today, two trillion galaxies should exist within our “observable Universe‘.
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- More than 90% of the galaxies within our Universe are beyond the detection capabilities of even humanity’s greatest observatory, even if we look for nearly a month at a time at a single spot.
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- Over time, galaxies merged together and grew, but small, faint galaxies still remain today. Even in our own Local Group, we’re still discovering galaxies that contain mere thousands of stars, and the number of galaxies we know of have increased to more than 70 galaxies.
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- The faintest, smallest, most distant galaxies of all are continuing to go undiscovered, but we know they must be there.
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- The next step in the great cosmic puzzle is to find and characterize as many of them as possible, and understand how the Universe grew up. Led by the James Webb Space Telescope and the next generation of ground-based observatories, including the Vera Rubin Observatory, the Giant Magellan Telescope, and the European Extremely Large Telescope, we’re poised to reveal the unseen Universe as never before.
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March 19, 2022 GALAXIES - how many are there? 3507
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