My Mak-Newt makes new friends in the dwarf galaxy world
Leo II
This is an easy search but not an easy glimpse. Just above Zosma on the crouching hip of the Lion, this dwarf galaxy is in a star-poor region in which you are forced to tiptoe along on mag 11–13 finder stars. Alvin Huey’s finder on p.57 of his Local Group Galaxies makes it easy to find. That doesn’t make this wan glow any easier to see. If you can’t resolve the stars like the accompanying SDSS picture, you may have to move up to a 60-inch scope; Leo II’s brightest red giants are mag 19–20, and its red clump is a wheezy mag 22.2.
If Leo II is a visual toughie, it’s because it is more than halfway to the Milky Way’s tidal radius, the surface where there is no net motion in or out of the galaxy. In numbers, that is 700,000 light years out, 3130 light years long and 2500 light years across the middle. It’s classified as a peculiar elliptical because its approx. 27 million solar masses of stars consists mainly of very old stars with low levels of the chemical elements which are signs of vigorous youth. Put a little more technically, Leo II’s CMD shows a –1.74 metallicity function indicating of an average stellar age of more than 9 billion years, in which there as been essentially no star formation for the last 6 billion years. Since stars in this category average 0.8 solar masses on average, the dim glow we see originated from roughly ±40 million suns, which used up its available mass of star-forming hydrogen. The term “red and dead” isn’t far off when characterizing this glow worm in the sky. But it begs the question, why have 40 million stars not had any children for half the age of the universe?
There are two main reasons Leo II’s long snooze. First is its real estate. As with here on Earth, real estate is all about location-location-location. Leo II lives out among the Milky Way’s 12 remote halo dwarfs, where the Galaxy is simply too far away to do much harm. Leo II joins Leo I (near Regulus), and the Fornax and Sculptor dwarf galaxies as part of the “FLS star stream”, an alignment of dwarf galaxies and debris that originated in the Milky Way’s tidal disruption of a larger galaxy called the Relic Fornax Galaxy many billions of years ago. (This stream is just one of several. Another is the Magellanic Stream, which is a trail of hydrogen gas stripped from the LMC and SMC during their fly-by encounter with the inner Milky Way halo some 65 million years ago.)
The second reason is more complex. Leo II’s dark matter mass is 100 times larger than its stellar mass—not unusual for dwarf galaxies far from a major galaxy. Leo’s star formation history shows a slow but steady build-up from 13 billion years ago to 6 billion, or z = 6 to z = 0.8. Forty million stars in 7 billion years comes out to roughly one new star every 100 years. Seems kinda slow for a galaxy that has roughly 3 billion solar masses of dark matter to hold itself together, and it is. Leo II’s slow senescence started shortly after the universe was born. Is original hydrogen was so hot that it was completely ionized—there were no hydrogen atoms, only protons and electrons whizzing around in tremendous numbers at tremendous speeds in the middle of a massive dark matter well. As the universe expanded, the gas eventually cooled to below 8,000 K, which is the ionization temperature at which hot protons and electrons lose enough energy to combine into an atom. While they were hotter than 8,000 K they emitted huge amounts of ultraviolet energy. UV is the most divisive of radiation energy levels. Gamma rays can crack an atom open, but UV prefers to simply tear their electron shells to shreds. In sum, UV thwarted star formation all across the then-small universe for billions of years, and low-mass objects like dwarf galaxies were the most severely affected. UV can expel ionized particles completely out of a galaxy if the galaxy is low-mass enough. (Learn about it all here.) Leo II fell into this category. Hence it formed stars steadily but slowly until it ran out of gas some 6 billion years ago. Now Leo II drifts slowly around out galaxy, neither affecting nor being affected by much of anything at all. As I looked at it in my Mak-Newt, I thought of eternity in Leisure World.
Leo A (aka Leo III)
When you finally spot Leo A it looks like the point of an arrow streaking eastward into the dawn. The Mak-Newt revealed it before I was done with the star hop from mag 3.9 Raselas up into Leo’s mane toward Cor (20 Leo Minoris). A fuzzy thing loomed into the field and I had no further need for steppingstone stars. Leo A is an easier sighting than its cousin Leo II over there on the Lion’s hip. It holds steadily in averted in an obvious triangular shape, and is framed nicely by an L-shaped line of mag 8.5 to 10 stars. A triangular galaxy was definitely a new one for me, and may be for you as well. All our suppositions about round & fuzzy go out the window with this one. Spotting it was made easier by the uncrowded region of our Galaxy in which it lies, 52 degrees above the Galactic plane and 2.6 million light years out. That is roughly a third of the way to Sextans A and B members of the NGC 3109 galaxy group. But Leo A doesn’t seem to belong to anything. It’s too far away from the Milky Way to be influenced by our galaxy, and is on the opposite side of the sky from Andromeda M31. It is three-quarters of a million light years removed from its nearest neighbour, the Antlia dwarf. How did it get so far away from the crowd? What’s it doing out there?
Leo A is the oddest duck in the Local Group. It is classified as a dwarf irregular galaxy. Leo A is weird in every way. It throws out all the rules about dwarf galaxies and introduces a few new rules of its own. Here’s the basics: (a) Leo formed only about 10% of its stars back in the beginning >12 Gyr ago, the very opposite of most dwarf galaxies which light the fireworks the minute they’ve got enough hydrogen; (b) It retained huge reserves of hydrogen gas for 8 billion years without any apparent reason or ability to do so; © It began a major starburst a mere 4 billion years ago which made 90% of its stars today and sports that rarest of star types in a dwarf galaxy, O and B supergiants <4 million years old; (d) It is not rotating, so its hydrogen and star clouds meander loosely inside; and (e) some boundary regions have abrupt cutoffs where stars end and space begins. Soft, warm, uniform, and fuzzy it is not.
Is there some kind of anthropic justice in the fact that the most contrarian galaxy we know was discovered by the most contrarian astronomer we know? Fritz Zwicky spotted Leo A in 1942 while searching for ultra-faint dwarf galaxies, a search partly prompted by his theories about dark matter’s effect on bar*****c matter. He referred to himself as a “lone wolf” and most astronomers heartily agreed with him. For one, he didn’t think the universe was expanding so formulated the idea of a neutron-induced drag effect on electrons which has been (incorrectly) renamed “tired light” by folks less keen on the realities of nuclear physics. Among his many contributions were the notion of dark matter and his invention of the word “supernova” to describe the events leading up to the formation of what was then a mere theory, the neutron star. Not suited to be one of those astronomers who discover by algorithms, he also discovered 120 supernovae all by himself the old-fashioned hard way of looking for them (albeit with a little help from Mt. Wilson and Palomar). He once tried to reduce the air turbulence above the Mt. Wilson dome by firing a gun through the slit above the telescope. It didn’t work. He then predicted gravitational lensing, which did work. He proposed that the entire solar system could be moved to Alpha Centauri by firing thermonuclear explosions into the sun to induce asymmetrical fusion, pushing us to Alpha Cen in 2500 years. No one put up money to give it a whirl. Then he proposed to create earth-made meteors by firing pellets downward from an Aerobee rocket at the peak of its ascent, some 60 miles up. He got money for that one, and it worked; the meteors could be seen at Mr. Palomar. He said he would believe in a God if Genesis had begun with, “Let there be electromagnetism.” He was the best free entertainment the astronomical community ever produced. And also the most prophetic: Except for gravitational waves, everything we know about the universe comes to us via electromagnetic waves.
Leo A would have delighted Zwicky had he known what we know. “Dunkle Materie” was his phrase for dark matter, which can as well mean “shadow matter” as “dark matter”. Shadowy indeed it is. Leo A’s mass is calculated at 80 million solar masses, of which only 4 million is stars and gas. The other 95% is murked in shadow. Nowadays, a 20-to-1 DM/BM ratio is not an unusual proportion of DM to bar*****c; some dwarfs like Segue I and KKs3 run to 4,000 DM masses to 1 bar*****c mass. Leo A is one of the most isolated galaxies in the Local Group. It exhibits no hallmarks of an interaction or merger across its many billions of years. It is nearly unique among irregular galaxies in that it formed only 10% of its stars in an initial burst more than 12 billion years ago. Then it went silent for 8 billion years, with very little star formation. Four billion years ago it abruptly came to life and fulminated into existence all the rest of its stars. No other dwarf has written such a late-bloomer’s biography. The question is how could a galaxy with such a low DM-to-BM ratio hold on to its hydrogen mass for so long, since there was no external shock to compress it to starburst densities and only dark matter to hold it in? The mystery is deepened by Leo A’s internal structure. Its neutral hydrogen occupies a volume similar to its optical extent, but is distributed in a squashed, uneven ring while the starry regions are clumpy, random aggregations that have no unit-body rotation. Leo A is built like a fruitcake instead of a spongecake like other dwarfs. The proportion of elements with higher atomic numbers than helium is only about 1-2% of the ratio in the Sun (i.e., [Fe/H] = –1.72). This points to an sputtery, inefficient conversion of gas into stars, but the cause is so far unexplained. Most disconcerting, Leo A’s sharp increase of star formation across the last 4 billion years dwindled to a halt, but the galaxy still has four H II regions powered by short-lived, O-class stars.
The faint arrow in my eyepiece gave no hint of this. Instead, it pointed to what fun the universe can be if we don’t take our place in it as the target. Gotcha # 3 for the night.
Carina Dwarf
One needs a lot of practice on faint, extended dwarfs before tackling Carina or Sextans. The Carina Dwarf, is a markedly tenuous enhancement of stars mag 18 and fainter. It is only 2.5° from Canopus so is best seen from southern mid-latitudes. It demands the most transparent, dark, non-LP skies you can find. An extended object like Carina with a surface brightness of 22.8 mag per sq. arcsec can be detected when one knows exactly where it is and what it should look like. It is a ne plus ultra observation for a 7-inch telescope. My 180mm scopes can reach mag 15.2 and the 8-inch Mak-Newt 15.6, yet I’ve glimpsed it numerous times over the past two years using a Santel 180mm f/10 and Intes 715. Divide the Sculptor Dwarf’s eyepiece impression by 10 and the Ursa Minor Dwarf’s by 5 and you get the idea. The stellar sparsity of ultradwarfs are such that one looks for a subtle overdensity instead of a tangible surface brightness. As with the Sculptor Dwarf, I’ve often gotten the feeling that I’m seeing a granular net rather than a stellar enhancement. RGB reality says that Carina is a sprinkle of mag <18 pinpoints popping phantom-like up to mag 15 as scintillation effects make air lenses that come and go in a second or two. All this takes much observing patience. A couple of phorescent glows across five minutes is all one can expect. But when they do drop their dark veils, they are THERE. Then poof, justlikethat, gone. It’s like the scent of a flower you’ve only seen once but whose memory has never left you.
The Carina Dwarf is quite large in the sky at 23’ x 15’, just under half the size of the full Moon. It lies around 300 000 light-years from Earth, which places it half again further than the Magellanic Clouds (the nearest galaxies to the Milky Way), but significantly closer to us than the Andromeda Galaxy. Carina is so dim and diffuse that astronomers only discovered it in the 1970s on plates taken by the UK Schmidt Survey of Southern Skies. Dwarf spheroidals like Carina are very common in the Universe, but they are difficult to observe. Their faintness and low stellar density make it easy to see right through them. In the attached image, the Carina Dwarf presents as many faint stars scattered across most of the central part of the picture. It is hard to distinguish between stars from the dwarf galaxy, foreground stars within the Milky Way, and faraway galaxies that quaver through the gaps: the Carina Dwarf is a master of cosmic camouflage.
Carina fascinates astronomers for many reasons. Its stars show an unusual extended gradient of age groups, with starform epochs at main sequence turnoff (MSTO) peaks of >10, 7, and 2 Gyr, accompanied by inexplicable enhancements of metals typical not from normal stellar evolution but rather gas picked up during orbits through the MW middle halo and the LMC’s outer halo. These populations formed in rapid bursts, with quiet periods lasting several billion years in between. Carina would pass through virtually empty space for very long periods and then blunder into dense, huge gas masses called CHVCs or cosmic high-velocity clouds, which compressed matter throughout Carina until star formation blossomed all across the galaxy instead of the typical starburst geography of either all in the core or all in the halo. Dwarfs are loosely categorized as outside-in or inside-out star formers; Carina is an unusual example of all-over, all-at-once starburst. (For more on Carina’s starform history, try these: 1 & 2.) While dwarf galaxies have only modest amounts of visible stars and gas, they have vast dark matter wells. This means they can bind stars much further out than one expects from an object of their bar*****c mass. Carina’s extended star halo reaches 2° out from its 23 x 15 arcmin half-light radius (the part we see). Even more astonishing, astronomers discovered that to catalog Carina’s age and metallicity bins they had to first decontaminate it of stars not only from our galaxy’s foreground stars but also stars belonging to the LMC over 20° away. The LMC is big, as we’ve all seen from the scads of amateur astrophotos of it on IIS and Astrobin, but the fact that it has bound stars five times further away than its visible patch is startling evidence of just how powerful a force dark matter can be.
Or you can just bask in the look of the thing and leave it at that. Peering through the starscape of a faraway heaven, seeing Carina is like living a dream you were always told was an allegory.
Sextans Dwarf
This all but unseeable object is seldom reported by an amateur. It’s very difficult in the visual waveband because of its large surface area and thinly spread stellar content. Its total visual luminosity of mag 10.4 is spread over a 30 x 12 arcmin lozenge making its surface brightness 23.5 mags per sq. arcsec. It’s like looking at a quarter-moon sized fragment of the vapourous inter-arm region between us and the outer Perseus Arm as seen between M46-47 and the Rosette Nebula. Inter-arm regions between spiral arms have very little molecular cloud HII activity and emit only the lustreless mat of ±1 solar mass red clump stars left behind by disbanded clusters and filamentary debris blending into the Galactic thin disc. Our eyes can detect extended patches like dwarf galaxies or faint globulars if the s.b. is greater than mag 25 sq. arcsec. In the Mak-Newt the Sextans Dwarf quietly whispered its luminous overdensity against a nearly starless background. (WikiSky locates it but doesn’t show anything there.) See Plate I and Figure 1 of the discovery paper here.
Five minutes of steadily observing the field at 60x, switching eyes every minute or so, for the galaxy became apparent as a kind of backwards Horsehead in which instead of darker on dark we look for not-quite-dark on fully dark. Once the patch’s location and subtle enhancement became familiar I could re-navigate to it easily on subsequent nights. It was discovered only in 1990 by an automated machine scanning process re-examining old glass plates from the U.K. Schmidt Telescope Survey that had previously yielded up the Carina Dwarf in 1977 by the old fashioned method of eyeball and blink comparator. In the plate comparison surveys, the Carina signal-to-noise ratio was 2.6 where Sextans was 1.4 (a S/N ratio of 1.0 in this context means the object emission equals the background emission). The decontaminated (i.e., pruned of intervening field stars) stellar density on the Schmidt plates was 6.0 stars per sq. arcsec for Carina but only 2.2 stars per sq. arcsec for Sextans. The Sextans dwarf held the record of the intrinsically faintest galaxy from 1990 till 2006 when Sloan Digital Sky Survey and its multifibre spectroscopes turned up galactic weirdoes like the dwarf Segue I which has only ~1,000 stars. For we owners of more modest 8-inch scopes the Sextans dwarf aces out the Carina dwarf as the most elusive and difficult object for an observer. Too bad: Carina is in a MUCH prettier field, so even if you come away empty-fielded, the view is reward enough.
Pal 8 Sagittarius
04:15 arrived accompanied by a choir of roosters, awakening lambs, burping frogs, and a drowsing me. Pal 8 was surprisingly easy. Alvin Huey’s chart p. 104 makes it an easy locate. Follow the curved line of 4 stars that act as a pointer to Bernard’s Galaxy NGC 6488 in Sagg near Cap, but go the other way. Gauge the distance between the two stars that parallel the teapot handle, and double this distance on a line going directly away and parallel to the teapot lid toward M25. Pal 8 is brightest at about 60x; it quickly loses its lustre with magnification. Look for an unmistakeable slender reverse-S chain of stars; Pal 8 is near the bottom of the chain on the teapot side. In the Mak-Newt with a 15mm 100° eyepiece, Pal 8 presented as a significantly reddened globular, colourless to the eye but betrayed by the dimness of the core compared with its diameter. The core has a flat luminosity profile about 4 arcmins in dia. The halo falls away rapidly, less than an arcmin thick. At 120x in a 10mm 100°, Pal 8 presents as a soft, dim, granular glow with an abrupt fall-off to dark sky. It’s such a star-rich part of the sky, filled with lines, loops, curls, and filagree, that the cluster seems almost the clasp on the necklace instead of the diadem we are supposed to marvel.
Envoi
The evening ended like the film Life of Pi, when the tiger, having been pummeled to near-death by the worst terrors of the sea, disappears into the jungle without even bothering to look back. At 05:00, deep prelight arrived in the form of turquoising black in the east and Jupiter past the meridian to the west. Some say pre-dawn in the loveliest time of the day. My drooping eyelids said this wasn’t the day to find out. Three new globular firsties and four new dwarf galaxies. I think the APM Wirth-Intes Mak-Newt is going to be with me awhile.
