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. It demands the most transparent, dark, non-LP skies you can find. An extended object 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 are there, 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 baryonic 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 only 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 slowly revealed itself as a kind of backwards Horsehead in which instead of darker on dark we look for pretty-dark on really-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.
