The Fornax Dwarf and Its Five Globulars
Part 1: Observations, Feb 21–25, 2012
By Dana De Zoysa
They’re all here. But where? (Source: ESO//Digital Sky Survey 2)
Dwarf galaxies are the smallest, least luminous, and most common galaxy systems in the universe. Of the 30-odd dwarf galaxies orbiting the Milky Way, the Fornax Dwarf Spheroidal PGC 10074 is intrinsically the brightest. But it is also 460,000 light years away, so actually seeing it is another matter. Its RA of 2h 40m places it directly beneath M77 (which neatly bisects the celestial equator at -00°00’48”). But its declination of -34° 27’ puts the Dwarf rather low for many northern observers—in the same declination band of Scorpius’s sting in summer and 10° more southerly than Lepus and Canis Major in winter. For me, though, Fornax is nearly overhead in November, and very accessible from October through early March. I observe from Weltevreden Farm, 18 km west of Nieu-Bethesda in the Great Karoo of South Africa. The climate is semi-desert and looks much like the American Southwest. NELMs are commonly 7.5, transparency 8 out of 10. My equipment is a two-saddle Alt-Az holding a Celestron C6 150/1500 SCT on one side and a Santel 180/1800 MCT on the other. These observations were made in mid-February, when the Dwarf was still 40° to 45° elevation above my western horizon at astronomical dark.
Harlow Shapley discovered the Dwarf nearly 75 years ago on glass photographic plates using the 24 inch reflector at Boyden Observatory in South Africa. 1938 was a good year for Shapley: he also discovered the Sculptor Dwarf Irregular galaxy using the same sets of plates. But it was a bad year for the classical Hubble “tuning fork” classification system for galaxies because these two galaxies fit nowhere on the tuning-fork branches.
Shapley noticed three globular clusters which he felt to be associated with the galaxy. These were confirmed and studied 1939 by Walter Baade and Edwin Hubble using the 100-inch Hooker reflector at Mt. Wilson. But due to its southerliness for the big North American telescopes, nobody paid much attention to the region till 1957—the year of both Sputnik and the introduction of Boeing 707s—when astronomer Paul W. Hodge set off “going like a Boeing,” to use the picturesque South African expression, to “Boyden in Bloemi” (Bleomfontein) to add two more globulars to the three then known. The five globulars ranged in visual magnitude from 12.6 to 13.9.
You can see Paul Hodge’s 1958 75-minute unfiltered blue/visual ADH plate taken in South Africa here. Hodge named them Hodge 1 through 5, in keeping with the nomenclature practice of the time, which associated discoverers of clusters and H2 regions with the larger objects of which the clusters were a part. Hodge was a busy man in the early 1960s. He and George Wallerstein founded the Astronomy Department at the University of Washington in 1965; Hodge is still there. Over the years his name appeared on at least 200 technical papers listed on the SAŘNASA Astrophysics Database. A great many other objects bore a Hodge # identity. A 1977 paper he authored about Barnard's Galaxy identified 16 star forming H II regions there, designated Hodge 1 to Hodge 16. M31 boasts 140 globular clusters and 413 open clusters bearing Hodge numbers, though many of the globulars were first charted by Edwin Hubble as far back as 1932 and described in his 1936 book Realm of the Nebulae. The Hodge system worked for its time, much to the dismay of catalogers and bibliographers of our time, who prefer tidier arrangements. But in honor of the man and his accomplishments, I’m going to keep to Hodge’s nomenclature system for the Fornax Dwarf globulars, duly noting the ESO designations for readers whose go-to paddles and handheld-device apps have fits when you enter the name “Hodge.”
Hodge’s Fornax five became a challenge for amateur and professional astronomers alike. Professionals wanted to know why the Fornax Dwarf was the only dwarf spheroidal galaxy then known to contain a globular cluster system. In the prevailing knowledge of the time, the Dwarf was the least massive galaxy known to have globulars and should have been too much of a lightweight to acquire or hold on to them. Why didn’t the globulars migrate to the center of the galaxy to boost its density profile? By 2005 a vast gob of dark matter was adduced to have been associated with the Dwarf’s unusual star-formation history. That solved the “digestion” problem, but it didn’t settle a lot of other issues. Which is why an arXiv.org search comes up with 243 technical papers associated with the Dwarf. Save these for a really rainy night when a game of Solitaire is a bit rich for your blood.
For amateurs the Fornax five are worth going after just because they are there. It’s not often one can see five faint, difficult globulars in the same field as a faint, difficult galaxy. The Fornax Dwarf clan is all the more worthy for anyone who wants to sharpen their spotting skills before going after the M31 globulars, whose brightest member G1 or Mayall II is half a magnitude fainter than the 13.9 vm of Fornax’s faintest globular, Hodge 1.
Aside from the challenge of just seeing them, what else is interesting about the Dwarf’s globulars? Five globulars is a large number for such a low-mass galaxy. They are a laboratory for comparing their physical and chemical evolutions with those of our own Milky Way and also the Magellanic galaxies. The histories of the Fornax globulars are complex. They average two to three billion years younger than the Milky Way globular cluster population, and their metal abundances vary by an order of magnitude. Lighter metals such as the oxygen, nitrogen, and carbon you’re breathing as you read this, are forged in stars with masses between 0.8 and 8 times that of the Sun when these stars evolve into red giants, blow their atmospheres off into space, and end as white dwarfs. Heavier metals like the iron which we think of when we use the word “metals,” are forged in core-collapse supernovas. There are significant metallicity differences between the Fornax Dwarf’s globulars and the Dwarf’s own stars. How does one account for a large spread in metal abundances in objects of seemingly uneventful origins?
First, let’s find them. Alvin Huey’s The Local Group (p.51) makes hunting much easier for Fornax fans. One can hop from the two brightest stars on Huey’s Mega Star-derived chart (named Lambda Fornacis 1 and 2 on some charts) directly to the Dwarf’s largest and brightest globular, NGC 1049. Despite its listed magnitude of 12.6, 1049 is not a quick hit in my Celestron C6 ‘finder’ at 50x and 1°36’ tfov. I need at least 100x to ID it. It helps that 1049 lies in an area with few distractingly bright stars—the nearest one is mag 8.4 SAO 193841 (HD 16690) 15' NNE. At first glimpse 1049 has the imprecise look of a star on a night of bad seeing. A close inspection reveals a dot 40” in diameter. 1049’s surface brightness is nearly half a magnitude brighter, 12.2/sq arcmin (21.1/sq arcsec). For some reason 1049 always seems faintly blue to me, resembling Neptune but four magnitudes fainter. It’s a curious illusion.
One reason for 1049’s initial soft look is that it is a Class X globular whose appearance to my eye resembles a starry form of solar rim-darkening. (The I - XII system was devised in 1927 by Harlow Shapley and Helen Sawyer.) South African observer Auke Slotegraaf used a 12 f/4.8 Dob near South Africa’s professional astronomy centre at Sutherland to note, “At 120x it is a small, very faint, haze with a very bright, starlike, nucleus. It was discovered in 1825 by John Herschel from Cape Town. He recorded it as ‘pretty bright; small; round; like a star 12th magnitude a very little rubbed at the edges, a curious little object and easily mistaken for a star, which, however, it certainly is not’." Australian observer Andrew Murrell’s 2007 IAAC report using a 20" dob described 1049 as a 40" patch with a bright almost stellar nucleus. “The main body of the cluster has an even surface brightness . . . . well detached from the background sky. At 300x it . . . looked like a faint globular when viewed through a small telescope.” Steve Gottlieb reported on it here, and also in a 30 October 2010 note on the Australian forum IceInSpace he reported Fornax 3 as “a moderately bright gc in the Fornax Dwarf. Appears small, round, ~30" diameter, gradually increases to a small brighter core.” An observer using Australia’s Coonabarabran Observatory 30” at 264x described 1049 as “very bright, moderately large, very sharply concentrated with a very small, very bright core surrounded by 1' halo that dims around the periphery.” If the Coonabarabran skies are especially good an observer with a 30” telescope might resolve a few of 1049’s four magnitude 18.4 red giants. (Note for Jimi Lowrey and his 48” Tyrannostarus Rexas [T*xas?], 1049’s 6th brightest is 19.7. Go for it!) The rest of us who log 1049 must console ourselves that we have seen a globular 460,000 ly out, nearly twice as remote as NGC 2419 the “Intergalactic Tramp” in Lynx at 280,000 ly. (In the 1950s & 60s, what we today call the “remote halo globulars” were collectively called “the intergalactic tramp globulars”; see Hodge’s 1961 paper, p.84, 1st paragraph.)
That’s as close as 1049 ever gets: a 2010 paper by Méndez et al. calculates that the entire Fornax Dwarf system is presently near its 460,000 light year apogalacticon point in a near-circular orbit (eccentricity 0.13 or roughly one-eighth thinner in the minor axis) that takes about 3.2 billion years and never brings it closer than 400,000 light years. The Méndez team found a handy cosmologically-distant quasar behind the Fornax Dwarf whose light is preferentially absorbed by the Dwarf, giving spectrographers all kinds of useful absorption features to fiddle with. (Quasars—what would we ever do without them?) The Dwarf’s orbit is also retrograde and highly inclined (approx. 101°) to the Milky Way’s disk. That its locale is so remote from the MW center, plus its near-circular, highly inclined orbit, have important ramifications in understanding how the Dwarf was formed and has evolved.
Despite the fact that the Dwarf visually presents as a faint ellipse, it is technically a dwarf spheroidal. Until the Andromeda Galaxy arrives in 3 billion years or so, the Fornax Dwarf will escape the assimilation fated to the Sagittarius Dwarf. While the shredded Sagg Dwarf’s gift to amateur astronomers was the globulars M54 in Sagittarius and Pal 12 in Capricornus, we won’t be getting any going-out-of-business globulars from the Fornax Dwarf anytime soon. The near-circularity of its orbit shields the Fornax Dwarf from most but not all of the tidal forces of crossing the Milky Way plane and having its star-forming gas stripped away by ram pressure as it plows through the Milky Way’s gas halo. Not that it makes much difference—the Dwarf has only 5,000 solar masses of star-forming gas left anyway. In the Milky Way that would make it a fairly peewee Barnard object.
To the naked-eye, the constellation Fornax can be an empty place. (The Hubble Ultra Deep Field was recorded close by, near NGC 1360.) I started my search with NGC 1049 because the Dwarf itself is spread out (roughly half the diameter of the moon as seen in my 180mm aperture) and faint at a mean surface brightness of mag 17.0. I assumed the average surface brightnesses of 12.7/sq. arcmin (21.6/sq. arsec) of its globulars would be an easier find. Also, the Dwarf’s star density is 9 per sq. arcmin down to visual magnitude 19.9, while the globulars’ visual densities are approx. 80 stars per sq. arcmin, though they average 0.6 mag fainter than the galaxy’s. Find the globulars first, my reasoning went, and the galaxy would be somewhere in the middle.
Visual astronomy is full of surprises. To paraphrase the humorist Woody Allen: “If you want to give the stars a laugh, tell them your plans.” After half an hour of obsessing over globulars at 100x to 269x in the 180mm Mak, I rechecked the 50x field in the C6 SCT and belatedly noticed a faint but unmistakable brightness in the sky covering a third of the field of the eyepiece. I hadn’t paid much attention to it because at the magnifications needed to locate the globulars, the glow seemed nothing more than a half-dozen stars in the mag 11 to 13 range in between a couple of 13th mag fuzzy dots. The two fuzzy dots turned out to be fuzzy for a good reason, but only with a larger exit pupil did the big hazy galaxy loom behind that little twinkly asterism. I felt a little like the “Pink Panther” scene where Inspector Clouseau notes down every detail of the crime scene except the presence of the gun on the table. I also gained more empathy for Messier’s “mistake” in confusing a close double for a nebula because information he received from Helvetius indicated he had observed a nebula there. My mistake was exactly the opposite, misidentifying a faint glow as a group of faint stars.
It was brighter and smaller to my eye than Andrew Murrell’s description, “a faint glow just above the sky background covering an area of more than 50'.” As measured against the 1°20’ and 49’ tfov in the 30mm and 18mm eyepieces of the 180 Mak, the glow was more like 20’ along the major axis. (The Deep Sky Browser M45.com gives 57.8' x 43.3' at position angle: 60°, visual mag 8.8.)
61 million solar masses, not counting the globulars. The numbers adjacent to certain field stars are their visual brightness to a tenth of a magnitude with the decimal point omitted. (Source: ESO//Digital Sky Survey 2 with labels added by the author of this article)
The Northern Hemisphere amateur forums can be a bit despairing over the Fornax Dwarf, the same way southern observers wring their hankies over Cepheus. Admittedly, 34° degrees south dec is not a great help whilst seeking an extended dwarf galaxy whose stars are vm 18 to 25. But Andrew Murrell also wrote, “One of my observing companions had a Televue Traveller scope with him and we tracked the galaxy down with this RFT. The Fornax Dwarf was visible as a faint haze.“ (Teaser: No matter what your aperture, an object being near-overhead in black skies enhances spotting prospects no end. South Africa has a beer and braai (barbie) tradition to match Australia’s, there’s always a spare room at Weltevreden (1, 2, 3, 4), and you can tote along an 8” SCT and alt-az on a 20 kg bag limit if you skimp on the wardrobe and toothpaste till you get here.)
I swept the field horizontally and vertically several times to confirm. The Dwarf was an unmistakable diffuse brightening each time. I revisited the field several times on subsequent nights with both scopes, throwing in sweeps of the nearby general fields as a check, and the verdict was constant: confirmed sighting.
The phrase “a faint brightening” seems rather a modest bowl of rice considering that the Fornax Dwarf weighs in at 61 million solar masses. Omega Centauri’s roughly 10 million stars is visible naked-eye at magnitude 3.7 (and a dec 14° further south, -47°28’). But Omega Cen is only 13,700 light years away, while the Fornax Dwarf is 33.5 times further out. If we lived 13,700 ly from the Fornax Dwarf it might rival the Large Magellanic Cloud in brightness, though not in all the exciting things going on there.
OK, two fuzzies down, four to go. Now Alvin Huey’s “The Local Group” was joined by my hard-copy edition of John C. Vickers and Alexander Wassilief’s “Deep Space CCD Atlas: South.” NGC 1049 is visual magnitude 12.6. The other four are listed as 13.4, 13.5, 13.6, and 13.9. In principle all five should be visible in 150mm apertures in clear, dark skies. Andrew Morrell’s IAAC report documents the five, but there are no reports known to me of them all being seen in single observing sessions. Hence this. The globulars are positioned as marked in the above image, which emulates the view in an SCT with diagonal at 33° South, with the Dwarf descending at 45° elevation above the western horizon.
Below are individual images in the object’s numerical order. Their surface brightnesses are brighter than their visual magnitudes. There’s no sorting through the tantalizing hints, glimpses, or glimmers the mind introduces into wide magnitude/surface brightness gaps when the eye is at the limit of its luminance threshold. Still, Hodge 1, marked on these charts as Fornax 1, is so difficult at mag 13.9 that you might want to read the following description for reference, then track down the other four globulars first to familiarize yourself with the overall field.
Hodge 1 aka Fornax 1 and ESO 355-SC029, is RA 02 37 02 dec -34 11 00 and is the most difficult. It is only 1.2’ dia. You also may want to check the DSS view of Fornax 1 (patience, it takes time to load). Lucky you if it ever looks like this in your telescope! Hodge’s 1961 paper, based on visual plates, not spectrographic data, states, “No. 1 is unusually open and poor in stars. It has a very irregular appearance, but is quite certainly a globular cluster, for its brightest stars are similar in magnitude and color to those of the other four globulars.”
Andrew Murrell describes Hodge 1 as “a very low surface brightness and the most challenging of the 5 main clusters. The cluster had a diameter of about 1' and had no central brightening. The low surface brightness made this a difficult cluster to locate without a chart. It was best viewed at 300x, where it could be seen easily with direct vision. This cluster lies to the NNW of the Fornax Dwarf.” To my 180mm eye it was so faint and diffuse that I detected it only momentarily three times across a cumulative half-hour looking at the exact spot where I knew it to be. As a tough object for a 180mm scope in good skies it compares to NGC 4730, one of the Centaurus Cluster galaxies at visual 14.1 (sb 13.8), or for northern observers using a 150mm scope, NGC 6675 near Vega in Lyra at mag 13.3 sb 13.9.
Hodge 1’s visual magnitude of 13.9 emanates from a spare 37,000 solar masses. (See the table at the top of p.2 of Angus & Diaferio’s paper for solar mass and orbital details for all of the Dwarf’s globulars.) That seems a modest mass for a globular. Indeed, it is exceeded in the flyweight department mainly by Pal 5, which has been tidally stripped till a mere 10,000 stars are left. Testifying to the virtues of life in a galactic exurb, Fornax 1 has been coherent as a single body for over 10 billion years. It is the remotest of all the Dwarf’s globulars at 1.6 kparsecs (5,200 ly) out—though its orbital details are still sketchy. Small-mass globulars remote from periodic galaxy crossings can retain structural integrity on Hubble-time scales of 13.8 billion years. For the Fornax Dwarf, that is three “perigalacticons” (the technical term for an orbiting galaxy’s closest approach to a larger one; “apogalacticon” happens at the far end; the sun happens to be near its perigalacticon now). Even as remote as it is, each perigalacticon subjects the Dwarf and its entourage to a certain amount of ram-stripping of gas (analogous to a hurricane leaving a vacuum behind) and to the gravitational analog of a tidal wave encountering an island, sweeping its sand away as the wave recedes. All this trial and tribulation is reversed at apogalcticon, in whose distant reaches the hot gas generated by earlier orbital compression and supernova shock fronts gradually radiates its high-energy photons away, cools, infalls to the galaxy’s center, and awaits the next perigalacticon to initiate another billion-year starburst cycle. Three energy purges and it’s no wonder things are so quiet out there in Fornax land.
It wasn’t always like that. While globulars form early and enjoy mostly graceful retirements, dwarf spheroidals can form later and undergo dramatic star formation histories. The similar “star ball” appearance of globulars and dwarf spheroidals belies composition and histories that are very different. All dwarf spheroidals, like globulars, contain a population of ancient stars, but most dwarf spheroidals have undergone one or more starburst episodes. Also, almost all new dwarf spheroidal star formation occurs at the center. The timing and intensity of the formation periods is traced by the chemical enrichment of their various star populations. Primordial hydrogen and helium gas clouds collapsed to form stars. Many evolved modestly, like the sun. Early solar-mass stars went through the planetary-nebula/white dwarf phase, while heavyweights went supernova. These happened so long ago the only traces are a richer array of heavier elements. These enriched clouds in turn were swept up into new cycles of star formation and subsequent enrichment. Stefania Salvadori et al. proposed a time frame of about 250 million years for a typical enrichment cycle. (Also see these papers: 1, 2, 3.)
These ancient star forming episodes show up today in the Fornax Dwarf as absorption dips in high-resolution spectrograms. The Dwarf itself is large enough that over 2,000 of its 47,000 photometrically logged stars can be sifted by hundredths of a magnitude and arc-second into orbital, age, and color-magnitude bins. The five globulars are a different story. High-resolution spectroscopy of such tiny, faint objects requires long exposure times in pricey 6-meter class telescopes hosting multiobject spectrographs. These transport photons via fibre-optic light threads instead of the pixel-based devices used by we folks lacking 6-meter budgets. But like pixel data, fibre optic data is degraded too, by crowding, source confusion, signal-to-noise ratios, and fiber crossings. These effects are as frustrating as bad seeing. The FLAMES instrument on the Very Large Telescope in Chile required four hours to obtain individual Fornax globular spectra to visual magnitude 18.5. That’s a lot of effort for a handful of stars from a globular 460,000 ly out.
Another method commonly used to determine object distances—proper motion studies—poses even more problems. An object with a transverse velocity of 100 km a second (roughly three times as fast as the Fornax globulars appear to move) at a distance of 325,000 light years traces a proper motion of 0.2 milli-arc-seconds per year. That comes to 0.03 pixels on the Hubble’s Advanced Camera for Surveys over the 15 year baseline during which the cluster data has been recorded (scroll through to the article’s section on the Leo I and II dwarf galaxies). Costly and arduous though it may be to acquire, the data reveal Fornax’s five globulars to be structurally simple compared with the galaxy itself. There are multiple gradients of carbon and intermediate-age stars in the galaxy, for example, but none in the globulars.
We can get an idea of what Fornax looked like to the Dwarflings residing there 3.5 billion years ago by panning across the northern reaches of the Large Magellanic Cloud, florid with pink supernova bubbles and magnetically twisted gas clouds. In a hundred million years these will be but traces. We would see a mass of new H2 and OIII bubbles much like the existing ones but richer in heavy elements. If these filaments and bubbles look disturbing, they are. Because of them, only a quarter or less of a collapsing gas cloud ends up in stars; the rest is enriched and redistributed; enriched and blown away; or enriched and turned into stars. (And we think OUR taxes get turned into a lot of hot air!)
While one marvels how objects like NGC 6791, the 6.9 billion-year-old open cluster in Lyra, can hang on for so imponderably long while porpoising above and beneath the gyres and eddies of the Milky Way, the answer shows up in 6971’s color-magnitude diagram: two distinct billion-year-long copious gestations of white dwarfs. In the case of the Fornax Dwarf’s globulars, the Angus & Diaferio paper analyzes several orbital models to find one that fits their known positions and orbital eccentricities, yet also supports the current information that their orbits are stable over Hubble time.
Circularization of elliptical orbits in consequence of dark matter dampening. (Source: Angus & Diaferio, p.6 bottom right)
The best-fit model results in a rosette-like orbital eccentricity pattern. Note the number of orbits concentrated in the lobate donut 1.5 to 2 kpc (4875 to 6500 ly) from the center. This is a tidal-radius orbital pattern that circularizes an eccentric orbit. (A tidal radius is the outer limit of a cluster’s gravitational potential energy well, outside of which its stars can be pulled away by more massive objects like a galaxy.) De-ellipticization implies the presence of a very large mass of dark matter centered on the Dwarf, whose gravitational well is sufficient to circularize orbits but not great enough to decay the orbits into the center.
Hodge 2, aka Fornax 2 and ESO 356-SC001, at RA 02 38 44 dec -34 48 33, stands out easily from its star field. The Harris Globular Catalog classifies it as a Class VIII globular (see section III of the catalog) 1.2’ dia with a visual magnitude of 13.5 and a surface brightness of 13.6. A mag 12.5 field star near its core improves your chances. Andrew Murrell writes, “Fornax 2 Is a 1' haze of even surface brightness much brighter than cluster 1. It was easily visible and well defined against the background sky. The cluster was seen through a 12" scope with direct vision and could be seen with averted vision in a 10". A 15th magnitude star lies just off the SW edge of the cluster. Cluster 2 lies to the SW of the Dwarf.” My notes state, “Forms a near-right triangle with the bright, close 40” pair of stars TYC 7014-766-1/2 of mag 10.6/12.1 (the close pair in the annotated image) and TYC 7014-50-1 at 11.5; easily found; seen direct vision throughout this logging; presents a faint, tight, hard-edged core and weak-edged outer halo, lacks the blue cast of N1049; seems rather loosely packed for a Class 8; looks like NGC 7006 Delph in a 80mm at 125x.” Hodge 2 radiates the light of 182,000 solar masses and orbits 1.05 kpsc (3410 ly) from the Dwarf at a rate of roughly 4 kms/second.
Hodge 3 aka ESO 356-SC003, Fornax 3 is NGC 1049, the globular most Fornax ferreters find easiest. It lies RA 02 39 48 dec -34 15 30. It is a tiny globular, only 0.8’ dia., yet is the brightest of the Fornax Five (thanks to 363,000 solar masses) at visual magnitude 12.6 and surface brightness 12.2. It is bright enough to feature in Northern Hemisphere observing reports using telescopes upward of 5.5 inches. Steve Gottlieb’s observing notes on NGC/IC mention, “Located 15' NNE of mag 8.0 SAO 193841”, which is out of the field in this 10’x10’ image. The brighter star adjacent to NGC 1049 at 6:30 o’clock in this image is mag 14.15 and the fainter, closer star at 7:00 is 16.2. Australian Andrew Murrell reported, “Appears as a 40" patch with a bright almost stellar nucleus. The main body of the cluster has an even surface brightness. . . . The cluster was well detached from the background sky. It was best viewed at 300x where it even looked like a faint globular when viewed through a small telescope.” My own notes added: “Once spotted at 100x, 1049’s appearance barely changed while ascending the magnification scale to 369x. As the sky background darkened in response to magnification, the bluish cast became more pronounced, till at 369x it looked like Neptune. The core seemed to become denser and the halo thinner as the power went up.”
We’re going to skip over the youthful and problematic Fornax 4 for a moment and deal with the more straightforward and easily findable Hodge 5.
Hodge 5, AKA ESO 356-SC008, Fornax 5 is at RA 02 42 21, Dec -34 06 05. This cluster is located away from the main body of the galaxy toward the North East. It is an easy star hop from the center of the Dwarf—look for the line of 4 stars marked “Pointers” on the annotated chart. The same diameter as 1049 but 0.8 magnitudes fainter at 13.4 (12.7 surface brightness), it radiates the luminosity of 178,000 solar masses. It is density Class III, with a tight core and faint, thin halo. With a mean distance of 4645 ly from the Dwarf’s center of mass and a moderately elliptical orbit with a perigee of 3900 ly, it whizzes along at the breathtaking (by Fornax Dwarf standards, anyway) speed of 34 km/second. To data-record anything more takes sophisticated equipment and photometry skills. Andrew Murrell writes, “Appears as the smallest of the clusters at an apparent diameter of just 30". My log states, “369x: N1049 lite, fainter nucleus, thinner halo.”
To professionals, Fornax 4 is the most interesting of the lot.
Hodge 4, AKA ESO 356-SC005 and Fornax 4 lies at RA 02 40 07, Dec -34 32 15 and is a tight Class IV globular 0.6’ dia. with a visual mag of 13.6, sb 12.1 /sq. armin (20.1 arcsec). Fornax 4’s high luminosity-to-size ratio makes for a visually dense core with fainter halo. Andrew Murrell confirms: “Lies just a few arc minutes SW of the 9th magnitude star [off-field on above image]. This cluster appears very similar to but fainter than NGC 1049 with the bright nucleus surrounded by an even surface brightness halo. It has a 15th magnitude star just off the northern edge of the cluster.” But while Fornax 1, 2, 3, and 5 are by-the-book color-magnitude globulars, Fornax 4 has astronomers busily writing papers. Sydney Van Den Bergh and Buonanno et al. (1999) used the Hubble Telescope to obtain Fornax 4’s color-magnitude diagram. Where 1, 2, 3, and 5 have horizontal branches that include a wide range of colors and many of the RR Lyrae variables used to evaluate globular distances, Fornax 4 combines a clumpy red giant horizontal branch with a narrower range of other colors. Its red branch stars are still evolving towards the planetary nebula/white dwarf stage, which the other Fornax globulars passed through billions of years ago. This and orbital path data puts Fornax 4 at three billion years younger than the other globulars.
More interesting, Fornax 4 formed in the center of the Dwarf, not out in the halo where dwarf galaxy globulars usually form coevally with the parent galaxy. Fornax is not alone in this, a number of other dwarf spheroidals have the same property. Van Den Bergh’s paper asked why is it that dwarf spheroidal galaxies can form globular clusters in the central regions long after globular formation has ceased in the galaxy’s halo. Buonanno’s paper asked how a globular cluster can form at all in the center of a low-mass galaxy that evolved billions of years earlier and shouldn’t have enough star-forming gas left. Where did the gas come from? Did the Fornax Dwarf somehow manage to accrete an even dwarfier dwarf? It’s not a far-fetched notion—the dwarf irregular NGC 4449 in Canes Venatici has elongated a nearby subdwarf (memorably named a Hobbit galaxy) into a stellar spindle and is now eating it like a noodle (see p.3 of the Martinez-Delgado paper).
“Zut alors!” exclaims Inspector Clouseau, “I smell something odd in this room. It smells like . . . smoke!”
Aaahh, is the Inspector on to something? The Fornax Dwarf system is a visual challenge. The thrill of seeing all five globulars dotted across the faint main galaxy is like seeing a rich Abell Cluster for the first time. Yet simply seeing it is a little like Inspector Clouseau noting everything at the crime scene except the evidence. Like the good Inspector, professional astronomers are devoting increasing telescope and computer modeling time to the Dwarf, and keep coming up with new questions as they answer old ones. Until recently the Fornax picture comprised two major star formation epochs, each consisting of several episodes. The first epoch, from metal-poor primordial gas, was coeval with the birth of globulars 1, 2, 3, and 5, ten billion years ago. Then a slow decline in star formation. Between 7 and 2.5 billion years ago Fornax formed the bulk of its present stellar populations from the gas enriched by the earliest stars. Star formation has continued at a declining rate in the central regions until as recently as 100 million years ago—and some reports now say 10 million years.
Hmmm, Inspector, is that a gun on the table?
“Zut alors!” Clouseau exclaims, “I smell le smoke! I see le gun. But what is ze connexion between les two? We must look for ze 2nd section of zis report!”
References & Copyrights:
ESO//Digital Sky Survey 2 copyright & fair-use notice here.
Hodge, Paul W., “The Fornax Dwarf Galaxy. I. The Globular Clusters,” Astronomical Journal, 66, 83-84 (1961). [This research made possible by NASA's Astrophysics Data System Bibliographic Services.]
NASA’s DataScope or Virtual Astronomical Observatory is hoisted by the Astrophysics Science Division and the High Energy Astrophysics Science Archive Research Center (HEASARC) at NASA/ GSFC under Cooperative Agreement AST0122449 with the Johns Hopkins University.
G.W. Angus & A. Diaferio, “Resolving the timing problem of the globular clusters orbiting the Fornax dwarf galaxy,” Mon. Not. R. Astron. Soc., 16 March 2009.
Further readings: arXiv, IOP Science, Scholarly & Archival papers