IC 1613 is the smoking gun on the table, but there’s no body on the floor. In fact, there’s no crime scene. But there is a mystery. After 11 billion years a nearby gas-starved galaxy has erupted into life. Several ten thousands of stars have popped into being apparently out of nowhere, and for no reason.
Source: DSS DR7, enhanced by me to bring out colours & structure.
It’s an oddball detective story when the victim and the perp mosey off into the sunrise. But right there in our eyepieces, IC 1613 sports a brand-new population — and a big one at that; we can easily see it even though it’s 2.38 million light years away. Can a galaxy just whisper “Presto-Chango” and thousands of stars pop out of empty space? Let’s take a look for ourselves.
Spotting this dwarf galaxy’s soft haze in my 6 to 8 inch scopes is one of the gentler charms of spring in the southern world. October and November are Dwarf High Season. In one gigantic arc lie Barnard’s Galaxy NGC 6822, IC 5152 Indus, Phoenix, Carina, Sculptor, Fornax, and the M31 Andromeda halo dwarfs Pegasus and Andromeda VI. (Just out of range for me, though, are NGC 147 and 185. Snif.)
Taken with tea at 02:00, IC 1613 is a frosted muffin to the eye while awaiting Sextans A and B to arrive amid first light. Eight to 10x per aperture inch is about right for many dwarf searches, and IC 1613 does not disappoint. Its muffin part is a soft elliptical phorescence whose edges ease into the core as though they are pushing up a pie plate — the characteristic luminosity profile of a dwarf spheroidal galaxy. The frosting part is a 5.5 arcmin NW patchwork of ill-defined glows that remind me of Stephan’s Quintet.
In an 8-inch Mak-Newt at 90x to 181x the thing blossoms. It looks every bit the dwarf irregular the catalogs classify it to be. The oval patch adjacent to the magnitude 11.1 star TYC 19-603-1 hints at a mottled surface density with hints of an east-west extinction band ¼ the width of the oval. The ellipsoid’s edges seem a bit ragged at times, but this may be a scintillation effect. Big-glass owners might have a look to confirm or deny my 8-inch impression of ragged edges and an extinction lane; for reference there’s a mag 16.4 star nearly centered in the spheroid.
At 180x the mottled patch 5.5 arcmin NE is very faint but still reveals an elongated quincunx (looks like the 5-dot side on dice) of hazy stellarings that resemble the Quintet all the more. The central and easternmost of the five patches resemble a ring galaxy with a core and detached halo. The other three patches look like 13th magnitude ellipticals. These effects are vexingly subtle and I won’t allege them as confirms in an 8-inch scope. Overall, the impression is a thin soup of an elliptical galaxy with a nearby irregular companion.
Up until October 11 this year, there were an estimated 350 billion large luminous galaxies in the visible universe. That is perhaps the most malleable number in astronomy today. On that day a team led by Christopher Conselice of Nottingham University published a paper that announced the number of luminous galaxies in the universe is ~10 times previous estimates. Conselice’s 2016 paper concludes, “We calculate that the cosmic variance on our measured counts is . . . Ntot = 2.47 x 1011 [247 billion], which is our final estimate of the number of galaxies we can observe today with current technology at HUDF depths in the universe.”
Anyone willing to lay odds on this being the last word on the matter?
Given current luminous-to-dwarf galaxy ratios in the Local Volume, Conselice’s number scales to 7 trillion dwarf galaxies in the same universe. IC 1613 is one of them. Its current classification as a dwarf irregular barred with spiral features (IB(s)m) is a chronological accident. For 99.997% of its ~11 billion-year life it was a quiet milquetoast of a dwarf spheroidal galaxy, minding its own business far far removed from anything else in the Local Group (LG). It lies in near-isolation, 755 kpc from the Milky Way, 758 kpc from M31 the Andromeda Galaxy, and 517 kpc from the LG barycentre. Its nearest neighbours are the tiny Pegasus, Cetus, Aquarius, and Antlia dwarfs. IC 1613 is hundreds of thousands of light years removed from any other significant mass that could interact with it. It is in such an immense volume of nearly empty space that it has been completely alone out there its entire life.
Without any neighbours to interfere with it, IC 1613 had evolved as a dwarf spheroidal galaxy, sipping slowly on its modest reservoir of birth gas, steadily turning the gas into stars like dropping pennies into a piggy bank at the rate of several per million years. This adds up if you’re 11 billion years old. All the while the rest of the Local Group went through dramas high and low, hot and cold, dense and vacuous — all conducted with the dining finesse of a school of sharks. Set the universe’s video projector at one frame per million years and run it at DVD speed, and our sedate local cosmology would look more like Mad Max than the present serenity shimmering across our nights.
IC 1613 is approaching us at ~254 km sec-1. The fact that it is approaching rather than receding from us suggests that internal motions in a galaxy group are much larger than the ~70 km sec-1 Mpc-1 bulk flow of the expanding universe. Could those unseen internal motions signal there is something lurking in our empty quarters of the sky that if it gets irritated enough is big and brusque enough to outshine a galaxy?
Ask IC 1613. About 30 million years ago, and for no apparent reason, it had a rude introduction to the messy side of life in the Local Group. It exploded (literally) into new life with a starburst far out in its hitherto placid suburbs. The quincunx we see in our scopes is actually a fluffy fizz of bubble-like shells, most with a hot young star cluster in their centres.
This inexplicable starburst* in its NE halo has since grown in size and complexity until today the fading old spheroid is escorted by two generations of brand-new stars grouped in an agglomeration of contiguous bubbles whose overall surface pattern suggests three immense blobs of near-emptiness larger than the galaxy itself.
* The term “starburst” is an ill-defined farrago of equations and properties useful for theorizing but not the imagination. For one, does “burst” really characterize an event that takes 30 million years? IC 1613’s starburst is more akin to a teakettle than a firecracker.
Today the ancient gas-depleted spheroid dawdles into genteel retirement while its boisterous NE progeny are creating a spectacular fuss out in the suburbs — so much of a stir that astronomers all over our distant planet are tracking IC 1613’s every move and writing scads of thoughtful papers about it. Let’s take a look at the old spheroid first, then its starburst, and finally, the revealing tale this galaxy tells us about the wondrous complexity of our universe.
IC 1613 is embedded in an HI envelope of ionized gas totalling 33.88 x 106 M☉. The major axis of the HI envelope is at an angle of 58° to the spheroid. The spheroid appears small in the centre of this image, but in fact, the dwarf’s core photometric limits extend beyond the red HI clouds. The galaxy’s carbon-star radius extends 2.4 scale lengths of the spheroid in a broad oblate ellipse. The HI gas is less dense but more uniformly distributed in the SE (upper right) quadrant, with clearly defined waveform clumps that curl inward reminiscent of spiral galaxy density waves. This suggests that the HI mass is rotating around the spheroid on a different axis than the spheroid’s rotation. The large hole in the NNE sector is unexplained, as is the monotonic surface density of the HI cloud at 250°–270° directly adjacent to the starburst region. Source: Lowell LITTLE THINGS Project.
Walter Baade classified IC 1613 as dIrr — dwarf irregular — in 1928. Baade’s images with the Mt. Wilson 100-in telescope resolved IC 1613’s stars to magnitude 17 (red) to 18 (blue). He noted strong signatures of HII gas and red giant stars. Baade compared it to Barnard’s Galaxy in Sagittarius. In 1978 Paul Hodge pointed out that there are really two objects here — the ancient spheroidal dwarf galaxy of metallicity [Fe/H] = –1.7, and the massive young HII star-forming region 550 pc (1956 light years) to the NE, with a metallicity of [Fe/H] = –1 (1/10 of solar).
The spheroidal’s very low non-hydrogen chemical content (2 percent of the sun’s) suggested that the ancient galaxy ran out of star-making gas very early in its history. Typically an [Fe/H] of –1.7 turns up in a generation of stars enriched by 2 – 3 generations of earlier stars that exploded as supernovae or shed their envelopes into the dwindling gas reservoir. Since then evidence has accumulated that IC 1613’s distinct components are evolving independently. There is some thermal transfer between the two caused by UV radiation from hot young stars, but little kinetic or angular momentum exchange.
Both the spheroid and starburst components are bathed in a clumpy halo of low-energy HI atomic hydrogen gas in a flocculent ball that encloses both components (pic above). The halo gas of 33 million M☉ has a metals content of [Fe/H] = –1.4 (1/25th or 0.04 that of the Sun). The HI halo features some regions which visually resemble spiral density waves, while others look like summer-day clouds. The outstanding feature of the HI halo is the very bright, blue starburst in the NE quadrant — the quincunx — which to our eyes is as luminous as the old spheroid.
IC 1613 is not unique in its off-centre starburst within a common HI envelope surrounding an ancient spheroid. Similar morphology is found throughout the Local Group and even has a name: starburst dwarfs. These range in mass from giants like the Magellanic Clouds and Barnard’s Galaxy, to middleweights like Phoenix and Sextans A, and even in extremely metal-poor (XMP) I Zwicky 18 and the recently discovered DDO 68 (UGC 5430 35.8 million light years (Mly) away in the Lynx–Cancer intergalactic void.
DDO 68. While IC 1613 has one large starburst region, DDO 68 has nine small ones. They clump mainly in the 0° to 90° and the 290° sectors, suggesting that the galaxy is interacting with massive gas clouds streaming in from the 70° to 90° sector. See also the ESA video zoom-in sim of DDO 68 using HST and ESA images.
Today IC 1613’s spheroidal core rotates as a unitary body, as revealed by an edge-to-edge velocity distribution of ± 10.8 km sec-1 in a classic Gaussian (bell-shaped) distribution. Its rotation is an ancient relic of when the galaxy’s dark matter core interacted with other dark matter cores, spinning them up as gravitational contraction accelerated angular momentum (spin speed) as the interacting DM blobs sought energy equilibrium. While the old spheroid rotates as a unit around its centre, the galaxy’s individual stars rotate at 40 km sec-1 on random paths like in an elliptical galaxy. The fact that the galaxy as a whole rotates on one axis while its stars rotate at random testifies that the galaxy itself was rotating long before its stars came into being. This is one clue that primordial dark matter cores were set into rotation by grazing encounters with other dark matter cores before any of the dark matter attracted enough hydrogen gas to initiate star formation. The added fact that IC 1613 has no globular clusters suggests that its dark matter nursery comprised other blobs its own size, with few or no globular-scale mini-blobs. Fascinating what we can deduce just by a galaxy’s star motions.
So just how big was IC 1613’s original DM blob? IC 1613 is not bulky as dIrr galaxies go. Its half-light radius is 1040 pc (3390 light years). The entire galaxy’s baryonic (normal) matter content is ~110 million solar masses (M☉) and it shines with 100 ±20 million solar luminosities (L☉). That implies most of its stars are less than one solar mass, and most about the same age. On the other hand, IC 1613’s mass-luminosity ratio (M☉/L☉−1 (the ratio of dark to luminous matter) is 2.2 ±0.5, so its dark matter mass is equivalent to ~220 million M☉. This is the lowest ratio of any dwarf galaxy in the Local Group — a bit of an oddity, since large galaxies like the Milky Way average 10 times as much dark matter as luminous, while dwarf galaxies have dozens to hundreds of times as much DM to baryonic. There is a fairly simple relationship between a dwarf’s luminous matter/baryonic matter ratio and its dark matter ratio. Even for mathophobes this one is easy: the total luminosity of the galaxy divided by the luminosity of the sun roughly equals 3200 times the total mass of the galaxy divided by the mass of the sun. Simple as it gets, right?
OK, so let’s take the easy way out: with a picture:
Back to our mystery. If IC 1613 didn’t hit another galaxy, what exploded to give us the eyepiece view we enjoy today? Who, exactly IS the butler around here?
The only other known star-making masses in a region as empty as IC 1613’s are random high-velocity clouds (HVCs). These are giant bubbles of atomic hydrogen (HI) gas left over from the origin of the universe. They are cold (3–10 K), thin (10 to 50 atoms per cc), and huge (50–300 light years in diameter). The numbers, masses, and distribution of these primordial clouds are difficult to trace at IC 1613’s distance because atomic hydrogen emits a very weak signal at 21 cm in the radio band. At IC 1613’s distance, it takes roughly 27,000 HI atoms to generate a signal energy level of 1 photon that can trigger a detector response (1 milliJansky, MJy) after the VLA radio telescope’s most recent receiver upgrade.
Hence the somnolent old spheroidal part of IC 1613 is undergoing a modern energy awakening similar to many other isolated irregular dwarfs which likewise have high-energy starburst regions not associated with their primordial core. In fact, most dwarf irregulars far removed from large galaxies like the Milky Way, M31, or M81 exhibit this same kind of unpredictable starburst behaviour. Phoenix Dwarf has been through five of them; Sextans A has been through three — I can see two of the remnant glows as galaxy-like even in my modest 8-inch scope.
Hence, when we gaze at IC 1613 and see that faint mottled patch 5.5 arcmin NE of the slightly larger ancient spheroid’s patch, we are really confronting the mystery, “How can an ancient isolated galaxy suddenly blossom with massive star formation of the kind usually seen in the arms of spiral galaxies? Where did the gas come from? What triggered this kind of stellar explosion so far out in the middle of nowhere?”
So after all this, we’re still no closer to what the butler didn’t do than we were in the first paragraph.
Look, lads, it's late. Time to turn of this telly and promise Part II anon.
Edited by WeltevredenKaroo, 15 October 2016 - 05:19 AM.