Weltevreden SA
20-11-2013, 03:43 AM
09/11/2013:
The Wolf-Lundmark-Melotte dwarf irregular galaxy (WLM) is not an easy target, but is very fascinating when you do find it. I am blessed with very dark and transparent skies and no neighbours for 10 miles, so I routinely spot it as a hesitant patch in my 180mm Mak, and as steadily visible with averted vision in a 200mm scope. The best exit pupil sizes are equivalent to 100-130x. The faintest stars my 200 mm scope can pull in are visual mag 15.5. A good warmer-upper is WLM’s nearby neighbour, the Class XII globular cluster NGC 7492 in Aquarius. It is visually larger than WLM but about the same surface brightness. My skies permit me to spot roughly 12 to 15 red giants dotting in and out across NGC 7492, all listed at 15.4 and fainter. If I spot those, WLM is on.
WLM is shown on Uranometria p.260 (2nd ed. p.121), and José Ramón Torres’ A series (http://www.uv.es/jrtorres/triatlas.html) #34. Star hopping to WLM is no easy affair. The most suitable hopping stars are mag 6 and below. The nearest visible star to WLM is 1 Ceti at mag 6.25, located 7 min 15 sec to the SE. The only field star brighter than the 11th mag is 25 arcsec due north. I have only a simple alt-az, so am stuck with it. This is not so awful, though—WLM lies 12° due east of NGC 7492. In northern skies it might be easiest to just find a buddy with a go-to. Key in RA 0h 01m 58s, Dec -15° 27’ 39”. WLM’s visual mag is listed as 11.0, which is about 1.5 magnitudes too optimistic given the galaxy’s size of 4.0’ x 1.6’ and surface brightness 14.9 sq arcminute (=23.8 sq arsec). However, the nearby globular NGC 7492’s listed magnitude is 11.4 and is twice as big, Hence its surface brightness is 17.6. So WLM is not such a visual ogre after all. For me WLM shows readily as a uniform oblong patch on nights when 7492 yields up at least a handful of quavery at-the-limit stars. It looks like NGC 147 in a 4-inch scope.
For us, the big thrill of WLM is just seeing it. For professionals, though, WLM has inspired a recent cottage industry of professional attention in the past few years, including $60,000-a-night observing sessions using the largest visual-band instruments in the world at Paranal and Mauna Kea (http://adsabs.harvard.edu/cgi-bin/bib_query?2013ApJ...767..131L). The interest lies not in WLM’s 3 million light year distance from us, but that it is 2.65 mlyr from the Local Group’s centre of mass. WLM is close to the surface where the Local Group exerts no net gravitational tug at all. Galaxy specialists are keen on WLM because it is both low-mass (430 million solar masses is ‘low-mass’ by galaxy standards) and because it is so isolated from all other Local Group galaxies. WLM’s nearest neighbour is the IC 1613 dwarf irregular galaxy in Pisces, a million light years away. If you long to get away from the rat race, WLM is the ticket.
For astrophysicists, though, WLM’s interest is that its evolution is driven solely by its own inner dynamics. The gravitational tides of massive galaxies like the Milky Way and M31 which eat hapless nearby dwarf galaxies for snacks, do not concern WLM one bit. Literally home-made and home-grown, WLM’s HI gas mass is 1/7th of its star mass, and the total star/gas mass is less than 1/10th of its dark matter mass. One revealing fact about WLM perks astrophysicist ears: the galaxy has many more observed carbon stars and oxygen-rich interstellar gas than any other dwarf irregular. That would occur if WLM’s primordial 0.8 to 2.1 solar-mass star population evolved without disruptive tidal forces. Today the galaxy is an unflustered populace of helium-poor, carbon-rich middleweights which have shed almost all their oxygen and nitrogen into space. Carbon-oxygen anticorrelation typically marks the final stage in a sub-2.1 solar-mass star population’s history before the stars fling off their remaining photosphere into a planetary nebulae and dwindle to a white dwarfs. That process is where Earth’s air and water came from, and it took a l-o-n-n-g time. So did we. A galaxy like WLM is likely to have more planetary formation than a twisty, magnetic, burst-shocked place like our local spiral arm because free gas is not tumbled about so much.
WLM has had a very leisurely history. Its first stars collapsed out of the Local Group’s modest little local filament in the Virgo Supercluster dark matter gravitational well more than 13 billion years ago. WLM’s one and only globular formed at the same time. It is visible at mag 16.5, good luck. WLM had plenty of company back in the early days—all of the dwarf galaxies of the Local Group formed in the same early era. Like many dwarf irregulars, WLM’s first stars formed uniformly all across the protogalactic gas well for over five billion years. The depletion of gas reserves by new star formation plus the heating effect of starburst clusters and supernovae slowly blew away the unused gas in the outer regions, so new star formation today has shrunk closer and closer to the remaining gas reserves in the core. This ‘outside-in’ pattern is the very opposite of spirals like the Milky Way. WLM’s old red stars are mostly in the halo, and its few young stars are all in the middle. There is a whole professional substudy of outside-in starform history called the LITTLE THINGS survey. In one of those acronyms astronomers must dream up after a couple of beers, there really is such a thing as the ‘Local Irregulars That Trace Luminosity Extremes—The H I Nearby Galaxy Survey’. Look it up here (http://arxiv.org/pdf/1208.5834v1).
Whimsical though the name is, it has important goals. One is why dwarf irregular galaxies form stars from the outside in while dwarf spherical galaxies do the opposite. This has to do with whether a protogalaxy’s original dark matter halo was core-shaped (like an open cluster), or cusp-shaped in a rapidly densifying centre (like M15). WLM’s star formation history indicates a core-shaped dark matter halo. But WLM contrarily has only one outsize globular cluster instead of numerous smaller globulars peppered about, which points to a cusp-shaped dark matter halo. This disagreement may have something to do with why WLM has spawned 20-odd papers in the last ten years (http://ned.ipac.caltech.edu/cgi-bin/TextSearch?words=WLM+dwarf&scope=Abstract+Collection&scope=Level5&scope=Thesis+Collection&Find=Search), authored by the most prominent names in galaxy development physics.
For us, though, it’s a fun star-hop with a faint fuzzy to jot in the log. Happy hunting, guys.
The Wolf-Lundmark-Melotte dwarf irregular galaxy (WLM) is not an easy target, but is very fascinating when you do find it. I am blessed with very dark and transparent skies and no neighbours for 10 miles, so I routinely spot it as a hesitant patch in my 180mm Mak, and as steadily visible with averted vision in a 200mm scope. The best exit pupil sizes are equivalent to 100-130x. The faintest stars my 200 mm scope can pull in are visual mag 15.5. A good warmer-upper is WLM’s nearby neighbour, the Class XII globular cluster NGC 7492 in Aquarius. It is visually larger than WLM but about the same surface brightness. My skies permit me to spot roughly 12 to 15 red giants dotting in and out across NGC 7492, all listed at 15.4 and fainter. If I spot those, WLM is on.
WLM is shown on Uranometria p.260 (2nd ed. p.121), and José Ramón Torres’ A series (http://www.uv.es/jrtorres/triatlas.html) #34. Star hopping to WLM is no easy affair. The most suitable hopping stars are mag 6 and below. The nearest visible star to WLM is 1 Ceti at mag 6.25, located 7 min 15 sec to the SE. The only field star brighter than the 11th mag is 25 arcsec due north. I have only a simple alt-az, so am stuck with it. This is not so awful, though—WLM lies 12° due east of NGC 7492. In northern skies it might be easiest to just find a buddy with a go-to. Key in RA 0h 01m 58s, Dec -15° 27’ 39”. WLM’s visual mag is listed as 11.0, which is about 1.5 magnitudes too optimistic given the galaxy’s size of 4.0’ x 1.6’ and surface brightness 14.9 sq arcminute (=23.8 sq arsec). However, the nearby globular NGC 7492’s listed magnitude is 11.4 and is twice as big, Hence its surface brightness is 17.6. So WLM is not such a visual ogre after all. For me WLM shows readily as a uniform oblong patch on nights when 7492 yields up at least a handful of quavery at-the-limit stars. It looks like NGC 147 in a 4-inch scope.
For us, the big thrill of WLM is just seeing it. For professionals, though, WLM has inspired a recent cottage industry of professional attention in the past few years, including $60,000-a-night observing sessions using the largest visual-band instruments in the world at Paranal and Mauna Kea (http://adsabs.harvard.edu/cgi-bin/bib_query?2013ApJ...767..131L). The interest lies not in WLM’s 3 million light year distance from us, but that it is 2.65 mlyr from the Local Group’s centre of mass. WLM is close to the surface where the Local Group exerts no net gravitational tug at all. Galaxy specialists are keen on WLM because it is both low-mass (430 million solar masses is ‘low-mass’ by galaxy standards) and because it is so isolated from all other Local Group galaxies. WLM’s nearest neighbour is the IC 1613 dwarf irregular galaxy in Pisces, a million light years away. If you long to get away from the rat race, WLM is the ticket.
For astrophysicists, though, WLM’s interest is that its evolution is driven solely by its own inner dynamics. The gravitational tides of massive galaxies like the Milky Way and M31 which eat hapless nearby dwarf galaxies for snacks, do not concern WLM one bit. Literally home-made and home-grown, WLM’s HI gas mass is 1/7th of its star mass, and the total star/gas mass is less than 1/10th of its dark matter mass. One revealing fact about WLM perks astrophysicist ears: the galaxy has many more observed carbon stars and oxygen-rich interstellar gas than any other dwarf irregular. That would occur if WLM’s primordial 0.8 to 2.1 solar-mass star population evolved without disruptive tidal forces. Today the galaxy is an unflustered populace of helium-poor, carbon-rich middleweights which have shed almost all their oxygen and nitrogen into space. Carbon-oxygen anticorrelation typically marks the final stage in a sub-2.1 solar-mass star population’s history before the stars fling off their remaining photosphere into a planetary nebulae and dwindle to a white dwarfs. That process is where Earth’s air and water came from, and it took a l-o-n-n-g time. So did we. A galaxy like WLM is likely to have more planetary formation than a twisty, magnetic, burst-shocked place like our local spiral arm because free gas is not tumbled about so much.
WLM has had a very leisurely history. Its first stars collapsed out of the Local Group’s modest little local filament in the Virgo Supercluster dark matter gravitational well more than 13 billion years ago. WLM’s one and only globular formed at the same time. It is visible at mag 16.5, good luck. WLM had plenty of company back in the early days—all of the dwarf galaxies of the Local Group formed in the same early era. Like many dwarf irregulars, WLM’s first stars formed uniformly all across the protogalactic gas well for over five billion years. The depletion of gas reserves by new star formation plus the heating effect of starburst clusters and supernovae slowly blew away the unused gas in the outer regions, so new star formation today has shrunk closer and closer to the remaining gas reserves in the core. This ‘outside-in’ pattern is the very opposite of spirals like the Milky Way. WLM’s old red stars are mostly in the halo, and its few young stars are all in the middle. There is a whole professional substudy of outside-in starform history called the LITTLE THINGS survey. In one of those acronyms astronomers must dream up after a couple of beers, there really is such a thing as the ‘Local Irregulars That Trace Luminosity Extremes—The H I Nearby Galaxy Survey’. Look it up here (http://arxiv.org/pdf/1208.5834v1).
Whimsical though the name is, it has important goals. One is why dwarf irregular galaxies form stars from the outside in while dwarf spherical galaxies do the opposite. This has to do with whether a protogalaxy’s original dark matter halo was core-shaped (like an open cluster), or cusp-shaped in a rapidly densifying centre (like M15). WLM’s star formation history indicates a core-shaped dark matter halo. But WLM contrarily has only one outsize globular cluster instead of numerous smaller globulars peppered about, which points to a cusp-shaped dark matter halo. This disagreement may have something to do with why WLM has spawned 20-odd papers in the last ten years (http://ned.ipac.caltech.edu/cgi-bin/TextSearch?words=WLM+dwarf&scope=Abstract+Collection&scope=Level5&scope=Thesis+Collection&Find=Search), authored by the most prominent names in galaxy development physics.
For us, though, it’s a fun star-hop with a faint fuzzy to jot in the log. Happy hunting, guys.