I’ve had three good viewings of Sextans A using my 180 & 200 mm glass. A half hour brings out pretty much all I can tease out. The overall galaxy’s extended surface is readily spotted at first glance in my 150mm MK66 fuzzy-finder at 60x and becomes more textured in the larger apertures. At 225x in a 200mm the A-3 OB association appears as a 1.5’ x 1.2’ patch roughly twice the surface brightness of the galaxy, about vm >13 star cloud >14 galaxy. Above 300x A-3 appears to segregate into two small concentrations of near-stellar appearance 50” separation at pa 145°. The 5 or 6 stars seen in a scope of my aperture are >mag 15 field stars. The brightest A-3 star is a mag 16.0 red supergiant three arsecs from the mag 16.25 field star USNAO2 0825-07118971 (10 11 06 / -04 42 13). At Sext A’s distance of 4.7 Mlyr an A-3 red supergiant of 20 solar masses is only mag 19.2, so the one visible at mag 16 must weigh in +60 solar masses. That's an amazing star to come out of a gas-poor dwarf galaxy—especially since it has so many cloud-mates. Sextans A’s primordial background population reflected in its zero-age main sequence turnoff is <mag 22. Put another way, every star except one 16th mag star, which a lucky soul with a 20-inch scope might see, is a field star. DSS images and WikiSky hint at a second, older star cloud on the eastern edge of Sext A at pa 90° (10 10 54 / -04 41 07). This patch forms an isosceles triangle with m16 and 17 field stars. However, 15 minutes staring at the position of this cloud revealed only two fleeting evanescences that could just as well have been the vagueries of tremulous air.
I don’t know how many of you follow up visual sightings with research in the literature, but Sext A is well worth an excursion. See
van den Bergh,
HI distribution,
http://arxiv.org/pdf/0801.2547v1.pdf (see esp §4.5), and the
Massey-Hodge et al photometry results. The search is not made any easier by the newly discovered Sextans Dwarf Spheroidal garnering most of the recent attention. Sext A, Sext B, the Sext dSph, the Antlia Dwarf, and NGC 3109 are their own small group ~4.7 Mlyr out, well beyond the Local Group’s zero velocity surface (no net motion in or out) at 3.85 Mlyr. Our Local Group’s mass distribution is a clumpy ellipsoid with a half-mass radius of only 1.14 Mlyr, so the zero-velocity surface further out is uneven.
It’s likely that the Antlia-Sextans Group formed in isolation and remained so ever since. All are Irregular or Dwarf Spheroidals. Their radial velocities are so modest (~400 kms/sec approaching) that it would seem unlikely they have interacted very much with each other. That leaves unexplained why Sext A has its fulminous A-3 O-B association plus some
red supergiants in three older clumps (visible in a WikiSky image), and Sext B has a relatively youthful 2.1 Gyr old globular cluster of
~800,000 solar masses. Dwarf Irregular galaxies like Sext A, B, and NGC 3109 are composed mainly of
young and intermediate-age star populations and are typically found at distances less than a million light years from a nearby massive galaxy. None of the Sextans Group has interacted with Local Group galaxies, hence their recent starform activity has been informed by purely local factors.
Why? Earlier HI survey mapping by the Very Large Array identified hundreds of dense, compact gas clouds free-floating in the Local Group medium. The Sextans Group apparently has its own massive gas clouds as well (Galactic Groupies?). Sext A’s star-forming cloud is visible to our modest amateur equipment only because A-3 (actually two adjoined supergiant groups) whose age is < 6 Myr. A-3 has twice the [Fe/H] metallicity ratio, -1.24, of the galaxy as a whole. This follows a Dwarf Irregular pattern of the redder stars being on the periphery and bluer ones in the centre (the opposite of large spirals). Nearby Sext B has its own youthful 2.1 Gyr-old massive globular of ~800,000 solar-masses. Such starform intensities are is at odds with the view that present-day massive gravitationally bound star clusters can only evolve near the centers of massive galaxies or in interacting galaxies. That a recent GC of Sext B’s mass can form in the ambient pressure well of a modest dwarf spheroidal whose gas mass is only 10% greater than its star mass says much about the number of unseen intergalactic gas masses available to bump into a dwarf galaxy with a flirty dark matter halo. As gas density is low in dwarf irregulars, earlier starbursts should have caused enormous gas outfows due to a dIrr’s shallow galactic gravitational potential well. This said, the Sextans Group’s evolution is still speculative because field star contamination is a very high ~85% of the 1516 stars surveyed.
Isn't it cheering to know that our small-telescope eyepieces and 4 metre Kitt Peak Mayall telescope show us the same thing: dwarf irregular galaxies can have three types of HI envelopes:
smooth, chaotic, and starless clumps? Messy for the galaxy, fun for us.
