Object of the Week, September 29th 2024 – The Sculptor Dwarf

[akarsh]

Sculptor Dwarf = Sculptor dSph = PGC 3589 = ESO 351-30
Dwarf Galaxy in Sculptor
RA (ICRS): 01 00 09
Dec (ICRS): -33 42 33
Mag: 8.6
Size: 40' × 31'

Hello from Hanle, Ladakh, India where a group of 13 of us are trying to observe and image the night-sky from 14000 feet altitude. Surprisingly, we have LTE connectivity here, and I'm able to make this OOTW post. This week, I chose the venerable Sculptor Dwarf. It was first reported by Harlow Shapley in 1938 in a report titled "Two Stellar Systems of a New Kind". The two stellar systems were the Sculptor Dwarf and the Fornax Dwarf – Shapley had just discovered the first two dwarf spheroidal galaxies. He first wrote that "There are no irregular nebulosities, no clumping of stellar images, no sharp or bright nuclei...". Whereas this statement did not hold up for the Fornax Dwarf which has globular clusters, till date it does as far as I know for the Sculptor Dwarf.

Steve Gottlieb features both the Sculptor and Fornax dwarf systems in his October 2023 Sky & Telescope article with the catchy title "The Milky Way and the Seven Dwarfs"! Therein he writes that the Sculptor Dwarf is about 270 kly away from us and that it has a substantial dark-matter halo.

scl.jpg
Sculptor Dwarf, POSSII Blue image, 1° x 1° field. Credit: MAST / STScI / DSS

SculptorDG_2022_L.jpg
Sculptor Dwarf as imaged by the Subaru telescope

Visually, the Sculptor Dwarf is a very tough target. I remember seeing it in Cartes du Ciel prominently marked as magnitude 8 and wanting to try it, only to later learn about its extremely low surface brightness. Observers in the southern hemisphere may have better luck, but I only know of maybe a dozen successful observations from the temperate northern latitudes. Depending on how you calculate it, the surface brightness is about 16.3 MPSAM.

Roger Clark, who pioneered a theoretical framework for visual observing, concluded from his framework that the Sculptor dwarf could never be detected visually. However, there were positive observations discussed on amastro. For many years, I was simply afraid to try this object, and then recently started consistently attempting it. My instrument of choice for this was my 25×100 binocular which has a large field of view and the low 25× magnification. I finally had a lucky break on the night of the October 2023 Annular Eclipse in the USA, when I had joined Jimi and Connie Lowrey, Steve and Patty Gottlieb, and Paul and Debbie Alsing at Bluff, UT to watch the spectacle. The same night after the eclipse, Steve and I stepped out in the front-yard of our host with our binoculars. Steve retired earlier, but I battled the cold for a little longer as I had a large aperture binocular under an extremely transparent Bortle 2 sky. I attempted the object for about an hour around its culmination. I managed to record a "weak" observation of the object. "I knew the general area to look at, but managed to refine the position by just studying the field. The refined position was a bit off but the elongation was correctly determined. After further studying a photograph, a very weak E-W elongated glow was detected and the maximum intensity confirmed to be at the exact location. The glow was sensed repeatedly. I tried hallucinating glows in other places but the only place where the glow was consistently detected with averted vision was at its correct location! It seems to be best detected when I place it at the left half of the field".

About two nights later, I found myself on the highway UT-12, a few miles west of the Natural Bridges National Monument. This is one of the darkest regions in the USA. I was observing again with my 25×100 binocular. I warmed up with a few Messier globulars until the harrowing Sculptor Dwarf climbed higher in the sky. This time, I already knew its exact position and shape, so I had to work harder to confirm the object. I wrote in my logs: "Pretty much at the limit of visibility, an elongated sensation of a brightening of the background sky appears intermittently at the expected location. The sensation is persistent, reproducible and is sensed to be above the background noise of my eye. Trying to develop similar sensations in other places did not yield, indicating that this sensation is real. The larger diameter of the glow is at least 16'. Chocolate, breathing, neck posture, and a persistent inquiry of the background helped. Sugar probably had the biggest impact."

There have been many successful observations from 30° N to 36° N latitudes posted on amastro and elsewhere. I'd love to hear your reports, so why not
GIVE IT A GO AND LET US KNOW

[Uwe Glahn]

Interesting fact that Clark concludes, that the Dwarf is not visible at all.

I only had one observation from Namibia with a 25x150 binocular under Bortle1 skies. I wrote: showy! brightening, steadily visible with direct vision, very diffuse and so difficult to estimate the real size, around 20' large and 3:2 E-W elongated

sketch: 25x150, NELM 7m5+
ESO351-30.jpg
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[Steve Gottlieb]

Here's what I wrote in the Sky & Tel article on Milky Way dwarf spheroidal satellites --

A few years ago, I observed the Sculptor Dwarf using my 18-inch telescope at 113× from a dark site at latitude 36.7° north, where the galaxy culminates 20° above the southern horizon. With some patience, I spied a large, ill-defined stain measuring 15? to 18? in diameter, with very little central concentration. Despite my difficulty, other amateurs have spotted the Sculptor Dwarf in 3- to 4-inch scopes. I’m not surprised — the visibility of dwarf spheroidal galaxies mainly depends on the darkness and transparency of the sky, along with the observer’s experience. A large telescope isn’t necessarily a requirement.

[kisspeter]

I can only contribute a negative observation using a 4" Newtonian from Namibia, 2012. It was only a quick half-hearted attempt with the dawn approaching and many other much more spectacular objects on my list. So I cannot really say anything apart from that it's certainly not very easy.