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Sextans A galaxy

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#1 timokarhula

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Posted 03 May 2013 - 01:21 PM

A month ago, I observed the dwarf galaxy Sextans A (MCG-1-26-30, UGCA 205, DDO 75) with my 10-inch Dob. It was a tough object culminating at only 25º altitude for me. I have searched on Internet for other visual observations but could not find much. Instead, I found that there is another dwarf galaxy in Sextans (besides Sextans B which I saw two years ago) which is spheroidal and a satellite galaxy to our Milky Way. It is the Sextans I galaxy and discovered as late as 1990.

In Sextans A, the brightest part was a "clump of stars", catalogued as A3 on the galaxy's SE edge. Otherwise the Sextans A was similar to a naked eye Gegenschein (which I, by the way, noted the same night, half way between Spica and Gamma Virginis). Sextans A lacked a brighter centre. My SQM-L meter showed a typical April value of 21.5 from my regular observing site here in Sweden. Have you seen the Sextans A and B dwarf galaxies? What is A3, a stellar burst region or an ordinary H-II region?

Edit: There was still snow on most of the ground. Otherwise the SQM value should have been at least 0.1 mag darker.

/Timo Karhula

#2 nytecam

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Posted 04 May 2013 - 01:40 AM

Well done Timo - excellent report - Sextans is a neglected constellation for DSOs! Ummm - so snow is hampering your SQM values - swap you my 15M neighbours here in LP London for a bit of snow :roflmao:

Here's the same 60sec shot of M57 at 21 deg altitude over central London to NE with and without the software LP filter engaged. Removing LP takes its toll but AP results acceptable whilst visual is total rubbish. The whole sky is the same dirty orange colour :rainbow:

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#3 timokarhula

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Posted 04 May 2013 - 03:42 AM

Hi Nytecam!

Thanx. I have always admired your images taken from London. It's unbelievable that you can image more from the City than most people can see visually with very large scopes from dark skies. My skies here in Virsbo are dark but once a year I travel to my apartment in Western Australia where the skies are darker than in Mauna Kea, for example. :-9

BTW, you can read my report from W.A. in the Webb Deep Sky Society magazine #154. I was awarded their annual prize last year! :jump: The skies in W.A. are just amazing!

/Timo

#4 nytecam

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Posted 04 May 2013 - 04:26 PM

Hi Nytecam! Thanx. I have always admired your images taken from London. It's unbelievable that you can image more from the City than most people can see visually with very large scopes from dark skies. My skies here in Virsbo are dark but once a year I travel to my apartment in Western Australia where the skies are darker than in Mauna Kea, for example. :-9 BTW, you can read my report from W.A. in the Webb Deep Sky Society magazine #154. I was awarded their annual prize last year! :jump: The skies in W.A. are just amazing! /Timo

Congrats on your Webb Soc award - haven't attended their UK meets for awhile.

Where in Western Australia do you observe? We visited OZ two decades ago eg Perth WA + Perth Obsy, Adelaide, Melbourne and Sydney to many local astro clubs and my [now late] brother :rainbow:

#5 WeltevredenKaroo

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Posted 04 May 2013 - 05:39 PM

Were you in Sweden when you observed Sext at 25° elevation? A. It's nearly overhead (in season) here at 33°S in S Africa—as in W. Oz. Sext A-3 is an OB *BLEEP*'n somewhat brighter than the galaxy itself, surf. brightness by my estimate of about 14. The listed vis. mags of Sext. A, Sext. B, and the Carina Dwarf are meaningless, esp. Carina, which is spread out over 23' x 15', is a vexatious search, takes half an hour and a lot of eyepiece tapping in my 200mm and then maybe two glimpses. Sext A is visually the most concentrated at 5.5' x 5.0. Both Sext A and B are easy to mistake for an aged PN, but are ID'd as dwarfs by a their null response to OIII and positive response to UHC. I don't perceive A's squarish shape in my 180 and 200mm scopes; just an irregular diffuse patch with the A-3 brightening to twice the overall galaxy's apparent luminosity. Looks like a Magellanic starform region without the stars. It's best for my equip't at 225x, above that it fades into airglow. Sext. B is of the same visual size and only 0.4 mag fainter. It diffuses more slowly into the ISM and takes a half hour of steady looking to become certain. Looks like a fainter version of a reddened Cl X-XII GC like NGC 6256 Sco. Both galaxies have been studied extensively in the professional literature. Try Yandex, arXiv, and IOP.

Next time you're in Oz, if Crux is high, try the nearby Circinus Galaxy if you haven't spotted it already. This galaxy is in a sort of Baade's Window amid a dense MW starfield. It's an eyepiece full of heaven.

Cheers, Dana in SA

#6 WeltevredenKaroo

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Posted 04 May 2013 - 05:45 PM

Ummm, I see that CN mother-henned my abbreviation of "Association" to BLEEP'n in the previous post. Good thing I added the "t" to contraction of Sextans.

BTW, huzzahs for seeing Sext. A from where you are. Impressive!

#7 timokarhula

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Posted 05 May 2013 - 05:53 AM

Next time you're in Oz, if Crux is high, try the nearby Circinus Galaxy if you haven't spotted it already. This galaxy is in a sort of Baade's Window amid a dense MW starfield. It's an eyepiece full of heaven.

Cheers, Dana in SA



Hi Dana,

Yes, I have seen the Circinus galaxy. I was using Akira Fujii's 30cm Mitaka telescope back in 1999. The galaxy was difficult to find amidst myriads of stars.

BTW, I did observe the stars in South Africa (Cederberg Mountains) in May 2001 while engaging in a solar eclipse safari in South Africa, Namibia, Botswana and Zambia.

A friend of mine, Christian Sasse, has been observing at Albert Janssen's observatory in the Great Karoo. Both very dark sites.

/Timo Karhula

#8 timokarhula

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Posted 05 May 2013 - 06:08 AM

Where in Western Australia do you observe? We visited OZ two decades ago eg Perth WA + Perth Obsy, Adelaide, Melbourne and Sydney to many local astro clubs and my [now late] brother :rainbow:


I stay in Geraldton, 430 kms north of Perth, where I have a 10-inch Dobsonian. To find extremely dark skies I just drive east or north. It's totally devoid of both light and radio pollution. That's why a part of the Square Kilometer Array, SKA, will be built here!

/Timo Karhula

#9 nytecam

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Posted 05 May 2013 - 04:25 PM

We visited Oz with our next door neighbours who have/had relatives in Geraldton - small world :grin:

#10 WeltevredenKaroo

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Posted 14 May 2013 - 02:14 PM

Timo:

I’ve had three good viewings of Sext A since your drawing attention to it in the CN forum. In 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 obj >14. 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 an +60 solar masses. The dwarf’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. Across ~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 if 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 clumpy and has a half-mass radius of only 1.14 Mlyr, so the zero-velocity surface is uneven. It’s likely that the Antlia-Sextans Group formed in isolation and remained so till now. 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. Earlier HI survey mapping by the Very Large Array identified hundreds of dense, compact gas clouds free-floating in the Local Group medium, patiently biding their time to infall into a galactic gravitational well. The Sextans Group apparently has its own massive gas clouds as well (Galactic Groupies?). Sext A’s A-3 cloud is visible to our modest amateur equipment because of A-3 (actually two adjoined supergiant groups) whose age is < 6 Myr. A-3 has twice the [Fe/H] metallicity ratio of the galaxy as a whole, which follows a Dwarf Irregular pattern of the redder stars being on the periphery and bluer ones in the centre (the opposite of large spirals). Sext B has its own youthful 2 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 sizable 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. The Sextans Group’s evolution is speculative because field star contamination is a very high ~85% of the 1516 stars surveyed. Even so, it’s 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.

#11 timokarhula

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Posted 15 May 2013 - 08:23 AM

Thanx very much for your detailed observations of the Sextans Dwarfs and analysis in the literature. I appreciate it. :) We up here in the north have now too bright nights until August to make any DS-observations.

Please have a look at the personal message I sent you.

/Timo Karhula

#12 sgottlieb

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Posted 16 May 2013 - 05:05 PM

Dana, thanks for the excellent summary of research on Sextans A and the Sextans/Antlia group just outside the local group.

I'm curious if you know where the source of the "A-3" designation for the main O-B association (2 principal clumps)? Hodge's 1974 paper "A Second Survey of H II Regions in Galaxies" in ApJS, 27, 113 1974 mentions 3 HII regions in Sextans A including "one large, bright HII region", but I'm not sure if this is the original source.

I'm also curious about the brightest star in Sextans A, as I'd like to try and track it down if possible. In Alan Sandage's 1982 paper "Distance and absolute magnitudes of the brightest stars in the dwarf galaxy Sextans A" (1982ApJ...258..439S), the brightest star he lists is V = 17.25 (star #15). Could the mag 16 value you mention refer to the bright end of a blue variable?

#13 WeltevredenKaroo

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Posted 19 May 2013 - 10:38 AM

Steve:

Your second question is the easiest. See the 1997 Phillip-Massey Survey of Local Group Galaxies, Table 9, line 1 for LGGS J101021.29-045018.6, RA 10 10 21.2, Dec -04 50 18.6, V 16.0. I had a second reference to the same star (probably derived from Phillip-Massey) that also included brightest-star mags for Sextans B and other members of the Sextans Group, I can’t seem to find it. If you need it in addition to the first reference above, I’ll look about a bit.

Now to A-3. The only place I have seen A-3 called by that name outside this thread is Alvin Huey’s chart of Sext A in his most recent The Local Group. You might ask him where he got it. A literature search reveals no such designation that I could find. But, it was an interesting excursion into the way astronomical data refines in step with better equipment. Hence the results may be of interest to anyone who takes pride in our achievements:

If Paul Hodge mentioned the aka A-3 association in 1978 (and also 1985b & 1986), I’m amazed we’re not calling it H-3. (Hodge is still going strong, most recently in 2012 in connection with Sext A.) Sandage & Carlson (ApJ 1982, 257,439-456 identified 17 “bright standards” named S1, S2, etc (Table 1 and esp. the detailed star nomenclature in Fig. 2 and image on p. 444). The paper also introduces the term “PM1, 2 etc” signifying “Proper Motion”. I find no reference to an A-3 in literature relying on the Cambridge Automated Plate Measuring Facility, which went into service in 1986(?). Its photometry accuracy fell off (p.1 r.col, & several other refs therein) because of crowding effects and what we call A-3 may not have been resolvable into a close pair of OB associations. The earliest mention of A-3 as two clumps is Dohm-Palmer 1997, Astron. J. 114(6) pp 2528ff, see esp Fig. 16. Four sets of CCD photometry were undertaken in the 1980s: Hoessel et.al 1983, 652 stars; Sandage & Carlson 1985, 67 stars (plate measurement, not CCD); and Aparicio et al 1987, 2279 stars. Distance moduli ranged from 25.6 to 26.2, so it’s understandable why ground-based resolution was not that fine-grained. Sext A’s starform nomenclature does not seem to have originated in synthetic Monte Carlo modeling between 1988-1995. This is understandable: until HST, photometry couldn’t go deeply enough at milliarcsec resolution to define individual bursting episodes on <Gyr scales. Radio studies at VLA , Dwingeloo, and Effelsberg also mention four radio-bright regions but not the A-3 name; see e.g. Skillman et al. A&A 1988, esp Fig 2. The last pre-HST CCD survey by Mateo et.al. AJ 101 (1991) has detailed plates and magnitude tables but no mention of an A-3. The term A-3 does not appear in the first (1997) deep HST photometry published in Dohm-Palmer; they mention only “several bright HII regions” and “the young association” in a context clearly referring to A-3. (The Palomar yellow plate PH-7117S in Dohm-Palmer provides a very pretty, highly resolved image of the galaxy which shows more fine detail than many subsequent images.) Post HST but still ground-based, Kurtev et.al. 2004 identified 36 OB associations but made no mention of an A-3 (Table 1 and Fig 3). It’s now established that Sext A and B’s starform history occurs in clumps 200-300 pc in size over periods of 100-200 Myr, the same massive young cluster pattern established by Portegeis-Zwart. See Dohm-Palmer as above, Figs 10 & 13.

All this is beginning to look like a mathematics conjecture in which the proof is a lot longer than the problem. Here we have a name but no namer. I wonder if a call to Sherlock Skiff is in order. Would you like to post a request on AmAstro and we’ll see what happens?

Let us know if you pick up that 16.0 star.

=Dana

#14 sgottlieb

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Posted 19 May 2013 - 12:21 PM

Thanks for all the details on Sextans A! I'll report back on observing LGGS J101021.29-045018.6. I don't know if this is a spectroscopically confirmed member, but at V = 16.0 it would be much more luminous the brightest LBVs in M31.

As far as the A-3 designation, I'll check first with Alvin. In Hodge's 1974 "A Second Survey of H II Regions in Galaxies", he notes "This small irregular galaxy shows one large, bright HII region near one side and two fainter ones. Weak H(alpha) plate." But the three HII regions are not listed in his table of offsets, so any assignment of designations seems arbitrary based on this paper.

#15 Alvin Huey

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Posted 21 May 2013 - 05:14 PM

As far as the A-3 designation, I'll check first with Alvin.


The A-3 designation, should have been A3, came from Hodge, Skelton and Ashizawa book titled "An Atlas of Local Group of Galaxies".

I'll list that as one of the books under recommended reading section. I also realized that I left out a small list of papers I found related to this class of objects. When I feel better, I'll update it.



#16 sgottlieb

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Posted 21 May 2013 - 10:30 PM

Thanks, Alvin. I checked the book out from the UC Berkeley astronomy library awhile ago, but never ran across a reasonably priced copy for sale online to purchase. Do you have own a copy?

#17 Alvin Huey

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Posted 22 May 2013 - 01:43 AM

Hi Steve,

No I do not own a copy, but used a friend's copy a while back. I need to look at it again, but lost contact with him.

#18 sgottlieb

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Posted 22 May 2013 - 12:12 PM

If anyone's curious, here's the identification chart for the associations in Sextans A from Hodge's "Atlas of Local Group Galaxies", which identifies A3 discussed above.

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