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deepskytraveler
May 5th, 2015, 02:32 AM
Object of the Week May 3, 2015 - NGC 3521 A Flocculent Galaxy


NGC 3521, UGC 6150, PGC 33550, MCG 0-28-30
Type: SAB(rs)bc
RA: 11h 05m 48.8s
Dec: -00° 02’ 06”
Size: 11.0’ x 5.1’
Mag: 9.0V 9.9B
Mean Surface Brightness: 22.1 mag/arcsec2

I recently came across a thread on another popular astronomy site where the discussion centered on contemporary print and electronic resources focused on deep sky observing. It was no surprise to me to find Deep Sky Forum (DSF) among those resources mentioned by several posters. However what did surprise me as one person’s comment that the many of the deep sky objects discussed here on DSF “require large telescopes” and “are not visible with modest aperture.” Others then chimed in to state that on the contrary many of the objects discussed here are indeed visible in 8” to 10” telescopes.

By conducting a non-scientific and non-statistical visual scan through the almost three and half years of Object of the Week (OOTW) postings there appears to be a slight bias toward apertures of 18” and greater. It is a fact that several of the long-time OOTW contributors do observe with telescopes in the range of 18” through 28”, and in one case, a 48” telescope. Perhaps this is the source of the slight bias.

Regardless, do not let these “large telescope” observation reports by certain members scare you off. Forget about whether size matters and instead focus on the tag line that concludes every OOTW article – “GIVE IT A GO AND LET US KNOW”

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Size Matters Not - Courtesy Uncle Rod (Mollise)

“GIVE IT A GO” This is the challenge you are given for each OOTW. I suggest you step up to this challenge each and every time you go searching for a deep sky object (DSO) to observe. Large aperture certainly helps, but there are many other factors that will contribute to or hinder a successful observation. A few examples are the sky and weather conditions at your observing site, and your selection of eyepieces and filters. Believe it or not your physical and mental acuity plays a huge role in the quality of your observation. The list goes on but I’ll not belabor the point further beyond suggesting that the road you take and whether or not you arrive at your destination, i.e., the target DSO, must begin with knowing your destination. Quite simply, if you don’t know where you are going, how are you going to get there? This I believe is the primary reason that many amateurs observe the Messier objects over and over and over. I know. I’ve been there and done that.

When DSF members share an exotic DSO they will typically note if the observation requires some minimum aperture. But even that isn’t and shouldn’t be written in stone. Just because a description may originate from an observation with a 25” scope doesn’t mean you shouldn’t try chasing the target with a 10” scope. That is the challenge – GIVE IT A GO – try it! Maybe you will surprise even yourself.

Now don’t forget the second half of the tag line – LET US KNOW. Share your experiences and observations. Tell us what could be seen and what couldn’t? How does the observation change if you take that 10” scope to a much darker site, or higher elevation? Share your results even if the target DSO wasn’t visible.

Regardless of the outcome remember to think about and enjoy the absolutely most awesome, cool and incredible deep sky objects that you have come to learn. Here are just a few of them from past months to whet your appetite - extragalactic messes, train wreck galaxies, superthin galaxies, mystical open clusters, a rare Hoag-type galaxy, a dissolving planetary, and a discordant quasar. And now to add to that list – a flocculent galaxy.

I used a new approach to select this week’s OOTW – NGC 3521. Using my recently acquired copy of Interstellarum Deep Sky Atlas, I picked a chart of Leo (#58) and then using the atlas’ unique visibility encoding, I randomly selected a DSO “that is visible in a 4” (100mm) scope under reasonably dark skies.” In fact after observing this beautiful spiral galaxy you will question why Charles Messier never cataloged it.

It was only in the year that Messier published the final version of his catalogue, 1784, that William Herschel discovered NGC 3521 early on in his more detailed surveys of the northern skies. Through his larger, 31-inch telescope, Herschel first observed NGC 3521 on February 22 [1784]. His discovery log entry (H I-13) reads, “Very bright, much extended in the direction of the meridian of the nebula, suddenly much brighter middle, 7’ or 8’ in length.” Herschel classified the object as a bright nebula.

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A negative image courtesy the NGC/IC Project

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Image by Dale Liebenberg

Today we know that NGC 3521 is a spiral galaxy seen at an oblique angle. At a distance of 23 million light-years, it is a large object measuring 72,000 light-years in true physical extent. With a mass equivalent to some 150 billion Suns, NGC 3521 is equal to or nearly as bright as many of the Messier galaxies in Leo, Virgo, and Coma Berenices. It is a full magnitude brighter than M108 and a half magnitude fainter than M82.

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In this ESO picture from the Very Large Telescope (VLT), colourful, yet ill defined, spiral arms replace Herschel’s “nebulosity”. Older stars dominate the reddish area in the center while young, hot blue stars permeate the arms further away from the core.

NGC 3521 is a mixed spiral system, having characteristics of both a barred spiral and a normal spiral galaxy. It has a morphological classification of SAB(rs)bc which indicates that it is a spiral galaxy with a trace of a bar structure (SAB), a weak inner ring (rs), and moderate to loosely wound arm structure (bc). Photographs show that NGC 3521’s spiral structure is choppy and disjointed. Systems with this type of patch fragmentation and discontinuous spiral arms are known as flocculent spiral galaxies. A few other well known flocculent galaxies are NGCs 2841, 4414, and 7793 as well as M101.

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Image courtesy SDSS-3

Use this chart to find NGC 3521 in southern Leo.
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In The Night Sky Observer’s Guide Volume 2 by George Kepple and Glen Sanner, you find this description of NGC 3521 in 8/10” telescopes at 100x: “NGC 3521 is 30’ east of 62 Leonis (m5.9) and 11’ NW of an 8th magnitude star. This fine galaxy shows a well concentrated oval core containing bright stellar nucleus surrounded by a much fainter somewhat diffuse 5’ x 2’ NNW-SSE halo.”

In their Observing Handbook and Catalogue of Deep-Sky Objects, authors Christian Luginbuhl and Brian Skiff call the galaxy beautiful and describe the view through a 12-inch telescope: “The ends are ragged, the core and halo mottled. The oval core is roughly centered, but the brightest parts become progressively more eccentric to the [western] edge, where a dark lane 20” wide passes.”

Again from The Night Sky Observer’s Guide Volume 2 you find this description of NGC 3521 in 16/18” telescopes at 150x: “NGC 3521 is a fabulous object for large instruments! The halo is bright, elongated 8’ x 3’ NNW-SSE, and contains a large, bright, extended core with a stellar nucleus. Both the core and halo are mottled. The halo’s periphery is irregular in brightness and extends further west then east from the galaxy’s center. With averted vision a patchy dark lane can be seen along the halo’s west edge. A 14th magnitude star is 3’ NW of the galaxy’s center with a 13th magnitude star just beyond.”

Now it’s your turn. Regardless of your telescope’s aperture…

“GIVE IT A GO AND LET US KNOW”

I look forward to reading everyone’s observation reports and seeing their sketches of the beautiful, flocculent spiral galaxy, NGC 3521.

GOOD LUCK AND GREAT VIEWING!

wvreeven
May 5th, 2015, 06:20 AM
Mark,

What a great and inspiring post! You are absolutely right that the large apertures of the members of this forum (including myself) shouldn't scare off members with smaller apertures. I observed this galaxy in February of this year but since I used a large aperture I'll refrain from posting my observation here to allow members with smaller apertures to step up first.


Clear skies,

Wouter van Reeven

Steve Gottlieb
May 5th, 2015, 06:36 AM
Also check out Jay GaBany's image (http://www.cosmotography.com/images/small_ngc3521.html) of NGC 3521 :shocked:

A 60mm should have no problem in a dark sky, but here's my 18-inch notes...

Very bright spiral with impressive structure, elongated at least 5:2 NNW-SSE, ~7.5'x3'. Contains a bright, elongated core highlighted by an intense, stellar nucleus. The halo is noticeably mottled at first glance and a dust lane runs NNW-SSE along the west side of the core. There is a strong impression of spiral structure with an arm attached on the east side that extends to the north and appears to curve towards the west in the outer halo. The halo appears more extensive or slightly brighter on the northern end. A more difficult arm appears to sweep to the south on the west side of the core.

RolandosCY
May 5th, 2015, 07:03 AM
I totally agree with Mark's comment about the slight bias towards hard objects in the OOTW series and especially in the "response" by the average deep-sky observer that "those objects in the DSF are always faint and for those with large scopes". I was told so by some of my astrobuddies here on the island of Cyprus where I live, and amongst them is a 16" Lightbridge user! I have personally made a distinct attempt to include objects that I can observe in my 6" achromatic refractor (even if only barely). True, lots of the objects require 12 or more inches to enjoy in all their splendor, but it is simply amazing on how many objects can be readily observed AND enjoyed in smaller scopes. Another common obstacle is that often observing books, guides, or internet sites can be a bit misleading in the MINIMUM equipment needed to observe some objects. When a SN went off in NGC 7250 three years ago, I read that the galaxy is "very difficult for 12" scopes". Yet, in my subsequent OOTW write-up of beautiful NGC 7250 you will note that I have included decent observations made with a SIX - inch scope. How many though were immediately put off observing this galaxy once read that is very difficult for much larger scopes? My study in the visibility of the Horsehead Nebula came as a direct result of this widespread (definitely beyond DSF) attitude. With a few precautions I could readily see the Horsehead in a 3" refractor, yet common knowledge says you need at least 10 inches. And my eyesight is far from perfect! It is in our hand to try to dispell these myths regarding both the general visibility of deep sky objects AND that the DSF caters only for huge apertures!

By the way, although I don't have a drawing of it, I did observe NGC 3521 at least two times with my 6" achro and indeed it does show nice detail: A star-like core within a bright, well defined halo. Quite a nice object, I'll sketch it next time I'm out...

deepskytraveler
May 5th, 2015, 10:34 AM
Wouter,

Thank you for your compliment on this OOTW post. However please go ahead and share your observation of NGC 3521. The last thing in the world I'd want to create is the perception that large aperture observations are less than welcome. The broader the diversity and range of observations, the more complete our visual picture and understanding of the object becomes. One of the things I really enjoy about star parties is the easy opportunity it provides to compare my views of an object through a broad range of aperture and instruments.

Without sounding like a broken record my point was simply: don't let aperture size stop you from giving it a go.

deepskytraveler
May 5th, 2015, 10:49 AM
Rolandos,

Obviously you have seen the light, and with "modest aperture" too. :-)

Thank you for sharing your supportive remarks and your experience with minimum equipment required to observe an object.

wvreeven
May 5th, 2015, 02:18 PM
OK, here's my observation with my 20" from SQM 21.5 skies. The transparency wasn't great though. The notes are translated from Dutch.

At 320x the galaxy looks a bit odd at first. A stellar nucleus is visible surrounded by a reasonably bright area that is sharply cut off on the north side but fans away quite far to the south. Close inspection reveals a spiral arm to the north and one to the south as well. Very impressive.


Clear skies, Wouter

deepskytraveler
May 6th, 2015, 02:30 AM
Total coincidence - came across this tonight and just couldn't resist.

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Size Matters Not - Courtesy Uncle Rod (Mollise)

Keep those observations rolling in.

RolandosCY
May 7th, 2015, 07:49 PM
Here is a very recent drawing (last night!) of 3521. We did have possibly the best conditions of the last few months, but only for 50 minutes due to moonrise! The drawing was made from my backyard using a 6" achromatic refractor equipped with my trusty Takahashi LE 7.5mm ocular. Although there were some lights around (being that early in the evening) the sky clarity more than made up for it. My only comment is that no way the surface brightness is 22. Maybe 12 or 13, but not 22!!!

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Steve Gottlieb
May 7th, 2015, 08:41 PM
My only comment is that no way the surface brightness is 22. Maybe 12 or 13, but not 22!!!

We're talking about different units here.

Mark gave a mean surface brightness of 22.1 mag/arcsec2, (per square arcsecond). A square arcminute = 3600 square arcseconds (60*60). This translates into a difference of log (3600)*2.512 = 8.9 magnitudes. So, 22.1 mag/arcsec = 13.2 mag/arcmin.

Of course, a mean surface brightness can be misleading as the core region is generally brighter and the outer halo fainter.

RolandosCY
May 7th, 2015, 09:18 PM
Oops! Did not notice the different units...:igotit: You are right Steve.... Should had read carefully before talking... Your second statement is totally correct, I occasionally find the reported surface brightnesses (per arc MINUTE!) somewhat misleading in very extended galaxies...