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Thread: Object of the Week September 1, 2013 - M74

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    Member Steve Gottlieb's Avatar
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    Object of the Week September 1, 2013 - M74

    M74 = NGC 628
    01 36 41.6 +15 47 03

    V = 9.4; Size 10.5'x9.5'; Surf Br = 14.2
    Distance: ~30 million light years


    As a bright supernova (2013ej) was discovered in M74 just over a month ago, many amateurs turned their gaze to this beautiful "grand design" spiral of type SA(s)c.

    Due to its face-on orientation, small core and low surface brightness halo, M74 is certainly among the most challenging Messier galaxies for those viewing in light-polluted conditions. I remember M74 giving me fits when I first starting logging the Messier list in 1977 with a 6-inch reflector from the bright skies over Berkeley, California and it was satisfying just to glimpse a dim glow. Once I traveled to dark skies a year later, I realized my finder scope would do the trick.

    In the 1976 paper "H II regions in NGC 628. I. Positions and sizes.", Paul Hodge cataloged no less than 730 individual HII knots in M74. Visually, the brightest "knot" (HII complex/stellar association) is #627 near the end of the southern arm. How small a scope will reveal this object?

    A 1980 photometric study of the HII regions by Hodge and Kennicutt "H II regions in NGC 628. III - H-alpha luminosities and the luminosity function" provided the H-alpha flux of 593 regions. This image from the paper shows how clumps of HII regions define the spiral arms. #627 is the large clump at the bottom of the image, though it is not circled.
    M74 HII.jpg

    Visually, M74 really comes to life in 16" and larger scopes with two main spiral arms wrapping around the core and some of the brighter HII regions starting to pepper the arms. Lord Rosse in December of 1848 first detected the spiral nature of this galaxy in 1848 (one of 14 "spiral or curvilinear nebulae" he discovered before 1850). Observing with his 72-inch f/8.8 speculum telescope, he simply noted

    13 Dec 1848. Rough sketch made. Spiral? M74 Rosse.jpg
    14 Dec 1848. Confirmed last night's observations; feel confident it is a spiral.

    In Jimi's 48-inch, the amount of detail was stunning and the notes below were scribbled during a few minutes at the eyepiece.

    Beautiful face-on spiral with long, graceful arms wrapping around an intense 1' core that increases towards the center, but there is no sharp nucleus. At first glance at 375x there appeared to be four arms, but with a more careful look there are two main arms that each wrap more than 360° around the core as well as a couple of side branches. Each arm is studded with a number of non-stellar HII regions that highlight the arms. In addition, a number of stars are superimposed, both in the inner region (two faint stars are within 25" of the center) and around the edge of the halo, which extends to 7'-8' diameter.

    The more prominent arm "southern" arm is very regular - emerging from the core on the south side and wrapping counterclockwise around the core to the north, unwinding gradually as it curves to the east and then pulls away from the central region more suddenly on the south side. This arm is very patchy and delineated by a large number of HII knots with the two most prominent ones near the outer southern end. The "northern" arm begins to emerge from north of the core, tightly wraps counterclockwise around the core, passing near or through a few superimposed stars on the south side of the core, unwinding more as it stretches again to the north. The arm structure is a bit more complex on the north side due to side branches and the embedded HII knots are more scattered.

    The HII regions were viewed more carefully at 610x. The following identifications are from Paul Hodge's 1976 paper. The brightest is #627, near the end of the outer southern arm 2.7' SSW of center. It appeared fairly bright, fairly small, round, ~20" diameter. Moving clockwise along this arm towards the core, the next prominent knot is #592-598, situated 2.2' SSE of center. It was slightly fainter than #627, round, 15" diameter. Next in line is #550-555, a faint round knot of 10" situated 1.8' SE of center. East of the core by 1.5' is #406, a very faint, round 10" knot situated 36" S of a superimposed mag 14.5 star. Just 30" W of this star and 1.2' NE of center is #292, a fairly faint, very small knot, ~8" diameter. Continuing inward along this arm, the next knot is #194-197, a very faint hazy knot 1.2' N of center. Finally, less than 1' NW of center is another very faint patch with Hodge numbers #260-268.

    There were no notable knots on the inner southern portion of the northern arm, but a noticeable clump of knots is on the NW portion of this arm. First was #167-168, a faint 10" knot 1.6' NW of center. Continuing outward 2.0' NNW of center is a faint, elongated patch, ~25" diameter, consisting of #85-101 and #49 at the north end of the glow. I didn't search the outer region of the halo for additional HII knots, except noted #330, a 10" knot situated between two mag 12-13 stars at the eastern edge of halo, 8' from center.

    The labeled image below is from the Gemini North Telescope on Mauna Kea where Howard Banich and I were fortunate to have an insider's tour last year (that's my wife, not Howard ;-).
    Gemini North.jpg

    I've generally only labeled a single listing from the Hodge paper, though visually each "knot" I noted above in the 48-inch probably includes multiple clusters and HII complexes.
    Labeled M74.jpg

    "Give it a go and let us know!
    Good luck and great viewing!"
    Last edited by Steve Gottlieb; September 2nd, 2013 at 10:32 PM.
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    Member Ivan Maly's Avatar
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    With my 16" I have only been able to see the Hodge 627/Ivanov 52 knot distinctly:

    http://ivm-deep-sky.blogspot.com/201...om-erupts.html

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    Member Steve Gottlieb's Avatar
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    It was interesting to take a look at the Ivanov et al paper "Stellar associations and aggregates in NGC 628".

    Matching up three of the visually brightest HII complexes with the Ivanov "Associations & Aggregates" --

    Hodge 627 = Ivanov 52 V = 15.9
    Hodge 598 = Ivanov 90 V = 16.6
    Hodge 049 = Ivanov 60 V = 16.6

    Hodge 554 = Ivanov 108 is listed at V = 16.4 but I found it fainter than the latter two above.

    If I'm interpreting the last column correctly, which gives the corresponding Hodge number, it's offset or shifted by one line on many or most objects.
    Last edited by Steve Gottlieb; September 3rd, 2013 at 04:10 AM.
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    Summer nights are finally over here and I started this season on last weekend with my 12" dobson. M74 was on my observing plan because of a supernova. The supernova was ~13 mag and easily visible. Otherwise M74 is pretty difficult object because of low surface brightness. I was able to see some hints of spiral structure but bigger telescope is needed to see details that are listed above. There are also several foreground stars.

    Iiro Sairanen, Finland
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    Member Ivan Maly's Avatar
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    Fine sketch, Iiro - the enhancements closest to the core showed well.

    I too noticed the shift in the Hodge number column in the Ivanov et al. paper.

    Indeed the V magnitudes they are reporting must account not just for starlight but also for the contribution to the visual spectrum of the glowing gases of the associated HII regions. I wonder if the discrepancy with the visual perception (if it is not due to any physiological effect of the knot size on perception) might be due to different relative areas of the star complexes and HII regions in different knots, and therefore the contribution of the gas within the area of measurement. After all their intent was to measure starlight only.
    Last edited by Ivan Maly; September 4th, 2013 at 02:11 PM. Reason: typo
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    Member Marko's Avatar
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    re: that's my wife, not Howard
    Very funny Steve!

    Great writeup on M74 with nice references and pictures.
    Thanks,
    Marko
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    Member Howard B's Avatar
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    I laughed too! That was a great tour on Mauna Kea and the sky was fabulous that day - so blue it was almost purple. Anyway, I made a quick sketch of M74 and it's new supernova 2013ej last month at the Oregon Star Party and only noted the magnification was 253x and SQM reading was 21.65:

    M74 and SN2013ej_crop.jpg M74 and SN2013ej_cropInvert.jpg

    M74 is a very dim spiral, as has been noted already, and although I didn't go looking for any of the HII complexes I think a very fine night would be needed to see them - it took some doing to see just the spiral arms when I made this sketch!
    Howard
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    Last new moon I tried to catch as much HII region as I can with the 27". Transparency was fairly good, Seeing unfortunately not the best. I'm sure that a few more HII region can be catch up when the Seeing is better and the observing time longer.

    27", 172x-244x, Seeing III, Nelm 7m+

    sketch with inverted version
    Clear Skies, uwe
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    Member Howard B's Avatar
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    Wow, now that's a great sketch and observation Uwe! It also represents much more than I've been able to see in M74 and is motivation to give it more effort next time - wonderful!
    Howard
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    Thanks Howard,

    yes it made a lot of fun to pick all HII regions. I need at least around 2 hours for the rough sketch. The outer regions were only visible with 4mm AP (172x) as subtle glowing around the main structure - a little bit to bright in the sketch.

    The faintest HII region was the stellar HII region [H76]-13 3,5' NW of the core. I missed the brighter [H76]-292 1,2' NE of the core, perhaps the background arm was to bright?

    Look also after the galaxy 2MASXJ01365368+1542071 around 5,6' SE. NED and LEDA did not mentioned a magnitude, VizierR note 17,3bmag for the USNO-B1.0. Seems to fit with my observation, the galaxy is the same brightness like the brighter to middle bright HII knots within the galaxy.
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    Member ScottH's Avatar
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    9/27/22 Messier 74. Inspired by this OOTW, I was able to see the H-II/SFRs H672/Ivanov 52 and H598/Ivanov 90 at 300x in the 16". The seeing wouldn’t allow me to go fainter nor allow me to confirm seeing H672/Ivanov 52 with the 10”.

    Scott

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