This Member of the Nearby M81 Group Shows an Intriguing Structure
that Amateur Astronomers can Observe Through their Telescopes
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| Figure 1. IC 2574, in constellation Ursa Major. Photo by the author. |
This article is devoted to IC 2574 (also UGC 5666, DDO 81, and VII Zw 330), an interesting dwarf galaxy discovered by the american astronomer Edwin Foster Coddington on a photographic plate using the Crocker Photographic Telescope at Lick Observatory (University of California, USA), in 1898. This object is part of the M81 group which contains 30 members, being Messier 81, NGC 2403, and NGC 4236 the brightest ones (book "The Cambridge Photographic Atlas of Galaxies" by Michael Kônig Stephan Winnewies). M81 group is one of the nearest prominent groups in the vicinity of the Local Group.
The IC 2574 galaxy is located at a distance D = 4.02 Mpc (1 arcsec = 19.5 pc) according to Karachentsev, Makarov & Kaisina (2013), paper "The Supergiant Shell with Triggered Star Formation in the Dwarf Irregular Galaxy IC 2574: Neutral and Ionized Gas Kinematics" O. V. Egorov,1‹ T. A. Lozinskaya,1 A. V. Moiseev1,2 and G. V. Smirnov-Pinchukov1 (2014).
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| Figure 2. IC 2574 and its surrounding field. |
IC 2574 hosts many HI holes and also an interesting structure to be observed through big telescopes, a Supergiant Shell1 (SGS) of ∼1000 × 500 pc in deprojected size (Walter et al. 1998; Walter & Brinks 1999; Cannon et al. 2005). The SGS, as well as many holes found in IC 2574, is surrounded with HII regions likely triggered by the stellar feedback related to the star-forming event that occurred in the hole center ∼25 Myr ago (Walter & Brinks 1999; Weisz et al. 2009). The SGS is located in the northwest region of IC 2574 and created by a significant burst of star formation in its center. It is likely responsible for triggering a ring of star formation around it (Walter et al. 1998; Walter & Brinks 1999; Pasquali et al. 2008) See paper "Hunting for Young Dispersing Star Clusters in IC 2574 ", Anne Pellerin, Martin M. Meyer, Daniella Calzetti, and Jason Harris (2012).
A few questions arise now, how much details can we see in this galaxy through an amateur telescope? Are the components of the supergiant shell visible?
This article was written based on observations I made using an 18" (45cm) f3.5 telescope. I first found this galaxy using low power (45x). The group of stars labeled with a circle on Figure 2 was useful to identify the surrounding field and to find the galaxy which was visible at this low power. With a visual magnitude of 8.2 HD 90820 is the brightest star in the 1 degree field of view.
At 45x, IC 2574 looks like a "rather smooth nebulosity" elongated northeast-southwest with a small area appearing slightly brighter, even though is faint and round. This area is indicated with an arrow in Figure 2 and is the aforementioned supergiant shell. We are talking of a rather faint object even for an 18-inch telescope, at least under the sky conditions in central Oregon in 2018 summer, when this observation was made. It is important to say that the sky conditions were good enough for carrying out this observation but it was not an excellent sky, so maybe observers with the same kind of telescope under a darker and more clear sky might be abble to see this galaxy and its features a little better or easier.
At 45x, IC 2574 looks like a "rather smooth nebulosity" elongated northeast-southwest with a small area appearing slightly brighter, even though is faint and round. This area is indicated with an arrow in Figure 2 and is the aforementioned supergiant shell. We are talking of a rather faint object even for an 18-inch telescope, at least under the sky conditions in central Oregon in 2018 summer, when this observation was made. It is important to say that the sky conditions were good enough for carrying out this observation but it was not an excellent sky, so maybe observers with the same kind of telescope under a darker and more clear sky might be abble to see this galaxy and its features a little better or easier.
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Figure 3. Working with a DSS image I tried here to reproduce
how the galaxy looked through the 18-inch telescope. Faint target, with the two "brighter" areas indicated with squares on the picture.
North is down. South is up.
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At 61x the galaxy looks again smooth and clearly elongated, faint in brightness. Using averted vision two areas are slightly brighter than the rest (see Figure 3). However, the whole object is hard to see.
While the structure to the south end looks smooth in brightness and bigger than the other one, the north patch appears round and smaller. Observing carefully, some star-like structures seems to be visible in this region. Figure 4 to the left is a picture taken from the paper "The supergiant shell with triggered star formation in Irr galaxy IC 2574: neutral and ionized gas kinematics". O.V. Egorov , T.A. Lozinskaya , A.V. Moiseev , and G.V. Smirnov-Pinchukov. This picture shows the giant HII regions on the HST/ACS F658N image of the SGS area according to the list of Stewart & Walter (2000). Bottom panel on Figure 4: location of the star clusters on the same image. The clusters identified by Pellerin et al. (2012) are shown by blue. The red circles denote the clusters from Cook et al. (2012). The four largest green circles show the star clusters identified by Yukita & Swartz (2012).
While the structure to the south end looks smooth in brightness and bigger than the other one, the north patch appears round and smaller. Observing carefully, some star-like structures seems to be visible in this region. Figure 4 to the left is a picture taken from the paper "The supergiant shell with triggered star formation in Irr galaxy IC 2574: neutral and ionized gas kinematics". O.V. Egorov , T.A. Lozinskaya , A.V. Moiseev , and G.V. Smirnov-Pinchukov. This picture shows the giant HII regions on the HST/ACS F658N image of the SGS area according to the list of Stewart & Walter (2000). Bottom panel on Figure 4: location of the star clusters on the same image. The clusters identified by Pellerin et al. (2012) are shown by blue. The red circles denote the clusters from Cook et al. (2012). The four largest green circles show the star clusters identified by Yukita & Swartz (2012).
121x. This is an intersting magnification to see IC 2574. It is a faint object so averted vision is always necessary to glimpse and identify its structures. A very challenging glimpse of a small nebulosity is possible for moments when observing carefully, I´m talking about [MH94a]3 IC 2574 203 and [MH94a] IC 2574 218, a small object composed of two HII regions. The bigger structure, to the south, looks again smooth (featureless). Although faint, the smaller northern feature is a little easier to detect, appearing slightly more detached against the surrounding stars. Some of the components on that zone (appearing star-like in shape) are barely visible. Using averted vision, PMC20122 13z, the brightest component according with what can see on a DSS image, seems to be the "easiest" part to pick up visually. PMC2012 13z is a star-forming complex – an extended (∼ 450 × 320 pc) fine-filamentary HII region, located in the northern wall of the SGS. Yukita & Swartz (2012) identified the young star cluster C1 inside Region.
162x. At this power the small object composed of two HII regions
is visible with averted vision (see Figure 5). It is a faint feature of the galaxy however. At this magnification the aforementioned areas of the galaxy seems to be connected by faint nebulosity, without a doubt part of the galaxy, appearing clearly elongated. Observing carefully, some components of the supergiant shell seem to jump to the view but it is not possible to see them clearly and discern and identify each one. The component of the supergiant shell, PMC2012 13z, an association4 of stars according to SIMBAD Database and the STScI Digitized Sky Survey web page is certainly visible appearing cuasi-stellar in size and with a hazy nature. The UHC filter slightly improves the view of the smaller patch of the galaxy. Even PMC2012 13z appears more nebulous.
162x. At this power the small object composed of two HII regions
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| Figure 4. Credit O.V. Egorov et al. |
Both the STScI Digitized Sky Survey web page and SIMBAD Database agree that CVL20095 UGC 5666-3, the brightest nebula in the SGS, is an HII region that looks rather
non-uniform and consists of a central nebula ≃ 120 pc wide, several more compact (≃ 10 − 20 pc) clumps that surround
it, and two faint external shells southeast and northwest of
the region with the sizes of ≃ 20 pc and 35 pc.
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| Figure 5. The supergiant shell and its main components. |
While some star-like structures seem to be visible briefly and with difficulty in this galaxy, as was mentioned above, the view is difficult and unclear, so I can not claim to have identified these last two features of the SGS visually. The only patch visible there seems to be the brightest one on Figure 5, PMC2012 13z.
IC 2574, one of the many outstanding and enigmatic galaxies that we can find in a dark sky, is a good target to put your eyes and telescope under proof and see how much you can detect under ideal observing conditions.
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1_ SGS Irregular galaxies reveal giant HI supershells and holes with sizes as large as 1 – 2 kpc and lifetimes up to several hundreds Myr. Giant supershells and holes in some galaxies represent the dominant feature of the ISM. Such large structures are usually called supergiant shells (SGS), or giant supershells. Formation mechanisms of supergiant shells have been discussed extensively in recent decades
2_ [PMC2012] - (Pellerin+Meyer+Calzetti+, 2012)
3_ [MH94a] (Miller+Hodge, 1994)= (MH)as appears in SIMBAD Database
4_ The concept of a stellar association was originally introduced in 1949 by V. A. Ambartsumian, who later separated them into OB and T associations (Ambartsumian 1968). Morgan, Sharpless, & Osterbrock (1952) considered as a stellar association any loose group of stars within an area where bright OB stars exist and with evidence of a common origin.
A recent definition of a stellar association (Kontizas et. al. 1999) refers to it as a single, unbound concentration of early-type luminous stars, embedded in a very young star forming region.
5_ [CVL2009] - (Croxall+van Zee+Lee+, 2009)as appears in SIMBAD Database
1_ SGS Irregular galaxies reveal giant HI supershells and holes with sizes as large as 1 – 2 kpc and lifetimes up to several hundreds Myr. Giant supershells and holes in some galaxies represent the dominant feature of the ISM. Such large structures are usually called supergiant shells (SGS), or giant supershells. Formation mechanisms of supergiant shells have been discussed extensively in recent decades
2_ [PMC2012] - (Pellerin+Meyer+Calzetti+, 2012)
3_ [MH94a] (Miller+Hodge, 1994)= (MH)as appears in SIMBAD Database
4_ The concept of a stellar association was originally introduced in 1949 by V. A. Ambartsumian, who later separated them into OB and T associations (Ambartsumian 1968). Morgan, Sharpless, & Osterbrock (1952) considered as a stellar association any loose group of stars within an area where bright OB stars exist and with evidence of a common origin.
A recent definition of a stellar association (Kontizas et. al. 1999) refers to it as a single, unbound concentration of early-type luminous stars, embedded in a very young star forming region.
5_ [CVL2009] - (Croxall+van Zee+Lee+, 2009)as appears in SIMBAD Database