sign in sign up
<<
Home , Irregular galaxy, Magellanic Clouds

The “ugly duckling”, in the realm of the

Domingos Soares

The galaxies that most call our attention to are the spiral galaxies — es- pecially because of the beauty of the spiral arms — and the elliptical galaxies, which present an uniform and smooth light distribution against the sky back- ground. There is, though, a type of , whose attractiveness is precisely the fact of not presenting harmony or regularity on its shape. It is the “ugly duckling” of the realm of the galaxies, and the representatives of that type are called very appropriately “irregular galaxies”. Irregular galaxies tend to be smaller than spirals and ellipticals and are even called “dwarf” galaxies. Actually, the term “” is applied both to dwarf spiral galaxies, with disturbed structure, as our companion, the , and to all other galaxies that do not fit on any of the types that have a regular structure. Incidentally, the — the Large and the Small — are satellite galaxies of our , being easily seen with the naked eye on the skies. They were discovered by the Portuguese navigator Ferdinand Magellan (1480-1521), in 1520, when he undertook his Earth’s circumnavigation voyage. When Edwin Hubble (1889-1953) put forward his classification scheme of galaxies — known as Hubble’s tuning fork —, he simply classified all galaxies that did not fit on the types spiral and elliptical as “irregular”. Presently, irregular themselves have an internal classification distinction, as we shall see. Now, Hubble called “Irr I” the irregular galaxies in which he could discern at least some sign of a structure, for example, indications of a spiral arm. And called “Irr II” those which show an extremely disorganized appearance. For Hubble, therefore, both Magellanic Clouds, the Large and the Small, fit the Irr I category. The Large Magellanic Cloud presents a prominent stellar bar and indi-

1 cations of a spiral arm. The presents an elongated structure with no signs of spiral arms whatsoever. Both are dwarf galaxies, i.e., their masses are thousands of times smaller than the mass of a typical as ours.

The Large Magellanic Cloud on left and the Small Magellanic Cloud. They are dwarf galaxies and satellites of the Milky Way. The Large Magellanic Cloud is the prototype of a subclass of irregular galaxies. Notice the stellar bar of the Large Cloud, which is the main structure seen in the image. The Large and the Small Magellanic Clouds were classified by Hubble as Irr I. The Small Cloud is likely a disk distorted by gravitational tidal forces (Images: LMC, Eckhard Slawik; SMC, David Malin).

The classification of the irregulars used by Hubble revealed itself very broad, as long as more observations of irregular galaxies became available. There was clearly the need of further detailing in the definition of types of irregular galaxies. The French astronomer, based in the United States, G´erardde Vau- couleurs (1918-1995) was responsible for a great improvement of the Hubble system of galaxy classification. Among other contributions, he introduced a modification, in the case of irregulars, with the objective of having a more discriminating classification. He proposed that the irregulars were classified as an extension of the classification of spiral galaxies. Hubble defined the types Sa, Sb and Sc for the spirals. The bulges diminish and the spiral arms

2 open up, along the sequence a-b-c. A spiral galaxy Sa, for example, presents a large bulge and spiral arms very close to the bulge. Hence, de Vaucouleurs created the new type Sd, in which arms do exist but they start to show ir- regularities in their definition. Next, he created the type Sm — “m” as in Magellanic —, already in the domain of irregulars, in fact, substituting Hub- ble’s Irr I, whose prototype, or model, was the Large Magellanic Cloud. He created yet the type Im, whose most illustrious representative is the Small Cloud, and finally created the type Ir, to include the truly irregulars, often also called “amorphous” galaxies, that is, literally galaxies “without a form”. The latter would include Hubble’s Irr II. We have then for spiral and irregular galaxies the new sequence Sa-Sb- Sc-Sd-Sm-Im-Ir. In practice, this corresponds to an extension of the branch of normal spirals on Hubble’s tuning fork. Likewise, we have the extension SBa-SBb-SBc-SBd-SBm-Im-Ir for the branch of barred spiral galaxies on the original Hubble’s classification. We can now reconsider the above figure, where we show the Magellanic Clouds. According to de Vaucouleurs’ revision, they are now classified as SBm — the Large Cloud — and Im — the Small Cloud. Irregular galaxies are also many times observed at great distances from us. This is shown in the next figure, which presents some of the irregular galaxies observed by the Hubble Space Telescope, whose times of exposure were very long.

3 Distant irregular galaxies observed by the Hubble Space Telescope. From the galaxy at the upper left corner, and in the clockwise direction, they may be ten- tatively classified as SBm, Ir, Ir and Sd, in de Vaucouleurs’ revised system. The blueish regions in the galaxies indicate regions of intense formation, a typical feature of irregular galaxies (Images: Richard Griffiths, NASA/HST).

Irregular galaxies have some characteristics which are exhibited in all subtypes, from Sd to Ir. In the first place, they have regions of intense . Such regions show up in the images as zones of predominantly blue color. The blue color is emitted by young with masses much larger than the Sun mass — 5 to 10 times larger — and that evolve rapidly, existing for “only” tens of millions of years. Stars with masses equal or smaller than the Sun mass exist for up to tens of billions of years. Another characteristic, intimately related to star formation, is the presence of gas and dust clouds. These clouds are seen in the images with small reddish patches. The red light comes from the energetically excited gas hydrogen. Hydrogen is not the only

4 component of these clouds, but is the most abundant element. The hydrogen excitation is caused by the intense radiation from neighboring stars. These stars, besides blue light, strongly emit ultraviolet radiation, that is highly energetic, and ionize and energetically excite the gas present in the clouds. From the structural point of view, irregular galaxies present many times a stellar bulge — the spheroidal structure present in the central region of spiral galaxies. But the bulge, when present, is always displaced from the central region of the galaxy. This is a sign of morphological disturbance, a basic feature of irregular galaxies. Irregular galaxies are frequently the result of collisions of two or more galaxies, or of disturbances originated from strong tidal gravitational interactions with a nearby galaxy. Our next irregular, M82, was probably disturbed by its companion, the beautiful spiral M81. M82 is a Ir. It is called the “Cigar” galaxy, due to its global appearance on short time exposure observations. But when we examine its internal structure we verify that it is really a true Ir, with little order in the stellar distribution.

On left, the pair of galaxies M81 and M82. M82 is seen beside in details. Notice the disturbed internal structure. The dark patches are dust clouds, which absorb visible light. The blueish color comes from the galaxy stars. The filamentary structure, perpendicular to the plane of the galaxy, is due to ionized hydrogen emitting its characteristic red light (Images: M81-M82, Takayuki Yoshida; M82, Subaru Telescope, Japan).

To finalize, we made a homage to the Large Magellanic Cloud, our clos- est galaxy, and easily visible to the southern hemisphere observers. With the naked eye, it is seen as a whitish luminous patch against the night sky

5 background, resembling an usual cloud of meteorological nature. Such a confusion is frequent amongst the less informed. But it is a true galaxy! And, for the inhabitants of the southern hemisphere, having this vision is an extraordinary privilege. The image presented next was obtained with long time exposure and shows that the Large Cloud indeed resembles a . The fragmented structure of the probable spiral arms leaves no doubt, nevertheless, about its true nature as an irregular galaxy. But this “ugly duckling” is very pretty, isn’t it?

Image of the Large Magellanic Cloud obtained with a long exposure time. Its classification as SBm is unquestionable. Notice the central bar and the fragmented spiral arms around it (Image: Yuri Beletsky, ESO).

6