HEIC: A Peculiar Compact Blue Dwarf Galaxy (NGC 5253)

[bystander]

A Peculiar Compact Blue Dwarf Galaxy
ESA/HEIC Hubble Picture of the Week | 2012 Nov 26

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The NASA/ESA Hubble Space Telescope provides us this week with an impressive image of the irregular galaxy NGC 5253.

NGC 5253 is one of the nearest of the known Blue Compact Dwarf (BCD) galaxies, and is located at a distance of about 12 million light-years from Earth in the southern constellation of Centaurus. The most characteristic signature of these galaxies is that they harbour very active star-formation regions. This is in spite of their low dust content and comparative lack of elements heavier than hydrogen and helium, which are usually the basic ingredients for star formation.

These galaxies contain molecular clouds that are quite similar to the pristine clouds that formed the first stars in the early Universe, which were devoid of dust and heavier elements. Hence, astronomers consider the BCD galaxies to be an ideal testbed for better understanding the primordial star-forming process.

NGC 5253 does contain some dust and heavier elements, but significantly less than the Milky Way galaxy. Its central regions are dominated by an intense star forming region that is embedded in an elliptical main body, which appears red in Hubble’s image. The central starburst zone consists of a rich environment of hot, young stars concentrated in star clusters, which glow in blue in the image. Traces of the starburst itself can be seen as a faint and diffuse glow produced by the ionised oxygen gas.

The true nature of BCD galaxies has puzzled astronomers for a long time. Numerical simulations following the current leading cosmological theory of galaxy formation, known as the Lambda Cold Dark Matter model, predict that there should be far more satellite dwarf galaxies orbiting big galaxies like the Milky Way. Astronomers refer to this discrepancy as the Dwarf Galaxy Problem.

This galaxy is considered part of the Centaurus A/Messier 83 group of galaxies, which includes the famous radio galaxy Centaurus A and the spiral galaxy Messier 83. Astronomers have pointed out the possibility that the peculiar nature of NGC 5253 could result from a close encounter with Messier 83, its closer neighbour.

This image was taken with the Hubble’s Advanced Camera for Surveys, combining visible and infrared exposures. The field of view in this image is approximately 3.4 by 3.4 arcminutes.

Credit: NASA/ESA/Hubble

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[Ann]

Very interesting. I have been interested in this galaxy since I read about it in Burnham's Celestial Handbook. That was a while ago.

This galaxy has a relatively large, smooth envelope completely dominated by the dime-a-dozen bright stars of the universe, the humdrum red giants. The non-Betelgeuse red giants, the kind of red giants that the Sun will become one day in the future. The red giants that evolve out of non-remarkable main sequence stars. The stars in this outer envelope of NGC 5253 could well be billions of years old. But the central part of the galaxy is brimming with hot bright young blue stars, probably a few tens of millions years at most.

It's fun to look at one of the high-resolution versions of this picture at the www.spacetelescope.org homepage. The red giants suddenly become so obvious and numerous. But the thumbnail versions of this picture make the galaxy look all blue.

The B-V index of this galaxy is +0.430. That's blue for a galaxy, and it's almost exactly the same color as well-known F-type star Procyon, whose B-V index is +0.432.

The caption that accompanies the new Hubble image of NGC 5253 wrote:

NGC 5253 does contain some dust and heavier elements, but significantly less than the Milky Way galaxy.

Well, there is not all that much dust in NGC 5253, but the galaxy isn't dust-free. The galaxy is brighter in the far infrared, which traces dust, than in blue light, which traces starlight. It is typical for starforming galaxies to be relatively bright in the far infrared. In the case of NGC 5253, its B magnitude is about 10.7 while its far infrared magnitude is 9.6. So it is about a magnitude brighter in the far infrared than in blue light. This is also typical of starforming galaxies.

A galaxy that is slightly similar to NGC 5253 is NGC 3077, which belongs to the M81/M82 group. Like NGC 5253 it has a red envelope surrounding an active, starforming center. But the red envelope of NGC 3077 is bigger and brighter than the red envelope of NGC 5253, and the overall luminosity of NGC 3077 is higher than the total luminosity of NGC 5253, too. Fascinatingly, even though NGC 3077 looks dustier than NGC 5253 to me, NGC 3077 is comparatively more modest in the far infrared than NGC 5253. NGC 3077 is "only" about half a magnitude brighter in the far infrared than in blue light. Well, assuming my software can be trusted, of course.

Another galaxy that should also be compared with NGC 5253 is M82. Like NGC 5253 and NGC 3077 M82 has a disk without any visible star formation, but its center is brimming with hot young stars. The overall color of M82 is rather red, +0.89, but an important reason for that is that the brilliant young blue stars of M82 are hidden by dust. M82 is dusty indeed: its far infrared magnitude, 5.6, is about three and a half magnitudes brighter than its B magnitude, which is about 9.2! It's true that in the present-day universe, dust is (or can be) a fountain of youth and a wellspring of star formation!

Ann

[neufer]

• The last really close Type Ia supernova prior to
  
  [list]SN 2014J in starburst galaxy M82 was
  SN 1972E in starburst dwarf galaxy.

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<<NGC 5253 is an irregular galaxy in the constellation Centaurus. It was discovered by John Frederick William Herschel on 15 March 1787. NGC 5253 is located within the M83 Subgroup of the Centaurus A/M83 Group, a relatively nearby group of galaxies that includes the radio galaxy Centaurus A and the spiral galaxy M83 (the Southern Pinwheel Galaxy). NGC 5253 is considered a dwarf starburst galaxy. Supernova 1972E, the second-brightest supernova visible from Earth (peak visual magnitude of 8.5, fainter only than SN 1987A) in the 20th Century, occurred in this galaxy. Another supernova, SN 1895b, also has been recorded in the galaxy.>>

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<<SN1972E was a supernova in the galaxy NGC 5253 that was discovered 13 May 1972 with an apparent B magnitude of about 8.5, shortly after it had reached its maximum brightness. In terms of apparent brightness, it was the second brightest supernova of any kind (fainter only than SN1987A) of the 20th Century. It was observed for nearly 700 days, and it became the prototype object for the development of theoretical understanding of Type Ia supernovae.

The supernova was discovered by Charles Kowal, about 56 arc seconds west and 85 arc seconds south of the center of NGC 5253. The position in the periphery of the galaxy aided observation, minimizing interference by background objects. Well-positioned for Southern Hemisphere observers, it was quite observable from Northern Hemisphere observatories as well. Attempts made to observe it in X-rays with UHURU and OSO-7 and to detect gamma rays from it via Cerenkov radiation showers gave at best equivocal results.

Photometric and spectroscopic measurements were made in the visible and near infrared by many observers, extending to about 700 days after maximum light. Interstellar absorption lines of ionized calcium due to gas both in our galaxy and NGC 5253 were observed, allowing an estimate of the interstellar extinction.

The extended length of the observed light curve found a remarkably uniform 0.01 magnitudes per day decline starting about 60 days after discovery. Translated into other units, this is almost exactly a 77-day half-life, which is the half-life of ⁵⁶Co. In the standard model for Type Ia supernovae, approximately a solar mass of ⁵⁶Ni is formed and ejected from a white dwarf which accretes mass from a binary companion and is raised over the Chandrasekhar limit and explodes. This ⁵⁶Ni decays with a half-life of about 6 days to ⁵⁶Co, and the decay of the cobalt provides the energy radiated away by the supernova remnant. The model also produces an estimate for the luminosity of such a supernova. The observations of SN1972e, both peak brightness and fade rate, were in general agreement with these predictions, and led to rapid acceptance of this degenerate-explosion model.>>