4 Attachment(s)
Object of the Week May 7th, 2023 – NGC 5253 in Centaurus
MCG-05-32-060, ESO 445-4, UGCA 369
Self-Contradictory Galaxy
Centaurus
Irregular Galaxy/Blue Compact Dwarf
RA 13:39:56
DEC -31:38:30
Mag +10.4
Size 5.0’ x 1.9’
William Herschel discovered NGC 5253 in Centaurus on the night of March 15, 1787. It’s a remarkable discovery considering that from his home in Slough, England, the magnitude +10.4 galaxy never rose higher in the sky than 8°. In fact, even though it resides 1.5° into the modern day boundary of Centaurus, apparently it’s the only deep-sky object he ever discovered in the constellation!
In my 8x56 binoculars, it can be found (on good nights) as a dim little glow 1.9° SSE of magnificent M83. At a distance of about 11.6 million light-years, it should come as no surprise that it’s a member (the fifth brightest, actually) of the Centaurus A/Messier 83 Group1. The difficult question for professional astronomers has been trying to decide which subgroup (Cen A or M83) the little galaxy is a member of.
NGC 5253 first began receiving attention in December 1895. That’s when Williamina Fleming serendipitously noticed a star with a peculiar spectrum on a spectrum plate she was examining2. That plate had been taken at Arequipa, Peru on July 18 and upon immediate investigation, had been found to be a magnitude +7.2 star just 25” northeast of NGC 5253’s bright core on photographic plates taken on July 8 and again on July 10. However, no trace of the star was found on 55 plates taken from May 21, 1889 to June 14, 1895.
Attachment 5121
July 1895 photographic plate with arrow pointing towards Z Cen with NGC 5253 barely visible on its southwest edge
This was actually the second new star Fleming had found on photographic plates taken in 18953. The first had appeared in the constellation of Carina in the spring. Its spectrum was also peculiar, but closely resembled that of Nova Normae (1893) and Nova Aurigae (1891). But in their discovery paper titled A New Star in Centaurus2, Fleming and Pickering noted that the Centaurus star’s spectrum “is unlike the new stars in Auriga, Norma, and Carina” and actually “resembles that of the nebula surrounding 30 Doradus [the Tarantula Nebula].” Within about a year of its discovery, the new star found in Centaurus was given the variable star designation Z Centauri since it was unclear if it was a bona fide nova.
On the morning of February 8, 1896 William W. Campbell (of Campbell’s Hydrogen Envelope Star fame) “carefully examined the spectra of the new star [Z Cen] in Centaurus and the adjacent nebula [NGC 5253], using the large spectroscope and 60° crown prism [on the 36-inch refractor at Lick Observatory].4” He found that the “nebula’s spectrum was of the usual type, the lines at angstrom 5007, 4959 and 4862 having their usual relative intensities.” This meant that he found that the observed spectrum of NGC 5253 was that of a nebula. He also described the spectrum of the new star as “very peculiar” and later wrote that “if the star is simply a variable, as some writers contend, its period must be very long, or its variations very irregular. If the star is to be classed with the ‘temporary’ stars, it is not analogous to the new stars in Cygnus, Auriga, and Norma, but rather to the star of 1885 in the Andromeda Nebula [S Andromedae].” In hindsight, Campbell was spot on when said that the new star was closer in comparison to (what is still to this day) the only observed supernova in the Andromeda Galaxy than to the novae that had been recently observed in our Milky Way.
However, the new star in Centaurus continued to be a subject of debate amongst professional astronomers, and things got messier before they got better. Possibly the worst muddying of the waters came in a 1923 paper by Edwin Hubble and Knut Lundmark5. While some believed that the spectra did not fit with that of other observed novae, they argued otherwise and dubbed it Nova Z Centauri. What was truly odd, though, was their opinion as to nature of NGC 5253. Upon images taken with the 100-inch telescope at the Mount Wilson Observatory, they seemed to conclude that it wasn’t a “spiral nebula” and noted that the central part “is very intense and seems to consist of a number of soft nuclei.” To get a spectrum of it, they used a 6° objective prism attached to the 10-inch Cooke telescope. From this they found that “it seems to belong to the kind typical of spirals and other nongalactic nebulae; a strong continuous spectrum of approximately the type G without bright lines or bands.” In other words, they found it to have the spectrum of a typical galaxy…though, oddly, they never once mentioned the word “galaxy”!
The problem with Hubble and Lundmark’s findings was that they were simultaneously correct and incorrect – NGC 5253 was a galaxy and yet Z Centauri wasn’t a nova. They almost had it. If only they’d been able to put it together that Z Centauri was like S Andromedae, they would’ve made a groundbreaking discovery ten years early. But the worst part was that they hadn’t been able to record the obvious spectrum of the H-II region that Campbell had visually seen and is easily recorded nowadays. This led Caldwell & Phillips (1989)6 to conclude that “it seems rather likely that Hubble and Lundmark misidentified the spectrum of NGC 5253 on their plate.” Brian Skiff of Lowell Observatory7 has surmised that it’s also possible that the emission lines just didn’t show up on such a low resolution spectrum or it was soft and the details washed out. To resolve the matter, he’s advised that I’d “have to dig out the Mount Wilson spectrum plate, assuming it exists and hasn’t decayed, and have another look.”
Hubble and Lundmark’s fumble allowed Cecilia Payne Gaposchkin of Harvard to score big in 1936 when she put it all together8. Her breakthrough came when she incorporating the idea of “super-novae”, a brilliant and groundbreaking idea that had been proposed by Walter Baade and Fritz Zwicky just two years prior9 and in the same year the neutron had been discovered10. By assuming that NGC 5253 was a galaxy and not a nebula, she could account for the tremendously broadened emission lines in the spectrum of Z Centauri as being created from the extreme velocities produced by a supernova. This finding was confirmed by David Evans of South Africa in 195211 and it would later be known as SN 1895B.
A galaxy with a large, bright emission nebula at its center is very unique. That’s why you’ll usually hear about NGC 5253 when the likes of Messier 82 or Henize 2-10 are being discussed. But it wasn’t always that way. In 1970 Gary Welch12 was the first to prove that the large H-II region at the heart of the galaxy was being ionized by clumps of young, hot stars. Before then, many theories existed as to why the galaxy displayed such a strong nuclear emission line spectrum. One was that radiation emitted from the supernova Z Cen was responsible for illuminating it. Another was the possible superposition of an irregular and an elliptical galaxy in our line of sight! Welch summed up NGC 5253’s peculiarity well when he called it a “self-contradictory object” and wrote “In a system whose large-scale stellar population appears similar to that of an elliptical galaxy there is an enormous, centrally condensed emission complex.”
In 1972, Sidney van den Bergh was the first to find direct evidence for star clusters in NGC 525313. By studying a blue plate taken under excellent seeing with a 24-inch telescope on Las Campanas in Chile, he was able to count about a dozen tight concentrations of knots. One of those knots then had its spectrum taken with the 200-inch at Palomar Observatory and in addition to a rich emission-line spectrum, an underlying early-type absorption-line spectrum was visible. He noted that “this absorption-line spectrum appears to resemble the spectrum of a bright knot (super star cluster) near the nucleus of M82 that had previously been obtained with the same spectrograph.”
Later, in 198014, van den Bergh speculated that a previous encounter with M83 might have been what triggered all the star-formation seen in NGC 5253 – include the intermediate-age star clusters he discovered in its halo. That theory has since fallen out of favor, with the current starburst possibly being triggerd by the infall of an H I cloud15 or ram pressure stripping as the galaxy moves through the dense intergalactic medium of the Centaurus A group16.
Attachment 5124
Image Credit:ESA/Hubble & NASA/Judy Schmidt, North is 11.7° clockwise from up
Observations of NGC 5253’s central region using the Hubble Space Telescope (HST) were published in 1996 by Varoujan Gorjian17. Using the WFPC2, six young star clusters were discovered. They were labeled 1-6, in order of brightness, with the brightest one (the “central cluster”) having an F606W magnitude of +16.9. Another set of observations of the galaxy’s core was made using the HST nearly 20 years later. However, in Calzetti et al (2015)18, they used the Advanced Camera for Surveys (ACS), which allowed them greater detail. With that, they were able to identify an additional five stars clusters. The most intriguing one is that which they believed to be the ionizing source for a massive ultracompact H-II region visible with radio telescopes and dubbed the “supernebula” by Turner and Beck (2004)19. It’s a super star cluster that lies 0.4” west of another super star cluster – which is the “central cluster” Gorjian had identified. The only major difference between them is that the one powering the supernebula is obscured by up to 50 magnitudes of extinction! These are two of the youngest known among the super star clusters detected in nearby galaxies such as Henize 2-10, NGC 1569, M82, SBS 0335-052, the Antennae, and NGC 1705, to name a few.
Attachment 5123
Image Credit: ESA/Hubble & NASA/Judy Schmidt, North is 25.4° clockwise from up
In my 130mm reflector, NGC 5253 is the brightest galaxy south of M83 and both fit into the same field of view at 27x. Adding my NPB filter, the galaxy stays bright while the surrounding stars dim. This is something that you will find doesn’t happen with 99.99% of galaxies. At 68x in my 16-inch, a large part (but not all) of the galaxy appears to get brighter when using the NPB filter. At 300x and with the filter removed, the bright central region is broad and bulges out to northwest. Using a 36-inch at 332x, I found that the bright, broad central region gave a phenomenal response to my NPB filter. Without the filter, I scrutinized the lumpy H-II/SFR central region for any individual glows that would indicate resolution of a star cluster and sadly came away with nothing concrete.
Attachment 5122
My drawing from 4/2023 of NGC 5253 at 332x in a 36-inch Dobsonian
Cluster [G96] 1 (+16.9 F606W) = [CJA2015] 5 ----- all inside emission nebula [WS83] 4
Cluster [G96] 2 (+17.9 F606W) = [CJA2015] 1/2 ----- all inside emission nebula [WS83] 1
Cluster [G96] 3 (+18.2 F606W) = [CJA2015] 6 ----- all inside emission nebula [WS83] 5
Cluster [G96] 4/5 (+17.80 F606W being the combined magnitude of +18.5 and +18.6) = [CJA2015] 10/9 ----- all inside emission nebula [WS83] 2
Cluster [G96] 6 (+19.3 F606W)
I look forward to the day when I can claim having seen my next super star cluster. Until then,
“Give it a go and let us know!”
Special thanks to Brian Skiff of Lowell Observatory for his insights!
[1] Arp 1968 (https://ui.adsabs.harvard.edu/abs/19....129A/abstract)
In order of brightness it goes Cen A, M83, NGC 4945, NGC 5102, and then NGC 5253.
[2] Pickering & Fleming 1896 (https://ui.adsabs.harvard.edu/search...e%20desc&p_=0)
[3] Pickering & Fleming 1895 (https://ui.adsabs.harvard.edu/abs/18....320P/abstract)
[4] Campbell 1897 (https://articles.adsabs.harvard.edu/...pJ.....5..233C)
[5] Hubble & Lundmark 1922 (https://ui.adsabs.harvard.edu/abs/19....292H/abstract)
[6] Caldwell & Phillips 1989 (https://ui.adsabs.harvard.edu/abs/19....789C/abstract)
[7] Private email communication 5/2023
[8] Cecilia Payne Gaposchkin 1936 (https://ui.adsabs.harvard.edu/abs/19....173G/abstract)
[9] Baade & Zwicky 1934 (https://ui.adsabs.harvard.edu/abs/19....254B/abstract)
Also a good, short article on the matter here (https://www.pnas.org/doi/pdf/10.1073/pnas.1422666112)
[10] Chadwick 1932 (https://ui.adsabs.harvard.edu/abs/19....312C/abstract)
[11] Evens 1952 (https://articles.adsabs.harvard.edu/...bs....72..164E)
[12] Welch 1970 (https://ui.adsabs.harvard.edu/abs/19....821W/abstract)
[13] van den Bergh 1972 (https://ui.adsabs.harvard.edu/abs/19....237V/abstract)
[14] van den Bergh 1980 (https://ui.adsabs.harvard.edu/abs/19....122V/abstract)
[15] Meier et al 2002 (https://ui.adsabs.harvard.edu/abs/20....877M/abstract)
[16] Karachentsev 2007 (https://ui.adsabs.harvard.edu/abs/20....504K/abstract)
[17] Gorjian 1996 (https://ui.adsabs.harvard.edu/abs/19...1886G/abstract)
[18] Calzetti et al 2015 (https://ui.adsabs.harvard.edu/abs/20.....75C/abstract)
[19] Turner & Beck 2004 (https://ui.adsabs.harvard.edu/abs/20.....85T/abstract)
