Category: Astrophotos

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No. This polar ring isn’t a phone call from Santa, but an unusual interacting galaxy designated as NGC 660. Located over 20 million light-years away, this member of the M74 group located in the constellation of Pisces is rare breed – a “polar ring” galaxy – a bizarre configuration of stars, gas, and dust orbiting in ring formations nearly perpendicular to the plane of a flat galactic disk. What caused it? Read on…

Polar ring galaxies are believed to have formed from a spectacular collision of two galaxies who shared a common past. While they may not have merged, the encounter could have left a debris trail which encircles the host galaxy’s disk. “High-resolution interferometric data of the H I and OH absorption in the nuclear region of NGC 660 reveal three distinct absorbing structures. The central disk of the galaxy with a large velocity gradient dominates the absorption signature. The gas in the warped outer disk appears in absorption close to the systemic velocity; the outer rings of the warp located at large radii are moving in front of the nuclear radio source.” says Willema Baan (et al), “Third, an outflowing feature can be seen at the center of the radio source at 100 km/s below the systemic velocity. This mostly molecular feature could be due to a perturbed spiral structure in the inner regions of the disk.”

But, when it comes to NGC 660, the explanation might not quite be as straight forward. Apparently from our line of sight, the area of the ring closest to us doesn’t cross that galactic plane in the middle- but off to one side. This gives us a unique opportunity to study the shape of this galaxy’s hidden dark matter halo by calculating the enigma’s gravitational influence on the rotation of the ring and disk – a mass of starburst activity! Within the ring itself are an estimated 500 clusters where stars are continuously being born with the youngest of the siblings estimated to be about 7 million years old.

“NGC 660 contains concentrated central star formation of power ~ 2 x 1010~ Lsun. Our 1.3 cm continuum image reveals a bright, compact source of less than 10 pc extent with a rising spectral index. We infer that this is optically thick free-free emission from a super star cluster nebula. The nebula is less than 10 pc in size, comparable in luminosity to the “supernebula” in the dwarf galaxy, NGC 5253.” says J.P. Naiman, “We estimate that there are a few thousand O stars contained in this single young cluster. There are a number of other weaker continuum sources, either slightly smaller or more evolved clusters of similar size within the central 300 parsecs of the galaxy.”

But that’s not all that’s hiding in NGC 660, its unusual profile gives us an opportunity to study what happens to molecular gas densities when galaxies collide, too. It opens the mysterious phenomena of megamasers and kilomasers. “Contrary to conventional wisdom, IR luminosity does not dictate OHM formation; both star formation and OHM activity are consequences of tidal density enhancements accompanying galaxy interactions. The OHM fraction in starbursts is likely due to the fraction of mergers experiencing a temporal
spike in tidally driven density enhancement.” says Jeremy Darling. “OHMs are thus signposts marking the most intense, compact, and unusual modes of star formation in the local universe. Future high-redshift OHM surveys can now be interpreted in a star formation and galaxy evolution context, indicating both the merging rate of galaxies and the burst contribution to star formation.”

But what about things we can’t see? Things far more powerful in the electromagnetic spectrum than those given off by Cassiopeia A… Compact radio sources! “Nuclei of starburst galaxies are often obscured by dust and hence are probed best in non-visual wavelength regimes such as the infrared and radio.” sys A. Wiercigroch (JPL). “A number of the compact sources appear to lie along a ring projected against the more diffuse radio emission in the galaxy’s nuclear region.”

Not bad for just another Christmas story….

Credits: Image processing Dietmar Hager and Immo Gerber. Image Acquistion at Tao Observatory. We thank you so much!

What accretes quietly in the night and can be a blast to observe? Try a FUor… These high accretion, high luminosity phase pre-main sequence stars may only last a few decades – but display an extreme change in magnitude and spectral type in a very short period of time. While FU Orionis may be the prototype you know about, there’s a lot more to learn and even more to observe! Step outside in the dark with me and let’s take a look…

What we know so far about FU Orionis-type stars is they flare with abrupt mass transfer from an accretion disc onto a young, low mass T Tauri-type star. In itself, this is very exciting because nearly half of T Tauri stars have circumstellar disks or protoplanetary discs. These could very well be the forerunners of planetary systems similar to our own solar system! How do we know there is a disc there? Try variablility. “Variable circumstellar extinction is pointed out as responsible for the conspicuous variations observed in the stellar continuum flux and for concomitant changes in the emission features by contrast effect. Clumpy structures, incorporating large dust grains and orbiting the star within a few tenths of AU, obscure episodically the star and, eventually, part of the inner circumstellar zone, while the bulk of the hydrogen lines emitting zone and outer low-density wind region traced by the [OI] remain unaffected.” says E. Schisano (et al), “Coherently with this scenario, the detected radial velocity changes are also explainable in terms of clumpy materials transiting and partially obscuring the star.”

While accretion rates for a FUor could range anywhere from 4 to 10 solar masses annually and its eruptions last up to a year or longer, astronomers believe their entire lifetimes only last a few decades. The proto-star itself may also be limited to undergoing an average of one to two eruptions each year. “The brightness of FUors increases by several magnitudes within one to several years. The currently favored explanation for this brightness boost is that of dramatically rising accretion from the disc material around a young star. The mechanism leading to this accretion increase is a point of debate.” says S. Pfalzner, “The induced accretion rates, the overall temporal accretion profile, the decay time, and possibly the binarity rate we obtain for encounter-induced accretion agree very well with observations of FUors. However, the rise time of one year observed in some FUors is difficult to achieve in our simulations unless the matter is stored somewhere close to the star and then released after a certain mass limit is transgressed. The severest argument against the FUors phenomenon being caused by encounters is that most FUors are found in environments of low stellar density.”

Surprisingly enough, even given the short period of time in which a FUor exists, no one has ever seen one phase out. “A cross-correlation analysis shows that FUor and FUor-like spectra are not consistent with late-type dwarfs, giants, nor embedded protostars. The cross-correlations also show that the observed FUor-like HH energy sources have spectra that are substantively similar to those of FUors.” says Thomas P. Greene (et al), “Both object groups also have similar near-infrared colors. The large line widths and double-peaked nature of the spectra of the FUor-like stars are consistent with the established accretion disk model for FUors, also consistent with their near-infrared colors. It appears that young stars with FUor-like characteristics may be more common than projected from the relatively few known classical FUors.”

Just how common and observable are these unusual characters? A lot more than you might think. According to Bo Reipurth (et al); “The original FUor class was defined by a small number (5-6) of pre-main sequence stars that had been observed to brighten up by 3-6 magnitudes on time scales of 1-10 years. The class has since been augmented by a comparable number of stars that have similar spectra or SEDs to the classical FUors, but that have not been observed to behave photometrically in that way. It is likely that the FUor phenomenon is recurrent, but it is not at all clear whether it is a property shared by ordinary T Tauri stars, or whether it is confined to a special minority among them. It is important that more examples be found, and found promptly, and as the result of systematic search rather than by accident as has been the case in the past. The goal would be to examine, on a regular monthly basis, all the molecular clouds within about 2 kpc that lie along the galactic plane and Gould’ s Belt for faint (or previously invisible) stars that had brightened up by a magnitude or more. It is essential that any such detections be followed up spectroscopically as soon as possible, to weed out interlopers: flare stars, cataclysmic variables, Miras, and EXors (the latter also being pre-main sequence but which unlike FUors soon return to their original brightness level, usually in a year or less). All of these objects are readily distinguishable from one another even at modest spectroscopic resolution. Such an on-going survey would serve also to follow the development of FUors.”

So let’s do the FUor dance!

According to CBET 2033 released on November 21, 2009 from the International Astronomical Union: “The discovery of a possible FU-Ori-type eruption (see Hartmann and Kenyon 1996, ARAA 34, 207) is located at R.A. = 6h09m19s.32, Decl. = -6o41’55”.4 (equinox 2000.0), and coincident with the infrared source IRAS 06068-0641. Discovered by the CRTS on Nov. 10, it has been continuously brightening from at least early 2005 (when it was mag 14.8 on unfiltered CCD images) to the present magnitude of 12.6, and may possibly brighten further. On recent images, a faint cometary reflection nebula is visible to the east. A spectrum (range 350-900 nm), taken with the SMARTS 1.5-m telescope at Cerro Tololo, on November 17, shows H-alpha in emission, all other Balmer lines and He I (at 501.5 nm) in absorption, and a very strong Ca II infrared triplet in emission, confirming it to be a young stellar object. The object lies inside a dark nebula to the south of the Mon R2 association, and is likely related to it. In addition, also inside this dark nebula, a second object at R.A. = 6h09m13s.70, Decl. = -6o43’55”.6, coincident with IRAS 06068-0643, has been varying between mag 15 and 20 over the past few years, reminiscent of UX-Ori-type objects with very deep fades. Also, this second object supports a variable cometary reflection nebula, extending to the north. The spectrum of this object also shows H-alpha and the strong Ca II infrared triplet in emission.”

Visible? Yeah. You know it. And here are the wide field results as taken by Joe Brimacombe…

“A smaller site of ongoing star formation in the Mon R2 molecular cloud are the objects associated with GGD 16 and 17. To the south of GGD 17, the T Tauri star Bretz 4 is probably associated with the GGD object. This star has been studied spectroscopically and was classified by as a K4 spectral type with a class 5 emission spectrum.” says Carpenter and Hodapp, “The infrared source IRS 2 is positionally coincident with Bretz 4, while the more deeply embedded IRS 1 has no optical counterpart and lies between the GGD objects. A detailed optical study showed that GGD 17 is part of a curved jet extending north of the star Bretz 4 and consisting of HH 271, and possibly also HH 273. Nebulosity close to the star shows the typical morphology of scattered light from an outflow cavity wall. The embedded infrared objects and optical reflection nebulosity in the general GGD 16-17 region is associated with 850 um emission.”

Capture a FUor… It may be the most unusual thing you’ve ever done!

Many thanks to Joe Brimacombe for the awesome images and awakening my ‘FUor’ curiousity!

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Far away in the constellation of Aries, in a 14th magnitude barred spiral galaxy designated as NGC 918… a star exploded with enough candlepower to briefly outshine its home. Discovered independently by Lick Observatory Supernova Search (LOSS) and Koichi Itagaki (Japan) on October 11, 2009, this Type II supernova might be hiding in the intergalactic dust, but it isn’t hiding from Joe Brimacombe.

So who is to blame for this poor intergalactic housekeeping condition, eh? Just exactly where did this film of dust come from that dims distant galaxies and cloaks supernova events? Try our own Milky Way. We’ve known since the first Palomar Sky Surveys that we’re looking through clouds and filaments of dust at high galactic latitudes. But it isn’t just our galaxy either… It’s our whole family! Chances are the entire local group is puffing out enough hydrogen to send up a smoke screen – possibly even with higher redshift extragalactic objects. And just who is the smoker of our group?

The Andromeda Galaxy – M31…

“Finally we come to the aspect which could most shake conventional beliefs about the Local Group and the nature of near space. Deep prints of a red sensitive Schmidt plate (Arp and Sulentic 1991) show unmistakable filamentary dust features reaching back along the minor axis direction toward M31.This filament is repeated in the blue photographs and 100 the hundred micron infra red scans. They have to be real. Although no one has cared to take a spectrum there is no hint of gaseous emission.” says Halton Arp.

“The ejection path across the whole Local Group sky from M31 to 3C120 must have carried material either dusty or capable of forming dust from the ejecting M 31. But that means dust and obscuration within the Local group of galaxies – a point which has never before been seriously advanced. But how can one escape the mult-iwavelength evidence? The most provocative object in the M31 minor axis line is NGC 918. The nebulous dust is most concentrated at the position of the galaxy but a region has been cleared on either side of the minor axis of the galaxy. Higher resolution images would give invaluable information on the process whereby ejections come out along the minor axis of galaxies. In addition the nebulosity is of such long extent across the sky and so coincident with the alignment along the M31 minor axis that it must be in the Local Group. Therefore interaction with the dust filament would represent direct evidence for a distance much smaller than NGC 918’s conventional redshift distance.”

“The filamentary features surrounding NGC 918 are well shown in this image. The outer features appear to be dust illuminated by the galaxy. Immediately around the galaxy the dust appears to cleared away. By either outflow of matter or radiation pressure from the galaxy.” explains Arp, “If the galaxy is not interacting with the nebulosity but just shining through a serendipitous hole we still have the remarkable inference that material has been ejected along the minor axis of M31 into the middle of the Local Group of galaxies. The question then arises as to how many other nearby galaxy groups contain intergalactic material and what this would do to our view of purportedly more distant galaxies.”

If dust is to blame for a clouded view here, is it possible that NGC 918 could be just as guilty of ignoring the Swiffer? Darn right it could. According to research done by E. E. Martinez-Garcia (et al), NGC 918 has its share of spiral density waves that present azimuthal color gradients that even an infrared passband won’t fully penetrate. “We believe that this effect may be due to the position of the dust lanes and stars with respect to the observer.” says Garcia, “More research needs to be done to understand the origin of this effect.”

In the meantime, we’ll thank Joe Brimacombe of Northern Galactic for being on watch and capturing this distant supernova within 24 hours of its discovery. Cuz’ another one bites the dust!

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The Crescent Nebula, also known as NGC 6888, is a very well renown and most intriguing object located in the constellation Cygnus in the northern hemisphere. At an apparent size of about 18 by 13 arc-minutes it is a very pale nebula. Even in a moderate amateur telescope you can’t quite see this one unless you have absolute dark skies (or narrow band filters) and a decent “light bucket”. So how do we get a chance to study it? Photographically, of course…

Spanning some 25 by 18 light years, gazing at NGC 6888 means we are looking 4700 years into the past, a past that renders a nebula fueled and excited by the blue star at the center. And not just any blue star – but a high mass super-giant star – one that depleted its fuel at “full speed”. Not only was it a super giant, but hot… in the class of “Wolf Rayet” stars (HD 192163). Now, after only a couple of million years the “stellar gas” is almost used up and the star is standing right before a significant change: a supernova candidate. Behold a star that vents its outer layers into space at terrific speed!

“Images are used to constrain models of the ionization structure of nebular features.” says Brian D. Moore (et al) of the Department of Physics and Astronomy, Arizona State University, “From these models, we infer physical conditions within features and estimate elemental abundances within the nebula. The results of our analysis, together with the degree of small-scale inhomogeneity apparent in the images, call into question the assumptions underlying traditional methodologies for interpretation of nebular spectroscopy. The thermal pressure of photoionized clumps is higher than the inferred internal pressure of the shocked stellar wind, implying that the current physical conditions have changed significantly over less than a few thousand years.”

While the central star sustains severe loss of mass, the gas is holding lots of oxygen and hydrogen… just before the individual big “bang” of the WR-star creating a “hot bubble” whose struture can’t quite be explained yet. “A detailed analysis of the H I distribution at low positive velocities allowed us to identify two different structures very probably related to the star and the ring nebula. From inside to outside they are: (1) an elliptical shell, 11.8×6.3 pc in size, that embraces the ring nebula (labeled inner shell); and (2) a distorted H I ring, 28 pc in diameter, also detected in IR emission (outer shell). The borders of the inner shell strikingly follows the brightest regions of NGC 6888, showing the sites where the interaction between the nebula and the surrounding gas occurs. A third structure, the external feature, is a broken arc detected at slightly higher velocities than the former shells.” says Christina Cappa (et al), “We propose a scenario in which the strong stellar wind of HD 192163, expanding in an inhomogeneous interstellar medium, blew the outer shell during the main sequence phase of the star. Later, the material ejected by the star during the LBV (or RSG) and WR phases created NGC 6888. This material encountered the innermost wall of the outer shell originating the inner shell. The association of the external feature with the star and the nebula is not clear.”

For a look inside, view the full size image!

Many thanks to Dietmar Hager and Immo Gerber of TAO-Observatory for sharing this incredible image!