Author: Tammy Plotner

Object Name: Messier 98
Alternative Designations: M98, NGC 4192
Object Type: Type Sb Barred Spiral Galaxy
Constellation: Coma Berenices
Right Ascension: 12 : 13.8 (h:m)
Declination: +14 : 54 (deg:m)
Distance: 60000 (kly)
Visual Brightness: 10.1 (mag)
Apparent Dimension: 9.5×3.2 (arc min)

Locating Messier 98: As part of the Virgo Cluster of Galaxies, M98 is best found by returning to our “galaxy hopping” ways we’ve learned. Begin with the bright M84/84 pairing located in the heavily populated inner core of the Virgo Cluster of galaxies about halfway between Epsilon Virginis and Beta Leonis. Once identified, stay at the eyepiece a move your telescope north until you locate M99 and continue at least 3 or 4 more eyepiece fields. This is what is known as “sweeping”. When you reach a star pattern you are certain that you can identify, shift the telescope one eyepiece field to the west. Now sweep south for several eyepiece fields. If you have not seen the slender scratch of M98, continue the process carefully one eyepiece field at a time. (Not all eyepieces have the same apparent field of view, but use your lowest magnification.) M98 is edge-on in presentation, so it will be a slender scratch of nebulousity that requires dark, clear skies and at least 4″ in aperture.

What You Are Looking At: M98 is nearly edge-on in presentation and displays a disturbed, misty elongated disk. There are some blue regions of new star formation, as well as a massive quantity of occulting dust which reddens the appearance of the small, bright nucleus. But where did all this dust come from?

“Debris sent into the intergalactic medium during tidal collisions has received much attention as it can tell us about several fundamental properties of galaxies, in particular their missing mass, both in the form of cosmological Dark Matter and so-called Lost Baryons. High velocity encounters, which are common in clusters of galaxies, are able to produce faint tidal debris that may appear as star–less, free floating HI clouds. These may be mistaken for Dark Galaxies, a putative class of gaseous, dark matter (DM) dominated, objects which for some reason never managed to form stars. VirgoHI21, in the Virgo Cluster, is by far the most spectacular and most discussed Dark Galaxy candidate so far detected in HI surveys. We show here that it is most likely made out of material expelled 750 Myr ago from the nearby spiral galaxy NGC 4254 during its fly–by at about 1000 km s?1 by a massive intruder. Our numerical model of the collision is able to reproduce the main characteristics of the system: in particular the absence of stars, and its prominent velocity gradient. Originally attributed to the gas being in rotation within a massive dark matter halo, we find it instead to be consistent with a combination of simple streaming motion plus projection effects.” say Piere Alain Duc.

“Based on our multi-wavelength and numerical studies of galaxy collisions, we discuss several ways to identify a tidal origin in a Dark Galaxy candidate such as optical and millimetre–wave observations to reveal a high metallicity and CO lines, and more importantly, kinematics indicating the absence of a prominent Dark Matter halo. We illustrate the method using another HI system in Virgo, VCC 2062, which is most likely a Tidal Dwarf Galaxy . Now, whereas tidal debris should not contain any dark matter from the halo of their parent galaxies, it may exhibit missing mass in the form of dark baryons, unaccounted for by classical observations, as recently found in the collisional ring of NGC 5291 and probably in the TDG VCC 2062. These “Lost Baryons” must originally have been located in the disks of their parent galaxies.”

So is it dust that dims M98’s core or is it something else? Something like maybe a Low Luminosity Active Galactic Nuclei (LLAGNs)? “Low-luminosity active galactic nuclei (LLAGNs) comprise 30% of all bright galaxies (B?12.5) and are the most common type of AGN . These include LINERs, and transition-type objects (TOs, also called weak- [OI] LINERs). These two types of LLAGNs have similar emission line ratios in [OIII]/HB, [NII]/H?, and [SII]/H?, but [OI]/H? is lower in TOs than in LINERs. LLAGNs constitute a rather mixed class and different mechanisms have been proposed to explain the origin of the nuclear activity, including shocks, and photoionization by a non-stellar source, by hotstars or by intermediate age stars.” says Rosa M. Gonzalez Delgado (et al). “Because we do not know yet what powers them and how they are related to the Seyfert phenomenon, LLAGNs have been at the forefront of AGN research since they were first systematically studied by Heckman (1980). Are they all truly “dwarf” Seyfert nuclei powered by accretion onto nearly dormant supermassive black holes (BH), or can some of them be explained at least partly in terms of stellar processes? If LLAGNs were powered by a BH, they would represent the low end of the AGN luminosity function in the local universe and would also establish a lower limit to the fraction of galaxies containing massive BHs in their centers. If, on the contrary, LLAGNs were powered by nuclear stellar clusters, their presence would play an important role in the evolution of galaxy nuclei. Therefore, it is fundamental to unveil the nature of the central source in LLAGNs.”

But that’s not all that’s hiding inside M98. Now let’s try type II LINERS. ” We present ASCA observations of low-ionization nuclear emission-line regions (LINERs) without broad H? emission in their optical spectra. The sample of “type 2″ LINERs consists of NGC 404, 4111, 4192, 4457, and 4569. We have detected X-ray emission from all the objects except for NGC 404; among the detected objects are two so-called transition objects (NGC 4192 and NGC 4569), which have been postulated to be composite nuclei having both an H II region and a LINER component. The images of NGC 4111 and NGC 4569 in the soft (0.5-2 keV) and hard (2-7 keV) X-ray bands are extended on scales of several kiloparsecs. The X-ray spectra of NGC 4111, NGC 4457, and NGC 4569 are well fitted by a two-component model that consists of soft thermal emission with kT ~ 0.65 keV and a hard component represented by a power law (photon index ~2) or by thermal bremsstrahlung emission (kT ~ several keV). The extended hard X-rays probably come from discrete sources, while the soft emission most likely originates from hot gas produced by active star formation in the host galaxy. We have found no clear evidence for the presence of active galactic nuclei (AGNs) in the sample.” says Yuichi Terashima (et al). “Using black hole masses estimated from host galaxy bulge luminosities, we obtain an upper limit on the implied Eddington ratios less than 5 × 10-5. If an AGN component is the primary ionization source of the optical emission lines, then it must be heavily obscured with a column density significantly larger than 1023 cm-2, since the observed X-ray luminosity is insufficient to drive the luminosities of the optical emission lines. Alternatively, the optical emission could be ionized by a population of exceptionally hot stars. This interpretation is consistent with the small [O I] ?6300/H? ratios observed in these sources, the ultraviolet spectral characteristics in the cases where such information exists, and the X-ray results reported here. We also analyze the X-ray properties of NGC 4117, a low-luminosity Seyfert 2 galaxy serendipitously observed in the field of NGC 4111.”

History: M98 was originally discovered by Pierre Mechain on March 15, 1781 and reported to Charles Messier who confirmed and logged it on April 13, 1781. In his notes he writes: “Nebula without star, of an extremely faint light, above the northern wing of Virgo, on the parallel and near to the star no. 6, fifth magnitude, of Coma Berenices, according to Flamsteed. M. Mechain saw it on Mar 15, 1781.”

Sir William Herschel would catch this great galaxy on December 30, 1783 with much detail. In his unpublished notes he writes: “”The difference [of Messier’s and Mechain’s observations on one hand, and Herschel’s on the other] will appear when we compare my observation of the 98th nebula with that in the Connoissance des Temps for 1784, which runs thus: [Messier’s description follows in French, as translated above]. My observation of the 30th December, 1783, is thus: A large, extended fine nebula. Its situation shews it to be M. Messier’s 98th; but from its description it appears, that that gentleman has not seen the whole of it, for its feeble branches extend above a quarter of a degree, or which no notice is taken. Near the middle of it are a few stars visible, and more suspected. My field of view will not quite take in the whole nebula.”

Obviously, taking your time and really “looking” at M98 makes a huge difference, as Admiral Smyth would point out about a hundred years later: “A fine and large, but rather pale nebula, between Virgo’s left wing and Leo’s tail; with the bright star, 6 Comae Berenices, following [East] in the next field exactly on the parallel. M. [Messier], who discovered it in 1781, merely registered it as “a nebula without a star, with an extremely faint light;” but on keeping a fixed gaze it brightens up towards the centre. It is elongated, in the direction of two stars, the one np [noth preceding, NW] and the other sf [south following, SE] of the object; with another star in the nf [north following, NE] quadrant pretty close. Differentiated with Beta Leonis, which star it follows by 6deg 1/2 in the direction of Arcturus; it lies on the outskirts of the vast region of nebulae that adorns the Virgin’s wing.”

Let your galaxy hunting skills take wing tonight!

Top M98 image credit, Palomar Observatory courtesy of Caltech, M98 2MASS image, M98 Spiral Structure (AANDA) M98 by Adam Block/NOAO/AURA/NSF, M98 Wikipedia courtesy of Ole Nielsen and M98 image courtesy of NOAO/AURA/NSF.

Object Name: Messier 97
Alternative Designations: M97, NGC 3587, Owl Nebula
Object Type: Type 3a Planetary Nebula
Constellation: Ursa Major
Right Ascension: 11 : 14.8 (h:m)
Declination: +55 : 01 (deg:m)
Distance: 2.6 (kly)
Visual Brightness: 9.9 (mag)
Apparent Dimension: 3.4×3.3 (arc min)

Locating Messier 97: Locating Messier 97 is fairly easy. You’ll find it one third the distance in a mental line drawn between Beta and Gamma Ursa Majoris and just slightly south of that line towards a dim star. Yep. The problem isn’t finding the Owl Nebula… It’s seeing it! Despite its billed combined magnitude of 9.9, this is one low surface brightness object and requires pristine skies to be seen with an average 4″ telescope. Nebula and light pollution filters do help, but sky conditions truly dictate. (This author has seen it in 16X65 binoculars, but from a guarded dark sky site.) What you are looking for is about the same diameter that Jupiter would be in the given eyepiece you are using and under average skies will appear only as the faintest contrast change. Large aperture, fast focal ratio telescopes improve your chances marginally.

What You Are Looking At: Messier 97 is a very unusual and dynamic planetary nebula whose shape may be considered that of a cylindrical torus shell viewed on the oblique. What we see photographically (and sometimes physically) as the “Owl’s Eyes” may be the projected matter-poor ends of the cylindrical shape, while the head could be a low ionization shell. Inside this 6,000 year old denizen of the night is a dying, now 16th magnitude star with a little bit more than half the mass of our own Sun. A star which – oddly enough – can sometimes be glimpsed easier than the nebula itself!

Why? Perhaps density? “We are able to evaluate the variation of excitation and electron density over the projected envelope of the source. We propose that the Owl Nebula consists of four primary shells: an internal, tilted, barrel-like component responsible for higher excitation emission; two much more uniform, spherically symmetric structures, CSCI and CSCII. These, finally, are enveloped by a much lower intensity, lower excitation halo, dubbed CSCIII. A large proportion of the low-excitation emission appears to be associated with the periphery of CSCI, and it is conceivable that this is, physically speaking, a relatively thin-shelled structure.” says L. Cuesta (et al). “[S II] density mapping appears to indicate that ne is preferentially enhanced toward the northern periphery of the shell, in a regime where low-excitation line strengths are also preferentially enhanced. We suggest that such trends may arise through northerly shocking of the shell CSC.”

So what gives with the holes we call eyes? Let’s ask R. L. M. Corradi (et al): “The haloes have been classified following the predictions of modern radiation-hydrodynamical simulations that describe the formation and evolution of ionized multiple shells and haloes around PNe. According to the models, the observed haloes have been divided into the following groups: (i) circular or slightly elliptical asymptotic giant branch (AGB) haloes, which contain the signature of the last thermal pulse on the AGB; (ii) highly asymmetrical AGB haloes; (iii) candidate recombination haloes, i.e. limb-brightened extended shells that are expected to be produced by recombination during the late post-AGB evolution, when the luminosity of the central star drops rapidly by a significant factor; (iv) uncertain cases which deserve further study for a reliable classification; (v) non-detections, i.e. PNe in which no halo is found to a level of ?10?3 the peak surface brightness of the inner nebulae.”

And what’s going on with the central star? “Einstein, EXOSAT, and ROSAT X-ray observations of planetary nebulae detected soft photospheric X-ray emission from their central stars, but the diffuse X-ray emission from the shocked fast stellar wind in their interiors could not be unambiguously resolved. The new generation of X-ray observatories, Chandra and XMM-Newton, have finally resolved the diffuse X-ray emission from shocked fast winds in planetary nebula interiors.” says Mart?n A. Guerrero. “Furthermore, these observatories have detected diffuse X-ray emission from bow-shocks of fast collimated outflows impinging on the nebular envelopes, and unexpected hard X-ray point-sources associated with the central stars of planetary nebulae. Here I review the results of these new X-ray observations of planetary nebulae and discuss the promise of future observations.”

Is it possible this is just one big planetary nebula bubble? According to Adam Frank and Garrelt Mellema: “We have presented radiation-gasdynamic simulations of aspherical Planetary Nebula (PN) evolution. These simulations were constructed using the Generalized Interacting Stellar Winds scenario where a fast, tenuous outflow from the central star expands into a toroidal, slow, dense circumstellar envelope. We have demonstrated that the GISW model can produce aspherical flow patterns. In particular we have shown that by varying key initial parameters we can produce a variety of elliptical and bipolar forward shock configurations. The dependence of the shock morphology on the initial parameters conforms to the expectations of analytical models (Icke 1988). We have demonstrated that including radiation-transfer, ionization, and radiative heating and cooling does not drastically alter the global morphologies. Radiative cooling does slow the evolution of the forward shock by removing energy from the hot bubble. The evolution of the forward shock configuration is independent of the ionization of the undisturbed slow wind. Also, radiation heating and cooling does change the temperature structure of the shocked slow wind material compressed into the dense shell.”

History: M97 was discovered by eagle-eyed Pierre Mechain on February 16, 1781. (That was back in the day where if you were complaining about light pollution that you asked your neighbor to “put out their candle”.) It was logged into record by Charles Messier on March 24, 1781 where he notes: “Nebula in the great Bear [Ursa Major], near Beta: It is difficult to see, reports M. Mechain, especially when one illuminates the micrometer wires: its light is faint, without a star. M. Mechain saw it the first time on Feb 16, 1781, and the position is that given by him.”

It was later noted by Sir William Herschel in his own celestial wanderings as: “The arguments that the nebulous matter is in some degree opaque which is given in the 25th article, will receive considerable support from the appearance of the following nebulae; for they are not only round, that is to say the nebulous matter of which they are composed is collected into a globular compass, but they are also of a light which is nearly of an uniform intensity except just on the borders. I give these nebulae in two assortments (incl. M97). Number 97 of the Connoissance is “A very bright, round nebula of about 3′ in diameter; it is nearly of equal light throughout, with an ill defined margin of no great extent.”

Top M97 image credit, Palomar Observatory courtesy of Caltech, M97 2MASS Image, M97 IR (NOAO), Owl Nebula – SEDS, “Owl Nebula” – Karen Kwitter (Williams College), Ron Downes (STScI), You-Hua Chu (University of Illinois) and NOAO/AURA/NSF, M97 (AANDA) and M97 images courtesy of NOAO/AURA/NSF.