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Slide1: 

A SUPPLEMENT TO A REVISED CLASSIFICATION OF GALACTIC MORPHOLOGIES BASED ON A PUTATIVE MECHANISM OF EPISODIC NUCLEAR MASS EXPULSIONS Wirt Atmar AICS Research, Inc PO Box 4691, University Park, NM 88003 atmar@aics-research.com August 2007

Slide2: 

Bars are now estimated to be present in 70-75% of all spiral galaxies, thus they can no longer be considered the unusual state in galactic morphology. In the revised taxonomy to be presented, the episodic formation of bars is considered central to the process of galactic evolution. Bars are argued to be formed by mass expelled from the galactic nucleus. During that time when these expulsions are underway, a bar is created as the first stage in the evolution of a spiral. When the nuclear expulsion loses potency, the bar decays and eventually disappears, evolving the galaxy into a “normal” spiral. The identification and classification of galactic types is based on this putative process. THE HYPOTHESIS

Slide3: 

In distinct contrast to this mass expulsion hypothesis, current theory argues a far simpler thesis: the evolution of spiral morphologies is wholly determined as the result of recurrent gravity density waves circulating through a relatively uniform disc of gas orbiting the galactic nucleus. Such an argument however appears to be incompatible with the bulk of observational data, as will be discussed. The mass expulsion hypothesis requires the invocation of two attributes, which on their face seem fantastic: (i) the episodic, highly collimated expulsion of mass from the galactic nucleus, and (ii) the presence of a viscous frame enveloping the evolving galaxy, presumably composed of dark matter, on which the emitted mass is “written” rather indelibly. However, this hypothesis is testable and supporting evidence will be presented throughout the presentation.  THE HYPOTHESIS

Slide4: 

THE COMMON ATTRIBUTES OF BARS NGC 1300 has often been described as the prototypical barred galaxy. It is a nearby (21 Mpc), single-armed, pole-oriented barred galaxy that has recently been well-resolved by the Hubble Space Telescope. Because of this image, NGC 1300’s structure can be used to illustrate those core features that are presumed to be common to all barred galaxies. 

Slide5: 

1. Linear, leading edge dust lanes appear to exist in all barred galaxies.

Slide6: 

1. Linear, leading edge dust lanes appear to exist in all barred galaxies. These lanes have been commonly argued to be regions of inflow, feeding an active galactic nucleus, transferring mass from the interstellar medium to the central engine of the galaxy, or creating central bulges that cause late-type spirals to evolve their Hubble type.  

Slide7: 

1. Linear, leading edge dust lanes appear to exist in all barred galaxies. However ~30% of all Seyfert galaxies, which are defined as a class by their active nuclei, do not currently possess bars. Moreover, NGC 1300, which clearly possesses dust lanes embedded in a bar, is not known to have an active nucleus, indicating that there is either no black hole at its center, or that it is not accreting matter. These two observations thus cast some doubt on the inflow hypothesis.   

Slide8: 

1. Linear, leading edge dust lanes appear to exist in all barred galaxies. Rather than indicative of inflow, the evidence to follow suggests that the leading edge dust lanes represent mass outflow from the nucleus to blades of the spiral.   

Slide9: 

2. 90° breakover points exist at the ends of the bar. Compelling evidence that the dust lanes cannot be inflow are the 90° breakover points that exist at the ends of the bar.  

Slide10: 

2. 90° breakover points exist at the ends of the bar. It would be difficult to hypothesize a physical process that could cause inflowing mass to follow such a trajectory, but the opposite is not true. If the dust lanes were associated with a collimated mass outflow from the galactic nucleus to the outer blades, the point at which the torque of the bar is exceeded would create such highly defined flow deflections.

Slide11: 

3. The center of the bar is thus similarly presumed to be a laminar, low-luminosity outflow.

Slide12: 

3. The center of the bar is thus similarly presumed to be a laminar, low-luminosity outflow. Little star formation occurs within the central bar, suggesting a collimated outflow, although the velocity of the flow is so low that it has proven quite difficult to measure.

Slide13: 

3. The center of the bar is thus similarly presumed to be a laminar, low-luminosity outflow. In order to provide the bar with its evident rigidity, the central bar is hypothesized to be constrained by a magnetic solenoid and to rotate as a rigid body.

Slide14: 

4. The bar arms tend to be highly symmetric, within measurement error, indicating the presence of balanced forces.

Slide15: 

4. The bar arms tend to be highly symmetric, within measurement error, indicating the presence of balanced forces. If the cause of the right-angle breaks at the ends of the bar are due to excessive torque, then the symmetry evident in the bar arms implies (i) a pervasive and equal force running throughout the collimating solenoid, and (ii) conditions of equal density in the medium encountered by the rotating bar.

Slide16: 

5. Regions of dust leakage exist on the trailing edges of a strong bar.

Slide17: 

5. Regions of dust leakage exist on the trailing edges of a strong bar. Multiple streams of dust are shed perpendicularly from the leading edge dust lanes, apparently as the result of a frictional wind created by the bar rotating through a static ISM, reinforcing the idea of a robust, rigid bar.

Slide18: 

5. Regions of dust leakage exist on the trailing edges of a strong bar. These trailing streams mirror the perpendicular trajectories at the ends of the bar of the primary dust lanes, similarly reinforcing the idea that the cause of the deflections is due to torque breakover in the face of an encountered wind.

Slide19: 

6. The primary dust lanes are surprisingly persistent and can be backtracked to the ends of the blades.

Slide20: 

6. The primary dust lanes are surprisingly persistent and can be backtracked to the ends of the blades. This persistence, when taken with other evidence, implies that the blade patterns are non-Keplerian in their motions. The blades instead appear as if they have been “painted” onto an unseen frame and possess a permanence that should not be expected if Newtonian considerations were dominant.

Slide21: 

7. Once the collimating torque of the bar has been exceeded, the bar breaks and the mass outflow shock-terminates, resulting in large regions of new star formation.

Slide22: 

8. The presence of the active mass-transport bar alters the nature and composition of the galaxy, fractionating the star species into distinctly separate geographic realms.

Slide23: 

8. The presence of the active mass-transport bar alters the nature and composition of the galaxy, fractionating the star species into distinctly separate geographic realms. province of intermediate Pop II stars The halo and nucleus of a spiral galaxy bears a strong resemblance to an elliptical galaxy, suggesting that it is the most ancient component of the galaxy.

Slide24: 

8. The presence of the active mass-transport bar alters the nature and composition of the galaxy, fractionating the star species into distinctly separate geographic realms. province of large-scale Pop I star formation province of large-scale Pop I star formation province of intermediate Pop II stars The halo and nucleus of a spiral galaxy bears a strong resemblance to an elliptical galaxy, suggesting that it is the most ancient component of the galaxy. The blades, in contrast, become the province of newer Pop I stars.

Slide25: 

8. The presence of the active mass-transport bar alters the nature and composition of the galaxy, fractionating the star species into distinctly separate geographic realms. province of large-scale Pop I star formation province of large-scale Pop I star formation province of intermediate Pop II stars The halo and nucleus of a spiral galaxy bears a strong resemblance to an elliptical galaxy, suggesting that it is the most ancient component of the galaxy. If the mass flow through the bar were inward, the expectation would be that these two provinces would be reversed.

Slide26: 

9. A secondary region of new star formation occurs in advance of the leading edge of the bar. The pressure wave indicated by this star formation is further evidence of the intrinsic strength of the bar.

Slide27: 

As will be discussed, in some barred galaxies, this leading edge region of shock star formation can be extensive. 9. A secondary region of new star formation occurs in advance of the leading edge of the bar. The pressure wave indicated by this star formation is further evidence of the intrinsic strength of the bar.

Slide28: 

THE EVOLUTION AND DECAY OF A BAR A sufficient number of nearby galaxies have now been imaged so that they may be considered an archipelago of replicates. All nearby galaxies are presumed to be of the same age, ~13 Ga, but are now found in different evolutionary stages, most likely because of their differing initial conditions. The evolution and decay of a bar can be illustrated using three nearby galaxies.

Slide29: 

A Bar at Full Strength NGC 1300 is a primary example of a galaxy with a bar at full strength. Mass expulsion is currently underway, and the presumed collimating solenoid is operating at full force. NGC 1300

Slide30: 

A Weakening Bar When the collimating solenoid, which is presumably driven by forces from within the nucleus, begins to weaken, the point of torque breakover and the distinct 90° dust lane deflection points at the ends of the bar begin to relax.   NGC 5236

Slide31: 

The result is threefold: NGC 5236

Slide32: 

The result is threefold: NGC 5236 a more gentle curvature in the flow pattern at the ends of the bar

Slide33: 

The result is threefold: NGC 5236 a more gentle curvature in the flow pattern at the ends of the bar a subsequent symmetric shortening of the bar as the bar continues to rotate

Slide34: 

The result is threefold: NGC 5236 a more gentle curvature in the flow pattern at the ends of the bar a significant increase in dust leakage from the trailing edges of the bar a subsequent symmetric shortening of the bar as the bar continues to rotate

Slide35: 

A Bar Fully Decayed As the bar arms continue to weaken, their radii contract, ultimately leaving no effective radius subsequent to the collapse of the solenoid. Mass apparently continues to flow, or was already in motion, during the contraction of the bar so that a complete spiral pattern is drawn. NGC 2997

Slide36: 

A Bar Fully Decayed The leading edge dust lanes remain prominent features after the bars have disappeared and are easily recognized as persistent attributes of the arms. primary dust lanes NGC 2997

Slide37: 

A Bar Fully Decayed Perhaps as importantly, the remnant trailing edge dust leakage lanes appear as persistent. primary dust lanes remnant dust leakage trails NGC 2997

Slide38: 

A REVISED CLASSIFICATION OF SPIRALS In Hubble’s original classification, galactic morphologies were assigned a linear sequence E0 to E7 for the ellipticals, while the spiral galaxies were broken into the two arms of a “tuning fork”, Sa to Sc for “normal” spirals and SBa to SBc for “barred” spirals.

Slide39: 

A REVISED CLASSIFICATION OF SPIRALS Hubble attached the labels “early,” “intermediate” and “late” to the non-committal letters “a,” “b” and “c,” respectively, although he made clear that he implied no evolutionary sequence by these labels. They were meant only to imply a progression from simple to complex forms.

Slide40: 

A REVISED CLASSIFICATION OF SPIRALS That view will be reversed in the material to follow, and the “tuning fork” will be collapsed into a single interwoven lineage. The consequence is that a specific evolutionary sequence is now prescribed in this fashion: SBc ® Sc ® SBb ® Sb ® SBa ® Sa

Slide41: 

A REVISED CLASSIFICATION OF SPIRALS The process that mandates this change are the hypothesized episodic mass expulsions from galactic nuclei. During that time when these presumptive expulsions are underway, a bar is formed as the first stage in the evolution of a spiral. When the expulsion loses potency, the bar decays and eventually disappears, transforming the morphology of the galaxy from a “barred” spiral into a “normal” spiral.

Slide42: 

A REVISED CLASSIFICATION OF SPIRALS SBc/Sc galaxies will be defined to be single-armed galaxies in which only one mass nuclear expulsion has occurred. As the number of expulsions increases, the individual galaxies climb in the sequence.

Slide43: 

M74 Type Sc In this revised classification, galaxies are typed by the number of nuclear mass expulsion events that they have experienced during the course of their evolutionary histories, and thus by the number of arms they now exhibit.  

Slide44: 

M74 Type Sc M74 is a prototypical structure composed of only one set of arms, indicating that only one nuclear mass expulsion event has occurred during its history, and is thus classified as a type Sc galaxy.  

Slide45: 

M101 Type Sb M101, a similar spiral, is composed of what appears to be a recent well-defined set of arms that has overwritten at least one older set of arms. The presence of these older, less distinct background arms defines the Sb type. 

Slide46: 

M101 Type Sb An older, fainter set of arms are traced by the dashed lines.

Slide47: 

M101 Type Sb Fainter, more ancient arms yet are traced by the dotted lines. If this interpretation is correct, then the persistence of these ancient arms would be impossible in a Newtonain framework unless they had been “painted” onto an unseen frame, perhaps composed of dark matter, and thus somehow immobilized.

Slide48: 

M101 Type Sb As many simulations have shown, a rapidly rotating galaxy, if its morphology were determined wholly by gravitational considerations, would very rapidly sweep up these extra arms. Yet the persistance of these extra arms appears somewhat permanent.

Slide49: 

NGC 4414 Type Sa “Flocculent” galaxies such as NGC 4414 have experienced a sufficiently large number of independent mass expulsion events, to the point that the individual arms can no longer be readily discerned, and thus become prototypical of the Sa form of galaxy in this classification.

Slide50: 

THE INTERPRETATION OF NGC 1530 NGC 1530 is representative of a modest class of barred galaxies (e,g., NGC 1365, NGC 2903, NGC 5383). These galaxies are single-armed, barred structures whose most salient feature is a bright “I” structure terminating their bars. The structure of NGC 1530 recapitulates the phenomena previously seen in NGC 1300, but now in a slightly more complex bar that is rotating into itself.

Slide51: 

NGC 1530 A bright “I” structure terminates the bar arms of NGC 1530.

Slide52: 

NGC 1530 The crossbar of the “I” results from two effects: (i) shock-terminated gas emanating from the ends of the bar producing a trailing line of new Pop I star formation A bright “I” structure terminates the bar arms of NGC 1530.

Slide53: 

NGC 1530 The crossbar of the “I” results from two effects: (ii) a leading edge compression wave creating a lesser region of star formation in advance of the rotating bar A bright “I” structure terminates the bar arms of NGC 1530.

Slide54: 

Rather than the single primary gas flow evident in NGC 1300, three distinct “spigots” of gas outflow are apparent at the right-hand end of NGC 1530’s bar. NGC 1530 outermost gas flow an intermediate, low volume gas flow innermost gas flow

Slide55: 

NGC 1530 outermost gas flow an intermediate, low volume gas flow innermost gas flow In addition to these flows, heavy dust leakage is occurring from the trailing edges of the bar. Rather than the single primary gas flow evident in NGC 1300, three distinct “spigots” of gas outflow are apparent at the right-hand end of NGC 1530’s bar.

Slide56: 

NGC 1530 outermost gas flow an intermediate, low volume gas flow innermost gas flow In addition to these flows, heavy dust leakage is occurring from the trailing edges of the bar. The combination of these various flows creates a near-ring, apparently ignited into star formation by a leading edge compression wave propagating through the deposited gas. Rather than the single primary gas flow evident in NGC 1300, three distinct “spigots” of gas outflow are apparent at the right-hand end of NGC 1530’s bar.

Slide57: 

NGC 1530 outermost gas flow an intermediate, low volume gas flow innermost gas flow Barred galaxies with internal rings (e.g., M95, M109, NGC 5850, etc.) are an extension of the NGC 1530 class. All of these galaxies possess the features common to active bars in the category: a rigid bar with pronounced leading edge dust lanes, 90° breakover points at their ends, and an evident “I” structure, which is now more often described as a “theta” design, due to a more fully formed ring of active star formation. Rather than the single primary gas flow evident in NGC 1300, three distinct “spigots” of gas outflow are apparent at the right-hand end of NGC 1530’s bar.

Slide58: 

BARRED GALAXIES WITH RINGS Ringed barred galaxies are the physical consequence of a bar persisting long enough and with sufficient strength for it to rotate into itself one or more times, and thus represents a culmination of the evolutionary sequence of these galactic classes: NGC 1300 ® NGC 1530 ® bars with rings

Slide59: 

Because the bar is now rotating into its previously deposited gas, the point at which torque breakover will likely appear occurs increasingly earlier with each rotation due to increased drag. NGC 3992

Slide60: 

NGC 1433 However, if the bar contracts only to a certain radius and remains that way for at least one half bar rotation, the result is a well-ringed barred morphology.

Slide61: 

A ringed galactic morphology appears to be an alternate, intermediate decay form to that of a “normal” spiral, but one that similarly results from the collapse of the generative bar. NGC 2217

Slide62: 

The ring is generally only the most interior portion of the galaxy’s morphology. Spiral arms almost always exist at much greater radii. NGC 2217

Slide63: 

NGC 6782 NGC 6782 is a ringed galaxy apparently nearing the final collapse of its bar. While the inner rings are undoubtedly Linblad orbital resonances, the major structure of the galaxy, which is not visible in this image, cannot be easily explained gravitationally.

Slide64: 

Illustration Credit: L. Frattare, STScI The outermost arms of NGC 6782, which are faint by comparison, are ancient and disjoint from the recent ring structure. The outer arms also appear to be immobilized against the background of the galaxy.

Slide65: 

MULTIPLANED GALAXIES Multiplaned galaxies are of special interest to the present hypothesis. All of the spiral galaxies in the Local Group are warped to some degree, but these warps have not yet been well explained by theoretical gas kinematics or other considerations. Nonetheless, as Nelson and Tremaine have written, warped galaxies are so common that they must have a commonplace explanation.

Slide66: 

M31 While galaxies are often warped in their HI disk, a region that extends further than the visible disk, many galaxies exhibit multiple planes within their visible disk, a phenomenon that is truly difficult to explain either by kinematic considerations or tidal interactions.

Slide67: 

M31 M31 exhibits at least two such planes in its visible disk. Plane A Plane B

Slide68: 

M31 Although a number of mechanisms have been proposed to explain the outer edge HI warps observed in galaxies, even these low-mass warps remain an unresolved puzzle in galactic dynamics. The two most plausible arguments advanced to explain the HI warps have been kinematic bending waves and tidal forcing. Plane A Plane B

Slide69: 

M31 Under the presumptions that a galactic disk is a self-gravitating structure, dominated by Newtonian/Keplerian constraints, Hunter and Toomre have shown that a thin disk in centrifugal equilibrium is stable to vertical perturbations, and should not persist in such a state for any length of time. Similarly, the Westerbork survey has demonstrated that tidal interactions cannot be a major cause of the warps. Many warped galaxies exist as isolated systems. Plane A Plane B

Slide70: 

NGC 7331 A self-gravitating, Newtonian disk would theoretically possess the qualities evident in Saturn’s rings: a self-restoring, very thin, very flat disc.

Slide71: 

To emphasize the flatness expected, Saturn’s disc has a ~170,000:1 width-to-depth ratio. When seen edge-on, the rings are so thin that they virtually disappear.

Slide72: 

NGC 7331 We should therefore not expect to find individual arm components significantly tilted out-of-plane if gravitational considerations alone dominate the formation of the galactic disk.

Slide73: 

NGC 7331 Nor could any easily imaginable tidal interaction lift only a few discrete rings, as evident here, and not perturb the others. But even if that were possible, such disturbances should not persist long under strict self-gravitating constraints.

Slide74: 

NGC 3190 The present hypothesis offers a simpler, alternate explanation. If the galactic nucleus should precess in its rotation between episodic bar formations, subsequent mass expulsions will occur out-of-plane with respect to the older, outer arms.

Slide75: 

NGC 3190 The innermost arms formed by these later expulsions will inherently be contained within the visible disk, but now oriented at an angle. Such wobble appears to have occurred in each of the galaxies illustrated in this section.

Slide76: 

ESO 510-G13 The right and left edges of ESO 510-G13, as seen in this edge-on image, exhibit the multiple flange pattern evident in M31.

Slide77: 

ESO 510-G13 This pattern, in the context of the current argument, is generated by a more recently emitted internal arms constrained by, and significantly tilted with respect to, an older, outer disk component.

Slide78: 

ESO 510-G13 The rings of Saturn are measured in terms of thousands of kilometers; galactic arms are measured in thousands of parsecs. Effects apparently too small to be influential on the size of Saturn’s rings become significant at galactic scales, and thus potentially offer another clue into the nature of dark matter, should it exist. Why should such a discrepancy in the behaviors of the rings of Saturn and the disks of these multiplaned galaxies exist? The only reasonable answer must ultimately invoke the nature of the space and scalar distances that each occupies.

Slide79: 

NGC 613

Slide80: 

M101 M101 may similarly be a multiplaned galaxy, but one that is being viewed pole on. Why do some of the older arms of M101 appear to cross the newest pair of arms without being disturbed? Nuclear precessions between mass expulsions would leave some of the more ancient expelled arms out of plane with others and thus not in contact, especially at larger radii.

Slide81: 

THE ALTERNATIVE ARGUMENT Virtually all current knowledge regarding the evolution of galaxies has been derived from N-body computational models. These models make the simplest of assumptions: that the evolution of galactic morphologies has been wholly dominated by gravitational considerations. It’s that assumption that is questioned in the mass expulsion hypothesis.

Slide82: 

COMPUTATIONALLY MODELED GAS FLOWS The computational model that has been in place for the last several decades, in its various forms, has invoked only Newtonian/Keplerian gravitationally driven orbital considerations, although with increasing realism. The models now include inelastic collisions for gas and collisionless stars. Within this context, this approach has been partially successful, but it has not yet accurately simulated the morphologies or gas flows commonly observed in galaxies, especially at larger radii, as will be discussed.

Slide83: 

Gas inflow towards the nucleus has been the consistent result of all recent attempts at modeling the evolution of barred galaxies using Newtonian N-body simulations operating within their own gravitational fields. In these numerical models, the evolution of a disk and bar structure is controlled by a feedback process involving recurrent waves propagating through the gas. The fact that a bar-like structure can be made to develop under these conditions is clearly a mark of success of the approach. after Athanassoula 2000

Slide84: 

Although the models vary somewhat in their constructions, the following conclusions are somewhat universal: o Ridge lines build up in the simulated bars. These ridge lines are taken to be analogous to the dust lanes in natural bars. o Of interest, these ridge lines represent the region of highest velocity flow in the computed results, which is always inwards towards the nucleus. o Natural dust lanes, on the other hand, are of such low velocity that their velocities have not yet been unambiguously measured, in either direction. after Athanassoula 2000

Slide85: 

Although the models vary somewhat in their constructions, the following conclusions are somewhat universal: o Flow circulates as a shearing wind on both edges of the bar. o This seems exceptionally unrealistic. While such a flow may be sufficiently laminar in front of the leading edge of a bar so as to be invisible, that cannot be true for the trailing edges of the bars, which are producing trailing edge streamers orthogonal to the calculated flow. The trailing edge dust streamers appear as if they are being shed from the bar as the consequence of an encountered wind and would be greatly disrupted by any such circulating flow, if it existed. after Athanassoula 2000

Slide86: 

Although the various models vary somewhat in their constructions, the following conclusions are somewhat universal: o The regions directly behind the calculated ridge lines that form in the simulations are the areas of lowest pressure. As a result, material is calculated to flow in from the shearing flows to equilibrate the deficit pressures. o This conclusion also seems unrealistic given that the calculated flows occur in the reverse direction of the trailing edge streamers commonly observed. after Athanassoula 2000

Slide87: 

Although the various models vary somewhat in their constructions, the following conclusions are somewhat universal: o The shape of the calculated bars is always that of a cat’s-eye. This too is in disagreement with the observed shapes of naturally occurring bars, which tend to be rectilinear when at full strength. o In contrast, natural bars are broad processes, possessing widths similar to the diameters of the nuclei themselves, and generally maintain that width to their ends. after Athanassoula 2000

Slide88: 

after Athanassoula 2000 NGC 5383 (i) bars are very linear structures, extending over many kiloparsecs at nearly constant widths they possess highly defined leading edge dust lanes dust leakage from the bars’ trailing edges is being shed as if it were encountering a frictional wind the two arms of the bar are always in nearly perfect symmetry right-angle breaks in the dust lanes exist at the ends of the bar contiguous dust lanes exist, running from the nucleus to deep into the blades of the galaxy regions of massive Pop I star formation exist at the points where the bars terminate and the blades begin. Ultimately, whichever model is deemed correct, that model must adequately explain the commonly observed attributes of barred galaxies. The following properties appear to be ubiquitous in all strong, young bars:

Slide89: 

DISTINGUISHING THE TWO HYPOTHESES Although the mechanisms underlying the presumed nuclear mass expulsions are completely unknown at the moment, the hypothesis nonetheless has physical consequences that readily distinguish it from the thesis that the morphologies of galaxies are gravity-determined. Some of those consequences are outlined here.

Slide90: 

THE CRITICAL TEST The mass expulsion hypothesis stands or falls on the eventual determination of the direction of mass flow through the bar. If the flow within bars is inward, the model as it has been outlined here cannot be correct. But if the flow is determined to be outward, then the consequences of that flow dictate some or all of the attributes of the revised taxonomy as they have been described here. To date, neither the rate nor the direction of such flows in galactic bars has been unambiguously determined. Nonetheless, slow flows can be important on an evolutionary timescale (1 km s-1 ~ 1 kpc Gyr-1), but such resolution lies beyond current technology.

Slide91: 

SIX PROXIES Although direct observation of mass flow in the bars is not available, several alternative, independent proxy measures of bulk mass flow can be defined that adequately parse the two hypotheses.

Slide92: 

1. GAS FLOW PATTERNS As discussed earlier, the flow patterns computationally evolved are not similar to those observed. The high-velocity shearing flows that are computed to appear on both the leading and trailing edges of the arms of the bar are quite dissimilar to the observations. Less realistic yet are the low-density regions created in the center of the bars immediately trailing the ridge lines, causing flow to back fill this region. Rather, the consistent pattern seen in high-resolution photographs of the nearby galaxies is that of dust streamers being orthogonally shed from the bars’ trailing edges, in a direction opposite to that predicted. Moreover, these trailing edge dust leakage patterns clearly are not transient phenomena and appear to persist long after the bar has decayed, as shown earlier in Slides 30-37. These discrepancies alone are sufficient to cast significant doubt on the validity of the computational models.

Slide93: 

2. EVOLVED METALLICITY Metallicity is the most certain diagnostic character of the evolved age of the gas and stars that constitutes a galaxy. It increases monotonically with each stellar generation, always unidirectionally, towards increasing metallicity. If it were true, as the computational models assert, that the outer disk is the source of the virgin gas flowing into the nucleus, then the metallicity of these two regions should be reversed. Gas flowing into a nuclear starburst region would operate to significantly accrue increased metallicity in the nucleus over time, but this is not the pattern observed. Only when the flow is reversed, so that the radial flow is outward, should we expect to see the metallicity patterns observed. This simple argument appears by itself to be a very strong rejection of the inflow hypothesis.

Slide94: 

3. THE MULTIPLE ARMS OF M101 Equally difficult to explain numerically within a Newtonian gravitationally dominated context are the overlain arms that exist in some galaxies. An image of M101 has been shown on which a presumed sequence of mass expulsions has been traced. The episodic bars are presumed to have decayed into spirals, such that the “spirals” now appear to be the results of decayed bar arms painted onto a cold dark matter canvas and thus immobilized. If the ancient arm tracings of M101 are correct, as they have been drawn, then it cannot emphasized strongly enough that such a multiarmed pattern cannot exist in a gravity-dominated context. Older, overlain galactic arms, should they have ever existed, would have been rapidly swept up and no longer visible.

Slide95: 

4. MULTIPLANED GALAXIES A fourth observation arguing against the domination of gravity in the evolution of galactic morphologies is seen in the edge-on images of the multiplaned galaxies. A self-gravitating disc would have the qualities evident in Saturn’s rings. In contrast, such galaxies, which are not rare, are clearly non-planar. They possess organized arms tilted with respect to one another, apparently immobilized in those planes, again as if painted onto a multidimensional canvas. No easily imaginable tidal interaction could lift only a few discrete internal rings and not perturb the others. But even if that were possible, such disturbances, once again, would not persist long in a Newtonian framework. Effects too small to be influential on the size of Saturn’s rings apparently become significant at galactic scales and thus potentially offer another clue into the nature of dark matter, should it exist.

Slide96: 

5. THE MULTIPLE SPIGOTS OF NGC 1530 If the outflows traced earlier for NGC 1530 are correct, no conceivable gravity-dominated, gas kinematic structure could create such a pattern of parallel inflows, all ending at a specific point. However, the obverse is not true. If mass is being transported through the bar outwards and deposited onto the surrounding frame at the bar’s ends, it is not difficult to imagine that multiple “spigots” at one or both ends spraying multiple outflows of trailing matter due to inhomogeneities in mass at those ends. If confirmed, this observation becomes a fifth important discriminator between the two hypotheses.

Slide97: 

6. BAR FRAGILITY The final proxy test is more aesthetic and subjective, and thus inherently less rigorous. Nonetheless, the bars that develop numerically are not reminiscent of the natural bars observed. In the simulations, the bars develop as a fragile consequence of the initial of conditions associated with infalling gas and are easily dissipated. Natural galactic bars, in contrast, do not give the impression of merely being a consequence, but rather the primary forcing function in the evolution of galactic morphologies. They appear to have a rigidity, persistence and strength that the simulated bars do not.

Slide98: 

CONCLUDING REMARKS The taxonomy of galactic evolution presented here is distinct from anything that has been proposed before, and it is clearly unconventional in its argument. The hypothesis is nevertheless testable, and it stands or falls on the eventual determination of the direction of mass flow through the bar, either directly or by proxy. If the flow within bars is inward, the proposed model cannot be correct. If the flow is determined to be outward, then the consequences of that flow dictate some or all of the attributes of the revised taxonomy as they have been described here. Whichever model of galactic evolution is eventually believed to be correct, that model must adequately explain the commonplace properties observed in strong bars as they have been outlined. Understanding the mechanism that creates the galactic bars appears to be fundamental to a proper understanding the evolution of galactic morphologies.

Slide99: 

PHOTO CREDITS NGC 1300: Hubble Heritage Team/ESA,/NASA NGC 5236: ESO NGC 2997: Martin Altmann/ESO M74: Gemini/GMOS M101: NASA/ESA NGC 4414: Hubble Heritage Team (AURA/STSci/NASA/ESA) NGC 1530: NOAO/AURA/NSF NGC 3992: AURA/NOAO/NSF NGC 2217: Alan Uomoto/Univ. Arizona M31: Jason Ware, Galaxyphoto.com NGC 3190: ESO NGC 7331: Paul Mortfield and Dietmar Kupke/Flynn Haase/NOAO/AURA/NSF Saturn’s rings: STScI/LPL/NASA/E. Karkoschka ESO 510-G13: NASA/Hubble Heritage Team

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