A SUPPLEMENT TO A REVISED CLASSIFICATION OF GALACTIC MORPHOLOGIES BASED ON A PUTATIVE MECHANISM OF EPISODIC NUCLEAR MASS

1. 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

2. THE HYPOTHESIS 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.

3. THE HYPOTHESIS 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. 

4. 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. 

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

6. 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.

7. 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.

8. 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.

9. 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.  

10. 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.

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

12. 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.

13. 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.

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

15. 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.

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

17. 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.

18. 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.

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

20. 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.

21. 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.

22. 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.

23. 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.

24. 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 intermediate Pop II stars province of large-scale Pop I star formation 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.

25. 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 intermediate Pop II stars province of large-scale Pop I star formation 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.

26. 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.

27. 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. As will be discussed, in some barred galaxies, this leading edge region of shock star formation can be extensive.

28. 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.

29. NGC 1300 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.

30. NGC 5236 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.

31. NGC 5236 The result is threefold:

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

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

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

35. NGC 2997 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.

36. NGC 2997 primary dust lanes 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.

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

38. 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.

39. 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.

40. 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

41. 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.

42. 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.

43. 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.  

44. 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.

45. 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. 

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

47. 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. M101 Type Sb

48. 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. M101 Type Sb

49. 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.

50. 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.