Morphology The Study of the Basic Pattern of Things

Transcription

1 Morphology The Study of the Basic Pattern of Things

2 Fundamental Considerations Different libraries of galaxies are liable to lead to the ID of different classes Images in library must be homogeneous (filter, redshift) Classifications are biased to what the classifier thinks is important The selection criteria must lead to a unique classification in each case Bottom Line: Exact classification is very difficult to impossible, but is generally still worthwhile

3 Classification Scheme 1: Hubble-Sandage Barrishness [E,S0,SB,Irr] Openness of arms / disk-bulge ratio [a,b,c] Degree of resolution of arms into stars

4 Elliptical Galaxies Early Type Galaxies Classified as E0 E7 En, where n is the axial ratio i.e., n = 10(1-[b/a]), where a is the semi-major axis and b is the semi-minor axis.

5 Spiral Galaxies Bulge/Disk ratio (light) Tightness of Spiral arms Degree to which arms are resolved into stars and individual emission nebulae (HI regions)

6 Spiral Galaxies Sa Sb Sb Sc Sc Sd

7 Barred Spiral Galaxies SBa SBb SBc SBd

8 Lenticular Galaxies S0 3 Smooth central brightness condensation surrounded by region of less steeply declining brightness S0 = no bar SB0 = bar S0 1 S0 3 : depends on strength of dust absorption in disk-like component SB0

9 What about smaller, fainter galaxies? Dwarf ellipticals (de) Dwarf spheroidals (dsph) Dwarf irregulars (di)

10 Globular Clusters will also be considered alongside galaxy classifications

11 Globular Cluster - an example

12 M B vs. Hubble Type

13 M(HI)/L B vs. Hubble Type HI = neutral hydrogen Gas fraction increases towards later Hubble Types

14 M(HI+H 2 )/M dynamical vs. Hubble Type Relative gas mass increases with increasing Hubble Type 5% for Sa to 25% for Scd

15 M(H 2 )/M(HI) vs. Hubble Type Decreases by a factor of 10 from Sa to Sd Actual quantity of HI does increase to later Hubble Types H 2 decreases to later Hubble Types

16 Classification Scheme 2: De Vaucouleurs Barrishness [E,SO,S,SA,SB,I] Openness of arms / disk-bulge ratio [a,b,c,d,m] Rings or S Shapes

17 Classification Scheme 2: De Vaucouleurs SA = unbarred SB = barred SAB = Weakly barred SO = Barrishness not clear (i.e., edge-on SA = unbarred galaxy)

18 Classification Scheme 2: De Vaucouleurs Nuclear ring r/a < 1 Inner ring r/a ~ 1 Outer ring r/a > 1 Lens: elliptical feature with shallow brightness gradient interior to a sharp outer edge

19 Classification Scheme 3: Morgan cd galaxy Central condensation of light [k,g,f,a] Barrishness/smoothness [E,R,D,S,B,I] The only part that has stuck cd Galaxies (i.e., supergiant elliptical galaxies with extended haloes)

20 Classification Scheme 4: van den Bergh Young star richness of disk [E,S0,A=anemic,S,Ir] Barrishness [B] Central condensation of light Quality and length of arms

21 Optical image of NGC 253

22 Once again, the library you choose the galaxies from matters

23

24 Comparison of M31 & The Milky Way

25 Galaxy Collapse Types Dissipationless collapse The stars form before the gas has time to dissipate away its energy and sink towards the center of the gravitational potential well Dissipational collapse The gas dissipates away most of its energy and becomes rotationally stabilized (in a disk-like structure) before forming stars.

26 Galaxy Features & Their Possible Formation Mechanism Halo Dissipationless collapse of? During early phase of galaxy formation Ellipsoidal (E) Dissipationless collapse + mergers (star formation mostly precedes collapse) - Elliptical - Bulge Disk (S) Dissipational collapse (stars form after collapse) - Thick disk - Thin disk Smooth Progression Smooth Progression

27 Features & Formation, cont. Bar (B) Dynamical Instability during collapse phase + later secular growth Lens (l) Mass from bar by destruction of resonance? Inner ring (r) Disk material rearranged by bar Outer ring (r) Disk material rearranged by bar

28 Nomenclature Galaxies are named after survey they were detected with Messier or M (bright local objects, many are star clusters) New General Catalogue or NGC (about 8000 objects, many are star clusters) Zwicky Catalogue or Zw (peculiar objects) Arp Catalogue or Arp (peculiar interacting galaxies) Markarian Catalog or Mrk (UV-bright systems) Index Catalog or IC Uppsala General Catalog or UGC IRAS Catalogue (Infrared luminous, discovered by IRAS satellite)

29 Nomenclature, cont. Galaxies can also be named by their coordinates Example: a galaxy at right ascension of 01 hour 30.5 minutes and declination +33 o 05 can be named Epoch of coordinates is also important. B and J are typically used today (i.e., J )