Lecture 2 Classification of Galaxies

Classification of galaxies Aim: Understand the different galaxy classification schemes. Learning Outcomes : -1- Distinguish the main structural features of different types of galaxies -2- Characterize the mass content of different types of galaxies -3- Describe the main features of...

Classification of galaxies Aim: Understand the different galaxy classification schemes. Learning Outcomes : -1- Distinguish the main structural features of different types of galaxies -2- Characterize the mass content of different types of galaxies -3- Describe the main features of each galaxy type  Galaxy luminosity, dwarfs and giants: Starting point for understanding galaxies: identify and classify their different morphologies. Later on, more classes were recognised: dwarfs, giants, low-surface brightness galaxies. Galaxy luminosity determined by: number of stars in the galaxy the stellar population : - young → luminous - old → faint Definition: L < 109 L⊙ (MB > –17 mag) → dwarf galaxy L > 109 L⊙ (MB < –17 mag) → giant galaxy Milky Way: 2x1010 L⊙ , MB ≈ –20.6 mag Spirals, Lenticulars, Ellipticals TODAY GALAXIES PRESENT THEMSELVES IN A VARIETY OF MORPHOLOGIES AND SPECTRAL FEATURES Lenticulars ‘Early’ Irregulars ‘Late’ type at type at this end this end 3 From Cosmo0-en.wikipedia Lecture 20  BASIC GALAXY FEATURES: 1-Morphology. Spirals have a stellar and gaseous disc with spiral arms, and a small or negligible stellar bulge. Lenticulars Have a disc but with no spiral features, and have a larger central bulge. Ellipticals are bulge-dominated, with the most massive ones having very small/negligible discs. 2-Content and Spectra. Spirals are generally younger systems, more gas-rich, with a higher (specific) Star Formation Rate, [ie SFR per unit mass] they are usually blue. Lenticulars have lower star formation rates and gas fractions, and are redder. Ellipticals have quite low gas fractions and very low specific star formation rates. The most massive ones are usually said to be “red and dead”. 3-Dynamics. Spirals are dominated by circular motions, Ellipticals by random motions, while Lenticulars are in between. Random motion is measured via the velocity dispersion, which is a measure of the statistical dispersion of the random velocities of stars about the mean velocity of the system. 4-Statistics. The stellar-mass luminosity function describes the number of objects per unit volume per unit mass. Spirals are most abundant, then come Lenticulars, and then Ellipticals. BASIC MORPHOLOGICAL CLASSIFICATIONS  Hubble classification: Ellipticals Smooth 3-dimensional shapes → appear in 2-dimensions on the sky as M87 ovals (round to elliptical shapes) Central light concentration Various degrees of apparent flattening: ellipticity ε (large ε - more flattened) b ε = 1− a, b: major, minor axes a In the Hubble scheme denoted with letter b E and a number n (ranging from 0 to 7) a which denotes increasing ellipticity: ⎛ b⎞ n = 10 ⎜1 - ⎟ = 10 ε ⎝ a⎠  Elliptical examples These images show an E0, E2, E3 and E6 galaxy. Which is which? NGC3377 E6 NGC3465 E3 M32 E2 M89 E0  Hubble classification: Lenticular galaxies Main components – a bulge and a disc. Resemble ellipticals in central region: central large bulge Smooth, structureless, flattened disc (rotating) lens-shaped when viewed edge-on Classified as S0 Barred lenticulars are classed as SB0. Further numerical notation (1 - 3) to lenticulars according to strength of bar or evidence of dust (not considered here)  Lenticulars example NGC 5866 – Spindle galaxy (HST) NGC 4664 – SB0 NGC 936– SB0 (SDSS)  Hubble classification: Spiral galaxies Type of disc galaxy Components: - central bulge - thin disc - surrounding halo Key feature: spiral structure in disc Spirals are denoted by letter S, and the form family a, b, c, depending on three parameters: Size of the bulge relative to disc length The tightness of the winding of the spiral arms The degree of resolution/ordered structure of the disc into stars, star clusters and gaseous starforming HII regions (this parameter is considered less often)  Hubble classification: Spiral galaxies Parameters are loosely correlated: spirals with large bulges tend to have tightly wrapped arms Form family a, b, c, depending on three parameters: a – big bulges, tight windings c – small bulges, open arms b – intermediate between a and c  Pitch Angle Tightness of spiral arm winding is measured by pitch angle: - Draw a circle at any radius around the nucleus - the angle between the tangent to the spiral arm and the tangent to the circle is the pitch angle Sa: pitch angle ≈ 5° Sc: pitch angle ≈ 20° In most spirals, the pitch angle is relatively constant for all arms regardless of radius http://beares.net/r/astro/galaxies/galaxies_docs/pitch_angle.htm  Spiral galaxies continued Hubble also noted that some spirals have elongated, bar- shaped bulges → Barred spiral galaxies, denoted SB Gerard de Vaucouleurs (1950s) expanded the Hubble classification system in the 1950s, to add Sd and Sm types. m - for Magellanic spirals named after the prototype Large Magellanic Cloud (LMC) which shows a hint of spiral structure.  Normal spirals: examples M81 type Sab M63 type Sbc NGC2997 type Sc NGC7793 type Sd Hubble classification: Spiral galaxies How would you classify our own galaxy, the Milky Way? This is tricky because we are embedded in it and it is thus difficult to extract an exact unbiased view as an external observer would do. However, given the variety of available data, an artist’s view of the MW would be some thing of the type: How would you classify our own galaxy, the Milky Way? Milky Way: SBbc Artist’s conception of the Milky Way galaxy. Credit: Nick Risinger  Projection effects and disk galaxy inclination We can only see a 2D projection of the galaxies on the sky. How do we know the true shape, and the inclination angle we are viewing them from? Ellipticals: we might just see the projection effect Statistically, the chances of seeing ellipticals head-on are small → only a few E0 expected But: we observe a lot of them → must be intrinsically round  Projection effects and disk galaxy inclination For disc galaxies, i.e. spirals and lenticulars, the situation is easier: inclination i = angle measured between the plane of the disc and the perpendicular to our line of sight ( i = 0 is face-on). For an undistorted (i.e. long) major axis a and distorted (short) minor axis b, we have: ⎛b⎞ −1 i = cos ⎜ ⎟ ⎝a⎠ b a  Projection effects and disk galaxy inclination i =0° face-on i = 90° edge-on NGC3184 NGC891  Hubble classification: Irregulars Irregulars are simply galaxies with shapes that do not fit into Hubble’s other types. complex irregular structure no order (like spiral arms), patchy star forming regions type I irregulars Irr I, or Im or Magellanic Irregulars after the Small Magellanic Cloud (prototype). no regular disk structure flattened systems nucleus non-barred (IA), barred (IB) or ovally distorted (IAB) tend to be smaller than ellipticals and spirals type II irregulars, or starburst galaxies massive star formation, which causes their irregular appearance tend to be giants, probably originally spirals  Irregulars: examples SMC type Irr I M82 type Irr II M110 E5/E6 M32 E2 M87 E0 M101 (Sombrero Galaxy) Sa M83 SBa  APOD, © Jason Ware M31 (Andromeda) Sb M91 SBb © Jyri Näränen, Tommy Grav and Kaare Aksnes, NOT M51 Sbc © Magnus Gålfalk, NOT M101 Sc APOD, © W. Keel, KPNO  www.starkenburg-sternwarte.de/deepsky/ngc.htm NGC3115 S0  APOD, © P. Challis M82 – The Cigar Galaxy IRR II LMC IRR I, SBm http://www.astrosurf.com/antilhue/lmc_&_smc.htm  To Recap: -Variety of morphological classes from bulge-dominated, gas-poor, less numerous Ellipticals to more numerous, more star-forming, gas-rich, spirals. -Variety intermediate stages, for Ellipticals En, Lenticulars S0, spirals Sa, Sb, Sc, etc… -Letter “n” in Ellipticals measures ellipticity. -Barred galaxies are denoted with a final “B”, SB, etc… -Pitch angle to measure winding of spirals. -Type I and II irregulars to denote their level of Star formation rate, morphology, and mass. ADDITIONAL MATERIAL (NOT ASSESSED)  Spiral galaxies continued De Vaucouleurs also added intermediate types, Sab, Sbc, Scd; and intermediate bar-types, the oval- shaped bulges → denoted SAB  Barred spirals: examples NGC210 type SABb NGC1365 type SBb M83 type SABc NGC2903 type SBd  Spiral galaxies continued Major types: Normal spiral galaxies: S or SA - round bulges Barred spiral galaxies: SB - elongated, bar-shaped bulges Oval-shaped bulges: SAB - intermediate between S(A) and SB Form families: S(A,B,AB)a – large bulge, tightly wrapped spiral arms S(A,B,AB)c – small bulge, open spiral arms S(A,B,AB)b,ab,bc,cd – intermediate types The form family is assessed by experience, there are no strict quantitative rules except by comparison with other galaxies.  Ring structures in spirals De Vaucouleurs looked at the spiral arm morphology in more detail and classified: arms spiral all the way into the nucleus → denoted s arms spiral in to a ring around the nucleus → denoted r intermediate features denoted rs or sr http://www.astro.queensu.ca/~courteau/Phys216/gal.html Occasionally, galaxies have a ring in the outer disk near the end of the spiral arms → indicated using an R preceding the Hubble type. Ring structure may be related to resonance of disk and spiral arms  Examples of ring structures NGC5921 type SB(r)bc NGC2859 type (R)SB(r)0  Peculiar galaxies Galaxies that show unusual, peculiar feature, can be otherwise “normal“ galaxies, like M87 All interacting galaxies are peculiar M87 – giant elliptical E0 with a jet APOD ©J.-C. Cuillandre (CFHT) – long exposure short exposure