Astrophysics - Galaxies PDF

PART 2: GALAXIES - **General Definition:** - Stars + remants (WD, NS, SBH) -- observed by optical, SMB observed by Xray too - Gas in all states (ionized, neutral, molecular) -- observed by millimetre/submillimeter - Dust: key role in redistributing the electro...

PART 2: GALAXIES - **General Definition:** - Stars + remants (WD, NS, SBH) -- observed by optical, SMB observed by Xray too - Gas in all states (ionized, neutral, molecular) -- observed by millimetre/submillimeter - Dust: key role in redistributing the electromagnetic radiation emitted by galaxies -- observed by infrared. - Dark matter -- not directly observable. Observations are made by quantities: masses, luminosities, star formation rate, etc. - **Hubble classification of galaxies** +-----------------------+-----------------------+-----------------------+ | **Types** | **Further types** | **Characteristics** | +=======================+=======================+=======================+ | **Spiral** | Normal: Sa, Sb, Sc | Spiral arms, | | | | which are prominent d | | | Barred: SBa, SBb, SBc | ue | | | | either to bright O-B | | | | stars or to dust | | | | lanes. | +-----------------------+-----------------------+-----------------------+ | **Elliptical** | E0 -- E7 En where n = | Smooth, featureless, | | | 10(a-b)/a (major, | much less gas and | | | minor axis) | dust than spiral | | | | galaxies. | | | | | | | | Stars are old red | | | | stars, no new episode | | | | of star formation | +-----------------------+-----------------------+-----------------------+ | **Lenticular** | | | +-----------------------+-----------------------+-----------------------+ | **Irregular** | Irr I, Irr II | Small blue galaxies l | | | | acking any organized  | | | | spiral | | | | structure; often | | | | shows signs of star | | | | formation | +-----------------------+-----------------------+-----------------------+ **Also: Dwarf Galaxies - ellip4cals / irregulars / spheroidals** A group of spirals with text Description automatically generated - **Elliptical galaxies** Red Old stellar population Smooth Profile Relaxed old system High surface brightness densely packed Egg-shaped massive Little or no dust lane reservoir exhausted Absorption lines only no star formation Many globular clusters formed via mergers No rotation formed via mergers Found in clusters formed via mergers Galaxies that appear ellip4cal on the sky may be intrinsically oblate, prolate, or triaxial, depending upon their symmetries:  Orbits of stars in ellip4cal galaxies differ substan4ally in different types:  axisymmetric galaxies (prolate/oblate) and triaxial galaxies. - **Spiral Galaxies** - Spiral arms, which are prominent due either to bright O-B stars or to dust lanes. - Flat, rotating disk and central concentration of star, bulge. - Stars found with equal density everywhere in the disk. Young type O-B stars are only found in the spiral shaped arms. High luminosity of these stars dominates the picture spiral - The concentration of dark dust lanes and cool molecular clouds is largest in the spiral arms.   - Hydrogen regions which glow red after being ionized by young O and B stars are mainly found in the spiral arms.  - Spiral arms are the sites of star formation. After being born, a star moves out of the arm Red bulge central bulge is old Bluish Arms/Disk Disk is intermediate Moderate surface brightness relaxing Dusty Star formation will continue Emission Absorption lines star forming ongoing    Rotating disk formed via collapse plus some merging  Numerous Globular Clusters  formed via collapse plus some merging (?) Seen in high- & low-density environments  collapse + merging (forming via collapsing vs merging?) - Density -- wave theory of spiral arms - Spiral Galaxies should be viewed as accretion disks  - Galaxies disks in perpetual evolution/reformation concentrate mass (to a least energy state) - Rotation prevents mass to concentrate more (angular momentum) - Energy dissipation reduces random motions but viscous torques insufficient. - The arms are simply density oscillations - Formation can be simulated only in the presence of cold dark matter.  - **Irregular galaxies** Blue young stellar population Strong Emission lines lots of star formation Very dusty star formation will continue Low surface brightness forming Highly Asymmetrical forming/low mass Rotating Formed via collapse Few Globular clusters formed via collapse A green and yellow rectangular box with black text Description automatically generated - **Spectrum**  A diagram of a graph Description automatically generated  - **Groups and clusters of galaxies** - Galaxies do not exist in isolation. Most galaxies belong to some larger bound structure. - The Milky Way has small satellite galaxies orbit around it. Together with Andromeda and some small galaxies it forms the Local Group (R \~1 Mpc) - Groups of galaxies have several to tens members.  - At large scales some galaxies are forming clusters of galaxies. A galaxy cluster may contain several hundred galaxies, in the volume of 2 -- 3 Mpc across. - **Formation of galaxies and galatic black holes** - Increasing evidence showed that early galaxies (universe less than 1 Gy old) were small and often colliding/merging in then much denser universe. - Less massive galatic black holes conceivably formed by merging of stellar black holes in the dense environments of galaxies centres. - More massive galatic black holes are more difficult to understand. Possible role of early galaxy collisions and merging. - Monolithic Collapse: giant balls of primordial gas stars forming central dense core with surrounding halo most massive galaxies formed first. Consistent with giant old inert red spheroids. - Hierarchical Scenario: small clumps smallest galaxies formed first merging massive galaxies. Ellipticals formed via mergers   - Secular evolution A diagram of gas and other gas systems Description automatically generated with medium confidence - **The Milky Way** - Clearest view is obtained in infra‐red: emission from cool stars (low mass, typically old) - Stellar disk diameter: 30 kpc - Disk galaxy, very similar to spiral galaxies: - Spiral arms: - Short distances: O-B stars and ionized hydrogen regions. - Larger distance: radio wave emitted by gas (HI, CO) which are not obscured by dust. - Most stars: within 15 kpc from the center (solar system: 8kpc) - Most H: within circle of radius 25kpc from the center. - **Galaxies Components** - **The bulge**: central spheroidal stellar component, R \~ few kpc - Milky Way: Lbulge: 5 x 10\^9 Lsun; M bulge = 2 x 10\^10 Msun - **The disk:** disk of stars, rotate about the galatic center, extends to at least 15 kpc from the Galatic center, centrally concentrated -- density of stars falls off exponentially, 95% of stars lie in a thin disk (300 -- 400 pc) - Milky Way: Ldisk = 15 - 20 x 10\^9 Lsun; M disk = 6 x 10\^10 Msun - **The halo:** outside the bulge and well above the plane of the disk, made up of: - Stars: old and random motions, low density - Globular clusters - **Dark Matter** - **Super Massive Black Hole:** at the center of galaxy - Milky Way: Srg A\*, M\~3x10\^6Ms - **Stars and Gas in Galaxies** - Gas is converted into stars in high-density regions  **Composition** Dark matter 90% --------------- -------------------------------------- Stars 5 -- 10% Neutral Gas 0.5% Molecular Gas 0 -- 1% Ionised Gas Hot & warm interstellar medium Dust \< 0.1% SMBH \< 0.1% WDs and BHs \< 1% - stellar population synthesis Planets \< 0.001% - **Andromeda:** - Nearest spiral galaxy, \< 800 kpc - In our global cluster, very similar to our own Milky Way - Visible by naked eye - **Galaxy collisions** - Collisions are believed to be common phenomena in the early denser Universe, play an important role in the process of galaxy formation. - Galaxy  collisions  usually  do  not  imply  direct  star  collisions  but  the  strongly  increased  gravity  field  enhances  the  collapse  of  hydrogen  clouds and the forma6on of new stars, many of which very massive. - Bigger and denser objects =\> easier to collide - Distances in a group of galaxies are relatively small compared to the sizes of the galaxies - Magellanic clouds A diagram of a galaxy Description automatically generated - When two galaxies collide, their dark matter, gas and dust clouds hit each other and can be flung out of the galaxies. - produces long tidal tails of matter - may produce large spiral arms. - strip the galaxies of their gas. - **Starburst Galaxies** - When galaxies come close to each other but miss, the tidal deformation can trigger a burst of star formation. -\> a star-burst galaxy is formed  A close-up of a text Description automatically generated