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Astronomía Extragaláctica y Cosmología Observacional

Depto. de Astronomía (UGto). Astronomía Extragaláctica y Cosmología Observacional. Lecture 6 Star Formation. The formation of stars gas clouds collapse Initial Mass Functions parameterizations Observational indicators of Star Formation (SF) recombination lines UV continuum

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Astronomía Extragaláctica y Cosmología Observacional

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  1. Depto. de Astronomía (UGto) Astronomía Extragaláctica y Cosmología Observacional Lecture 6 Star Formation • The formation of stars • gas clouds collapse • Initial Mass Functions • parameterizations Observational indicators of Star Formation (SF) • recombination lines • UV continuum • FIR from dust emission • radio continuum • CO from molecular clouds Lyman Break Galaxies Star Burst (SB) galaxies The SF along the cosmic time

  2. The formation of stars • star formation process is not well established due to: non hydrostatic equilibrium state of the proto-star entities, the complexity of ISM and the unknown internal dynamics of clouds (that prevents us of choosing which initital condition to use), importance of the magnetic fields, interactions of borning stars with their neighbours in clusters, etc • young star clusters (OC) are invariably associated with with dense interestellar clouds and with spiral arms

  3. The formation of stars • Gas clouds collapse: • gas in Giant Molecular Clouds (molecular and atomic gas, with some dust, and 106-107M) may be eventually compressed by shock fronts • fragmentation and gravitational collapse take place, against some combination of resisting turbulent and magnetic energies (as the ionization level of material, governed by high energy radiation and cosmic ray particles, becomes small, the coupling of magnetic field weakens and it dissipates) • as the cloud fragment free-falls, gravitational energy is released as radiation (luminosity rises rapdly but temperature remains quite low, ~ 10-20K, but growing in the center) • the dense core (proto-star) gradually becomes opaque, trapping the radiation and heating up

  4. The formation of stars • at about 2000 K, H2 molecules dissociates into HI, sinking energy and accelerating the collapse of the core • temperature continues rising and H becomes ionized (HII) • free e- pressure starts to balance the collapse that eventually halts in the core • convective proto-star reachs the Hayashi limit in the CM diagram and enter the hydrostatic equilibrium regime (becoming a T-Tauri star) • the core starts to burn D and Li and becomes radiative, the envelope continue contracting at nearly constant luminosity, the effective temperature continues rising, violent surface activity and strong protostellar winds occur (cleaning the remaining envelope) • ultimately the core temperature rises to the point at which the thermonuclear fusion of H into He becomes possible, and the star settles on to the ZAMS

  5. The formation of stars

  6. The formation of stars • Associations: • chain reaction

  7. Initial Mass Functions • distribution of masses of a freshly formed (just after a burst of SF) stellar population dN = N0Ж(M) dM where N0 is the normalizing constant – the number of solar masses contained in the burst, and Ж(M) is the IMF • we have no a priori reason to suppose that Ж is a universal function that applies to all SB, but observations indicate consistency at least for M M • determinations of the IMF are more difficult and unreliable for low mass stars, because these stars are slow to settle on the MS and their spectra deviate significantly from that of a black-body

  8. Parametrizations of the IMF: • E. Salpeter[1955, ApJ 121, 161] proposed a simple power law for the IMF: Ж(M)  M –2.35 • More recently, J. Scalo [1986, Fundam. Cosmic Physics 11, 1] presented new data on IMF, which can be adequately fitted by three power-law segments: Ж(M) M –αα = 2.45 for 10 < M < 100M 3.27 for 1 < M < 10M 1.83 for 0.2 < M < 1M • Kroupa, Tout & Gilmore [1993, MNRAS 262, 545] also advocated a three power law form for Ж, but one that falls off more steeply at the high mass end: α = 2.7 for 1 < M < 100M 2.2 for 0.5 < M < 1M 1.3 for 0.08 < M < 0.5M Minimum mass (for H burning) → 0.08 M Maximum mass (for equilibrium) → 100 M Salpeter Scalo • Initial Mass Functions

  9. Initial Mass Functions [Salpeter1955, ApJ 121, 161] [Miller & Scalo 1979, ApJS 41, 513] [Scalo1986, Fundam. Cosmic Physics 11, 1] [Kroupa, Tout & Gilmore 1993, MNRAS 262, 545]

  10. Initial Mass Functions • Parametrizations of the IMF: • Kennicutt, Tamblyn & Congdon[1994, ApJ 435, 22] argued, from the position of their galaxies in the [B-V, W(Hα)] plane, that the IMF in these systems has to be about as rich in massive stars as the Salpeter IMF predicts [Kennicutt 1983, AJ 88, 1094] – IMF close to Salpeter at high masses [Kennicutt, Tamblyn & Congdom 1994, ApJ 435, 22]

  11. Observational indicators of Star Formation • Past SF: • colors • Present SF: • recombination emission lines • UV continuum from hot stars • thermal far infra-red (FIR) from dust • radio continuum • CO emission from molecular clouds • Future SF: • amount of gas available

  12. Observational indicators of Star Formation

  13. M33 • Observational indicators of current Star Formation • Recombination emission lines: • line emission is characteristic of HII regions (zones of ionezed gas, around young star clusters) Mechanism: H is ionized by absorption of Lyman continuum photons (λ < 912 Å, energy above 13.6 eV) produced by the hot OB stars  the line radiation we detect arises from the recombination of the e– so released with another p+, and a cascade toward the ground state

  14. NGC 5427 (Hα) H Orion Nebula OII OIII OIII NII H SII NII • Observational indicators of current Star Formation • Recombination emission lines: • predominantly Balmer lines, especially Hα (the strongest and easiest to deal with) • narrow band imaging at Hα is usually used to find HII regions • SF rates may be calculated from the intensity of Hα lines (and by measuring equivalent widths) SFR [M/year] = 8.93  1042L(Hα) [erg/s] • SFR [M/year] = 1.4  1041L(OII3727) [erg/s] [Kennicutt 1983, ApJ 272, 54]

  15. Observational indicators of current Star Formation • Recombination emission lines: • not surprinsingly, the SFR per unit starlight climbs to later Hubble types [Kennicutt & Kent 1983, AJ 88, 1094] 

  16. Observational indicators of current Star Formation • Ultraviolet continuum: • stars not massive or hot enough to produce HII regions, but also young (less than 109 years for early A stars, p.e.) can be traced by their brightness in UV Mechanism:  they produce a UV continuum in SF galaxies at wavelengths longer than the Lyman limit (λ = 912Å) • this continuum is remarkbly flat, as can be modelled by spectral synthesis codes (first noted by Lilly & Cowie [1987, Infrared Astronomy w/ Arrays] and Cowie [1988, ApJL 332, L29]); this is due to the fact that, although these luminous stars have short lifetimes, they are constantly being replaced by new stars • the intensity of the flat part of the UVcontinuum is directly proportional to the rate of formation of heavy elements, since these stars are the ones that produce supernovaes • reddening (extinction) is a strong effect and correspondly serious uncertainty SFR [M/year] = 1.7  1028L(UV1250-2500) [erg s1 Hz1] [White 1989, The Epoch of Galaxy Formation]

  17. Observational indicators of current Star Formation • Thermal far infra-red from dust emission: • SF galaxies are also strong emitters in the FIR waveband because of the presence of dust in the SF regions (on average, SF galaxies are stronger emitters in FIR than in the UV), as was first shown by IRAS survey Mechanism: dust grains are heated by absorption of starlight, which operates most efficiently in the blue and UV (as the waveband comes closer to the characteristic grain size)  dust cools again by (approximately) black-body emission, with peak about 20-40 K • although the mechanism is not well understood, the total UV-optical energy from young stars, removed by dust absorption, must emerge in FIR (the luminosity from 10-300 μm may equals that emitted originally from 912-3000 Å)m • since there is an empirical tight relation between total FIR emission and Hα, this radiation may be strongly coupled to current SFR SFR [M/year] = 1.3  1029L(FIR60m) [erg s1 Hz1]

  18. Observational indicators of current Star Formation M82 • Radio continuum: • SF galaxies also emit in cm radio waveband, much of which must be connected, directly or no, with SF Mechanism: emission is nonthermal, from synchrotron process (accelerated particles, perhaps in SNe remnants, radiating while spiralling through large-scale magnetic fields) • radio emission is a valuable tracer in heavily obscured regions: VLA mapping proved to be a useful tool in identifying IR-loud galaxies at faint optical magnitudes since its positions are much more accurate than IRAS centroids. SFR [M/year] = 5.9  1029L(HI1.42GHz) [erg s1 Hz1]

  19. Observational indicators of current Star Formation • CO emission lines from molecular clouds: • since Giant Molecular Clouds are the immediate precursors of SF, their mapping is also an indicator of SF • CO molecules, the most abundant after H2, detected at 1.3 and 2.6 mm (230 and 115 GHz) are usually the tracers (H2 cannot be observed in radio domain because it is symmetric and does not possess an eletric dipole) Antenae

  20. Lyman Break Galaxies • Photometric search of high z • SF galaxies: • a color-selection technique for identifying high redshift galaxies (with flat rest UV continuum, like SF galaxies, and small extinction) was first used by Steidel & Hamilton [1992, AJ 104, 941],by Lilly et al. [1995, ApJ 455, 108] for CFRS data and Madau et al. [1996, MNRAS 283, 1388] for HDF data, the last ones to measure SF at earlier times. • since Lyman break is observed at 912(1+z) Å, for a z ≥ 2.5 it is found in the optical – galaxies with z ~ 3 will be seen in B, V and R filters, with similar magnitudes, but will not be seen in U filter. These are called Lyman break galaxies [Steidel 1999, PNAS USA 96, 4232]

  21. Mark 357 • Star Burst (SB) galaxies • some galaxies show evidence of a recent and transient increase in SFR by as much as a factor of 50 (hundreds of M/year) • since the galaxy gas is rapdly consumed in the SF, exhaution timescales are of order 107-108 years • the burst is often confined to a few hundred pc near the nucleus, although disc-wide bursts are also common • SB are usually found in interacting galaxies, merging systems and bursting dwarves • global (or super) winds are also found, powered by energy of starlight, stellar winds and supernovae • associated very luminous compact star clusters (up to 108 L) frequently occur (if these objects have a normal IMF, and remain gravitationally bound after the mass loss from massive members is complete, they will eventually become GC) • it is possible that all galaxies may pass anytime a SB phase LMC (IRAS) 30 Dor

  22. Star Burst (SB) galaxies • Observational characteristics of SB: • large Balmer lines luminosity and equivalent width • high ratio LFIR/LB • unusual strong radio continuum emission • optical spectra resembles those of HII regions • Possible SB mechanisms: • cloud collisions in a perturbed disc • collisions between clouds originally belonging to different galaxies • channelling of gas through bars towards the center • tidally induced density waves • disk instabilities produced by perturbations in the gravitational potential • physical transfer of gas during encounter (over a critical limit) • direct impact of gas rich dwarf satellites into disks [Poggianti et al. 1999, ApJ 518, 576]

  23. Star Burst (SB) galaxies M82 – X-rays, Chandra M82 – Hα + SII, WIYN M82 – UV M82 – IR, SAO M82 – radio, VLA M82 – opt, HST

  24. [Gallego et al. 1995, ApJL 455, L1] [Lilly et al. 1995, ApJ 455, 108] [Connally et al. 1997, ApJL 486, L11] [Madau et al. 1996, MNRAS 283, 1388 and 1998, ApJ 498, 106] • SF along the cosmic time • The “Madau-Lilly” plot: • the greatest rates of global SF occurred at z ~ 1-2! [Steidel 1999, PNAS USA 96, 4232]

  25. Star Burst (SB) galaxies • SB  AGN: • for very luminous galaxies, which are dusty enough that most of their power emerges in the FIR (once known as IRAS galaxies, now sharing such acronyms as LIRGs, ULIRGs, PIGs or ELFs) it is difficult to determine whether the dominant energy source is a SB or a AGN • many galaxies exhibit both nuclear activity and considerable SF activity, specially interacting and merging systems • SB • energy supplied by OB stars • more diffuse radio emission • lack of high ionization species • strong PAH features (6.2 μm, p.e.) • flat UV continuum • AGN • energy comes from continuum produced by • central accretion disk • compact, flat radio spectrum • high ionization species • PAH features are destroyed by the intense • hard radiation • UV continuum inclined

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