Evolution of Galaxies

With the advent of larger ground-based and space borne telescopes, covering a broad spectral range, galactic evolution has grown into a major research area. SOFIA will significantly contribute to, or solve, several of the key issues, some of which are briefly discussed here.

Interacting Galaxies and Clusters

Evidence is growing that interaction between galaxies may be the dominant driving force behind the evolution and diversification of galaxies (e.g. Moore et al. 1996). Most of the luminous and ultraluminous galaxies including quasars and other AGN's known today are part of interacting or merging binaries or multiple galactic systems (e.g. Sanders et al. 1988, Disney et al. 1995, Miles et al. 1996, Sanders & Mirabel 1996 and references therein). Such interaction almost always destroys the previous structure of the interstellar gas and dust in the participating galaxies. Angular momentum transfer easily triggers the infall of giant molecular gas and dust clouds towards the central regions of the galaxies where they collide with each other and eventually start a powerful starburst (SB). New populations of stars are generated by such events and the morphological type of the galaxy may change completely. Numerical calculations show that the majority of cluster galaxies have already gone through one or more such interactions (Moore et al 1996).

Although interaction processes between galaxies leading to SB events seem to be crucial for the understanding of galactic evolution as a whole, our knowledge of the phenomena involved is yet preliminary. The reason is that most signposts for such interaction processes show up first and most clearly in the infrared. Existing stars are not very much affected by interactions since stellar collisions are negligible. Collisions between molecular clouds, however, generate a plethora of shocks, turbulences, heating of dust particles and excitation, most of which can only be traced in the infrared. The possible aftermath of such an event, a powerful SB or eventually an active galactic nucleus (AGN), will create a lighthouse beam in the far infrared in the form of the huge spectral bump in the wavelength range 60µm to 100µm. It resembles reradiation of dust exposed to the powerful source of energy in the nucleus and it's bolometric luminosity can exceed that of our galaxy by more than 4 orders of magnitude. Since the detection of such galaxies by IRAS (Soifer, Houk, & Neugebauer 1987 and references therein), SOFIA will become the first MIR/FIR observatory sensitive enough to efficiently observe such targets. It will thus be possible for the first time:

• to analyze these events on a more statistical basis in particular in isolated clusters,
• to determine the spectral energy distribution (SED) of the sources involved,
• to measure their dust temperatures and separate components of different temperatures,
• to spatially resolve and investigate nearby merging systems in the MIR & FIR,
• to detect interacting systems at larger redshifts.

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Medium and high-z galaxies

If all the galaxies we observe today underwent violent episodes to form their different populations of stars, we should be able to see the footprints of those events in the history of the observable universe. Looking at the spectral energy distribution (SED) of normal and luminous infrared galaxies (LIRG) it is obvious that galaxies with high redshifts and the highest luminosities will be detected most easily in the MIR and FIR spectral range. There has already been a successful attempt using the ISO satellite at 6µm and 15µm to detect and identify luminous infrared galaxies in the Hubble Deep Field (Rowan-Robinson et al. 1997). Nine of the 10 objects with predicted 100µm fluxes are either stronger than SOFIA's (5Sigma, 1h, 40 mJy) sensitivity or fainter within a factor of 3, which still is practicable.

With SOFIA's predicted sensitivity one can determine the largest distance at which galaxies of a given luminosity can be detected with SOFIA at 100µm. Ultraluminous infrared galaxies (ULIRG) with L = 10¹² L_o can be observed out to z ~ 1 and galaxies comparable to the most luminous galaxies known can be detected out to z ~ 3. The sensitivity is high enough to study the evolution of IR bright galaxies in clusters at around z = 0.3 ~0.4, which are typical distances for so called "Butcher-Oemler" clusters (Couch et al. 1994). Although the atmospheric background will limit SOFIA's sensitivity generally to galaxies of z <= 1, the spatial resolution of SOFIA between 30µm and 250µm will be unprecedented. It will allow us

• to disentangle crowded field much better than ISO and SIRTF,
• to detect and complete the sample of IR bright galaxies out to a higher redshift, at wavelengths where ISO and SIRTF are confusion limited,
• to identify IR counterparts of deep optical and NIR images of the Hubble Space Telescope (HST),
• to search for galaxies in an early phase of their evolution (e.g. blue galaxy clusters),
• to determine their SEDs if their 100µm flux density exceeds ~100mJy,
• to compare the interaction rate and characteristics of the star formation events on different z scales.

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In addition it is very likely to observe or even discover new classes of objects. The so-called ERO's, for example, seem to be a population of very red galaxies discovered in the near infrared (e.g. Elston, Rieke, & Rieke 1988, 1989; Cimatti et al. 1997). Very little is yet known about their nature, their SED's, their sources of far infrared emission, and even their redshifts. Graham & Dey (1996) speculate that their 200µm flux density may reach 100mJy, well within the range observable with SOFIA.

The nature of the recently discovered extragalactic FIR-background (e.g. Guiderdoni et al. 1997, Harwit 1999) is still under debate. SOFIA's cameras will be ideally suited for follow-up observations and should be able to prove whether the FIR-background is being emitted by high-z galaxies and what the characteristics of those objects are.

ISO follow-up

The Infrared Space Observatory (ISO) has already delivered a wealth of data, in particular many new results on the evolution and nature of starburst galaxies, ULIRG's and active galactic nuclei (AGN)(see A.&A. Lett., 315, (1996), special ISO issue). Providing a mirror diameter more than 4 times larger than ISO, SOFIA is ideally suited to follow-up on those targets in much greater spatial detail and with an improved spectroscopic sensitivity.

In the case of SB galaxies it will thus be possible to proceed from ISO's determination of integral SB parameters to a spatially detailed analysis in the waveband between 15µm and 40µm on many sources. Individual SB regions will be localized and their dust temperature and distribution will be determined. Ratios of spectral line maps, such as [NeIII]/[NeII] (15µm/13µm) or [SIV]/[SIII] (18µm/33µm/), will be used to determine the radiation field, the excitation conditions and the dynamics in these regions and thus analyze the SB parameters. Dust components of low temperature will be searched for. These trace cold masses, which are important contributors to the galaxies total mass budget; the latter is still not well defined for the majority of the galaxies.

The Seyfert activity still bears two unsolved questions: The existence of the so called unified scheme that would allow us to understand different types of Seyfert activity within the frame of a single model, and the type of physical and/or evolutionary connection between Seyfert- and SB activity which are often closely associated with each other. The study of the small Seyfert nuclei, however, is difficult. AGN are often so closely associated with SBs or even surrounded and obscured by them that the nuclei themselves are not directly accessible in the optical and near-infrared (NIR) or their emission cannot be separated from the SB component. In the MIR, however, the situation is much improved because the combined emission of starlight and dust emission from the SBs reaches a minimum there, whereas the emission of the active nuclei has its maximum.

SOFIAs excellent spatial resolution will enable us for the first time to isolate the nuclear emission reliably from the rest of the galaxy, to study the nuclear energetic properties, the SED of the hot dust and its heating mechanism as a function of different Seyfert types and SB environments. High excitation lines like [NeV] (24µm) and [OIV] (26µm) are much less obscured compared to the NIR and trace the narrow line regions.

The content of this page was excerpted from a PDF document "SOFIA Astronomy and Technology in the 21st Century" by Alfred Krabbe and Hans-Peter Roeser.

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