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Title: Science Case Reference: MUSE-MEM-SCI-052 Issue: 1.3 Date: 04/02/2004 Page: 41/100 The following three features can be identified as clear signatures of very metal-poor or PopIII starbursts (primeval galaxies): • A larger Lyman continuum flux, and a continuum dominated by nebular emission, leading to a flatter intrinsic SED compared to normal galaxies. • For young bursts the maximum Ly α equivalent width increases strongly with decreasing metallicity from W(Ly α ) ~ 250-350 Å at Z >~ 1/50 Z sun to 400-850 Å or higher at Z between 10 -5 and 0 (Pop III) for the same Salpeter IMF. This is illustrated in Fig. 2-22 considering also various IMF at low metallicity. • Strong HeII recombination lines (1640, 4686 Å) are a quite unique signature due to hot massive main sequence stars of PopIII/very low metallicity. Significant HeII emission is, however, only expected at metallicities below 10 -5 solar (figure 2-21). Figure 2-22: Predicted Ly α equivalent widths for bursts of different metallicities and IMFs (from Schaerer 2003). The squares are for IMF 50-500Mo, the triangles 1- 500Mo, and the circles 1-100 Mo. The various curves correspond to increasing metallicity from top to bottom. The second characteristic could also be found in AGNs, but enough spectral resolution (R>1000) will allow to distinguish between the two possibilities. With the help of the Table 4 from Schaerer (2003), the Ly α line emission can be estimated, for a constant star formation of SFR= 2 M sun .yr -1 , and a photon escape fraction of 0.5, with an assumed metallicity of 10 -3 solar (and a non extreme, intermediate, IMF), to be 4 10 42 erg.s -1 , corresponding for a proto-galaxy at z=7, to the line flux of about 2 10 -17 erg.s -1 .cm -2 . For the HeII 1640 line, the line luminosity in the same conditions is 10 39 erg/s, and will give a flux of about 10 -20 erg.s -1 .cm -2 , for a redshift z=5. This last line will be too faint to be detectable. However, at even Figure 2-23 : Predicted hardness as a function of metallicity for starbursts between PopIII and normal metallicities (from Schaerer 2003). The 3 curves are for the 3 different IMF indicated (Mup= 100 or 500Mo, Mlo=1 or 50Mo)
Title: Science Case Reference: MUSE-MEM-SCI-052 Issue: 1.3 Date: 04/02/2004 Page: 42/100 lower metallicities, for truly primordial objects, the HeII 1640 line is favoured relative to Ly α (see Figure 2-21); for some assumed IMF, the HeII 1640 line luminosity could reach 4.10 41 erg.s -1 at zero metallicity, corresponding to a flux of 3.10 -18 erg.s -1 .cm -2 , at z=5. Such a line would be easily detectable in the MUSE deep field. Finally, the MUSE ultra deep field described in section 6 (using gravitational amplification by a factor 3 in average), with its enhanced detection limit of 8.10 -20 erg.s -1 .cm -2 , should be able to detect simultaneously Ly α and HeII lines in the 2.8-4.7 redshift range for a metallicity lower than 10 -5 solar. Based on the models of Scanapieco et al (2003), the fraction of Ly α emitters that are Pop III objects could under certain scenarios be as high as 10% at z ~ 5 and L Lyα ~ 10 43 erg s -1 cm -2 . The point is that this is highly model dependent, and the detection of such objects (which is quite plausible) would greatly add to our understanding of the early chemical enrichment of the Universe. References Scanapieco, E., Schneider, R., Ferrara, A., astro-ph/0301628. Schaerer, D., 2002, A&A, 382, 28 Schaerer, D., 2003, A&A, 397, 527 Tumlinson, J., Giroux, M.L., Shull, J.M., 2001, ApJ, 550, L1 Tumlinson, J., Shull, J. M., Venkatesan, A.: 2003, ApJ, 584, 608 2.10. Active galactic nuclei at intermediate and high redshifts In a dramatic change of paradigm over the last years, active galactic nuclei (AGN) have altered their status from interesting but somewhat exotic objects into fundamental components of galaxy formation and evolution. This change was mainly triggered by the recognition that supermassive black hole (SMBHs) are ubiquitous in massive galaxies (Magorrian et al 1998). The striking near-equality between the local black hole mass density and the total density of matter accumulated through accretion in AGN (Yu & Tremaine 2002) suggests that periods of nuclear activity may in fact be common phases within galaxy evolution. An intricate link exists between black hole growth, the formation of galaxy bulges, and nuclear activity cycles; but most details are still missing from this picture. This is a challenge to theory and observers alike. In particular, the host galaxy and environmental properties of AGN at redshifts around and beyond z~1 will allow one to set strong constraints on formation scenarios. This is an area where MUSE will provide significant progress, because of its capability to reach very faint flux levels at good spectral resolution, combined with a high multiplex factor over an astrophysically relevant field size of ~ 250 kpc. A fundamental advantage of MUSE over existing or other planned instruments is the integration of the traditional multi-stepped approach of imaging, low-resolution and high-resolution spectroscopy into a single observation.
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