Detecting population III galaxies with HST and JWST
A small fraction of the atomic-cooling halos assembling at may form out of minihalos that never experienced any prior star formation, and could in principle host small galaxies of chemically unenriched stars. Since the prospects of detecting isolated population III stars appear bleak even with the upcoming James Webb Space Telescope (JWST), these population III galaxies may offer one of the best probes of population III stars in the foreseeable future. By projecting the results from population III galaxy simulations through cluster magnification maps, we predict the fluxes and surface number densities of pop III galaxy galaxies as a function of their typical star formation efficiency. We argue that a small number of lensed population III galaxies in principle could turn up at –10 in the ongoing Hubble Space Telescope survey CLASH, which covers a total of 25 low-redshift galaxy clusters.
address=Department of Astronomy, Stockholm University,
Oscar Klein Center, AlbaNova, Stockholm SE-106 91, Sweden
The first generation of population III stars (hereafter pop III) is predicted to form in isolation or in small numbers within dark matter minihalos (–) at (e.g. Trenti and Stiavelli, 2009). However, even with the upcoming James Webb Space Telescope (JWST), the prospects of detecting such stars appear bleak (Rydberg et al., 2012), unless they attain masses – due to a prolonged dark star phase (e.g. Freese et al., 2010; Zackrisson et al., 2010a, b; Ilie et al., 2012). Larger numbers of pop III stars could in principle form at within HI cooling halos (–) that have remained chemically pristine (e.g. Johnson et al., 2009; Stiavelli and Trenti, 2010), and such pop III “galaxies” (a.k.a. pop III star clusters) should be significantly easier to detect. Fig. 1 schematically illustrates how such objects might form.
2 Detection prospects – unlensed fields
Using the Yggdrasil spectral synthesis code (Zackrisson et al., 2011a) to derive the minimum star formation efficiency
3 Detection prospects – lensed fields
The use of foreground galaxy clusters as gravitational telescopes has been advocated as a possible way to detect individual pop III dark stars with JWST (Zackrisson et al., 2010a), and a similar strategy can be used for pop III galaxies as well. While lensing boosts the fluxes of background objects by a factor equal to the magnification , the background volume that is probed in such cluster lensing surveys is at the same time reduced by the same factor. This converts into a lower limit on the surface number densities for all source populations that can efficiently be detected using this method. In Zackrisson et al. (2012), we project the pop III galaxy simulation cubes of Trenti et al. (2009) through the cluster magnification maps of J0717.5+3745 (J0717) – the galaxy cluster with the largest Einstein radius so far detected (Zitrin et al., 2009). This allows us to predict the lensed pop III galaxy source counts as a function of their typical star formation efficiencies. We find that cluster lensing should allow the detection of pop III galaxies with star formation efficiencies an order of magnitude lower than in unlensed fields, thereby pushing objects with (Safranek-Shrader et al., 2012) into the range detectable with JWST. If the actual pop III would be even higher ( to -1.5), lensed pop III galaxies could even be within reach of the ongoing HST CLASH survey (Postman et al., 2012), which targets a total of 25 low-redshift galaxy clusters, including J0717. Pop III galaxy candidates at can in principle be identified in photometric surveys because of their unusual broadband colours (e.g. Pello and Schaerer, 2003; Inoue, 2011; Zackrisson et al., 2011a, b). In the case of HST data, one such tell-tale colour signature can be produced by anomalously strong Ly emission (Zackrisson et al., 2011b). Of course, this requires that a substantial fraction of the Ly photons can be transmitted through the intergalactic medium, for instance due to outflows, patchy reionization or source clustering (e.g. Dijkstra et al., 2011; Dayal and Ferrara, 2011).
4 Summary and future outlook
Depending on their typical star formation efficiencies, lensed pop III galaxies may be within reach of JWST and possibly even the HST. Such objects may, under certain circumstances be identified in multiband photometric surveys because of their unusual broadband colours. A search for such objects in the HST/CLASH survey, targeting 25 foreground galaxy clusters, is currently underway.
- here defined as the fraction of halo baryons turned into pop III stars
- M. Trenti, and M. Stiavelli, ApJ 694, 879–892 (2009), 0901.0711.
- C.-E. Rydberg, E. Zackrisson, P. Lundqvist, and P. Scott, ArXiv e-prints (2012), 1206.0007.
- K. Freese, C. Ilie, D. Spolyar, M. Valluri, and P. Bodenheimer, ApJ 716, 1397–1407 (2010), 1002.2233.
- E. Zackrisson, P. Scott, C.-E. Rydberg, F. Iocco, B. Edvardsson, G. Östlin, S. Sivertsson, A. Zitrin, T. Broadhurst, and P. Gondolo, ApJ 717, 257–267 (2010a), 1002.3368.
- E. Zackrisson, P. Scott, C.-E. Rydberg, F. Iocco, S. Sivertsson, G. Östlin, G. Mellema, I. T. Iliev, and P. R. Shapiro, MNRAS 407, L74–L78 (2010b), 1006.0481.
- C. Ilie, K. Freese, M. Valluri, I. T. Iliev, and P. R. Shapiro, MNRAS 422, 2164–2186 (2012).
- J. L. Johnson, T. H. Greif, V. Bromm, R. S. Klessen, and J. Ippolito, MNRAS 399, 37–47 (2009), 0902.3263.
- M. Stiavelli, and M. Trenti, ApJL 716, L190–L194 (2010), 0911.3368.
- E. Zackrisson, C.-E. Rydberg, D. Schaerer, G. Östlin, and M. Tuli, ApJ 740, 13 (2011a), 1105.0921.
- M. Trenti, M. Stiavelli, and J. Michael Shull, ApJ 700, 1672–1679 (2009), 0905.4504.
- P. Kroupa, MNRAS 322, 231–246 (2001), arXiv:astro-ph/0009005.
- C. Safranek-Shrader, M. Agarwal, C. Federrath, A. Dubey, M. Milosavljevic, and V. Bromm, ArXiv e-prints (2012), 1205.3835.
- E. Zackrisson, A. Zitrin, M. Trenti, C.-E. Rydberg, L. Guaita, D. Schaerer, T. Broadhurst, G. Ostlin, and T. Strom, ArXiv e-prints (2012), 1204.0517.
- A. Zitrin, T. Broadhurst, Y. Rephaeli, and S. Sadeh, ApJL 707, L102–L106 (2009), 0907.4232.
- M. Postman, D. Coe, N. Benítez, L. Bradley, T. Broadhurst, M. Donahue, H. Ford, O. Graur, G. Graves, S. Jouvel, A. Koekemoer, D. Lemze, E. Medezinski, A. Molino, L. Moustakas, S. Ogaz, A. Riess, S. Rodney, P. Rosati, K. Umetsu, W. Zheng, A. Zitrin, M. Bartelmann, R. Bouwens, N. Czakon, S. Golwala, O. Host, L. Infante, S. Jha, Y. Jimenez-Teja, D. Kelson, O. Lahav, R. Lazkoz, D. Maoz, C. McCully, P. Melchior, M. Meneghetti, J. Merten, J. Moustakas, M. Nonino, B. Patel, E. Regös, J. Sayers, S. Seitz, and A. Van der Wel, ApJS 199, 25 (2012), 1106.3328.
- R. Pello, and D. Schaerer, “Looking for the First Galaxies with GTC + EMIR,” in Revista Mexicana de Astronomia y Astrofisica Conference Series, edited by J. M. Rodriguez Espinoza, F. Garzon Lopez, and V. Melo Martin, 2003, vol. 16 of Revista Mexicana de Astronomia y Astrofisica, vol. 27, pp. 225–228, arXiv:astro-ph/0203203.
- A. K. Inoue, MNRAS 415, 2920–2931 (2011), 1102.5150.
- E. Zackrisson, A. K. Inoue, C.-E. Rydberg, and F. Duval, MNRAS 418, L104–L108 (2011b), 1109.1556.
- M. Dijkstra, A. Mesinger, and J. S. B. Wyithe, MNRAS 414, 2139–2147 (2011), 1101.5160.
- P. Dayal, and A. Ferrara, MNRAS 417, L41–L45 (2011), 1102.1726.