天文現象及活動

光從哪裡去了？哈勃望遠鏡前緣場中透出受抑高紅移星系形成的證據

Public Seminar of RPg Student:- Where’s the Light? – Evidence for Suppressed High-redshift Galaxy Formation from the Hubble Frontier Fields

The standard Cold Dark Matter (CDM) model predicts that galaxies form hierarchically, resulting in an increasing number of galaxies towards lower masses and hence fainter luminosities at all redshifts since the onset of galaxy formation. In the Local Universe, such a dependence had been found for luminosities as low as dwarf spheroidal galaxies, the least massive galaxies known. A similar trend had also been confirmed in the early universe through blank field surveys, which provide a limiting rest-frame UV absolute magnitude of MUV ~ -17 at redshifts z > 4. Does this trend of increasing galaxy space density continue to even lower luminosities as predicted by the CDM model, or eventually turn over as predicted by competing models such the Wave Dark Matter (ψDM) model?

To probe even fainter luminosities than is possible through blank field surveys, we took advantage of gravitational lensing provided by massive galaxy clusters that boosts otherwise undetectable faint galaxies to above the intrinsic survey flux limit. Our sample comprises ~600 dropout galaxies identified in five galaxy cluster fields and their adjacent blank fields (observed simultaneously) from the Hubble Frontier Fields program. These galaxies are magnified in brightness by as high as ~100×, thus allowing us to probe approximately five magnitudes deeper than the intrinsic survey limit. Because a given solid angle in the sky is similarly magnified, the volume of space probed decreases in proportion to the magnification. This effect, however, is countered by the expected increase in space density of less luminous galaxies as predicted by the CDM model.

Contrary to such expectation, we discovered a significant deficit of highly magnified galaxies in the data relative to the CDM prediction, reflecting that the diluted apparent galaxy density from the magnified sky area heavily outweighs the addition of faint galaxies brought above the flux limit. This result was found to be in ＞4σ tension with the standard CDM model, indicating a slow rollover in the faint-end UV luminosity function as is predicted by the ψDM model. In this alternative model, the formation of low-mass DM halos, which seed the growth of faint galaxies, is suppressed by the quantum pressure arising from interfering ultralight dark matter waves with parsec-scale de Broglie wavelengths.

Through a comparison with state-of-the-art ψDM simulations, we found that our results are compatible with the model predictions for a DM boson mass of ~10^-22 eV. The inferred boson mass is consistent with that deduced independently for producing the large constant-density DM cores of local dwarf spheroidal galaxies. In addition, our results also show a marked transition (stronger decline towards higher redshifts) in the cosmic star formation rate density at z ~ 8, a similar redshift as the instantaneous epoch of reionization at z = 8.2 reported by Planck.　
　
時間：2017.05.04(四) 11:30
地點：香港大學莊月明物理樓105室
講者：Yi Lok Enoch LEUNG (香港大學)
語言：英語

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#119

此活動由香港大學物理系主辦。

2017-05-04