Dr Aprajita Verma

Abstract:

High redshift galaxies play a key role in our developing understanding of galaxy formation and evolution. Since such galaxies are being studied within a Gyr of the big bang, they provide a unique probe of the physics of one of the first generations of large-scale star-formation. We have performed a complete statistical study of the physical properties of a robust sample of z∼5 UV luminous galaxies selected using the Lyman-break technique. The characteristic properties of this sample differ from LBGs at z∼3 of comparable luminosity in that they are a factor of ten less massive (∼few10⁹ M) and the majority (∼70%) are considerably younger (<100Myr). Our results support no more than a modest decline in the global star formation rate density at high redshifts and suggest that ∼1% of the stellar mass density of the universe had already assembled at z∼5. The constraint derived for the latter is affected by their young ages and short duty cycles which imply existing z∼5 LBG samples may be highly incomplete. These intense starbursts have high unobscured star formation rate surface densities (∼100s M yr¹ kpc²), suggesting they drive outflows and winds that enrich the intra- and inter-galactic media with metals. These properties imply that the majority of z∼5 LBGs are in formation meaning that most of their star-formation has likely occurred during the last few crossing times. They are experiencing their first (few) generations of large-scale star formation and are accumulating their first significant stellar mass. As such, z∼5 LBGs are the likely progenitors of the spheroidal components of present-day massive galaxies (supported by their high stellar mass surface densities and their core phase-space densities). © 2008 International Astronomical Union.

Abstract:

We determine the ensemble properties of Lyman-break galaxies (LBGs) selected as V-band dropouts to in the Chandra Deep Field-South using their rest-frame UV-to-visible spectral energy distributions. By matching the selection and performing the same analysis that has been used for samples, we show clear differences in the ensemble properties of two samples of LBGs which are separated by 1 Gyr in look-back time. We find that LBGs are typically much younger (<100 Myr) and have lower stellar masses than their counterparts (which are typically ∼ few × and ∼320 Myr old). The difference in mass is significant even when considering the presence of an older, underlying population in both samples. Such young and moderately massive systems dominate the luminous LBG population (≳70 per cent), whereas they comprise ≲30 per cent of LBG samples at. This result, which we demonstrate is robust under all reasonable modelling assumptions, shows a clear change in the properties of the luminous LBGs between and. These young and moderately massive LBGs appear to be experiencing their first (few) generations of large-scale star formation and are accumulating their first significant stellar mass. Their dominance in luminous LBG samples suggests that witnesses a period of wide-spread, recent galaxy formation. As such, LBGs are the likely progenitors of the spheroidal components of present-day massive galaxies. This is supported by their high stellar mass surface densities, and is consistent with their core phase-space densities, as well as the ages of stars in the bulge of our Galaxy and other massive systems. With implied formation redshifts of, these luminous LBGs could have only contributed to the UV photon budget at the end of reionization. However, their high star formation rates per unit area suggest these systems host outflows or winds that enrich the intragalactic and intergalactic media with metals, as has been established for LBGs. Their estimated young ages are consistent with inefficient metal-mixing on galaxy-wide scales. Therefore these galaxies may contain a significant fraction of Population III stars as proposed for LBGs by Jiminez & Haimann. © 2007 The Authors. Journal compilation © 2007 RAS.