Joseph Silk

Abstract:

We analyze the spectral energy distributions (SEDs) of Lyman break galaxies (LBGs) at z ≃ 1-3 selected using the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) UVIS channel filters. These HST/WFC3 observations cover about 50 arcmin2 in the GOODS-South field as a part of the WFC3 Early Release Science program. These LBGs at z ≃ 1-3 are selected using dropout selection criteria similar to high-redshift LBGs. The deep multi-band photometry in this field is used to identify best-fit SED models, from which we infer the following results: (1) the photometric redshift estimate of these dropout-selected LBGs is accurate to within few percent; (2) the UV spectral slope β is redder than at high redshift (z > 3), where LBGs are less dusty; (3) on average, LBGs at z ≃ 1-3 are massive, dustier, and more highly star forming, compared to LBGs at higher redshifts with similar luminosities (0.1L* ≲ L ≲ 2.5L*), though their median values are similar within 1σ uncertainties. This could imply that identical dropout selection technique, at all redshifts, finds physically similar galaxies; and (4) the stellar masses of these LBGs are directly proportional to their UV luminosities with a logarithmic slope of ∼0.46, and star formation rates are proportional to their stellar masses with a logarithmic slope of ∼0.90. These relations hold true - within luminosities probed in this study - for LBGs from z ≃ 1.5 to 5. The star-forming galaxies selected using other color-based techniques show similar correlations at z ≃ 2, but to avoid any selection biases, and for direct comparison with LBGs at z > 3, a true Lyman break selection at z ≃ 2 is essential. The future HST UV surveys, both wider and deeper, covering a large luminosity range are important to better understand LBG properties and their evolution. © 2013. The American Astronomical Society. All rights reserved.

Abstract:

In order to understand the physical mechanisms at work during the formation of massive early-type galaxies, we performed six zoomed hydrodynamical cosmological simulations of haloes in the mass range 4.3×1012 ≤Mvir ≤8.0×1013M at z=0, using the AdaptiveMesh Refinement code RAMSES. These simulations explore the role of active galactic nuclei (AGN), through jets powered by the accretion on to supermassive black holes on the formation of massive elliptical galaxies. In the absence of AGN feedback, large amounts of stars accumulate in the central galaxies to form overly massive, blue, compact and rotation-dominated galaxies. Powerful AGN jets transform the central galaxies into red extended and dispersion-dominated galaxies. This morphological transformation of disc galaxies into elliptical galaxies is driven by the efficient quenching of the in situ star formation due to AGN feedback, which transform these galaxies into systems built up by accretion. For galaxies mainly formed by accretion, the proportion of stars deposited farther away from the centre increases, and galaxies have larger sizes. The accretion is also directly responsible for randomizing the stellar orbits, increasing the amount of dispersion over rotation of stars as a function of time. Finally, we find that our galaxies simulated with AGN feedback better match the observed scaling laws, such as the size-mass, velocity dispersion-mass, Fundamental Plane relations and slope of the total density profiles at z 0, from dynamical and strong lensing constraints.© 2013 The Authors.

Abstract:

We study ~330 massive (M* > 109.5M⊙), newborn spheroidal galaxies (SGs) around the epoch of peak star formation (1 < z < 3) to explore the high-redshift origin of SGs and gain insight into when and how the old stellar populations that dominate today's Universe formed. The sample is drawn from the Hubble Space Telescope (HST)/WFC3 Early-Release Science programme, which provides deep 10-filter (0.2-1.7 μm) HST imaging over one-third of the GOODS-South field. We find that the star formation episodes that built our SGs likely peaked in the redshift range 2 < z < 5 (with a median of z ~ 3) and have decay time-scales shorter than ~1.5Gyr. Starburst time-scales and ages show no trend with stellar mass in the range 109.5 < M* < 1010.5 M⊙. However, the time-scales show increased scatter towards lower values (<0.3 Gyr) for M* > 1010.5M⊙, and an age trend becomes evident in this mass regime: SGs with M* > 1011.5M⊙ are ~2 Gyr older than their counterparts with M* < 1010.5M⊙. Nevertheless, a smooth downsizing trend with galaxy mass is not observed, and the large scatter in starburst ages indicates that SGs are not a particularly coeval population. Around half of the blue SGs appear not to drive their star formation via major mergers, and those that have experienced a recent major merger show only modest enhancements (~40 per cent) in their specific star formation rates. Our empirical study indicates that processes other than major mergers (e.g. violent disc instability driven by cold streams and/or minor mergers) likely play a dominant role in building SGs, and creating a significant fraction of the old stellar populations that dominate today's Universe. © 2012 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.