The morphologies of massive galaxies at 1 <z< 3 in the CANDELS-UDS field: compact bulges, and the rise and fall of massive discs

Abstract

We have used high‐resolution, Hubble Space Telescope, near‐infrared imaging to conduct a detailed analysis of the morphological properties of the most massive galaxies at high redshift, modelling the WFC3/IR H160‐band images of the ≃200 galaxies in the CANDELS‐UDS field with photometric redshifts 1 < z < 3, and stellar masses M* > 1011 M⊙. We have explored the results of fitting single‐Sérsic and bulge+disc models, and have investigated the additional errors and potential biases introduced by uncertainties in the background and the on‐image point spread function. This approach has enabled us to obtain formally acceptable model fits to the WFC3/IR images of >90 per cent of the galaxies. Our results indicate that these massive galaxies at 1 < z < 3 lie both on and below the local size–mass relation, with a median effective radius of ∼2.6 kpc, a factor of ≃2.3 smaller than comparably massive local galaxies. Moreover, we find that bulge‐dominated objects in particular show evidence for a growing bimodality in the size–mass relation with increasing redshift, and by z > 2 the compact bulges display effective radii a factor of ≃4 smaller than local ellipticals of comparable mass. These trends also appear to extend to the bulge components of disc‐dominated galaxies. In addition, we find that, while such massive galaxies at low redshift are generally bulge‐dominated, at redshifts 1 < z < 2 they are predominantly mixed bulge+disc systems, and by z > 2 they are mostly disc‐dominated. The majority of the disc‐dominated galaxies are actively forming stars, although this is also true for many of the bulge‐dominated systems. Interestingly, however, while most of the quiescent galaxies are bulge‐dominated, we find that a significant fraction (25–40 per cent) of the most quiescent galaxies, with specific star formation rates sSFR < 10−10 yr−1, have disc‐dominated morphologies. Thus, while our results show that the massive galaxy population is undergoing dramatic changes at this crucial epoch, they also suggest that the physical mechanisms which quench star formation activity are not simply connected to those responsible for the morphological transformation of massive galaxies into present‐day giant ellipticals.

Sizes, colour gradients and resolved stellar mass distributions for the massive cluster galaxies in XMMUJ2235-2557 atz= 1.39

“…Note that we have also tried to derive a TinyTim PSF composite by adding PSF models using the TinyTim code (Krist 1995) into the raw data and drizzling them as science images. Nevertheless, we notice that the TinyTim drizzled PSF does not match well the empirical PSF in the outer part: a much stronger outer envelope (as well as diffraction spikes) can be seen in the empirical PSF (see also, Appendix A in Bruce et al 2012, for a similar description). On the other hand, van der Wel et al (2012) produced hybrid PSF models by replacing the central pixels of the median-stacked star by the TinyTim PSF.…”

Galaxy And Mass Assembly (GAMA): $\mathcal {M_\star }\text{--}R_{\rm e}$ relations ofz= 0 bulges, discs and spheroids

“…While the number of studies of large samples which employ bulge + disc decomposition to explore the nature of galaxies and their components is growing, the analysis is challenging (see e.g., Allen et al 2006;Gadotti 2009;Simard et al 2011;Bruce et al 2012;Bruce et al 2014;Lang et al 2014;Tasca et al 2014;Meert et al 2015;Salo et al 2015). This is because multi-component fitting is notoriously difficult, especially when trying to automate it for large samples.…”

“…Simard et al 2011;Bruce et al 2012;Lackner & Gunn 2012;Meert et al 2015). This reduces the number of free parameters and ensures the fitting process is more robust but it restricts the possible interpretations of the fitting outcomes, e.g.…”

“…For example, at high-redshift galaxies might look disc-like or elliptical/spheroidal but their physical properties are unlike any discs or ellipticals in the local Universe (see e.g. Bruce et al 2012;Buitrago et al 2013;Mortlock et al 2013). Galaxies at high redshifts are typically more irregular with thick slab-like disc structures and clumpy star-forming regions (Wisnioski et al 2012).…”

Evolution of galaxy shapes from prolate to oblate through compaction events

“…This leads to an inside-out quenching process into a compact, passive "red nugget", the likely progenitor of the centre of an early-type galaxy today. The observational basis for this picture is becoming solid, both for the red-nugget phenomenon (Trujillo et al 2006a,b;van Dokkum et al 2008;Damjanov et al 2009Damjanov et al , 2011Newman et al 2010;Bruce et al 2012;Whitaker et al 2012;van Dokkum et al 2014) and for their potential bluenugget progenitors (Barro et al 2013(Barro et al , 2014aBruce et al 2014;Nelson et al 2014;Williams et al 2014;Tacchella et al 2015a). The theory, partly based on the same simulations that we analyse here, is also becoming robust (Dekel & Burkert 2014;Tacchella et al 2015b,c;Zolotov et al 2015).…”