Bulgeless disks, dark galaxies, inverted color gradients, and other expected phenomena at higher z

Abstract

The spectral energy distribution (SED) of galaxies varies both between galaxies and within them. For instance, early-type spiral galaxies have a red bulge surrounded by a bluer star-forming disk with H II regions within. When observing redshifted galaxies, a given photometric filter probes light at a bluer rest frame, and in relating the observed magnitudes to the rest frame of the filter, so-called k corrections are commonly applied to account for the relative dimming or brightening in addition to the pure distance effect. The amount of correction depends on the shape of the spectrum (SED), so different k corrections apply to galaxies of different spectral types. This is, however, only part of the story, since any galaxy with a spatially non-homogeneous SED will experience a spatially varying relative dimming or brightening as a function of observed wavelength. Also, the morphological appearance of galaxies will therefore change with redshift. For instance, an early spiral galaxy observed in the V band would show a prominent bulge at z = 0, whereas, if at redshift z ∼ 1, the V filter probes emission in the rest-frame near-ultraviolet where the bulge is faint and the disk relatively brighter, thus the galaxy may appear as bulgeless. One popular way of studying spatial variations in the stellar population and dust content of galaxies is the use of color maps. For star-forming galaxies that have an appreciable contribution from nebular emission (lines and continuum), an additional effect is that the shifting of strong features in or out of filters will result in a non-monotonous color evolution with redshift. Hence, unlike the effects of distance, cosmological surface brightness dimming, and gravitational lensing, which are all achromatic, the fact that most galaxies have a spatially varying SED leads to a chromatic surface brightness modulation (CMOD) with redshift. While the CMOD effects are in principle easy to grasp, they affect multicolor imaging surveys and photometric properties derived from such surveys in a complex fashion. Properties such as the bulge-to-disk ratio, Sérsic exponent, light concentration, asymmetry index and effective radius, radial color gradients, and stellar mass determinations from SED fitting will depend on the redshift, the filters employed, and the rest-frame 2D SED patterns in a galaxy and will bias results inferred on galaxy evolution across cosmic time (e.g., the evolution of the mass-size, bulge-supermassive black hole, and Tully-Fisher relation), and potentially also weak lensing, if these effects are not properly taken into account. In this article we quantify the CMOD effects for idealized galaxies built from spectral synthesis models and from galaxies with observed integral field spectroscopy, and we show that they are significant and should be taken into account in studies of resolved galaxy properties and their evolution with redshift.

MIDIS: JWST/MIRI Reveals the Stellar Structure of ALMA-selected Galaxies in the Hubble Ultra Deep Field at Cosmic Noon

“…While the number of AGN in the sample is limited, we do not see a clear difference in Figure 6 between the ALMA galaxies that are identified as AGN and those that are not (note this already excludes the two AGN best fit with a point-source model). In the same vein, the MIRI emission may also trace nebular continuum and/or high equivalent width emission lines in case of very vigorous young star formation, which can have a complicated impact on the morphology (e.g., Papaderos et al 2023). This would most strongly impact galaxies with a low stellar mass and/or high specific star formation rate.…”

Morphological Evolution of Disk Galaxies and Their Concentration, Asymmetry, and Clumpiness (CAS) Properties in Simulations across Toomre’s Q Parameter

“…The visual inspection, however, has limitations for distant galaxies, as their surface brightness drops due to the cosmological dimming effect. Features such as spiral arms may become too faint to be detected at high redshift (e.g., Papaderos et al 2023). In fact, distant star-forming galaxies do not share the same structure or morphology as the local spiral galaxies (Yuma et al 2011(Yuma et al , 2012.…”