Quantifying the inside-out formation of disk galaxies at z = 0.8-1.2

Laura DeGroot

Galaxies that we observe in our local neighborhood today have relatively low star formation rates and are already assembled into massive galaxies. However, when the Universe was only about 6 billion years old (z~1), the Universe was around the peak of star formation and galaxies were rapidly evolving. Depending on where this star formation was taking place in the galaxy, it would have a large impact on the observed structural properties of that galaxy. It has also been observed that galaxies transition from increasingly clumpy morphologies at 𝑧 = 2 to fairly regular disk and spheroid morphologies at 𝑧 < 1. The morphological transformation, increase in galaxy sizes, and overall morphological evolution offer additional insights into the physical processes involved in the build-up of galaxies over cosmic time. 

Figure 1: UV and Hα size dependence on morphology of galaxies at z~1.

Based on previous work by Nelson et al. 2012, high-resolution, two-dimensional hydrogen (Hα) emission maps of the galaxies can show where the star formation is occurring as well as how the galaxy is growing. By continuing to study the two-dimensional maps of galaxies at z~ 1, including UV and optical analysis of the galaxies, we can further study if galaxies do form inside-out or outside-in. This is done by analyzing Hubble Space Telescope (HST) images from the Hubble Deep UV Legacy Survey (HDUV: Oesch et al. 2018) and from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS: Grogin et al. 2011; Koekemoer et al. 2011), programs that consist of hundreds of orbits of HST data providing high resolution images. Analysis of these images enables us to detect where star formation is occurring within a galaxy. To do this, we use GALFIT (Peng et al. 2002, 2010) to measure the UV and optical sizes of the galaxies in this dataset to determine where star formation is occurring within a galaxy, indicating if disk growth occurs from the inside out or outside in (Fig. 1). We also aim to incorporate galaxy mass information to further investigate quenching of star formation across different galaxy masses (Nelson et al. 2021). 

While UV and optical sizes of galaxies can be compared to see where star formation is occurring within a galaxy, the observed color gradients of galaxy disks may also be affected by radial gradients in age, metallicity, or dust extinction. By using multi-wavelength HST images to constrain the impact of dust extinction and metallicity gradients on the observed colors of galaxies, we construct 2D color maps (Fig. 2) as well as radial profiles of galaxies (Fig. 3) to infer the spatial variation of dust and star formation histories, and to quantify how this impacts the measured UV and optical sizes of galaxies. By including mass information of the galaxy sample, we also aim to further understand the impact of quenching (Mosleh et al. 2020). 

Other aspects we aim to include in our study in the future include using additional data such as 3-D HST grism spectra (Brammer et al. 2012; van Dokkum et al. 2011) or publicly available Multi Unit Spectroscopic Explorer (MUSE) Integral Field Unit (IFU) data to further map detailed star formation within the galaxy sample. Structure maps of the galaxies may also be created, which may be used to detect star forming clumps in the galaxies and compare the galactic properties to determine how the galaxy mass is growing in time. 

Figure 2: Optical and UV images, and color maps of six massive, passively evolving galaxies. (Guo et al. 2011)

Figure 3: Radial profiles of the galaxies shown in Figure 1. (Guo et al. 2011)