[Paper Review] Color Gradients in Galaxies Out to z~3: Dependence on Galaxy Properties

N. Welikala, Jean‐Paul Kneib|arXiv (Cornell University)|Feb 2, 2012

Galaxies: Formation, Evolution, Phenomena4 references3 citations

TL;DR

This study analyzes color gradients in 3,248 galaxies up to redshift z~3 using HST/ACS data, finding that bluer cores relative to outskirts increase with redshift, peaking at z~2.5 with a 0.4-magnitude difference, indicating central star formation. These gradients, dependent on magnitude, luminosity, and morphology, introduce systematic biases in weak lensing shape measurements, necessitating correction models for future cosmological surveys.

ABSTRACT

Using HST/ACS observations, we measure the color gradients of 3248 galaxies in the GOODS-South field out to z~3 and i_{AB}<25.5 and characterize their dependence on galaxy properties (luminosity, apparent magnitude, galaxy size, redshift and morphological type). The color gradient is measured by the difference of v-i color outside (R_{50}1, galaxies show increasingly bluer cores while the color of the outskirts does not change as rapidly. At z~2.5 and -22.0=2 is concentrated in their central regions. These color gradients and their dependence on galaxy properties could also have a significant impact on shear measurements in upcoming weak lensing cosmological surveys.

Motivation & Objective

• To characterize color gradients in galaxies out to z~3 and quantify their dependence on observable properties such as magnitude, luminosity, size, redshift, and morphology.
• To investigate how radial color variations evolve with redshift, particularly whether star formation is preferentially concentrated in galactic centers at high redshift.
• To assess the impact of intrinsic color gradients on weak lensing shape measurements, especially in upcoming space- and ground-based surveys.
• To provide a statistical basis for correcting PSF-induced shape biases in cosmological weak lensing surveys using galaxy color and photometric redshift information.

Proposed method

• Measures v-i color gradients by comparing the color difference between the inner region (r < R50) and outer region (R50 ≤ r < 2R50) of galaxies.
• Uses HST/ACS imaging from the GOODS-South field to obtain high-resolution multi-band photometry for 3,248 galaxies with i_AB < 25.5 and z ≤ 3.
• Applies stacking techniques to volume-limited subsamples to probe radial color variations as a function of redshift and absolute magnitude.
• Analyzes the dependence of color gradients on galaxy properties including apparent magnitude, luminosity, morphological type, and redshift.
• Models the impact of color gradients on PSF smearing in weak lensing by simulating shape distortions due to wavelength-dependent PSF and spatially varying galaxy spectra.
• Proposes a correction framework for shape measurements that incorporates galaxy color gradients and photometric redshifts, applicable to both space- and ground-based observations.

Experimental results

Research questions

• RQ1 How do color gradients in galaxies evolve with redshift from z~0 to z~3?
• RQ2 What is the dependence of color gradients on galaxy luminosity, apparent magnitude, size, and morphological type?
• RQ3 Are color gradients stronger in galaxies with bluer observed colors, and what does this imply about star formation location?
• RQ4 To what extent do color gradients introduce systematic biases in weak lensing shape measurements?
• RQ5 Can color gradients be modeled effectively using photometric redshifts and integrated colors to correct for PSF smearing in weak lensing surveys?

Key findings

• Color gradients increase from z~1 to z~2, peaking at z~2.5, with a median gradient of 0.4 magnitudes between the center and outskirts for galaxies with -22.0 < M_I ≤ -21.0.
• At z~2.5, galaxies show a mean 0.4-magnitude difference in v-i color between their centers and outer regions, indicating strong central star formation.
• Bluer galaxies have bluer cores relative to their outskirts, suggesting enhanced central star formation activity in high-redshift systems.
• The color gradient is strongly correlated with apparent magnitude, reaching a median of 0.24 magnitudes at i_AB ~ 25.5.
• At z < 0.5, galaxy centers are slightly redder than their outskirts, but this reverses at z > 1, with increasingly bluer cores.
• The observed color gradients imply that star formation at z > 1 is preferentially concentrated in galactic centers, consistent with self-regulated feedback mechanisms.