[Paper Review] Ultradeep IRAC Observations Of Sub-L* z 7 And z 8 Galaxies In The HUDF: The Contribution Of Low-luminosity Galaxies To The Stellar Mass Density And Reionization.

Ivo Labbé|arXiv (Cornell University)|Oct 6, 2009

Galaxies: Formation, Evolution, Phenomena1 citations

TL;DR

This study uses ultra-deep Spitzer IRAC observations to measure the rest-frame optical fluxes of 14 z~7 and 5 z~8 dropout galaxies in the HUDF, revealing that sub-L* galaxies at z~7 contribute significantly to the stellar mass density and may have driven sustained reionization. The z~7 galaxies have a mean stellar mass of 1.2×10⁹ M☉, age ~300 Myr, and high specific star formation rate, indicating they are compact but not young, while z~8 galaxies show a steep decline in mass density consistent with (1+z)⁻⁶ evolution.

ABSTRACT

We study the Spitzer IRAC mid-infrared (rest-frame optical) fluxes of 14 newly WFC3/IR-detected z~7 z_{850}-dropout galaxies and 5 z~8 Y_{105}-dropout galaxies. The WFC3/IR depth and spatial resolution allows accurate removal of contaminating foreground light, enabling reliable flux measurements at 3.6mum and 4.5mum. None of the galaxies are detected to [3.6]=26.9 (AB, 2 sigma), but a stacking analysis reveals a robust detection for the z_{850}-dropouts and upper limit for the Y_{105}-dropouts. We construct average broadband SEDs using the stacked ACS, WFC3, and IRAC fluxes and fit stellar population synthesis models to derive mean redshifts, stellar masses, and ages. For z~7 z_{850}-dropouts, we find z=6.9^{+0.1}_{-0.1}, (U-V)_{rest}=0.3, reddening A_V=0, stellar mass =1.2^{+0.3}_{-0.5} x 10^9Msun (Salpeter IMF). The best-fit ages ~300Myr, M/L_V=0.2, and SSFR~1.7Gyr^{-1} are similar to values reported for luminous z~7 galaxies, indicating the galaxies are smaller but not younger. The sub-L* galaxies observed here contribute significantly to the stellar mass density and under favorable conditions may have provided enough photons for sustained reionization at 7<z<11. In contrast, the z=8.3^{+0.1}_{-0.2} Y_{105}-dropouts have stellar masses that are uncertain by 1.5 dex due to the near-complete reliance on far-UV data. Adopting the 2 sigma upper limit on the M/L(z=8), the stellar mass density (to M_{UV,AB}<-18) declines from 3.7^{+0.9}_{-1.6} x 10^6Msun Mpc^{-3} (z=7) to < 8 x 10^5Msun Mpc^{-3} (z=8), following \propto(1+z)^{-6} over 3<z<8. Much lower masses at z=8 would signify much more dramatic evolution, which can be established with deeper IRAC observations long before the arrival of JWST.

Motivation & Objective

• To measure the rest-frame optical fluxes of faint, high-redshift galaxies at z~7 and z~8 using deep IRAC data to assess their stellar mass contributions.
• To determine the stellar population properties—redshift, mass, age, and SED shape—of sub-L* galaxies in the early universe.
• To evaluate the contribution of low-luminosity galaxies to the cosmic stellar mass density and their potential role in sustaining reionization at 7 < z < 11.
• To quantify the evolution of stellar mass density from z~7 to z~8 using stacking and upper limits, especially given the limited data at z~8.

Proposed method

• Utilized ultra-deep WFC3/IR and Spitzer IRAC imaging to achieve high spatial resolution and depth, enabling accurate foreground contamination subtraction.
• Performed stacking analysis on 14 z~7 z_{850}-dropout and 5 z~8 Y_{105}-dropout galaxies to detect faint mid-infrared fluxes despite individual non-detections.
• Constructed broadband SEDs using stacked ACS, WFC3, and IRAC fluxes to model stellar population properties.
• Fitted stellar population synthesis models to the stacked SEDs to derive mean redshift, stellar mass, age, and extinction (A_V).
• Applied a Salpeter initial mass function (IMF) and used the M/L_V ratio and SSFR to interpret galaxy evolution and reionization potential.
• Calculated stellar mass density at z~7 and z~8 using the 2σ upper limit on IRAC fluxes at z~8 to constrain mass estimates with 1.5 dex uncertainty.

Experimental results

Research questions

• RQ1 What is the mean stellar mass and age of sub-L* z~7 z_{850}-dropout galaxies, and how do they compare to more luminous z~7 galaxies?
• RQ2 To what extent do low-luminosity galaxies at z~7 contribute to the cosmic stellar mass density and reionization photon budget?
• RQ3 How does the stellar mass density evolve from z~7 to z~8, and what does the observed decline imply about galaxy evolution and reionization?
• RQ4 How reliable are stellar mass estimates at z~8 when relying primarily on far-UV data and upper limits in mid-IR bands?
• RQ5 Does the observed mass density evolution at z~8 follow a (1+z)⁻⁶ dependence, and what does this imply for future observations with JWST?

Key findings

• The z~7 z_{850}-dropout galaxies have a mean redshift of z = 6.9⁺⁰.¹₋₀.¹, consistent with their photometric redshifts, and exhibit a (U-V)_{rest} color of 0.3 with no reddening (A_V = 0).
• The mean stellar mass of z~7 galaxies is 1.2⁺⁰.³₋₀.⁵ × 10⁹ M☉ (Salpeter IMF), with a best-fit age of ~300 Myr and a high specific star formation rate of ~1.7 Gyr⁻¹.
• Despite being smaller in size, the z~7 galaxies are not younger than more luminous z~7 counterparts, indicating similar evolutionary stages.
• The stellar mass density at z~7 (to M_{UV,AB} < -18) is 3.7⁺⁰.⁹₋₁.⁶ × 10⁶ M☉ Mpc⁻³, declining to < 8 × 10⁵ M☉ Mpc⁻³ at z~8, following a (1+z)⁻⁶ evolution from 3 < z < 8.
• The z~8 Y_{105}-dropout galaxies have highly uncertain stellar masses due to reliance on far-UV data and IRAC upper limits, with a 1.5 dex uncertainty range.
• The observed mass density decline suggests a steep evolution at z~8, which could be confirmed with deeper IRAC data before the launch of JWST.