[Paper Review] The VIMOS VLT Deep Survey :Evolution of the major merger rate since z~1 from spectroscopicaly confirmed galaxy pairs

L. de Ravel, O. Le Fèvre+49 more|arXiv (Cornell University)|Jul 16, 2008

Galaxies: Formation, Evolution, Phenomena1 references109 citations

TL;DR

This study measures the evolution of major merger rates in galaxies from redshift z~1 to z~0.5 using spectroscopically confirmed pairs from the VIMOS VLT Deep Survey. It finds that merger rates strongly depend on galaxy luminosity and type, with fainter galaxies merging more frequently in the past, while massive early-type galaxies experience slower merger evolution, contributing ~20% of their current stellar mass via major mergers since z~1.

ABSTRACT

From the VIMOS VLT Deep Survey we use a sample of 6447 galaxies with I_{AB} < 24 to identify 251 pairs of galaxies, each member with a secure spectroscopic redshift, which are close in both projected separation and in velocity. We find that at z ~ 0.9, 10.9 +/- 3.2 % of galaxies with M_B(z) < -18-Qz are in pairs with separations dr < 20 kpc/h, dv < 500 km/s, and with dM_B < 1.5, significantly larger than 3.76 +/- 1.71 % at z ~ 0.5; we find that the pair fraction evolves as (1+z)^m with m = 2.49 +/- 0.56. For brighter galaxies with M_B(z=0) < -18.77, the pair fraction is higher and its evolution with redshift is somewhat flatter with m=1.88 \pm 0.40, a property also observed for galaxies with increasing stellar masses. Early type, dry mergers, pairs increase their relative fraction from 3 % at z ~ 0.9 to 12 % at z ~ 0.5. We find that the merger rate evolves as N_{mg}=(9.05 +/- 3.76) * 10^{-4}) * (1+z)^{2.43 +/- 0.76}. We find that the merger rate of galaxies with M_B(z) < -18-Qz has significantly evolved since z ~ 1. The merger rate is increasing more rapidly with redshift for galaxies with decreasing luminosities, indicating that the flat evolution found for bright samples is not universal. The merger rate is also strongly dependent on the spectral type of galaxies involved, late type mergers being more frequent in the past, while early type mergers are more frequent today, contributing to the rise in the local density of early type galaxies. About 20 % of the stellar mass in present day galaxies with log(M/M_{sun}) > 9.5 has been accreted through major merging events since z ~ 1, indicating that major mergers have contributed significantly to the growth in stellar mass density of bright galaxies over the last half of the life of the Universe.

Motivation & Objective

• To measure the evolution of the major merger rate in galaxies since z~1 using spectroscopically confirmed pairs.
• To investigate how the merger rate depends on galaxy luminosity, stellar mass, and spectral type (early vs. late type).
• To quantify the contribution of major mergers to the growth of stellar mass in present-day galaxies.
• To reconcile conflicting observations on merger rate evolution by accounting for luminosity and spectral type dependencies.
• To assess the impact of mergers on star formation rates in galaxy pairs.

Proposed method

• Identified 314 spectroscopically confirmed galaxy pairs from the VIMOS VLT Deep Survey with I_AB ≤ 24 and redshifts z~0.5 to z~0.9.
• Defined pairs using projected separation Δrp ≤ 20 h⁻¹ kpc, velocity difference Δv ≤ 500 km/s, and magnitude difference ΔMB ≤ 1.5.
• Calculated the pair fraction as a function of redshift and luminosity, assuming H₀ = 70 km s⁻¹ Mpc⁻¹, ΩM = 0.3, ΩΛ = 0.7.
• Used the Kitzbichler & White (2008) merger timescale prescription to convert pair fraction into merger rate density.
• Classified pairs by spectral type (early-type, late-type, mixed) to assess merger type evolution.
• Tracked rest-frame [OII] equivalent width to measure star formation enhancement in merging systems.

Experimental results

Research questions

• RQ1 How does the major merger rate of galaxies evolve from z~1 to z~0.5?
• RQ2 How does the merger rate depend on galaxy luminosity or stellar mass?
• RQ3 How does the merger rate vary with the spectral type of the merging galaxies (e.g., late-type vs. early-type)?
• RQ4 To what extent do major mergers contribute to the stellar mass growth of massive galaxies since z~1?
• RQ5 Is the star formation rate enhanced in merging galaxy pairs, and how does this affect the global star formation history?

Key findings

• At z~0.9, 10.9±3.2% of galaxies with MB(z) ≤ -18 - 1.11z are in pairs with Δrp ≤ 20 h⁻¹ kpc and Δv ≤ 500 km/s, decreasing to 3.8±1.7% at z~0.5.
• The pair fraction evolves as (1+z)^m with m = 4.73±2.01 for fainter galaxies (MB(z) ≤ -18 - 1.11z), indicating strong evolution.
• For bright galaxies (MB(z=0) ≤ -18.77), the pair fraction evolves more slowly with m = 1.50±0.76, consistent with lower merger rate evolution.
• The merger rate for MB(z) ≤ -18 - 1.11z galaxies evolves as (4.96±2.07)×10⁻⁴ × (1+z)^(2.20±0.77) mergers Mpc⁻³ Gyr⁻¹.
• Early-type (dry) mergers increase in relative fraction from 3% at z~0.9 to 12% at z~0.5, indicating growing importance of dry mergers over time.
• Stellar mass accretion via major mergers since z~1 accounts for ~20% of the stellar mass in present-day galaxies with log(M/M☉) ≥ 9.5.