History of Galaxy Interactions and their Impact on Star Formation over the Last 7 Gyr from GEMS (0903.3700v1)

Abstract: We perform a comprehensive estimate of the frequency of galaxy mergers and their impact on star formation over z~0.24--0.80 (lookback time T_b~3--7 Gyr) using 3698 (M*>=1e9 Msun) galaxies with GEMS HST, COMBO-17, and Spitzer data. Our results are: (1) Among 790 high mass (M*>=2.5e10 Msun) galaxies, the visually-based merger fraction over z~0.24--0.80, ranges from 9%+-5% to 8%+-2%. Lower limits on the major and minor merger fractions over this interval range from 1.1% to 3.5%, and 3.6% to 7.5%, respectively. This is the first approximate empirical estimate of the frequency of minor mergers at z<1. For a visibility timescale of ~0.5 Gyr, it follows that over T_b~3--7 Gyr, ~68% of high mass systems have undergone a merger of mass ratio >1/10, with ~16%, 45%, and 7% of these corresponding respectively to major, minor, and ambiguous `major or minor' mergers. The mean merger rate is a few x 1e-4 Gyr-1 Mpc-3. (2) We compare the empirical merger fraction and rate for high mass galaxies to a suite of Lambda CDM-based models: halo occupation distribution models, semi-analytic models, and hydrodynamic SPH simulations. We find qualitative agreement between observations and models such that the (major+minor) merger fraction or rate from different models bracket the observations, and show a factor of five dispersion. Near-future improvements can now start to rule out certain merger scenarios. (3) Among ~3698 M*>=1e9 Msun galaxies, we find that the mean SFR of visibly merging systems is only modestly enhanced compared to non-interacting galaxies over z~0.24--0.80. Visibly merging systems only account for less than 30% of the cosmic SFR density over T_b~3--7 Gyr. This suggests that the behavior of the cosmic SFR density over the last 7 Gyr is predominantly shaped by non-interacting galaxies.

An Examination of Galaxy Interactions and Star Formation over a 7 Gyr Time Span

The paper "History of Galaxy Interactions and their Impact on Star Formation over the Last 7 Gyr from GEMS" presents a comprehensive analysis of the frequency and impact of galaxy mergers over a significant cosmic era. Utilizing data from the Galaxy Evolution from Morphology and SEDS (GEMS) survey, the paper focuses on approximately 3600 galaxies with stellar masses greater than $1 \times 10^9 M_{\odot}$, over the redshift range $z \sim 0.24$ to $0.80$.

Key Findings

1. Merger Frequency and Classification: The merger fraction among high mass galaxies ($M \ge 2.5 \times 10^{10} M_{\odot}$) remains relatively constant, ranging from 9% at lower redshifts to 8% at higher redshifts. This stability is notable over lookback times of 3-7 Gyr. The paper distinguishes between major mergers (mass ratios greater than $1:4$) and minor mergers (mass ratios between $1:10$ and $1:4$), providing a more detailed picture of merger types.
2. Merger Rates: Assuming a visibility timescale of approximatively 0.5 Gyr, about 68% of high mass systems have experienced mergers with mass ratios greater than $1:10$ over the past 7 billion years.
3. Star Formation Rates (SFRs): Visibly merging systems were found to have only modest enhancements in star formation rates compared to non-interacting systems. The average enhancement was by factors of about 1.5 to 2, slightly contradicting the traditional view that interactions lead to significant starburst activity.
4. Cosmic SFR Density: Interacting systems account for less than 30% of the cosmic star formation rate density over the examined period. This finding suggests that non-interacting galaxies predominantly govern the cosmic star formation activity over the last 7 Gyr.

Implications and Comparisons

The results support existing hierarchical models in $\Lambda$CDM cosmology, showing qualitative agreement with theoretical predictions from various semi-analytic and hydrodynamic models, although with a factor of five dispersion. This underscores the complexity and variability in galaxy evolution modeling.

Given that previous studies have produced a range of results depending on methodology, this work emphasizes the importance of using diverse methods (both visual and quantitative) to refine estimates of merger rates and star formation impacts. For future investigations, the role of observational methods, such as modifications to existing ones or the introduction of new techniques, could provide further clarity in understanding galactic evolution.

Future Prospects

There is significant potential for further refinement in both theoretical models and observational methodologies. As simulations and observational datasets improve, it will be possible to narrow down the mechanisms governing galaxy interactions and their consequences with greater precision. Future research could benefit from more robust datasets, perhaps incorporating data from emerging or planned telescopes capable of deeper and higher resolution analyses, such as the James Webb Space Telescope. This could enable precise modeling of merger histories and their variations with redshift, contributing to a more comprehensive understanding of galactic evolution.