Evolution of the dust emission of massive galaxies up to z=4 and constraints on their dominant mode of star formation

Abstract: We aim to measure the average dust and molecular gas content of massive star-forming galaxies ($\rm > 3 \times 10^{10}\,M_\odot$) up to z=4 in the COSMOS field to determine if the intense star formation observed at high redshift is induced by major mergers or caused by large gas reservoirs. Firstly, we measured the evolution of the average spectral energy distributions as a function of redshift using a stacking analysis of Spitzer, Herschel, LABOCA, and AzTEC data for two samples of galaxies: normal star-forming objects and strong starbursts, as defined by their distance to the main sequence. We found that the mean intensity of the radiation field $< U >$ heating the dust (strongly correlated with dust temperature) increases with increasing redshift up to z$\sim$4 in main-sequence galaxies. We can reproduce this evolution with simple models that account for the decrease of the gas metallicity with redshift. No evolution of $< U >$ with redshift is found in strong starbursts. We then deduced the evolution of the molecular gas fraction (defined here as $\rm M_{\rm mol}/(M_{\rm mol}+M_\star)$) with redshift and found a similar, steeply increasing trend for both samples. At z$\sim$4, this fraction reaches $\sim$60%. The average position of the main-sequence galaxies is on the locus of the local, normal star-forming disks in the integrated Schmidt-Kennicutt diagram (star formation rate versus mass of molecular gas), suggesting that the bulk of the star formation up to z=4 is dominated by secular processes.

Evolution of Dust Emission in Massive Galaxies and Star Formation Modes

The study explores the dust emission characteristics of massive star-forming galaxies, specifically those with stellar masses exceeding $3 \times 10^{10}\,M_\odot$, extending our understanding up to a redshift of z=4 within the COSMOS field. The work's primary goal is to distinguish whether major mergers or substantial gas reservoirs primarily fuel the pronounced star formation activities observed at higher redshifts.

Key Findings

1. Spectral Energy Distribution (SED) Analysis:

   • The study employs a stacking analysis across various infrared and submillimeter datasets (from Spitzer, Herschel, LABOCA, and AzTEC) to assess the average SEDs of two galaxy categories: normal star-forming galaxies adhering to the 'main sequence' and pronounced starbursts exhibiting elevated specific star formation rates (sSFRs).
   • A notable finding is the progressive elevation of the mean intensity of the radiation field $\langle U \rangle$, associated with the dust temperature in main-sequence galaxies, up to z=4. This trend is consistent with models that consider a decrease in gas metallicity with increasing redshift. Contrastingly, strong starbursts do not show a redshift-dependent evolution in $\langle U \rangle$.

2. Molecular Gas Fraction:

   • The study deduces an increasing molecular gas fraction (molecular gas to total baryonic mass) with redshift for both normal and starburst galaxies, reaching approximately 60% at z$\sim$4.
   • The position of main-sequence galaxies within the integrated Schmidt-Kennicutt diagram aligns with that of local, normal star-forming disks, indicating that secular processes primarily drive star formation up to z=4.

3. Implications for Star Formation:

   • The findings suggest that the extreme star formation rates in massive galaxies at high redshift are primarily attributed to large gas reservoirs rather than an increased star formation efficiency.
   • The analysis provides robust evidence against significant evolutionary changes in the mode of star formation in main-sequence galaxies across cosmic times up to z=4, advocating for a consistency with secular star-forming processes.

Implications and Future Prospects

The implications of this research extend to the broader understanding of galaxy evolution and the primary mechanisms driving star formation over cosmic time scales. The indication that secular processes dominate star formation, even at high redshifts, challenges previous assumptions that major mergers were the main drivers. This research underscores the role of vast gas reserves in fueling star formation and how metallicity evolution impacts dust properties and thermal emission.

Future Directions

Further studies should aim to refine the gas-to-dust ratio models, factoring in varying environmental conditions across cosmic epochs and potential offsets in metallicity-dust relationships. Additionally, advancements in high-resolution infrared and millimeter-wave observations, such as those possible with the Atacama Large Millimeter Array (ALMA), will provide deeper insights into the physical conditions of high-redshift galaxies' interstellar media.

Ultimately, this paper contributes significantly to our understanding of galaxy evolution, particularly at high redshifts, by dissecting the interplay between dust emission, molecular gas content, and star formation dynamics. As observational capabilities grow, continued research in this direction will sharpen our understanding of the universe's infancy and the life cycle of galaxies.