The Most Luminous Galaxies Discovered by WISE

Abstract: We present 20 WISE-selected galaxies with bolometric luminosities L_bol > 10^14 L_sun, including five with infrared luminosities L_IR = L(rest 8-1000 micron) > 10^14 L_sun. These "extremely luminous infrared galaxies," or ELIRGs, were discovered using the "W1W2-dropout" selection criteria which requires marginal or non-detections at 3.4 and 4.6 micron (W1 and W2, respectively) but strong detections at 12 and 22 micron in the WISE survey. Their spectral energy distributions are dominated by emission at rest-frame 4-10 micron, suggesting that hot dust with T_d ~ 450K is responsible for the high luminosities. These galaxies are likely powered by highly obscured AGNs, and there is no evidence suggesting these systems are beamed or lensed. We compare this WISE-selected sample with 116 optically selected quasars that reach the same L_bol level, corresponding to the most luminous unobscured quasars in the literature. We find that the rest-frame 5.8 and 7.8 micron luminosities of the WISE-selected ELIRGs can be 30-80% higher than that of the unobscured quasars. The existence of AGNs with L_bol > 10^14 L_sun at z > 3 suggests that these supermassive black holes are born with large mass, or have very rapid mass assembly. For black hole seed masses ~ 10^3 M_sun, either sustained super-Eddington accretion is needed, or the radiative efficiency must be <15%, implying a black hole with slow spin, possibly due to chaotic accretion.

• The paper identifies 20 Extremely Luminous Infrared Galaxies with bolometric luminosities exceeding 10^14 Lsun using the WISE 'W1W2-dropout' method.
• It demonstrates that these ELIRGs exhibit strong mid-infrared hot dust emission around 450 K, setting them apart from typical quasars.
• The research implies that rapid supermassive black hole growth at z>3 may involve super-Eddington accretion or massive initial seeds despite heavy dust obscuration.

Overview of "The Most Luminous Galaxies Discovered by WISE"

The paper by Tsai et al. presents an analysis of a class of galaxies referred to as Extremely Luminous Infrared Galaxies (ELIRGs). This research exploits data from the Wide-field Infrared Survey Explorer (WISE) to identify twenty galaxies with extraordinary bolometric luminosities exceeding $10^{14}\,L_{\sun}$. The study underlines the discovery of five galaxies with infrared luminosities also surpassing this threshold. ELIRGs tend to be dominated by hot dust emission at rest-frame mid-infrared wavelengths, attributed to obscured active galactic nuclei (AGNs).

Luminosity and Selection Criteria

The authors adopt the WISE "$W1W2$-dropout" selection criterion to pinpoint these galaxies. This method involves identifying objects with minimal detections in the 3.4 and 4.6 $\mu \rm{m}$ bands ($W1$ and $W2$), yet strong detections in the 12 and 22 $\mu \rm{m}$ bands, indicative of large amounts of hot dust and potential AGN activity.

Spectral Energy Distribution and Dust Characteristics

The spectral energy distributions (SEDs) of identified ELIRGs show prominent emission in the 4–10 $\mu$m range, suggesting the presence of hot dust with temperatures near 450 K. The study contrasts these galaxies with existing optically selected quasars reaching similar luminosity levels. Notably, the rest-frame mid-infrared luminosities of ELIRGs are significantly higher compared to unobscured quasars, despite sharing comparable bolometric luminosities. This disparity underscores the role of dust obscuration in ELIRGs.

Implications on AGNs and Supermassive Black Holes

The discovery of ELIRGs at redshifts $z > 3$ aligns with the hypothesis that supermassive black holes are either born with significant mass or experience rapid mass accumulation. For instance, the presence of AGNs at such high luminosities implies either a phase of super-Eddington accretion or reduced radiative efficiency which points towards slower spinning black holes, potentially due to chaotic accretion processes.

Gravitational Lensing and Beaming

The study explores whether gravitational lensing or relativistic beaming might contribute to the observed luminosities. The absence of variability in WISE data and lack of strong gravitational lensing signatures suggest that the high luminosities of the ELIRGs are indeed inherent rather than apparent.

Potential Growth Trajectories and Evolutionary Scenarios

The investigation highlights the potential growth trajectories of supermassive black holes, especially in high-redshift quasars, and discusses radiative efficiencies needed to explain ELIRG characteristics. If growth through super-Eddington accretion or via massive initial seeds is assumed, radiative efficiencies would be less than traditionally assumed values, especially for sources at higher redshifts.

Conclusion and Future Directions

The recognition of these ELIRGs contributes significantly to our understanding of the co-evolution of galaxies and their central black holes under extreme conditions. The study establishes a framework prompting further exploration into the mass assembly history of black holes and the nature of high-redshift luminous infrared galaxies. Upcoming surveys and deeper observations in various spectral ranges (e.g., with the James Webb Space Telescope or future submillimeter observatories) could provide additional insights into the growth mechanisms of these enigmatic systems. The existence of ELIRGs prompts questions about the limits of AGN luminosity and the culmination of black hole growth, potentially revealing constraints on accretion physics under highly obscured conditions.