Four hot DOGs in the microwave (1510.04179v1)

Abstract: Hot dust-obscured galaxies (hot DOGs) are a rare class of hyperluminous infrared galaxies identified with the Wide-field Infrared Survey Explorer (WISE) satellite. The majority of them is at high redshifts (z~2-3), at the peak epoch of star formation in the Universe. Infrared, optical, radio, and X-ray data suggest that hot DOGs contain heavily obscured, extremely luminous active galactic nuclei (AGN). This class may represent a short phase in the life of the galaxies, signifying the transition from starburst- to AGN-dominated phases. Hot DOGs are typically radio-quiet, but some of them show mJy-level emission in the radio (microwave) band. We observed four hot DOGs using the technique of very long baseline interferometry (VLBI). The 1.7-GHz observations with the European VLBI Network (EVN) revealed weak radio features in all sources. The radio is free from dust obscuration and, at such high redshifts, VLBI is sensitive only to compact structures that are characteristic of AGN activity. In two cases (WISE J0757+5113, WISE J1603+2745), the flux density of the VLBI-detected components is much smaller than the total flux density, suggesting that ~70-90 per cent of the radio emission, while still dominated by AGN, originates from angular scales larger than probed by the EVN. The source WISE J1146+4129 appears a candidate compact symmetric object, and WISE J1814+3412 shows a 5.1-kpc double structure, reminiscent of hot spots in a medium-sized symmetric object. Our observations support that AGN residing in hot DOGs may be genuine young radio sources where starburst and AGN activities coexist.

• The paper presents VLBI observations of four hot DOGs, finding weak radio features mainly associated with AGN activity rather than just star formation.
• Structural analysis identified potential young AGN features like compact symmetric objects and medium-sized symmetric objects in some hot DOGs.
• Results indicate radio emission is mainly AGN-driven, positioning hot DOGs as a phase where young AGN and star formation coexist, relevant for galaxy evolution models.

Overview of "Four Hot DOGs in the Microwave" by S. Frey et al.

The paper "Four Hot DOGs in the Microwave" presents a detailed investigation of hot dust-obscured galaxies (hot DOGs), a rare class of hyperluminous infrared galaxies, using very long baseline interferometry (VLBI). The paper focuses on four such galaxies, providing insights into their compact radio emissions and the potential for active galactic nuclei (AGN) within these systems. The researchers utilized the European VLBI Network (EVN) at 1.7 GHz to conduct high-resolution observations of these galaxies, uncovering crucial aspects of their structure and activity.

Key Findings

1. VLBI Observations and Results:
   • The four hot DOGs observed exhibited weak radio features detectable by VLBI, signifying these structures are associated with AGN activity rather than mere starburst phenomena.
   • In two cases, WISE J0757+5113 and WISE J1603+2745, a significant portion ($\sim$70-90%) of the radio emission originated from angular scales larger than those probed by the EVN, indicating diffuse AGN-related emission.
2. Structural Characteristics:
   • WISE J1146+4129 appears as a candidate compact symmetric object (CSO), showcasing a radio morphology suggestive of young AGN characterized by double compact lobes.
   • WISE J1814+3412 exhibits a 5.1-kpc double structure, analogous to medium-sized symmetric objects (MSOs), supporting the notion of a burgeoning AGN amidst these galaxies.
3. Radio Emission and Star Formation Rates (SFR):
   • The researchers estimate the contribution of starburst activities is insufficient to account for the radio emissions observed; instead, the emissions are predominantly AGN-driven.
   • SFR estimates imply that these sources host intense star-forming activities co-existing with nascent AGN phases, although the balance tilts towards AGN domination.

Implications and Future Directions

• Practical Implications:
  • Hot DOGs, especially those with detectable radio emissions, present a unique opportunity to paper the interplay between AGN activity and star-forming processes. The observations underscore the critical role of AGN in shaping galaxy evolution, especially in heavily obscured environments.
• Theoretical Implications:
  • The paper supports theories of galaxy evolution involving transitions from starburst-driven to AGN-driven phases. Hot DOGs may represent an ephemeral stage where these transformations are actively occurring, potentially linked with significant feedback mechanisms affecting star formation rates and stellar mass build-up.
• Speculative Future Developments:
  • Future research is likely to explore the young AGN stages within hot DOGs through multi-frequency and time-domain observations to confirm expansion rates and derive the kinematic ages of these radio sources.
  • Enhanced radio surveys could identify additional radio-emitting hot DOGs, facilitating a better statistical understanding of these rare objects and elucidating their role in the grand narrative of galaxy evolution.

Conclusion

The investigation by S. Frey et al. elevates the understanding of hot DOGs as composites of intense star formation and AGN activity. By scrutinizing their radio structures, the paper paves the way for further explorations into the dynamics of young AGN and the evolutionary phases of massive galaxies. The integration of high-resolution radio data with other spectral observations will be pivotal in advancing this field.