A concurrent SFRD and BHARD in COSMOS: Blending disjointed samples & Testing AGN SED Decomposition methods
Context: The possible co-evolution between galaxies and their central supermassive black holes is supported by the similarity in shape between the Star Formation Rate Density (SFRD) and Black Hole Accretion Rate Density (BHARD) of the universe. Both trends (upper right) peak around z~2 and decline to present day, however, due to selection effects they do not trace the same galaxies and thus represent a similarity in the global evolution of these two quantities, rather than local. To trace the local causal connection, if any, requires constructing both trends for the same sample in a self-consistent manner.
Black Hole Accretion
What's new?: We will construct the SFRD and BHARD for the exact same sample of galaxies for the first time, also exploring a full range of AGN template parameter space. Previous BHARDs use ~35% of IR-selected samples to ensure strong AGN, however, nearly ~50-70% of IR samples can contain sources with measurable AGN contribution. This Herschel-250um-selected sample from COSMOS includes ~7,000 galaxies and we perform a series of AGN SED Decomposition tests with the MIT Supercloud Supercomputer to map systematics associated with decomposition of weak AGN, allowing for a statistically larger sample that does not conservatively omit these potentially bolometrically significant sources. The tests also probe how sparse broadband MIR data (a limitation of large high-z samples) impacts SED decomposition, along with AGN SED model parameters, paying close attention to the presence/depth of the Si 9.7um absorption feature in AGN SED.
Method: We use a test-sample of~100 24um-Selected sources with Spitzer/IRS MIR spectroscopy, 0.4 <z <2.7, and ranging in MIR AGN strength from Kirkpatrick+2012. Allowing the full range of AGN SED theoretical models from Fritz+2006 and Feltre+2012, commonly used in the literature, we study broadband decomposition results (SED3FIT; Berta+2013) and compare best-fit AGN fractions and parameters with those derived from MIR spectroscopy in Kirkpatrick+2012 to identify systematics. We see how these fits perform with a limited set of AGN templates and a wider set, also identifying where in the rest-frame MIR broadband coverage is most sensitive/crucial to constraining results.
Results: Paper I details SED fitting testing and is in prep, while the remainder of this thesis work (early 2021, Paper II) will compute both trends. Paper I preliminary results are as followed:
1. Higher optical depth AGN models (with extended FIR emission) offer better fits to AGN sources, suggesting high amounts of AGN dust-heating from either the nuclear region or the galaxy outskirts
2. AGN MIR fractional contribution can be underestimated by ~30% via SED fitting if there is no broadband constraint in the rest-frame ~5-12um region.
3. Absence of a broadband constraint in the narrow rest-frame ~5-8um region can falsely overestimate MIR AGN fraction by ~20-30%.
4. Weaker AGN can be well-fit by both high optical depth AGN models (strong Si absorption) and weak Type-1 AGN, resulting in AGN luminosity difference of ~2 dex. This demonstrates the sensitivity of allowed AGN models in fitting and supports JWST MIRI follow-up to understand the nature of these sources.