Exploring Color Magnitude Diagrams for AGN Accretion Disks with Red Shifts between 1.5-1.75

Fellow researchers: Nicole Granucci of Quinnipiac University; Varoujan Gorjian of NASA Jet Propulsion Laboratory; Thomas Rutherford of East Tennessee State University and King University; Grace Gogliettino of Colgate University; and Christian Jones of King University

Abstract

Active Galactic Nuclei (AGN), particularly quasars, are characterized by their extremely high luminosities, making them important tools for measuring cosmic distances. The accretion disks of quasars can be modeled as black bodies, allowing them to be employed as "standard candles". Standard candles are objects with known luminosities that enable the calculation of distances through redshift. This research examines quasars extracted from the COSMOS database within the redshift range of 1.5 to 1.75, focusing on the relationship between absolute magnitude and color index. Color-magnitude diagrams and color-color plots were generated to explore these relationships.

Introduction

Active Galactic Nuclei (AGN) are a classification of galaxies that are characterized by their highly concentrated galactic centers that are driven by a central supermassive black hole (SMBH) with a surrounding accretion disk².

Unlike typical galaxies, AGN black holes actively accrete gas, dust, and other materials. This influx of matter forms a swirling accretion disk that surrounds the black hole. As matter enters the black hole, it is subjected to intense gravitational forces which convert it into thermal radiation. This causes extremely high luminosities even at large redshifts of 1.5-1.75³. Current methods for determining galactic distances primarily rely on standard candles, such as Type 1A supernova. Although these methods have been effective in measuring objects at shorter distances, it becomes more inaccurate and difficult to measure deeper and fainter objects in the universe. Although Hubble’s Law can be applied to AGN, the linearity of this law diminishes at larger scales, which can lead to errors in distance measurements. Compared to Type 1a Supernova, AGN are newer objects in the universe since the Big Bang. As some of the most distant objects in the universe, their extreme luminosities measured at high redshifts offer a unique opportunity to determine cosmic parameters⁴.

Materials and Methods

Redshifts between 1.5 and 1.75 were selected based on the Spectral Energy Distribution (SED) of AGN, where UV emission shifts into the optical spectrum with minimal contamination, as seen in the COSMOS database (Figure 3). The contamination, visible as peaks in the SED in Figure 3, are indicative of electron transitions between energy levels rather than the thermal radiation from AGN accretion disks, which is the primary focus of this study. 56,590 AGN sources, spanning IR and optical data, were extracted, and the relevant columns for data reduction are shown in Table 1. Using the Ned Wright Cosmology Calculator, galactic distances were modeled, and initial color magnitude diagrams (CMDs) were generated. No significant correlations were found (Figure 4). To ensure only galaxies remained in the dataset, 446 nongalaxy sources identified by the column “lp_type” were removed, leaving 56,144 data points. After this adjustment, new CMDs were generated, again showing a poor R² correlation (Figure 5). A template from the Lephare library was used to classify AGN based on ID numbers, and after refining further with the "lp_modq" column, 22,177 type 1 AGN were identified. The resulting CMDs still showed poor R² correlations (Figure 6).

Due to inconclusive results across the entire redshift range, binning was introduced to observe correlations over smaller intervals. Binning was done in increments of +0.01 and +0.05. While results showed slight improvements, R² values remained unsatisfactory (Figure 7). This led to a shift in approach, focusing on confirming that the data contained only type 1 AGNs by exploring color-color plots. These plot color indices of specific bands on both axes. The subgroups “Blue Narrow”, “Red Narrow”, “IR Narrow”, “Wide Optical”, “Full Optical”, and “Wide IR were examined, and further binning by redshift (1.5-1.59, 1.6-1.69, 1.7-1.75). Results from revealed stronger R² correlations (Figure 8).

Table 1. Initial columns selected from COSMOS database for data reduction, representing both infrared (IR) and optical bands