Probing down to early cosmic epochs using limited redshift ($z\lesssim 1$) optical surveys with host galaxy age and lookback time analysis

TL;DR

This study tackles how to identify AGNs and track galaxy evolution in limited redshift optical surveys by coupling traditional and extended diagnostic diagrams (BPT, MEx, CEx) with a novel spectral fitting pipeline that extracts host-galaxy ages via BC03 templates. By deriving both $ au_ ext{lookback}$ from redshift and $ au_ ext{age}$ from the spectrum, the authors form $ au_ ext{tot}= au_ ext{lookback}+ au_ ext{age}$ to map galaxy life histories up to $ ext{cosmic epochs}$ of $ oughly 13$ Gyr, even at $z\lesssim 1$. Key findings include that AGN hosts tend to be older and that optical diagnostics miss a notable fraction of X-ray-selected AGNs, underscoring the value of multi-wavelength data; the MEx diagram best recovers X-ray AGNs among optical classifications. The work demonstrates that host-galaxy age is a crucial parameter for understanding AGN and star-formation co-evolution, and that the age–lookback framework provides a powerful lens for probing galaxy evolution across cosmic time. Together, these methods enable meaningful insights into the role of the cosmic environment, such as background temperature, in shaping AGN activity and star formation.

Abstract

We present a detailed analysis of AGN identification diagnostics and host galaxy evolution using optical spectral diagnostics using ESO-GOODS-S data. We employ traditional Baldwin-Phillips-Terlevich (BPT) diagrams along with their modern extensions, the Mass-Excitation (MEx) and Colour-Excitation (CEx) diagrams, to classify AGNs from among up to 600+ candidates. We extract the spectral properties utilising an indigenous spectral fitting code. In addition, the code also incorporates inputs from a state-of-the-art stellar population synthesis model (Bruzual and Charlot, 2003, MNRAS, 344, 1000), extracting the host galaxy ages and metallicities as model parameters. Redshift values for all the studied spectra are obtained from the literature. Using these redshift values, we are able to assign a 'lookback time' (using standard $Λ$CDM cosmological parameters). When this lookback time is combined with the galaxy ages (from spectral fitting), it enables us to study galaxy evolution even up to early cosmological epochs ($\sim$13 Gyr), despite restricting the studied field of galaxies to $z\sim 1$ (lookback time of $\sim$8 Gyr). We conclude that galaxies ultimately evolving into AGN hosts originate earlier ($\sim$8 Gyr ago) than non-AGN host galaxies ($\sim$6 Gyr ago). We relate this fact, that AGN hosts date back to an earlier epoch than non-AGN galaxies, as being likely due to higher cosmic temperatures at that time, probably enhancing black hole accretion. We emphasise that host galaxy age is an additional crucial parameter (apart from cosmological redshift) in defining our understanding of AGN and star formation co-evolution across cosmological time scales.

Figures (8)

• Figure 1: Spectral model fitting results for four sources: (a) GDS J033245.68-275534.4 ($z=0.102$), (b) GDS J033332.40-274350.3 ($z=0.321$), (c) GDS J033227.11-274922.0 ($z=0.559$), (d) highlights GDS J033237.09-274941.0 ($z=1.569$). Across the subfigures, we have shown the redshift-corrected spectra (black curve) and the overall model fit (red curve, including discrete emission lines), as well as baseline contributions, i.e. host galaxy contribution (magenta), AGN power-law continuum (cyan), and narrow and broad Fe lines (blue and red dashed). Subfigure (e): log-scaled version of (c), emphasizing all the spectral components for clarity.
• Figure 2: (a) The [OII] BPT diagram ('blue diagram'), (b) The [NII] BPT diagram, (c) The [SII] BPT diagram. Across subfigures, blue dots represent star-forming galaxies (SFs), cyan dots indicate composite galaxies, red dots correspond to Seyferts (AGN), and green dots denote LINERs (AGN). Square markers (blue, cyan, red, and green) indicate classifications from the [OII] BPT diagram, providing a comparison across subfigures.
• Figure 3: (a): [OII]-BPT diagram of \ref{['fig_2']}(a), limited to sources in two fitted age ranges — 0.5--1 Gyr (cyan stars) and $>$8 Gyr (magenta squares) — representing relatively young and old stellar populations, respectively. While AGNs are predominantly found in the older age bin, a small number of AGNs also appear in the younger age range 2014_Heckman2003_Kauffmann_MNRAS_a. (b): Fractional distribution of [OII]-classified star-forming galaxies (blue) and AGNs (red) as a function of fitted host galaxy age. A clear trend is seen where the fraction of AGNs increases with stellar age. The two age bins shown in (a) are indicated by shaded boxes for reference.
• Figure 4: MEx diagram for the present data. The bottom-left region indicates star-forming galaxies (SF, blue dots), the composite region is located between the solid and dashed lines (Comp., cyan dots), and the top-right region corresponds to AGN activity (AGN, red dots). The empirical lines are adapted from 2014_Juneau.
• Figure 5: CEx diagram for the present data. Star-forming galaxies (SF, blue dots) occupy the region with low [OIII]$\lambda$5007/H$\beta$ ratios and bluer $U-B$ colors, composite galaxies (Comp., cyan dots) are located in the intermediate region, and AGN-host galaxies (AGN, red dots) are found in the region with high [OIII]$\lambda$5007/H$\beta$ ratios and redder $U-B$ colors. The empirical boundaries are adapted from 2011_Yan_APJ.