A Little Red Dot by any other name would smell just like a Green Pea!?

Title: Discovery of Local Analogs to JWST’s Little Red Dots

Authors: Ruqiu Lin, Zhen-Ya Zheng, Chunyan Jiang, Fang-Ting Yuan, Luis C. Ho, Junxian Wang, Linhua Jiang, James E. Rhoads, Sangeeta Malhotra, L. Felipe Barrientos, Isak Wold, Leopoldo Infante, Shuairu Zhu, Xiang Ji, Xiaodan Fu

First Author’s Institution: Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, China

Status: Submitted to the Astrophysical Journal Letters [open access]

The universe is home to many fascinating objects! In today’s bite, let’s take a closer look at two of them:

Little Red Dots (LRDs): These compact red galaxies are peppered throughout the early universe (as seen in JWST images), and evidence suggests they host an active black hole (Active Galactic Nuclei, AGN) at their center. This raises essential questions about how black holes got so huge in a very short time since the formation of the universe. You can read more about LRDs here and here.

Green Pea Galaxies (GPs): These are small and green (resembling peas!) and are found in the nearby universe (0.1<z<0.4). They appear green because a large fraction of light from these galaxies originates from bright, glowing gas clouds that emit light at specific wavelengths (such as [OIII], which falls in the part of the electromagnetic spectrum that corresponds to green color) rather than the broad spectrum of light and continuous colors emitted by stars in other galaxies. The presence of broad emission lines in the spectrum of GPs suggests that these galaxies could also host an AGN. Interestingly, GPs were first discovered in 2007 by citizen scientists through the Galaxy Zoo Project.

Wait, are these the same thing?

The similarities between LRDs and GPs suggest that GPs could be early-universe LRDs that evolved with little change and survived to the present-day universe. The authors of today’s paper set out to explore this possibility. A good approach is to list the characteristics of LRDs and determine whether GPs share those same traits. The authors start with a larger sample of around 2000 GPs and then systematically select the one that has the closest resemblance to LRDs.

What are the defining characteristics of LRDs?

1. They have broad H-alpha lines. One of the characteristic features of LRDs is the presence of broad hydrogen lines, which indicates that they host an AGN. Thus, only the GPs with signatures of broad lines (Full width at half-maximum of the emission line is greater than 1000 km/s) were selected, narrowing the list down to 19 GPs with broad H-alpha lines.
2. They are little! If the name wasn’t clear enough, LRDs have compact sizes. 14/19 broad line GPs were compact, with the radius of the image being less than 2.5 arcseconds, which is similar to LRDs.
3. They have a ‘V’ shaped SED. Perhaps the most defining feature of LRDs is that they have a Spectral Energy Distribution (SED) that looks like a ‘V,’ with a sharp rise in the ultraviolet (UV) part of the SED, which gradually slopes downwards and then rises steeply again in the infrared (IR). While the increase in the IR can be attributed to dust reddening, the ultraviolet spike is still poorly understood. Nevertheless, seeing that this is the defining characteristic of LRDs, the authors only select compact, broad-line GPs that have a similar (based on the calculated slopes from LRDs) V-shaped SED, using UV data from the GALEX survey and optical data from SDSS (Figure 1).

After applying all these cuts, the authors narrowed down 7 V-shaped, broad-line, compact GP galaxies, which they believe are the best candidates for local analogs of the LRDs. They find similarities between their sample of GPs and LRDs regarding other properties, such as UV magnitudes, and similar correlations between the slope and the black hole mass found for LRDs. They also find that the black holes in LRDs and GPs are likely more massive (Figure 2) than predicted for their small galaxy sizes when using established scaling relations. 

Local analogs to high redshift galaxies are very useful to study because they offer an unprecedented way to study the galaxies of the early universe. Being at low redshifts, we can study their spectrum from UV to IR at very high sensitivity, allowing us to fully understand how these galaxies form and evolve. Many of these GPs need to be studied in greater detail to know if they are indeed local analogs of LRDs. This will enable us to investigate the early growth and evolution of supermassive black holes in a local context.

Astrobite edited by Erica Sawczynec 

Featured image credit: NASA, C. Cardamone (Sloan Digital Sky Survey)