Reversal of Fortune: Confirmation of an Increasing Star Formation-Density Relation in a Cluster at z=1.62

Abstract: We measure the rest-frame colors (dust-corrected), infrared luminosities, star formation rates, and stellar masses of 92 galaxies in a Spitzer-selected cluster at z=1.62. By fitting spectral energy distributions (SEDs) to 10-band photometry (0.4 micron < lambda(obs) <8 micron) and measuring 24 micron fluxes for the 12 spectroscopically confirmed and 80 photometrically selected members, we discover an exceptionally high level of star formation in the cluster core of ~1700 Msun/yr per Mpc^2. The cluster galaxies define a strong blue sequence in (U-V) color and span a range in color. We identify 17 members with L(IR)>10^11 Lsun, and these IR luminous members follow the same trend of increasing star formation with stellar mass that is observed in the field at z~2. Using rates derived from both the 24 micron imaging and SED fitting, we find that the relative fraction of star-forming members triples from the lowest to highest galaxy density regions, e.g. the IR luminous fraction increases from ~8% at Sigma~10 gal per Mpc^2 to ~25% at Sigma>100 gal per Mpc^2. The observed increase is a reversal of the well-documented trend at z<1 and signals that we have reached the epoch when massive cluster galaxies are still forming a substantial fraction of their stars.

Reversal of the Star Formation-Density Relation at High Redshift

The paper by Tran et al. explores a significant alteration in the established star formation-density relation within the context of galaxy clusters at a high redshift of $z=1.62$. Utilizing data from the Spitzer Space Telescope and other astronomical resources, the authors investigate a galaxy cluster known as ClG J0218.3-0510, presenting findings that contrast with established trends observed at lower redshifts.

Overview of Findings

In galaxy clusters at redshifts less than $z=1$, the star formation rate typically declines with increasing galaxy density, leading to environments dominated by older, spheroidal galaxy populations. However, Tran et al. provide evidence that this relationship is reversed in ClG J0218.3-0510 at $z=1.62$. The study measures various parameters like infrared luminosities, star formation rates, and stellar masses of cluster galaxies, resulting in some noteworthy observations:

1. Star Formation Rate Density: The core of the studied cluster at $R_{proj}=0.5$ Mpc exhibits an exceptionally high star formation rate density of $\sim1700$ Mpc$^{-2}$, indicating robust star-forming activity contrary to the relative quiescence observed in cluster cores at lower redshifts.
2. Blue Sequence and Color Distribution: Unlike traditional expectations wherein clusters exhibit a marked red sequence, ClG J0218.3-0510 demonstrates a strong blue sequence and a broader range in galaxy color, aligning more closely with field galaxies at $z\sim2$.
3. IR Luminous Galaxies: The paper identifies 17 IR luminous galaxies within the cluster, including ULIRGs and LIRGs. These galaxies are significant due to their luminosity and the trend they follow regarding increased star formation with larger stellar mass, similar to field galaxies at $z\sim2$.

Implications of Research

The study provides compelling evidence to suggest a period during which massive cluster galaxies were still undergoing substantial star formation, a process typically quenched in comparable environments at later cosmic epochs. This could imply that theoretical models accounting for galaxy evolution must adapt to incorporate variable epochs of star formation activity influenced by intricate dynamics of galaxy density and environment at high redshift.

Future Directions

Future research should consider deeper infrared surveys of additional clusters at similar or higher redshifts to verify whether the presented reversal is a universal phenomenon or peculiar to specific clusters. Additionally, integrating datasets from emerging astronomical observations could refine existing models concerning star formation and provide insights into the environmental factors driving these changes.

In summary, the findings by Tran et al. challenge existing perceptions of galaxy evolution within dense cluster environments, providing a complex view into the dynamic astrophysical processes at play during a crucial epoch in cosmic history. This work invites further exploration into the conditions facilitating ongoing star formation activity in ancient cluster cores, reshaping our understanding of galaxy evolution trajectories across cosmic time.