Spectrophotometrically Identified stars in the PEARS-N and PEARS-S fields

Deep ACS slitless grism observations and identification of stellar sources are presented within the Great Observatories Origins Deep Survey (GOODS) North and South fields which were obtained in the Probing Evolution And Reionization Spectroscopically (PEARS) program. It is demonstrated that even low resolution spectra can be a very powerful means to identify stars in the field, especially low mass stars with stellar types M0 and later. The PEARS fields lay within the larger GOODS fields, and we used new, deeper images to further refine the selection of stars in the PEARS field, down to a magnitude of mz = 25 using a newly developed stellarity parameter. The total number of stars with reliable spectroscopic and morphological identification was 95 and 108 in the north and south fields respectively. The sample of spectroscopically identified stars allows constraints to be set on the thickness of the Galactic thin disk as well as contributions from a thick disk and a halo component. We derive a thin disk scale height, as traced by the population of M4 to M9 dwarfs along two independent lines of sight, of h_thin = 370 +60/-65 pc. When including the more massive M0 to M4 dwarf population, we derive h_thin = 300 +/- 70pc. In both cases, we observe that we must include a combination of thick and halo components in our models in order to account for the observed numbers of faint dwarfs. The required thick disk scale height is typically h_thick=1000 pc and the acceptable relative stellar densities of the thin disk to thick disk and the thin disk to halo components are in the range of 0.00025<f_halo<0.0005 and 0.05<f_thick<0.08 and are somewhat dependent on whether the more massive M0 to M4 dwarfs are included in our sample.

💡 Research Summary

The paper presents a comprehensive analysis of stellar sources identified in the PEARS (Probing Evolution And Reionization Spectroscopically) program, which obtained deep ACS slitless grism observations over the GOODS‑North and GOODS‑South fields. By exploiting the low‑resolution (R ≈ 100) spectra produced by the slitless grism, the authors demonstrate that even modest spectral detail is sufficient to distinguish low‑mass stars, especially those of spectral type M0 and later, thanks to prominent molecular absorption bands (TiO, VO). A new “stellarity” parameter, calibrated on deeper imaging, was employed to refine morphological selection down to mz = 25, reducing contamination from compact galaxies. The final spectroscopically and morphologically vetted sample comprises 95 stars in the north and 108 in the south.

The authors use this well‑characterized sample to probe the vertical structure of the Milky Way. When considering only the coolest dwarfs (M4–M9), the thin‑disk scale height derived from the two independent sight‑lines is hthin = 370 +60/‑65 pc. Including the more massive M0–M4 dwarfs lowers the estimate to hthin = 300 ± 70 pc, reflecting the different scale heights of younger, more massive populations. Pure thin‑disk models underpredict the observed counts of faint dwarfs; satisfactory fits require an additional thick‑disk component with a scale height of roughly 1000 pc and a halo component. The relative density ratios that best reproduce the data are 0.05 < fthick < 0.08 for the thick‑disk to thin‑disk and 0.00025 < fhalo < 0.0005 for the halo to thin‑disk. These values are consistent with, but more tightly constrained than, previous star‑count studies because the PEARS data provide spectroscopic confirmation of stellar types.

Methodologically, the work validates slitless grism spectroscopy as a powerful tool for large‑scale stellar classification, especially for low‑mass stars that dominate the faint end of the Galactic stellar population. The approach is directly applicable to upcoming missions such as JWST and the Roman Space Telescope, where slitless spectroscopy will be a key capability for wide‑field surveys. By combining spectroscopic identification with refined morphological criteria, the study delivers robust measurements of the Galactic thin‑disk scale height and quantifies the necessary contributions from thick‑disk and halo components, thereby offering new constraints on the Milky Way’s vertical structure and its formation history.