Ultra-Flat Galaxies Selected from RFGC Catalog. III. Star Formation Rate

TL;DR

This study investigates star formation rates in ultra-flat galaxies from the RFGC catalog, revealing their isolated nature, higher gas content, and historical star formation activity compared to thicker disk galaxies.

Contribution

It provides the first detailed analysis of star formation properties specifically in ultra-flat galaxies, highlighting their gas content, star formation history, and differences from thicker disk galaxies.

Findings

UFGs are isolated and have higher H I mass-to-stellar-mass ratios.

Specific star formation rate increases from early to late types, with no mutual difference within types.

UFGs' past star formation rate was about three times higher than current.

Abstract

We examine the star formation properties of galaxies with very thin disks selected from the Revised Flat Galaxy Catalog (RFGC). The sample contains 333 ultra-flat galaxies (UFG) at high Galactic latitudes, $|b|>10^{\circ}$, with a blue major angular diameter of $a\geq 1.2^{\prime}$, blue and red apparent axial ratios of $(a/b)_b > 10$, $(a/b)_r > 8.5$ and radial velocities within 10000~km s$^{-1}$. As a control sample for them we use a population of 722 more thick RFGC galaxies with $(a/b)_b > 7$, situated in the same volume. The UFG distribution over the sky indicates them as a population of quite isolated galaxies. We found that the specific star formation rate, sSFR FUV, determined via the FUV GALEX flux, increases steadily from the early type to late type disks for both the UFG and RFGC-UFG samples, showing no significant mutual difference within each morphological type T. The population of UFG disks has the average H,I-mass-to-stellar-mass ratio by $(0.25\pm0.03)$ dex higher than that of RFGC--UFG galaxies. Being compared with arbitrary orientated disks of the same type, the ultra-flat edge-on galaxies reveal that their total H,I mass is hidden by self-absorption on the average by approximately 0.20 dex. We demonstrate that using the robust stellar mass estimate via $\langle B-K \rangle$-color and galaxy type T for the thin disks, together with a nowaday accounting for internal extinction, yields their sSFR quantities definitely lying below the limit of $-9.4$ dex,(yr$^{-1}$). The collected observational data on UFG disks imply that their average star formation rate in the past has been approximately three times the current SFR. The UFG galaxies have also sufficient amount of gas to support their observed SFR over the following nearly 9 Gyrs.