Astronomers Studied An Extremely Low-metallicity Dwarf Compact Star-forming Galaxy: J2229+2725 (Astronomy)

Using the Large Binocular Telescope (LBT)/Multi-Object Dual Spectrograph (MODS), Izotov and colleagues in their recent paper, have obtained optical spectroscopy of one of the most metal-poor dwarf star-forming galaxies (SFG) in the local Universe, J2229+2725. Their study recently appeared on Journal Monthly Notices of the Royal Astronomical Society.

This galaxy with a redshift z = 0.0762 was selected from the Data Release 16 (DR16) of the Sloan Digital Sky Survey (SDSS). This local SFG possesses extraordinary properties (in particular a very low metallicity and an extremely high O32 ratio), which makes it stand apart from other galaxies. They derived these properties of galaxy from the LBT observations which are most extreme among SFGs in several ways. LBT observations have been supplemented by medium-resolution spectroscopy with the Dual Imaging Spectrograph (DIS) mounted on the 3.5 meter telescope of the Apache Point Observatory (APO).

They also found that its (galaxy) oxygen abundance 12 + logO/H = 7.085±0.031 is among the lowest ever observed for a SFG. With its very low metallicity, an absolute magnitude Mg = −16.39 mag, a low stellar mass M⋆ = 9.1×106 M and a very low mass-to-light ratio M⋆/Lg ∼ 0.0166 (in solar units), J2229+2725 deviates strongly from the luminosity-metallicity relation defined by the bulk of the SFGs in the SDSS.

In addition they found that, J2229+2725 has a very high specific star-formation rate sSFR ∼ 75 Gyr¯1, indicating very active ongoing star formation. Moreover, they also found three other features of J2229+2725 which are most striking, being the most extreme among lowest-metallicity SFGs: (1) a ratio O32 = I([O iii]λ5007)/I([O ii]λ3727) ∼ 53, (2) an equivalent width of the Hβ emission line EW(Hβ) of 577 Å, and (3) an electron number density of ∼ 1000 cm¯3. These properties imply that the starburst in J2229+2725 is very young.

Figure 1. The a) (log R23) – (12 + logO/H), b) log(R23 – a0O32) – (12 + logO/H), and c) log(R23 – a1O32) – (12 + logO/H) relations, where a0 = 0.080 and a1 = 0.080 – 0.00078O32. In all these relations 12 + logO/H is derived by the direct method, R23 = ([O iii]λ4959+λ5007 + [O ii]λ3727)/Hβ and O32 = [O iii]λ5007/[O ii]λ3727. The galaxy J2229+2725 is shown by a red filled star, while J1631+4426 is represented by a blue filled square. Other lowest-metallicity SFGs, the galaxy Little Cub, the galaxy J1234+3901 and the H ii region DDO 68#7 are shown by filled circles. SFGs with the highest O32 ratios in the range ∼ 20 – 40 are shown by asterisks. Samples of SFGs used for the He abundance determination are shown by open circles. Grey dots represent SFGs at z > 0.02 from the SDSS, with the [O iii]λ4363 emission line measured with an accuracy better than 20 per cent. The solid lines in b) and c) are linear fits to the data with log(R23 – a0O32) ≤ 7 and log(R23 – a1O32) ≤ 7, respectively, and excluding J1631+4426. © Izotov et al.

Using the extremely high O32 in J2229+2725, they have also improved the strong-line calibration for the determination of oxygen abundances in the most metal-deficient galaxies, in the range 12 + logO/H < 7.3.

According to authors, the proposed strong-line method can be used to search for SFGs at the extreme end of oxygen abundances 12 + logO/H < 7.0, where other methods, including the direct Te method, cannot be applied because of the weakness of emission lines. This method can also be applied to verify oxygen abundances derived by the direct Te method.

In particular, using our new calibration of the strong-line method, we find that the oxygen abundance for the galaxy J1631+4426 is 0.28 dex higher than the value 12 + logO/H = 6.90±0.03 derived by Kojima et al. using the direct Te method, but it is consistent with their value derived using the strong-line method.

— told Izotov, first author of the study

Featured image: 12 arcsec × 12 arcsec region with the color composite SDSS image of J2229+2725. The 2 arcsec SDSS spectroscopic aperture and 1.2 arcsec wide LBT/MODS slit are indicated by a white circle and yellow parallel lines, respectively. © Izotov et al.


Reference: Y I Izotov, T X Thuan, N G Guseva, J2229+2725: an extremely low-metallicity dwarf compact star-forming galaxy with an exceptionally high [O III]λ5007/[O II]λ3727 flux ratio of 53, Monthly Notices of the Royal Astronomical Society, 2021;, stab1099, https://doi.org/10.1093/mnras/stab1099


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