Astronomers Studied Lenticular Galaxy, “PGC 38025” (Astronomy)

Chen and colleagues presented optical integral field spectroscopic (IFS) observation with the 3.5 meter telescope at Calar Alto (CAHA) Observatory, and high-resolution millimeter observation with the NOrthern Extended Millimeter Array (NOEMA), of the SDSS J120237.07+642235.3 (PGC 38025), a star-forming field lenticular galaxy with an off-nuclear blue core. They estimated star formation rates and metallicities of the galaxy and its blue core. Their study recently appeared on Journal Arxiv.

Lenticular galaxies (S0s) are morphologically situated between ellipticals and spirals, e.g., armless and with dense cores (bulges) as ellipticals, while having disk component as spirals do. They are traditionally considered as having used up most of their gas reservoir which is critical for star formation.

S0s in a group and/or cluster (i.e., dense environment) are considered to be transformed from spirals due to environmental effects, e.g., ram-pressure stripping in hot intergalactic medium (IGM), gravitational tidal effect and harassment, and encounter of galaxies. However, most rare field S0s cannot be fitted into the common formation scenario (environmental triggered formation) of S0s in groups/clusters, and may contribute to the diversity in the properties of S0s. Studying the evolution of S0 galaxies is essential to understanding formation and evolution of all different types of galaxies.

Now, Chen and colleagues carried out study on SDSS J120237.07+642235.3 (also called PGC 38025), a star forming field lenticular galaxy with an off-nuclear blue core, located ∼ 1 kpc away from the galaxy centre (as shown in Featured image). Its early-type morphology, together with the extended star-formation and off-nuclear blue core make PGC 38025 to be an ideal target in studying formation and evolution of S0s.

Table 1: Properties of PGC 38025 © Chen et al.

At first, astronomers obtained CAHA optical IFS observation and compared it to single aperture spectroscopy such as Sloan Digital Sky Survey (SDSS), which is highly competent in the study on spatial extended star formation mechanism of PGC 38025 and nature of its off-nuclear blue core. In order to study the origin of the star forming activity in this galaxy, they also carried out the interferometer radio observation from IRAM NOEMA to reveal molecular gas content and spatial distribution of PGC 38025.

They found that star formation rate of PGC 38025 and its accompanying blue core are 0.446 and 0.023 M yr¯1, respectively. While, the metallicity 12+log(O/H) of them are 8.42 and 8.41, respectively. The metallicity of off-nuclear core is the same within the errors, which indicates that this blue core may be the same origin with the galaxy PGC 38025, or their ISM is already well-mixed during galaxy evolution.

“The ionized gas and cold molecular gas in PGC 38025 show the same spatial distribution and kinematics, whilst rotating misaligned with stellar component.”

— told Chen, first author of the study

They also found that the redshift of off-nuclear blue core is identical to PGC 38025, and the spectrum of the off-nuclear blue core has strong emission-line feature and nearly zero continuum. In addition, they determined through BPT diagnostics that, this off-nuclear blue core is an H ii region excited by star formation. CO detection further revealed the molecular gas served as reservoir (gas fraction MH2 /M∗ reaches ∼ 10%) of PGC 38025, supplying for star formation on an extended region, including this blue core. The external gas (ionized and molecular) built a polar disk in PGC 38025. Astronomers suggested that, this blue core as a part of this disk, relative denser and thermal cold (with low velocity dispersion), generated during misaligned gas accretion and rotation.

Moreover, they suggested that star formation in PGC 38025 is triggered by a gas-rich minor merger, and the off-nuclear blue core might be a local star-formation happened during the accretion/merger process.

Figure 1. PV maps of 3DBarolo modelling, alonging the major and minor axis. Using CO data, is shown in grey and blue contour. The model is shown with red contours, and yellow dots shows the resulting rotation curve of the galaxy, projected on the data. © Chen et al.

“Finally, we found that there is another high velocity component decreasing from the centre to the blue core, which may also be an indication of bipolar outflows. We will further explore the case in a future work in the light of granted VLA HI data.”

— concluded authors of the study

Featured image: Optical grz composite image from DESI Legacy Imaging Surveys © Chen et al.

Reference: Chen Zhengyi, Gu Qiu-Sheng, Rubén García-Benito, Zhang Zhi-Yu, Ge Xue, Xiao Mengyuan, Yu Xiaoling, “PGC 38025: A Star-forming Lenticular Galaxy With an Off-nuclear Star-forming Core”, ArXiv, pp. 1-16, 2021.

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