BIBECHANA 18 (2) (2021) 43-49 43 Study of star formation rate and metallicity of the low redshift (z < 0.02) dwarf galaxies S. P. Gautam1*, A. Silwal2, N. Lamichhane3, A. K. Jha1, B. Aryal1 1Central Department of Physics, Tribhuvan University, Kirtipur, Nepal 2Patan Multiple Campus, Tribhuvan University, Lalitpur, Nepal 3Prithvi Narayan Campus, Tribhuvan University, Pokhara, Nepal *E-mail: astrosujan@gmail.com Article Information: Received: January 1, 2021 Accepted: February 22, 2021 Keywords: Dwarf galaxies Star formation rate Spectra Hα measurement SDSS ABSTRACT In this paper, we have presented an analysis of emission lines from two dwarf galaxies. We analyzed the strongest emission lines of wavelength ranging from 4100 Å to 6700 Å. Among these emission lines, Hα and OIII have the highest intensities with 113.09×10-17 erg/s/cm2/Å and 142.12×10-17 erg/s/cm2/Å in the galaxies SDSSJ222726.64+120539.8 and SDSSJ162753.47+482529.3, respectively. The Gaussian fit carried out in these emission lines showed the perfect fits with regression coefficient greater than 98 %, and full width half maximum (FWHM) of less than 4 Å. The line ratios calculated between Hα and Hβ for SDSSJ222726.64+120539.8 and SDSSJ162753.47+482529.3 were 2.78 and 2.85, respectively, suggesting that the galaxies are starburst galaxies. The measurement of the Hα line from both galaxies was then used to assess the rate of star formation. The star formation rate of the galaxies SDSSJ222726.64+120539.8 and SDSSJ162753.47+482529.3 was found to be 0.010 M☉ year-1 and 0.016 M☉ year-1, respectively, indicating a low rate of star formation, and the emission line metallicity was derived using the Hα and NII line, which were measured to be 8.23 dex and 8.70 dex, respectively. DOI: https://doi.org/10.3126/bibechana.v18i2.34067 This work is licensed under the Creative Commons CC BY-NC License. https://creativecommons.org/licenses/by-nc/4.0/ 1. Introduction Galaxies evolve by colliding with their neighboring galaxies through the interaction of their environment. These collisions and mergers of galaxies due to induced gravitation has been the best-known fact about the evolutionary mechanism [1]. In extragalactic research, the pivotal question remains to find to what extent the properties of galaxies are regulated by the initial condition and environment. Indeed, it has been found that the BIBECHANA ISSN 2091-0762 (Print), 2382-5340 (Online) Journal homepage: http://nepjol.info/index.php/BIBECHANA Publisher: Department of Physics, Mahendra Morang A.M. Campus, TU, Biratnagar, Nepal mailto:astrosujan@gmail.com https://doi.org/10.3126/bibechana.v18i2.34067 https://creativecommons.org/licenses/by-nc/4.0/ http://nepjol.info/index.php/BIBECHANA S. P. Gautam et al. / BIBECHANA 18 (2) (2021) 43-49 44 evolution of galaxies relies heavily on their environment [2]. In understanding the evolution of the universe, studying the star formation rate in the universe or the individual galaxies can be essential knowledge. But in extragalactic astronomy, assessing the star formation and birth of a star is a difficult task [3]. The importance of star formation bursts in the low mass galaxies was not unlocked until the modeling of blue galaxies and interacting systems [4]. Our primary objective is the study of galaxy formation and evolution is to gain insight into the interaction between the gaseous and stellar components of the galaxies. So, Dwarf galaxies with low surface brightness but gas-rich, low-mass, low-metallicity are ideal for studying the rate of star formation, as it may give us an understanding of how gases turned into a star within that system that might imitate the early universes [5]. In addition, due to a simpler dynamic and morphological system than spiral galaxies, dwarf galaxies are considered the ideal laboratory for studying the formation of stars within the low gas surface density domain along with lower rotational shear rates [6]. While dwarf galaxies have received less attention than spiral galaxies and elliptical galaxies, they may have more cosmological importance. These dwarf galaxies are the most diffuse type of galaxies in our modern universe and are expected to be more than one in the past. As for Cold Dark Matter (CDM), dwarf galaxies are the first to yield after Big Bang and are often thought to be the residue of the building blocks of massive galaxies [7]. Dwarfs are the most dominant type of galaxies in the universe with relatively low mass and are the regular companion of other galaxies. They are often found in clusters of galaxies [8]. There are mainly three types of dwarf galaxies: i) Spheroidal; gas-poor dwarf which is not supporting any star formations, ii) irregulars; gas-rich dwarf which supports star formation but at relatively low rates, iii) starbursting dwarf; forming star at a surprising rate [9]. Blue Compact Dwarf (BCD) is known as an irregular dwarf since it has no uniform shape. The composition of BCD is comparable to the material type during the formation of the first stars in the early universe, making it an apt object to research and understand the process of formation of the primordial stars [10]. The paper is structured as follows: the sample chosen for this study is addressed in Section II. The methods of the study was presented in Section III. Section IV contains the result and discussion part. Finally, we have concluded our results in Section V. 2. Sample Selection We have taken two dwarf galaxies for our study namely SDSSJ222726.64+120539.8 and SDSSJ162753.47+482529.3. These galaxies are among the merging dwarf galaxies listed in the catalog of Paudel et al. [11]. SDSS J222726.64+120539.8 is positioned in the sky at R.A.: 22h 27m 26.64, Dec.: 12d 05m 39.8s with a redshift value 0.0118 whereas, SDSS J162753.47+482529.3 is located in the sky at RA.: 16h 27m 53.47s, Dec.: 48d 25m 29.3s with the redshift value 0.0134. The g & r band magnitudes of SDSSJ222726.64+120539.8 and SDSSJ162753.47+482529.3 are 15.68 & 15.65 mag, and 16.47 & 16.19 mag, respectively. The optical view of selected dwarf galaxies is shown in Figure 1. Figure 2 shows the SDSS optical spectrum of the galaxies, wavelength (for both galaxies) ranging from 4100 Å to 6700 Å. The strongest among all those emission lines are OIII and Hα in these galaxies distinctly. 3. Methods We have analyzed the emission lines featured in the 2-D spectrum to study the properties such as line ratio, star formation rate, and metallicity of the dwarf galaxies. These images and spectrums were well-calibrated and no further reductions were required. The SDSS fibers had an arc diameter of 3 seconds. Therefore, these spectrums only cover the small area of these dwarf galaxies. We downloaded the spectrum data from the SDSS server (http://skyserver.sdss.org/dr14). The observed http://skyserver.sdss.org/dr14 S. P. Gautam et al. / BIBECHANA 18 (2) (2021) 43-49 45 emission lines from the spectrum were then studied using the software Origin8.5. The strongest emission lines, as shown in Figure 2, were then Gaussian fitted for the estimation of different statistical parameters. We also noted the parameter like FWHM, total flux, and best fits coefficients. In order to get an accurate corresponding flux of the emission line, we subtracted the continuum flux. Fig. 1: Optical view of SDSS J222726.64+120539.8 (upper) and SDSS J162753.47+482529.3 (lower). Fig.2: 2-D optical spectrum of SDSS J222726.64+120539.8 (upper) and SDSS J162753.47+482529.3 (lower) with several prominent emission lines. The Gaussian function is defined as [12, 13]: fG(x) = 1 √2𝜋σ2 𝑒 −(x−μ)2 2σ2 , (1) where, x, μ, and σ are normal random variables, mean deviation, and standard deviation respectively. The Gaussian fitting of each emission lines studied with higher intensities and their respective FWHM, offset, and regression coefficients were tabulated and discussed in the later section. FWHM is the value at which the dependent variable is equal to half of its maximum value. Kennticutt (1998) [4] S. P. Gautam et al. / BIBECHANA 18 (2) (2021) 43-49 46 obtained the qualitative estimation of total star formation (SFR) in the galaxies through photometry of coalescing Hα lines from a large sample of galaxies. The star formation rate can be calculated as: SFR (M☉year-1) = 7.9× 10-42 Σ L (Hα) (ergs s-1), (2) where ΣL(Hα) is the total luminosity of the Hα line after Gaussian fitting. Σ L(Hα) = Area of Gaussian fit × 10-17 × 4𝜋R2 (ergs s-1). R is the radius of the sphere and is calculated as R = D × 3.08 × 1024 cm. Here, D is the luminosity distance the galaxy is from the earth which is calculated using the redshift value in Mpc [13]. We used a line ratio between Hα and NII to derive the emission line metallicity given by [14], 12 + log (O/H) = 8.743 + 0.462 × log (NII/ Hα), (3) 4. Results and Discussion Among the various methods used to estimate the rate of star formation, we have adopted the method of Hα measurement estimated by Kennicutt, 1998, easiest, commonly used method of SFR determination as it provides estimation of SFR rate from the Hα emission lines of the spectra of the galaxies. We have analyzed the Balmer series i.e., Hα, Hβ, Hγ, and Hδ, these lines are primarily associated with the gas-mass and star formation rate in the galaxies. The other strongest emission lines like HeI, OIII, OIII, and NII were also identified to study the chemical parameters of the galaxy. The OIII is doubly ionized oxygen. Figures 3 and 4 display the example of Gaussian fits of two emission lines i.e. Hα and NII for the galaxies SDSSJ222726.64+120539.8 and SDSSJ162753.47+482529.3, respectively. The red line displays the Gaussian distribution with the error bars in the Gaussian curve. We pointed out that the data observed and the Gaussian distribution are in close agreement, which suggests the effectiveness of the findings. In the same way, we made the Gaussian fit for other emission lines as well. Tables 1 and 2 list the entire identified element with the strongest emission lines. The noted emission lines are redshift calibrated to get the corresponding emitted wavelength and are subjected to the baseline subtracted to get accurate flux. The Gaussian parameters of all eight Gaussian distributed emission lines are listed with their offset values. The average coefficient of regression was found to be 0.98. SDSS J222726.64+120539.8 Fig. 3: Example of Gaussian fitting of two emission lines Hα (left) and NII (right) of the galaxy SDSSJ222726.64+120539.8. From Table 1, we observed that the wavelength of 6564.63 Angstroms had the highest intensity of 113.09 x 10-17 erg/s/cm2/Å corresponding to the Hα line of the Balmer series. Similarly, the second peak S. P. Gautam et al. / BIBECHANA 18 (2) (2021) 43-49 47 had an intensity of 103.05 x 10-17 erg/s/cm2/Å which corresponds to the doublet OIII lines. The intensity at 5877.24 Angstrom had a low value of 5.63 x 10-17 erg/s/cm2/Å which resembles the HeI emission line. For the calculation of the star formation rate, we used the formula described in Equation 2. Redshift value (z) = 0.0118 from the catalog of Paudel et al. (2018) [11] was used to calculate the distance to the galaxy (D). We calculated the value of D as 49.17 Mpc. The star formation of the galaxy was found to be 0.010 M☉year−1. We also estimated the line ratio of Hα and Hβ. It was found to be 2.78, which is slightly less than the theoretical value (Hα/Hβ = 2.8) [13] for the star-forming galaxies modeled by Dessauges-Zavadsky et al. [15]. The ratio of NII and Hα was used to estimate the emission line and metallicity. Metallicity indirectly traces the history of star formation, demonstrating the stability of many of the galaxies' essential physical mechanisms, such as the expulsion of metals into the ISM by supernova and stellar winds, the deportation of gas through galactic outflows, and the accretion of gas from local environments in the galaxy [16]. The physical mechanism that controls the formation of stars and, most importantly, the evolution of galaxies can be understood by analyzing metallicity evolution in relation to the other essential properties of galaxies [16]. The value of emission line metallicity, calculated using Equation 3, was found to be 8.23 dex. Table 1: Statistical parameters obtained from the Gaussian fit of the emission lines of the spectrum of the galaxy SDSSJ222726.64+120539.8. Emissions Wavelength (Å) Peak Intensities (10-17 erg/s/cm2/Å) FWHM (Å) Area (10-17 erg/s/cm2/Å) Offset R- Square Hδ 4102.89 7.67 2.21 19.14 -0.113 0.97 Hγ 4341.69 18.32 2.32 47.26 -0.424 0.99 Hβ 4862.69 40.64 2.84 40.64 -0.348 0.99 OIII 4960.295 36.89 2.75 103.05 0.142 0.98 OIII 5008.24 103.05 3.01 327.83 -0.170 0.99 HeI 5877.24 5.63 3.74 23.29 0.307 0.98 Hα 6564.63 113.09 3.77 458.81 -0.319 0.99 NII 6585.27 8.68 3.77 35.63 -0.041 0.99 Fig. 4: Example of Gaussian fitting of two emission lines Hα (left) and NII (right) of the galaxy SDSSJ162753.47+482529.3. S. P. Gautam et al. / BIBECHANA 18 (2) (2021) 43-49 48 SDSS J162753.47+482529.3 From Table 2, we noticed that the 4960.295 Angstrom had the highest intensity of 142.12 x 10-17 erg/s/cm2/Å corresponding to the doublet OIII emission line. Similarly, the observed second peak had an intensity of 135.76 x 10-17 erg/s/cm2/Å which corresponds to the Hα line. The intensity had a low value of 8.65 x 10-17 erg/s/cm2/Å which belongs to the Hδ line. In a similar way as in the previous galaxy, the formula described in equation 2 was to find the star formation rate of this galaxy. The distance (D) to the galaxy was estimated to be 55.83 Mpc using redshift value z = 0.0134 [11]. The star formation rate of the galaxy was found to be 0.016 M☉year-1. The line ratio between Hα and Hβ for this galaxy was 2.85, which is estimated to be slightly greater than the theoretical value (Hα/Hβ = 2.8), which is due to internal extinction. The emission line metallicity for this galaxy was also calculated using Equation 3 and was found to be 8.7 dex. Table 2: Statistical parameters obtained from the Gaussian fit of the emission lines of the spectrum of the galaxy SDSSJ162753.47+482529.3. 5. Conclusion In this paper, we performed the Gaussian fit of the strongest emission lines from the spectrum archived from the SDSS server of two dwarf galaxies namely SDSSJ222726.64+120539.8 and SDSSJ162753.47+482529.3 to estimate the star formation rate, line ratio of Hα and Hβ, and emission-line metallicity of the galaxies. In both of the galaxies, the FWHM value corresponding to each element was below 4 Å. This shows that the observed characteristic lines are in close agreement with Gaussian fits. Also, we calculated the star formation rate from their Hα measurement as 0.010 M☉year-1 and 0.016 M☉year-1(indicating low star formation rate) and the metallicity as 8.23 dex and 8.7 dex for SDSSJ222726.64+120539.8 and SDSSJ162753.47+482529.3, respectively. In the future, the star formation rate will be estimated using FUV measurement and compared with these results. In addition, a photometric study of these galaxies will be carried out in order to observe other physical and morphological properties based on previous models. Acknowledgment We would like to acknowledge D.N. Chhatakuli, Ph.D. Scholar, Central Department of Physics, Tribhuvan University, for sharing his knowledge in this work. His previous studies inspired us to work in this field. We are also thankful to Sanjay Paudel, Centre for Galaxy Evolution Research, Department of Astronomy, Yonsei University, Emissions Wavelength (Å) Peak Intensities (10-17 erg/s/cm2/Å) FWHM (Å) Area (10-17 erg/s/cm2/Å) Offset R- Square Hδ 4102.89 8.65 2.16 26.93 -0.114 0.98 Hγ 4341.69 18.56 3.86 112.57 -0.576 0.94 Hβ 4862.69 47.64 3.08 161.71 -0.234 0.99 OIII 4960.295 46.50 2.82 142.94 0.245 0.99 OIII 5008.24 142.12 2.94 449.64 -0.113 0.99 HeI 5877.24 7.30 3.61 27.04 0.041 0.95 Hα 6564.63 135.76 3.79 549.59 0.386 0.99 NII 6585.27 14.89 3.96 60.58 0.097 0.99 S. P. 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