BIBECHANA Vol. 22, No. 2, August 2025, 171-180 ISSN 2091-0762 (Print), 2382-5340 (Online) Journal homepage: http://nepjol.info/index.php/BIBECHANA Publisher:Dept. of Phys., Mahendra Morang A. M. Campus (Tribhuvan University)Biratnagar A spectroscopic study of the merging dwarf galaxy PGC 007782 Shankar Timalsina, Daya Nidhi Chhatkuli∗ 1Department of Physics, Tri-Chandra Multiple Campus, Tribhuvan University, Ghantaghar, Kathmandu, Nepal ∗Corresponding authors. Email: chhatkulidn@gmail.com Abstract We conducted a detailed spectroscopic study of PGC 007782, an interacting emission- type dwarf galaxy. This study utilized optical spectra of emission lines to explore the system of interacting dwarf galaxies. Gaussian fitting techniques were used to examine the main spectral lines (Hα, Hβ , Hγ , SII, OIII, and NII) and determined their Gaussian parameters. Our investigation mainly focused on nine prominent emission lines with wavelengths ranging from 4342 Å to 6733 Å. Among which, Hα is the most prominent line with a peak intensity of 72.16 × 10−17 erg/s/cm²/Å corresponds to the central wavelength of 6564 Å. With the flux of 2.93 × 10−17 erg/s/cm²/Å, the NII line is the weakest at the central wavelength of 6550 Å. The SII line has the highest FWHM value of 4.04 Å, indicating a higher gas temperature. Each of these emission lines exhibits a nearly perfect Gaussian profile, with a coefficient of determination exceeding 92%. Utilizing the Hα line, the star formation rate of PGC 007782 is calculated to be 0.014 M⊙ yr-1, indicating that active star formation is concentrated at the galaxy's core. The calculated value of Balmer decrement is 4.02 which is higher than the normal value indicating the light coming out from this galaxy is being blocked by the dust particles. With oxygen abundance of 8.31dex, the physical and morphological characteristics of PGC 007782 confirmed this galaxy as a metal-poor, actively star forming blue compact dwarf (BCD) galaxy. Keywords Dwarf Galaxy, Galaxy merger, Balmer decrement, Star formation rate Article information Manuscript received: December 13, 2024; Revised: January 8, 2025; Accepted: May 20, 2025 DOI https://doi.org/10.3126/bibechana.v22i2.72618 This work is licensed under the Creative Commons CC BY-NC License. https://creativecommons. org/licenses/by-nc/4.0/ 1 Introduction Galaxies frequently come together and merge to form bigger and complex ones. This process (hier- archical) of structure formation, happens frequently in regions of many galaxies, like clusters or groups. According to Smith et al. (2018) [1], galaxies inter- act and collide frequently in this crowded region. When galaxies merge, gas clouds are compressed causing new stars to form quickly leading to a burst of star formation [2]. Merging galaxies also cre- ate long streams of gas and stars known as tidal, 171 http://nepjol.info/index.php/BIBECHANA chhatkulidn@gmail.com https://doi.org/10.3126/bibechana.v22i2.72618 https://creativecommons.org/licenses/by-nc/4.0/ https://creativecommons.org/licenses/by-nc/4.0/ Shankar Timalsina and Daya Nidhi Chhatkuli/ BIBECHANA 22 (2025) 171-180 172 pulled out by gravity. These tails come together and change the shape of the galaxy [3]. Dwarf Galaxies are small but significant for study as they can trigger starbursts-short periods of intense star formation. Galaxy formation can be affected by these starbursts, particularly in nearby galaxies [4]. These bursts of stars are very helpful for understanding how galaxies change over time and give information about dark matter [5]. When a small galaxy merges with bigger ones, the merging process mixes the gas inside the galax- ies spreading heavy elements (metals) through- out the galaxy. However, the amount of metal (metallicity) can be temporarily decreased when two galaxies of similar size merge [6]. Scientists use spectra of dwarf galaxies to know about them. Dwarf galaxies contain stars of dif- ferent gas and chemical compositions as studied by Lee et al. (2018) and Smith et al. (2020) [7, 8]. Hα and OIII emission lines can reveal areas where stars are still forming [9,10]. Information about how the galaxy's stars and metals have changed over time can be provided by absorption features [11,12]. These studies show a wide range of histories of dwarf galaxies. As star formation rate (SFR) measures how quickly gas and dust in the galaxy turn into stars, SFR is essential for understanding how galaxies grow and evolve. Star formation rate of the uni- verse has changed significantly over time. It reached its peak around redshifts z∼ 2. This period of in- tense star formation is called cosmic noon [13]. The star formation rate reached its maximum at that time due to the dense gas contained in the galax- ies. About 75% of all stars in the universe formed between the redshift of z∼2.5 to z∼0. The SFR steadily declined after this peak. Studying these periods provides valuable insight into galaxy evolu- tion. Many factors affect SFR. Under the effect of gravity, higher gas density and pressure cause gas to collapse, leading to faster star formation [14]. When the metallicity is high, that is, if the amount of elements heavier than Hydrogen and Helium is high, higher metallicity cools the gas better, making it easier to form stars [15]. Barnes & Hernquist in 1996 [16] studied the SFR in galaxies. According to them, when galaxies merge, tidal forces squeeze gas clouds, causing bursts of star formation. Magnetic fields have positive as well as negative influences on SFR. Magnetic fields either slow down by hold- ing gas clouds together or gathering gas to those places where stars can form easily [17]. Another factor that affects the SFR is Active Galactic Nuclei (AGN). AGN affects the SFR by either pushing gas to form stars or heating and removing gas, stopping star formation [18,19]. To study galaxy evolution of the universe, un- derstanding SFR is crucial. A very high SFR can finish the gas reservoir of the galaxy, stopping fu- ture formation of stars and changing the galaxy's shape and types of stars [20]. Elements like Iron and carbon are released when Supernova explodes, mix- ing these elements with surrounding gas and dust, enriching these materials in the galaxy. Next gen- eration of stars and even planets use this enriched gas and dust [21]. Studying SFR overtime provides valuable information about the forces that shape them, helping us to know how galaxies and the uni- verse have changed [22]. In this project, we present the spectroscopic study of a low redshift (z = 0.0180), metal-poor, antenna type actively star-forming merging Blue Compact Dwarf (BCD) galaxy PGC 007782 and calculate its SFR and oxygen abundance. 2 Sample Selection Due to their small size, low surface brightness, and dimness, dwarf galaxies are challenging to detect since they are hard to separate from surrounding stars and background noise. While massive galaxy interactions have been extensively studied, there's a lack of research on interactions between low-mass dwarf galaxies. For this reason, studying dwarf galaxies is crucial. Scientists can also learn much about dark matter—an unknown substance that makes up a large fraction of the universe. Not only that, because of their more easily studied stellar populations, these areas provide important infor- mation on the formation and evolution of stars. Dwarf galaxies, the building component of larger galaxies, provides information about the ori- gin and evolution of galaxies as well as how they interact with one another and with larger galaxies. To delve deep into, we have selected a low redshift (z = 0.0180) merging dwarf galaxy PGC 007782 from a catalog by Paudel et al., (2018) [23] located at R. A. (J2000): 02 h 02 m 38.7600 s, Dec.(J2000): -09 d 22 m 13.080 s. Morphologically, PGC 007782 is an antenna type galaxy 77.06 Mpc far from us. Its (g – r) color is 0.30 mag and has no satellite. The apparent g and r band AB magnitude of the galaxy are 15.43 mag and 15.13 mag respectively. The radial velocity of the galaxy taken from NED is 5396 km/s. Only one neighbor is found within the search criteria (within a sky-projected distance of less than 700 kpc, and a relative line-of-sight radial velocity of less than ±700 km/s). The SDSS DR 12 optical image of the galaxy PGC 007782 and its spectrum are shown in Fig. 1 (a) and 1 (b) respectively. We can see that the galaxy is elongated along the N-S direction and has an antenna in the southern region. The antenna indicates that it is an interacting galaxy. In the op- tical wavelength range of 3500 Å to 7500 Å, there Shankar Timalsina and Daya Nidhi Chhatkuli/ BIBECHANA 22 (2025) 171-180 173 are many emission lines which are due to different elements like Hydrogen, Helium, Oxygen, Nitrogen, Sulphur and so on. The spectrum shows that the galaxy is an emission type. Figure 1: (a) The SDSS DR 12 optical image of interacting galaxy PGC 007782. (b) The optical spectrum of the galaxy PGC 007782. 3 Data Analysis To study the star-forming properties of the merg- ing dwarf galaxy PGC 007782, this galaxy has been selected as a sample. The study will utilize data from the Sloan Digital Sky Survey (SDSS), obtain- ing its spectrum and analyzing it using Origin soft- ware. The galaxy spectrum will be extracted by using Aladin software. Aladin software provides a flexible viewer for astronomical images. With it, users can interactively analyze and explore as- tronomical objects and their attributes by seeing multi-wavelength photos and superimposing cata- logs. Aladin 2.5 is used to extract the archival data of the dwarf galaxy PGC 007782 from its FITS file. The data will be used to plot the optical spectrum by using Origin software of version 8. Origin soft- ware is widely used in the field of astronomy and is one of the simplest method to create high-quality graphs, performs statistical analysis, and supports curve fitting and automation and analyze graphs. A Gaussian fitting technique will be used to model data that follows a Gaussian distribution or sim- ply normal distribution. It involves fitting a Gaus- sian function to a dataset to calculate parameters as mean, standard deviation, which characterizes the central tendency and spread of the data respec- tively. This model is also applied to curve fitting, signal processing and experimental analysis in a va- riety of fields. We measure the physical parameters of each emission line by Gaussian fits, including to- tal flux, Full Width Half Maximum (FWHM) and best-fit coefficient. Mathematically, Gaussian dis- tribution function is given as; FG(x) = 1√ 2πσ2 e−(x−µ) 2σ2 2 Where, x represents a normal random variable, µ represents mean deviation and, σ represents the standard deviation of the distribution. FWHM is the width at which maximum amplitude drops to half a width. A width referred to as the full width half maximum (FWHM) is reached at this point. We choose nine emission lines and their Gaussian parameters based on the greater intensity, which will be described in this work along with their Gaus- sian parameters. We will calculate the hydrogen line ratio and the star formation rate (SFR). Kenni- cutti's [24,25] empirical formula is used for calculat- ing the SFR providing a reliable, empirical methods to calculate the star formation rate based on ob- served Hα luminosity. Mathematically, Kennicutt's empirical formula is given by equation (1). SFR (M⊙year-1) = 7.9 × 10-42 Σ L (Hα) (ergs s-1) (1) where L(Hα) represents the total luminosity of Hα line which will be calculated by using Gaussian fits. L(Hα) = Area of Gaussian fit × 10-17 × 4R2 (ergs s-1). R is the radius of the sphere which is calculated as: R = D × 3.08 × 1024 cm. Here, D is the luminosity distance of the galaxy in Mpc. The gas phase metallicity will be calculated by using the empirical formula given in equation (2) provided by Marino et. al. (2013) [26]. 12 + log(O/H) = 8.743 + 0.462 × log (NII/ Hα) (2) Shankar Timalsina and Daya Nidhi Chhatkuli/ BIBECHANA 22 (2025) 171-180 174 4 Results and Discussion Gaussian fitting curves of the main emission lines, i.e., Hα, Hβ , Hγ , SII, OIII and NII of the dwarf galaxy PGC 007782 are obtained. Among these, SII, OIII and NII are in duplet. Hα, Hβ , Hγ , are the Balmer lines. Only the prominent emission lines Hα, Hβ , OIII5008 and NII6585 are shown in Fig. 2. Observed data are presented by the black dots and the red solid line represents the Gaussian fit for the observed data. It shows that the observed data fol- lows the Gaussian distribution more accurately. Figure 2: The Gaussian fitting curves of four prominent emission lines Hα, Hβ OIII and NII are shown. Gaussian parametric values of the dwarf galaxy PGC 007782 obtained after fitting of Gaussian curves are presented in Table 1. The names of the chosen emission lines are shown in the first col- umn whereas, column two and three show the wave- length corresponding to peak intensity (λp) in Å and the peak intensity λp in (×10-17 erg/s/cm2/Å) respectively. The fourth, fifth, sixth, seventh and eighth columns represent FWHM in Å, area of Gaussian curve in (×10-17 erg/s/cm2/Å), height of Gaussian curve in (×10-17 erg/s/cm2/Å), off set and coefficient of determination value respectively. Shankar Timalsina and Daya Nidhi Chhatkuli/ BIBECHANA 22 (2025) 171-180 175 Table 1: Gaussian parameters of the observed emission lines of dwarf galaxy PGC 007782. Elements λp (Å) Ip FWHM (Å) Area Height Offset R-square Hγ 4342 26.23 2.49 68.18 25.72 +0.0036 0.97 Hβ 4863 26.35 2.77 77.75 26.37 -0.0043 0.98 [O III] 4960 19.97 2.79 65.12 21.95 +0.0042 0.99 [O III] 5008 64.74 2.91 211.73 68.38 +0.0004 0.99 [N II] 6550 2.93 3.65 10.79 2.78 +0.0004 0.92 Hα 6564 72.16 3.85 312.60 76.20 +0.0025 1.00 [N II] 6585 8.48 3.84 36.54 8.94 -0.0049 0.98 [S II] 6718 15.70 4.04 50.78 11.80 -0.0013 0.99 [S II] 6733 10.74 3.85 66.75 16.30 -0.0046 1.00 Through a careful investigation of the emission lines, significant new information about the physi- cal property and underlining process has been ob- tained. Hα emission lines come out with highest peak intensity of 72.16 × 10-17 erg/s/cm²/Å, cor- responding to the wavelength of 6564 Å suggest- ing active star formation within the galaxy. This prominent line implies that necessary hydrogen gas is being ionized by young hot stars. Similarly, second and third peak intensity of 64.74 × 10-17 erg/s/cm²/Å and 26.35 × 10-17 erg/s/cm²/Å respectively for lines OIII and Hβ lines at 5008 Å and 4863 Å, shows high-excitation regime that are probably driven by energetic pro- cesses near hot star or an active galactic nucleus (AGN). Balmer series lines Hγ and HHβ show nearly comparable peak intensity. These lines give important information on the temperature and den- sity of the ionized gas, as well as the constant ion- ization state throughout the galaxy [27]. NII lines are the weakest emission lines, indicating a reduced excitation or amount of nitrogen in the ionized part of the galaxy with a peak intensity of 8.48 × 10-17 erg/s/cm²/Å and 2.93 × 10-17 erg/s/cm²/Å, cor- responding to the wavelength 6785 Å and 6750 Å respectively. The SII curve shows the greatest value of FWHM that is 4.04 Å, indicating that the SII line has the widest width at half of its maximum inten- sity, compared to the other lines in the above table. Also, a larger FWHM means the spectral line is wider which indicates that a larger range of wave- lengths are being occupied by the line's emissions process or transition. This happens for a number of reasons such as increased temperature, increased turbulence, or a wider velocity dispersion of the substance that is emitting or absorbing [28, 29]. A large FWHM value, sometimes indicates a wider spread of velocity within the emitting or absorbing gas or a more complicated physical environment. The table also provides measurement of the FWHM for various other spectral lines observed in optical spectra, like the Balmer line Hγ has the low- est FWHM of 2.49 Å, indicating its width when its intensity is at half of its peak. This smaller FWHM indicates that the spectral line is narrower. This narrower line indicates that the substance, that is emitting or absorbing, is limited to a narrower range of temperature or velocity. This can happen in those environments where the motion or temper- ature of the gas is more ordered or where there is less turbulence. The greatest Gaussian area corresponds to the Hα emission lines, which is 312.60 × 10-17 erg/s/cm2/Å. A larger area denotes a stronger or more intense emission or absorption feature, since it suggests a greater total energy emitted or ab- sorbed over the whole width of the spectral line. In contrast, NII has the smallest area that is 10.79 × 10-17 erg/s/cm2 /Å, suggesting a lower total en- ergy flux or intensities, showing weaker or less in- tense spectral features. OIII has the largest offset of 0.0042 representing a greater discrepancy between the major data point and the expected value from the models or the theoretical prediction. Similarly, NII has also the smallest offset of -0.0049, indicat- ing there is least discrepancy between the measured data point and the expected values. Shankar Timalsina and Daya Nidhi Chhatkuli/ BIBECHANA 22 (2025) 171-180 176 Figure 3: Comparison of the physical parameters such as area, height and FWHM for emission lines. Different colors represent different parameters. Green color represents height of the emission lines, sky blue color indicates area of the fitting curve and red color represents the FWHM value, for their respective emission lines. Graph plot Fig. 3 compares three Gaussian pa- rameters such as area, height and FWHM values of the emission lines. These features indicate total en- ergy emitted by the lines (flux), velocity dispersion or broadening of the gas (turbulence) and, peak in- tensity of the line. Emission lines with larger areas usually have peak intensity [30]. For example, Hα line has the largest area and highest peak intensity. Above plot also shows a broad FWHM value can still have a high area. However, their peak heights are lower due to the distribution of energy over a wider wavelength range. For example, SII lines have moderate area but SII lines' broader FWHM re- duces their peak heights compared to lines like OIII (5008), which has a narrow width [31]. Similarly, emission lines having narrower width might still have higher peak height. The OIII (5008) line has a small value of FWHM but still has a high peak, maybe due to this line emitting gas which is less turbulent or moving uniformly. A combination of high area, peak height, and broad FWHM suggests intense star formation ac- companied by significant gas turbulence. Modest area, modest height of the peak and narrow FWHM indicates less active regions with lower gas densities velocities. Lastly, low area, low height of the peak and broad FWHM indicates low density gas with significant turbulence or outflow. Through the careful examination of emission lines of dwarf galaxy PGC 007782, significant de- tails on the physical parameters and star-forming activity are revealed. The line ratio of Hα to H, is calculated to be 4.2. This is the significant de- viation from the expected value of 2.8. This vari- ation indicates the possibility of internal dust red- dening within the galaxy, as dust dims the light from both lines differently, making the Hα line a comparatively greater appearance. Dust-rich envi- ronments within the galaxy might have affected the observed features of the emission lines as suggested by the high Hα to Hβ line ratio [32]. The star formation rate for dwarf galaxy PGC 007782 after extinction correction is determined to be 0.0136 M year1 by using equation (1). This cal- culated value indicates that the galaxy is actively producing stars at the faster rate, but as compared to the larger more massive galaxies, this galaxy forms new stars at a slower pace. The calculated value of SFR also indicates that this galaxy's cur- rent star formation is well after the cosmic noon and is still forming stars at a lower rate in the local uni- verse. In contrast, as galaxies have consumed much of their gas and external fueling mechanism are less frequent, the local universe (z∼0) shows the steady decline in SFR. This rate for PGC 007782 while which is not as high as some other star forming galaxies nevertheless, indicates a strong star form- ing activity particularly for a dwarf galaxy. The emission line metallicity is calculated to be 8.31, using the line ratio of NII and Hα emission lines in equation (2). It shows that PGC 007782 is a metal-poor galaxy, indicating that this is a young or not fully evolved dwarf galaxy, which is typi- cally a blue compact dwarf galaxy. These BCDs, small, irregular galaxies with rapid star formation rates appear blue, due to the presence of a larger Shankar Timalsina and Daya Nidhi Chhatkuli/ BIBECHANA 22 (2025) 171-180 177 number of young hot stars [33]. The metal poor state of PGC007782 indicates that it has not expe- rienced significant chemical enrichment, a charac- teristic common to star-forming galaxies that has not yet accumulated a major heavy element accu- mulation. The calculated parameters of the galaxy are summarized in the Fig. 3. Figure 4: Figure showing the horizontal bar graph summarizing the calculated parameters. Initially, by measuring the area under the Gaus- sian fitting for the Hα line, the calculated value for SFR is 0.0084 MM⊙yr1. The light emitted from these galaxies has to pass through dust, which blocks some of the light, especially at shorter wave- lengths. The Balmer decrement i.e., the ratio of Hα and Hβ , indicates the presence of dust. The theoret- ical value for Balmer decrement is 2.28, but what we calculate is 4.02, which is significantly higher. The high ratio of Hα to Hβ indicates that much of the Hβ line is blocked by the dust, whereas Hα line is not affected by the dust, making it stronger and indicating high star formation rate [34]. The ex- tinction coefficient was applied to the Hα flux and calculated to be 0.291, indicating dust along the line of sight to the observed object. This calculated value shows a moderate amount of dust between the observer and the object. After accounting for extinction, which reflects the true star formation ac- tivity, the calculated SFR is 0.014MM⊙ yr-1, which means dust hides some of the star formation activ- ity. We use the line of [NII] and Hα to calculate the metallicity i.e., the abundance of elements heavier than hydrogen and helium, which is calculated to be 8.31 dex. This value suggests that PGC 007782 has fewer heavy elements, indicating that this galaxy is a young or not fully evolved. 5 Comparison with Other Works The SFR obtained from spectroscopic study for SDSSJ222726.64+120539.8 and SDSSJ162753.47+482529.3 is 0.010 MM⊙ yr-1 and 0.016 MM⊙ yr-1 respectively [35], which is similar to the SFR of PGC 007782 showing that all have moderate star formation activity. Metallicity of these galaxies are nearly the same showing all these Galaxies are metal poor. The SFR of CG 0315 is calculated to be 0.051 MM⊙ yr-1 [36], which is higher than the SFR of PGC 007782, indicating star forming activities are happening more in CG 0315. With an effective radius of 4.5 arcsecond and half-light radius of 2.60 arcsecond, CG 0315 is declared to be BCD similar to PGC 007782. The SFR for NGC 5194 (M51a) is calculated to be 3.4 MM⊙ yr-1 [37]. This shows that NGC 5194 (M51a) forms stars at the rate of around 261 times the star formation rate of PGC 007782, indicating rapid star formation process. SFR using the Hα line method calculated for SDSS J134326.99+431118.7 Shankar Timalsina and Daya Nidhi Chhatkuli/ BIBECHANA 22 (2025) 171-180 178 is 0.019 MM⊙ yr-1 [38] which is slightly higher than that of PGC 007782 and has a more active star forming environment. This was also conducted by using the optical spectrum of the major emission lines. SFR of galaxy UGC 4483 is calculated to be 0.008 MM⊙yr-1 [39]. This value of 0.008 MM⊙-1 is slightly lower than the SFR of PGC 007782. That means, PGC 007782 forms stars approximately 1.6 times higher. This indicates that the higher SFR galaxy has more active star forming regions or pos- sibly more efficient star formation processes. Con- sequently, the galaxy PGC 007782 with the higher star forming rate undergoes more rapid evolution- ary changes compared to UGC 4483. This comparison highlights that galaxies can vary significantly in how fastly the formation starts, influenced by factors like their size, type and the amount of gas available for Star formation. The higher rate observed in this galaxy suggests that it is currently undergoing more active star formation compared to the standard reference value. These measurements are important for understanding the process of star formation across different galaxies and their evolution over cosmic time. 6 Conclusion The system of interacting dwarf galaxies is ex- amined through optical spectra of emission lines. Strongest or major emission lines are Gaussian fit- ted in this project. We present the result of a thor- ough study of the emission lines of the dwarf galaxy PGC 007782 with great attention given to the prin- cipal spectral lines Hα, Hβ , Hγ , SII, OIII, and NII. Gaussian fitting techniques were used to determine the FWHM, wavelength, peak intensities and other relevant parameters of these lines. Based on the observation and calculations, the following conclusions are drawn. (a) The Hα emission line is the most prominent. Similarly, the NII emission line is weakest reflecting reduced energy releases from this element. (b) The SII line has the largest FWHM in- dicating that, SII line is wider than other emis- sion lines, possibly caused by higher temperature or more movement of the gas in the galaxy. (c) Areas were represented by total flux under the gaussian curves, with Hα having the largest area, indicating strong emission. Similarly, the NII line has the smallest area indicating weak emission. (d) PGC 007782 is classified as a metal-poor actively star-farming Blue Compact Dwarf (BCD) galaxy indicated by SFR (0.0136 MM⊙yr-1) and its metallicity (8.31dex). 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