Monitoring broad emission-line components in spectra of the two low-metallicity dwarf compact star-forming galaxies SBS 1420+540 and J1444+4840

We report the discovery of broad components with P-Cygni profiles of the hydrogen and helium emission lines in the two low-redshift low-metallicity dwarf compact star-forming galaxies (SFG), SBS 1420+540 and J1444+4840. We found small stellar masses of 10^{6.24} and 10^{6.59} M$_\odot$, low oxygen abundances 12+log O/H of 7.75 and 7.45, high velocity dispersions reaching $\sigma$ ~700 and ~1200km/s, high terminal velocities of the stellar wind of ~1000 and ~1000-1700km/s, respectively, and large EW(H$\beta$) of ~300A for both. For SBS 1420+540, we succeeded in capturing an eruption phase by monitoring the variations of the broad-to-narrow component flux ratio. We observe a sharp increase of that ratio by a factor 4 in 2017 and a decrease by about an order of magnitude in 2023. The peak luminosity of ~10^{40}ergs/s of the broad component in $L$(H$\alpha$) lasted for about 6 years out of a three-decades monitoring. This leads us to conclude that there is probably a LBV candidate (LBVc) in this galaxy. As for J1444+4840, its very high $L$(H$\alpha$) of about 10^{41}ergs/s, close to values observed in active galactic nuclei (AGNs) and Type IIn Supernovae (SNe), and the variability of no more than 20 per cent of the broad-to-narrow flux ratio of the hydrogen and helium emission lines over a 8-year monitoring do not allow us to definitively conclude that it contains a LBVc. On the other hand, the possibility that the line variations are due to a long-lived stellar transient of type LBV/SNIIn cannot be ruled out.


INTRODUCTION
The Luminous Blue Variable (LBV) stage is a very short high luminous phase in the evolution of the most massive stars, with masses greater than ∼20 -30M⊙, during their thansition from early young O stars to Wolf-Rayet (WR) stars and/or supernovae/black holes (SN/BHs) (Crowther 2007;Solovyeva 2020).Besides being very hot and luminous massive stars, LBVs show significantly greater variability of brightness, observed both photometrically and spectroscopically, than other variable stars (Humphreys & Davidson 1994;Vink 2012).They exhibit strong enhancement in emission and absorption lines, in the continuum level and in the blue shape of the UV and optical continuum.The most prominent spectral features are broad components of hydrogen and often of helium lines, with P-Cygni profiles.The source of the broad emission can be stellar winds or eruptions of massive stars propagating in dense circumstellar envelopes (Humphreys & Davidson 1994;Smith et al. 1994;Drissen, Roy & Robert 1997;Drissen et al. 2001).There are two types of variability in LBV stars: (1) the first type consists of moderate irregular quasi-periodic variations in brightness, due to variations in photospheric temperatures and to strong and variable stellar wind massloss on timescales from several years to several decades (see e.g.Massey et al. 2000;Humphreys et al. 2013Humphreys et al. , 2017;;Humphreys 2019;Grassitelli et al. 2020;Weis & Bomans 2020).Photometric variability of up to ∼0.5 -2 mag on a relatively short time-scale of several years is characteristic of the S Dor type of LBV stars.About a hundred such stars are known in the Galaxy and in the nearest galaxies of the Local Group.Their proximity, especially for the LBVs in our Galaxy, allows a more detailed study of the individual stars and their envelopes.However, these young stars are located in the Galactic plane and are subject to large extinction.In addition, the parameters of these massive stars, as well as those of the interstellar environment in more distant galaxies, can be quite different.In particular, the metallicity, the interstellar medium density, the star-formation rate (SFR) and the specific SFR (sSFR) can vary; (2) the second type of variability is found among a special class of rare LBVs with giant eruptions and brightening of more than 2.5 -3 mag, on timescales of up to thousands of years (Davidson & Humphreys 1997;Smith et al. 2011;Vink 2012;Weis & Bomans 2020).These are very rare events.Only two well established variable stars of this type are known in our Galaxy: η Carinae and P-Cygni (Davidson 1999;Lamers et al. 1983).
Until now, the origin of LBV variability remains uncertain and many questions concerning the nature of LBVs are left open: do all most massive stars go through a LBV phase in their evolution?Can a giant eruption and a S Dortype variability occur in the same star or only in different ones?We are also interested in investigating possible metallicity effects on the properties of LBV stars.The well-studied nearest star-forming dwarf galaxies Small Magellanic Cloud (SMC) and Large Magellanic Cloud (LMC) have respective metallicities of 0.2 and 0.5 that of the Sun, so they do not offer very metal-deficient environments.Investigating the LBV phenomenon in star-forming galaxies (SFGs) with lower metallicities will allow us to better study the abundance dependence of stellar wind and mass loss properties of LBVs.
The scarcity of known LBVs in distant galaxies is due to the difficulty of detecting them.While it is impossible to obtain the spectrum of an individual LBV star in a distant galaxy, it is still quite a challenge to find signs of LBVs in the integral spectra of far-away galaxies.The chance of finding a short-lived LBV in a distant galaxy increases with the number of massive stars in it.The latter depends in turn on the age of the starburst and hence on the value of the equivalent width of the Hβ emission line.Up to thousands and tens of thousands young massive stars may be present in the super-star clusters contained in SFGs with EW(Hβ) > 100 Å, corresponding to a starburst age of less than 4 Myrs at a metallicity 12 + log(O/H) < 8 (see e.g.Schaerer & Vacca 1998;Schaerer et al. 2000).Broad component luminosities of 10 36 -10 38 ergs s −1 are observed in such SFGs which are attributed to massive stars with strong stellar winds or to LBV/SN (Drissen et al. 1997(Drissen et al. , 2001;;Izotov et al. 2007;Guseva et al. 2022).
In this paper, we present long-slit spectrophotometric observations with high signal-to-noise ratio of two compact dwarf SFGs, SBS 1420+544 and J1444+4840, which have respectively EW(Hβ) = 328 Å and 288 Å. SBS 1420+544 was selected from the Second Byurakan Survey (SBS) and J1444+4840 from the Sloan Digital Sky Survey (SDSS) Data Release 16 (DR16) (Ahumada et al. 2020).Both show strong emission lines with a strong broad component in the hydrogen and helium lines.These broad features are often used to find LBV stars in the integral spectra of galaxies in the local Universe.
The observations were obtained with the Large Binocular Telescope (LBT), equipped with the Multi-Object Double Spectrograph (MODS).In addition, SBS 1420+544 was also observed with the LBT Utility Camera in the Infrared (LUCI).We use the LBT observations to derive the physical and chemical properties of the two galaxies with high accuracy.In addition to the LBT data, we have also collected SDSS data and other observations of the two objects from previous studies.We have also added several new observations obtained with the 3.5m Apache Point Observatory (APO) telescope.This allows us to extend our monitoring baseline of the two galaxies to as long a duration as possible.
The LBT observations and data reduction of SBS 1420+544 and J1444+4840 are described in Section 2. General properties of the studied galaxies are considered in subsection 3.1.Subsection 3.2 discusses their locations in various emission-line diagnostic diagrams.Derived element abundances are presented in subsection 3.3.Subsection 3.4 describes the decomposition of the broad emission-line profiles of hydrogen and helium lines into various components.In subsection 3.5 we discuss the evidence for the presence of LBV candidates in the two galaxies.Subsubsections 3.5.1 and 3.5.2discuss the properties of SBS 1420+540 and J1444+4840 in detail.We summarize our main results in Sect. 4.  (Kauffmann et al. 2003).

LBT and APO observations
Coordinates, redshifts, and other characteristics of SBS 1420+544 and J1444+4840 obtained from the SDSS photometric and spectroscopic databases are shown in Table 1.
We have obtained long-slit LBT spectra of SBS 1420+544 and J1444+4840 in 2013 and 2019, respectively.The observations were done in the twin binocular mode, using simultaneously the MODS1 and MODS2 spectrographs, equipped with CCDs (8022 pix × 3088 pix).The G400L blue grating (dispersion of 0.5 Å/pix), and the G670L red grating (dispersion of 0.8 Å/pix) were used.The SBS 1420+544 spectrum covered the wavelength range ∼3150 -9500 Å.With a 1.0 arcsec wide slit, this results in a resolving power R ∼ 2000.The seeing during the SBS 1420+544 observations was ∼ 1.0 arcsec.Two subexposures were obtained for that galaxy, resulting in an effective exposure time of 2×1800s, after addition of MODS1 and MODS2 spectra.
As for J1444+4840, the wavelength range is ∼3500 -8200 Å with a 1.2 arcsec wide slit.The seeing during the J1444+4840 observations was also ∼ 1 arcsec.We obtained eight subexposures for this galaxy, resulting in a total exposure time of 2×7200s.The logs of the observations are presented in Table 2.
Spectra of the spectrophotometric standard stars BD+28d4211 and Feige 34, were also obtained during the night of observations with a 5 arcsec wide slit.They were used for flux calibration and correction for telluric absorption lines.We obtained also calibration frames of biases, flats and comparison lamps.Bias subtraction and flat field correction was done using the MODS Basic CCD Reduction package modsCCDRed (Pogge 2019).Other reductions, in particular flux and wavelength calibration were performed with iraf.Finally, all MODS1 and MODS2 subexposures were co-added, one-dimensional spectra were extracted along the spatial axis by summing all the pixels with emission in the aperture of 2.5 arcsec (Fig. 2a,b and Fig. 3).
In addition, a near-infrared (NIR) long-slit LBT spectrum of SBS 1420+544 was obtained in 2016 in binocular mode, using LUCI1 and LUCI2.Our aim is to check whether the emission lines of hydrogen and helium in the near-infrared range of 0.9-2.5µmcovered by LUCI, have P- Cygni profiles.In addition, the NIR spectrum also allows the use of the density-sensitive infrared He i λ10831 Å emission line for electron number density determination (Izotov et al. 2014).
The NIR observation was performed under good sky condition.We used the N1.8 camera with 0.25 arcsec pixel −1 coupled with the 60×1.0arcsec slit and the G200-LoRes grating, resulting in a spectral resolving power of R ∼ 2000.Spectra have been reduced using iraf and following the standard long-slit reduction procedures described above.The near-infrared spectrum of SBS 1420+544 is shown on Fig. 2c in the wavelength range from 9400 to 13200 Å.
In addition to the LBT observations, we also performed supplementary observations with the 3.5m Apache Point Observatory (APO) telescope.The Dual Imaging Spectrograph (DIS) was used prior to 2023, and the Kitt Peak Ohio State Multi-Object Spectrograph (KOSMOS) in 2023.All APO observations were carried out in the long slit mode.For DIS we use the red channel, with occasionally the blue channel.Slit widths were in the range 0.9 -2.0 arcsec for DIS and equal to 1.18 arcsec for KOSMOS.The average value of the airmass was 1.2.The DIS B1200 grating with a central wavelength of ∼6600 Å and a linear dispersion of 0.56 Å pixel −1 was used in the red range, providing a medium resolving power R = 5000.iraf routines were used for background subtractions, bias-and flat-field corrections.Onedimensional spectra were extracted after removal of cosmic particle trails and wavelength and flux calibration.All spectra used to study the spectral temporal variability are shown in Figs. 6 and 7.

Extinction-corrected emission-line fluxes
The most striking feature of all spectra in Figs. 2 and 3 is the presence of many strong narrow emission lines.Among a Rest-frame wavelength in Å. b Observed flux in units 10 −16 erg s −1 cm −2 .c Extinction-corrected luminosity in 10 39 erg s −1 .d Full width at half maximum in Å. e Full width at half maximum in km s −1 .f Velocity dispersion in km s −1 .g The terminal velocity, vterm = λmax(br)λ min (abs) in km s −1 .h Broad-to-narrow component flux ratio.i Broad means the sum of the broad and very broad emission components.
them, the hydrogen and helium lines stand out, the brightest of which showing also broad emission components, with the presence of blueward absorption, i.e. they display P-Cygni profiles.The P-Cygni profile is seen in the He i λ5876 line.However, it is absent in the He i lines λ4471 in the optical range and λ10831 in the near-infrared range.Note, however, that the NIR observation was carried out three years later than the optical ones and the absence of the P-Cygni profile may be due to flux variabilty.The [O iii] λ4363 emission line is clearly seen in these spectra.It allows to reliably determine the electron temperatures and the chemical composition of the two galaxies by the direct method.
Emission-line fluxes and their errors were measured with the iraf splot routine.For the determination of extinction from the Balmer decrements, we use the Hα, Hβ, Hγ, Hδ, H9 -H12 emission lines.Note that only the narrow components of hydrogen emission lines were measured in the cases when these lines also contain broad components.This means that the derived chemical composition is that for the ionized gas of the entire galaxy, excluding expanding circumstellar envelopes.
All hydrogen line fluxes were corrected for extinction and underlying hydrogen stellar absorption, derived simultaneously in an iterative procedure, using the method described by Izotov et al. (1994).The equivalent widths of the underlying stellar Balmer absorption lines, EW(abs), in our iterative method are assumed to be the same for all hydrogen lines.The other lines were corrected only for extinction.The ratios of the extinction-corrected nebular emission line fluxes to that of the narrow Hβ line, together with the extinction coefficient C(Hβ), the observed Hβ emission-line flux F (Hβ), the rest-frame equivalent width EW(Hβ), and the equivalent width of the Balmer absorption lines EW(abs) are shown for both galaxies in Table 3.
It is seen in the table that the extinction-corrected relative fluxes of the hydrogen emission lines are close to the theoretical relative recombination line intensities (Table 8, p.86 in Hummer & Storey 1987).Thus, we can be confident that the extinction coefficient C(Hβ) and the equivalent width of the Balmer absorption lines EW(abs) were correctly calculated.

General properties
The two studied galaxies are both dwarf SFGs, based on their low absolute magnitudes Mg ∼ -17 mag, derived from the SDSS g magnitude corrected for extinction in the Milky Way, and their low stellar masses M⋆ ∼ 2×10 6 and 4×10 6 M⊙ (Table 1), derived from SED fitting of the aperture-and extinction-corrected SDSS spectra.They have low total extinction AV ∼ 0.4 and 0.6 mag for SBS 1420+540 and J1444+4840, respectively (Table 3).Strong emission lines in the spectra, very high equivalent widths of hydrogen emission lines [EW(Hβ) is equal to 328 Å and 288 Å for the two galaxies, as seen in Table 3] suggest active star formation with the presence of a very young stellar population, with an age of ∼ 3 Myr.This is also evidenced by high star-formation rates and sSFRs (Table 1) derived from extinction-and aperture-corrected Hα luminosities, obtained from LBT spectra, and using equation 2 in Kennicutt (1998).

Emission-line diagnostic diagrams
Strong emission lines are often used to construct diagrams to diagnose different photoionization mechanisms in galaxies (Steidel et al. 2014;Cullen et al. 2014;   In the BPT diagram, low-z SFGs define a tight relation called "main sequence".Such diagrams used to estimate the metallicities, ionization states, galaxies evolution with redshift and others. The locations of SBS 1420+544 and J1444+4840 in the BPT diagram are indicated in Fig. 1a.For comparison, compact SFGs from the SDSS (Izotov et al. 2016) are also represented.The two blue lines show the relations calculated with the cloudy code c13.04 (Ferland et al. 1998(Ferland et al. , 2013) ) for metallicities 12 + log O/H = 7.3 and 8.0, starburst ages varying from 0 to 6 Myr and other input parameters typical of low-metallicity SFGs (see for details Izotov et al. 2018a).
Our two galaxies are located to the extreme left of the SFG main sequence, mainly because of their very low metallicities.This region to the left is also characteristic of objects with high ionization parameters (see e.g.Steidel et al. 2014), i.e. with young starburst ages.For comparison, two previously studied galaxies with LBV candidates, PHL 293B and DDO 68 (Pustilnik et al. 2008;Izotov & Thuan 2009;Guseva et al. 2022), have also been plotted in Fig. 1.However, note that the LBV in DDO 68 is located in the H ii region number 3 (hereinafter #3).The position of the most outlying point (filled circle in the figure), DDO 68 #3, is determined by its extremely low metallicity 12 + log O/H ∼ 7.1 and its low equivalent width of Hβ, less than 100 Å (Izotov & Thuan 2009).The position of PHL 293B, with a metallicity 12 + log O/H ∼ 7.7, is close to those of the two galaxies discussed here, but with its relatively low EW(Hβ) = 37 Å.
Another type of diagram is the O32 -R23 one (Fig. 1b), where O32 5007)/Hβ.Our two galaxies are similarly shifted from the main-sequence of compact SDSS SFGs to regions of lower metallicity, young age and high O32, the latter being an observational indicator of the ionization parameter.For comparison, we have also shown in Fig. 1b the locations of PHL 293B and H ii region #3 in DDO 68 (Guseva et al. 2022).As for SBS 1420+540 and J1444+4840, their locations are shifted in both diagrams (Figs.1a and b).DDO 68 #3 has the largest offset due to its extremely low metallicity.

Element abundances
The large wavelength range of our LBT spectra, from ∼3200 Å to ∼9500 Å for MODS, and from ∼3200 Å to ∼13000 Å for MODS and LUCI, gives numerous bright emission lines.This allows us to derive abundances for many heavy elements.
The presence of fairly intense oxygen emission lines [O iii] λ4363 in both objects makes it possible to use the direct Te method for determining the metallicity of galaxies (expressed in units of 12 + logO/H) with a high precision.We calculate the physical conditions and element abundances following the method described by Izotov et al. (2006), based on the photoionization models of Stasińska & Izotov (2003).
Briefly, the electron temperature Te(O iii) is calculated from the ratio of oxygen emission line fluxes [O iii] λ4363/(λ4959 + λ5007).The electron temperatures Te(O ii) and Te(S iii) are then obtained from equations of Izotov et al. (2006): (1) where t(O ii) = 10 To obtain the total oxygen abundance, we sum the abundances of the ions O + , O 2+ and O 3+ .To take into account unobservable ionization states of other ions of heavy elements, we use the ionisation correction factors ICF s to obtain their total element abundances.The physical conditions in the H ii regions of the galaxies, i.e. the electron temperatures Te in the high-ionization zone O 2+ , in the low-ionization zone O + , and in the intermediate zone S 2+ , together with their electron number densities Ne(S ii), are given in Table 4.We provide also in this table the ionic and total heavy element abundances for oxygen, nitrogen, neon, sulfur, chlorine, argon and iron, as well as their corresponding ionisation correction factors.
We do find an anomaly for J1444+4840: its N/O ratio is enhanced by ∼0.4 dex, compared to the lowest value ∼ −1.7 -−1.6 for this metallicity.Can we understand this N/O enhancement?In the metallicity range 12 + logO/H 8, the production of nitrogen is mainly primary so that the N/O ratio remains fairly constant, at a plateau value of about -1.5.However, more and more SFGs with an enhanced N/O value have been found at the low metallicity end, both for low-z (see e.g.Amorin et al. 2012;Sanders et al. 2016;Vincenzo et al. 2016;Guseva et al. 2020) and highz (see e.g.Kojima et al. 2017) galaxies.Several mechanisms have been proposed to account for this enhancement.Izotov et al. (2006) proposed to explain the observed N/O enrichment by a local N/O enhancement in dense clumps of H ii regions, created by winds of evolved WR stars.It has also been proposed that the N/O ratio increases as a result of the inflow of low-metalicity intergalactic gas onto the galaxy (Amorin et al. 2010(Amorin et al. , 2012;;Loaiza-Agudelo, Overzier & Heckman 2020).Another proposal is the enhancement of nitrogen in rotating massive stars, as found in numerical simulations by Roy et al. (2020), Grasha et al. (2021), Roy et al. (2021).Roy et al. (2022) also explain N/O enrichment in galaxies at high-redshifts using models with pre-supernova wind yields.Lastly, both observations and models also suggest an enhanced nitrogen abundance due to the presence of LBVs in galaxies (see e.g.Weis & Bomans 2020).

Broad emission
The most noticeable manifestations of a LBV eruption in the integrated spectrum of a star-forming dwarf galaxy are the observed strong broad emission components of hydro-gen and sometimes of helium and metal lines.The spectrum also often shows P-Cygni profiles.These broad, highvelocity components, superimposed on the narrow emission lines, are thought to arise in the circumstellar envelope and/or in a gas-dust nebula around a LBV star (see e.g.Weis & Bomans 2020).The narrow components are formed predominantly in the ionized interstellar medium of a galaxy and they very weakly increase in brightness during the eruptive LBV phase (see e.g., the temporal variations of the narrow Hα flux, as compared to the broad one for DDO 68 #3, shown by thin and thick black dotted lines in Fig. 7b by Guseva et al. 2022).We can thus follow the temporal spectral variations caused by a LBV outburst by using as a tracer the flux ratio of the broad-to-narrow components of the brightest hydrogen and helium lines.Using that ratio allows us to bypass differences in exposure time, aperture and seeing when comparing observations taken with different telescopes in the course of the spectral monitoring (Figs. 6  and 7).
We note that the luminosity L(Hβ) given in Table 5 for J1444+4840, is nearly 2 times lower than the value derived from the SDSS spectrum, both in the DR14 and DR16 releases.A closer examination showed that the standard star for J1444+4840 was observed during non-photometric conditions, which can result in smaller calibrated fluxes for the galaxy.However, it should be remembered that the SDSS spectrum was obtained in 2016, while the LBT spectrum was taken in 2019, so the flux could have varied over this period.
We have performed a decomposition of the strongest emission lines by fitting their profiles with multiple Gaussians (narrow, broad emission and absorption and also very broad emission, when needed), using the deblending routine of the iraf/splot software package.Other authors (see e.g. in Izotov et al. 2011;Terlevich et al. 2014) have also fitted high S/N Hα profiles with more than two components.The results of the decomposition of the brightest hydrogen Hα and Hβ line profiles in the LBT spectra of the two galaxies are presented in Figs. 4 and 5.For J1444+4840, while the decomposition of the bright hydrogen lines can be made by using only a two-component fit, one narrow Gaussian and one broad Gaussian, the best fit to the high signal-to-noise profile of Hα is given by a four-component fit.In addition, for J1444+4840, decomposition into two or three Gaussians has also been performed for the He i 5876 and 7065 Å lines.For the weaker lines of hydrogen and helium, only narrow and possible broad components can be distinguished.
The results of the profile decompositions in the LBT/MODS spectra are also presented in Table 5.Here, we give the observed fluxes, extinction-corrected luminosities and FWHMs for the two strongest hydrogen emission lines in both galaxies, and for the helium lines He i 5876 and 7065 Å in J1444+4840.

Signatures of LBVs
The properties of the two galaxies suggest that they contain a large number of massive stars, the most massive of which can rapidly evolve to become LBV stars.As remarked before, intense quasi-periodic variations in spectral features and photometric brightness, the so-called S Dor variability or S Dor cycle which can last for years or decades, are the clearest characteristic features that distinguish LBVs from other massive evolving stars (see e.g.van Genderen et al. 1997;Weis & Bomans 2020).In the spectra, these transient LBV phenomena manifest themselves by broad emission features with blueward absorption in hydrogen and often in helium and Fe ii lines.Our aim then is to search for time variations of these broad features during the monitoring period of the studied galaxies.We will discuss them in turm.
As for the blueward absorption in P-Cygni profiles near Hα, it is not seen in every spectrum, but only in those with the highest S/N (Figs. 4 and 5).Furthermore, for this particular galaxy, the absorption by the extended envelope occurs at wavelength ∼ 6545 Å, close to the [N ii] λ6548 Å emission, making it difficult to measure both with accuracy.Fig. 8a shows the time variation of the broad-to-narrow line flux ratio of Hα.The flux ratio is seen to be nearly constant at 0.05 from 1993 to 2013, then to sharply increase by a factor of ∼ 4 after 2013, reaching a maximum of ∼ 0.2 during the period 2016 -2017, before decreasing back to a level of about 0.02.This behavior means that we have caught SBS 1420+540 in a LBVc outburst phase, from beginning to end.
It is seen from Fig. 8b which displays the temporal variations of the fluxes of the narrow, broad and very broad components of the Hα emission-line, and their sum, that the outburst is characterized by an increase in luminosity of the whole broad bump, i.e. of the sum of broad and very broad components.The luminosity jumped by about one order of magnitude, from 1.3 to 12.0 × 10 39 erg s −1 , typical of LBV luminosities.
We have also examined the temporal variations of the kinematics of the ionized gas by measuring the full width at half-maximum (FWHM) of each Gaussian.The gas velocity dispersion σ is then obtained by dividing the FHWM by 2.355 (Fig. 8c).The FWHM measurements show that the velocity dispersion of the fastest-moving gas (the very broad component) reaches the high value σ ∼ 700 km s −1 (or a FWHM ∼ 1600 km s −1 ) (Fig. 8c).It is interesting that the velocity dispersion of the fastest moving ionized matter remains fairly constant with time, during the period 1993-2019.However, on 2019-04-08, the high-velocity emitting gas vanished abruptly after the outburst, its intensity becoming so low that it could not be detected anymore by our observations (Fig. 6).Let us remember that after the burst of activity peaks in 2017, the broad-to-narrow flux ratio also became very low.
The latest observations in April 2023 show that the flux intensity (Fig. 8b) and the velocity dispersion (Fig. 8c) of the whole broad bump has sharply declined, after some fluctuation (Fig. 6).As a result, the broad-to-narrow flux ratio has decreased by an order of magnitude, compared to its maximum value (Fig. 8a).A similar picture of a sharp decrease of the velocity dispersion of the moving gas, shortly after the peak of activity, was also observed for the LBVc in DDO 68 #3, with a decrease of σ from 500 km s −1 to 140 km s −1 , and then to a complete disappearance of the broad component (Guseva et al. 2022).
We calculate the stellar wind terminal velocity, vterm.It is related to ∆λ = λmax(br)λmin(abs), the difference between the wavelength at maximum intensity of the broad component and that of the absorption component of a spectral profile.We found the stellar wind terminal velocity to be ∼1000 km s −1 for SBS 1420+540.
In summary, the LBVc eruption in SBS 1420+540, with a peak luminosity of ∼ 10 40 erg s −1 , lasted for about 6 years, from 2013 to ∼2019, after which the intensity of the broad component returned back to its low original value, of about two hundredths of the intensity of the narrow component.However, the broad emission bump in the strong hydrogen and helium lines lasted much longer, persisting over the whole three-decade monitoring and still not disappearing completely (Figs. 2 and 6).Thus, our spectroscopic monitoring of SBS 1420+540 indicates that the LBVc has probably passed through the maximum of its eruption activity and is now likely on its way to the minimum.With continuing monitoring observations of SBS 1420+540, we will be able to further constrain the scenario of LBV stars undergoing periodic or quasi-periodic luminosity variations in this galaxy.

J1444+4840
A strong broad component, with a flux of approximately 40 per cent that of the narrow component, is clearly detected in the spectra of the compact SFG J1444+4840, during the whole period of its monitoring from 2016 to 2023.This bright broad component is present not only in hydrogen lines, but also in helium lines.This is clearly seen for the He i λ5876 Å and λ7065 Å emission lines in Fig. 3.The broad-to-narrow flux ratios are even slightly higher in helium lines than in hydrogen lines.We note that while all the broad-to-narrow flux ratios in J1444+4840 are one order of magnitude higher than those in SBS 1420+540, the broad-to-narrow flux ratios of helium lines in the latter are also similar to those of hydrogen lines.
The temporal variations of the broad-to-narrow flux ratio in Hα is shown in Fig. 9a.We see very small variations of this ratio over the 6 years from 2016 to 2022, with some small rise of activity in 2019, followed by a slight decrease of about 20 percent in 2023.Further monitoring is needed to know whether this signals the beginning of a decline in activity of a possible LBV star in this galaxy.
Over the same period, the Hα flux of the narrow component increased by a factor of 5 in 2019 compared to that in 2016 and 2022 (Fig. 9b), going from 40 × 10 −16 erg s −1 cm −2 to ∼193 × 10 −16 erg s −1 cm −2 .The flux of the broad component increased in 2019 by even more, ∼8 times.There is also an increase of the Hα flux in the very broad com-ponent, compared to 2016 and 2022.The flux increase of all velocity components in 2019 cannot be explained by an aperture effect, given that the slit width during LBT observations was 1.2 arcsec, and subsequent APO observations were made with comparable slit width of 1.5 arcsec (2022) and 1.18 arcsec (2023).
From 2019 to 2023, the velocity dispersion of the fastestmoving gas (very broad component) decreased by a factor of ∼2, from ∼1200 km s −1 to ∼600 km s −1 (Fig. 9c).A similar behavior is seen for the broad component.The FWHM of the fastest-moving gas, corresponding to velocities of 1400 -2800 km s −1 , stayed very high throughout the entire monitoring of the galaxy.
We now discuss the types of physical phenomena that can account for the above properties of J1444+4840: are they due to LBVc stars or to AGN/SN IIn?We first discuss dust extinction.LBVs are often associated with emission gas-dust nebulae as shown by both observations (see e.g.Kniazev et al. 2015Kniazev et al. , 2016) ) and hydrodynamical simulations (van Marle et al. 2011).The extinction coefficients derived from the observed hydrogen Balmer line ratios are small for our galaxies.They are C(Hβ) ∼ 0.2 (or E(B − V ) ∼ 0.1) for both (Table 3).However, these extinction coefficients were obtained only from the narrow emission component.The C(Hβ) derived from the broad Hα and Hβ emission are significantly higher, by a factor of ∼5 -6 times, in any case if broad components are used or if the sum of the broad and very broad components is used (see Table 5).Note, however, that a high Balmer decrement of the broad component can be caused not only by a high dust content, but also by a high density and thus by a high collisional excitation which is much higher for the Hα emission line.By contrast, C(Hβ) for the broad component of SBS 1420+540 is approximately the same as that for the narrow component.
A significant number of LBVs exhibit the presence of enhanced nitrogen (see e.g.Walborn & Fitzpatrick 2000;Weis & Bomans 2020).This appears to be the case for J1444+4840.The [N ii] λ6548 Å emission line near Hα is very weak and was not measured in J1444+4840 due to low metallicity (12 + logO/H = 7.45) and due to the bright and broad emission and blueward absorption, which muffles the [N ii] nebular line.Nevertheless N/O ratio is enhanced by ∼0.4 dex, as mentioned above in subsect.3.3.The increased dustiness of the high velocity component of circumstellar ionized gas together with the enhanced ratio of N/O in J1444+4840 may argue for the presence of LBVs in this galaxy.
Permitted Fe ii emission lines can act also as indicators of LBV stars.However, these permitted lines are absent in the LBT spectrum of J1444+4840.Only forbidden [Fe ii] λ4287 Å and [Fe v] λ4227 Å emission lines have been found whereas [Fe iii] lines seen in SBS 1420+540 are not detected.
In summary, the spectral properties discussed above for J1444+4840 can not definitely rule out the presence of a LBV star in it, but neither can they confirm it.During the whole 8-year monitoring period of J1444+4840, we have not observed significant variations of the broad-to narrow flux ratio, characteristic of eruptive events in LBVs.If we nevertheless adopt the LBV hypothesis, we would have to assume that the period of its eruptive cycle is larger than 8 years.Again, further monitoring observations are needed to confirm that hypothesis.

CONCLUSIONS
We report the discovery and the time-monitoring of broad components with P-Cygni profiles of the hydrogen and helium emission lines in two compact star-forming galaxies (SFG): SBS 1420+540, selected from the Second Byurakan Survey (SBS), and J1444+4840, selected from the Sloan Digital Sky Survey (SDSS) Data Release 14 (DR14).The spectrophotometric data have been obtained with the 2 × 8.4 m Large Binocular Telescope (LBT) with the spectrographs MODS1, MODS2, LUCI1 and LUCI2 and the 3.5 m Apache Point Observatory (APO) telescope with the spectrographs DIS and KOSMOS.We collected also our different observations of the two galaxies obtained in previous studies.The spectroscopic data cover a time baseline of three decades for SBS 1420+544 and nearly one decade for J1444+4840.Our main results are the following: 1) The two SFGs, SBS 1420+540 and J1444+4840, have low oxygen abundances 12 + logO/H = 7.75 ± 0.02 and 12 + logO/H = 7.45 ± 0.02, respectively, derived by the direct Te method.The N/O, Ne/O, S/O, Cl/O, Ar/O and Fe/O ratios are similar to those usually derived for other low-metallicity SFGs.The N/O ratio for J1444+4840 with lower oxygen abundance is enhanced by ∼0.4 dex compared to its average value.for given metallicity.
2) We decompose the profiles of the brightest hydrogen and helium lines into the sum of three (four) Gaussians representing the narrow, broad, (very broad) and absorption components.We use the flux ratio of the broad-to-narrow components to trace the temporal variations of possible LBV outbursts.
3) For SBS 1420+540, our spectroscopic monitoring has captured the eruption phase of a LBVc (c for candidate) that lasted for about 6 years.The broad-to-narrow flux ratio sharply increased by a factor of 4 in 2017 and decreased by about an order of magnitude in 2023, with a peak luminosity of the broad component L(Hα) ∼ 10 40 erg s −1 .The velocity dispersion of the fastest moving gas reaches the value σ ∼ 700 km s −1 (or FWHM ∼ 1600 km s −1 ), and the terminal velocity of the stellar wind, derived from the P-Cygni features, is vterm ∼1000 km s −1 .Our spectroscopic monitoring of SBS 1420+540 indicates that the LBVc has probably passed through the maximum of its eruption activity.
4) For J1444+4840, the fluxes of the strong broad components of hydrogen and helium lines are quite high, amounting to ∼40 per cent that of the narrow components.The Hα broad component shows a high luminosity L(Hα) about 10 41 ergs s −1 , a value that is more similar to those observed in active galactic nuclei (AGNs) and Type IIn Supernovae (SNe) than in LBVs.We found the variability of less than 20 per cent of the broad-to-narrow flux ratios, the high velocity dispersion σ ∼600 -1200 km s −1 , and the high terminal velocity vterm ∼1000 -1700 km s −1 that has persisted over 8 years of monitoring.The results obtained thus far do not allow us to definitively conclude that a LBVc is present in J1444+4840.The hypothesis of a long-lived stellar transient of type AGN/SN IIn would work as well.

Figure 2 .
Figure 2. The rest-frame a) blue and b) red LBT/MODS and c) LBT/LUCI spectra of SBS 1420+544 uncorrected for extinction.Blue absorption in P-Cygni profiles of hydrogen and helium lines are marked by blue and red arrows, respectively.Wavelengths are in Å and fluxes are in units of 10 −16 erg s −1 cm −2 Å−1 .

Figure 3 .
Figure3.The rest-frame LBT/MODS spectrum of J1444+4840 uncorrected for extinction.Blueward absorption in P-Cygni profiles of hydrogen and helium lines is marked by blue and red arrows, respectively.Wavelengths are in Å and fluxes are in units of 10 −16 erg s −1 cm −2 Å−1 .

Figure 4 .
Figure 4. Decomposition in the LBT spectrum by gaussians of a) Hβ and b) Hα narrow and broad emission lines and absorption lines of SBS 1420+544 (blue dotted and solid lines for emission, and magenta dashed lines for absorption, respectively).Additionally, two blue dotted lines in b) also represent [N ii]λ6548 and λ6583 Å emission lines.The fitted profiles in a) and b) are shown by the red solid lines whereas the observed spectra are represented by black solid lines.Wavelengths are in Å and fluxes are in units of 10 −16 erg s −1 cm −2 Å−1 .

Figure 5 .
Figure 5. Decomposition of a) Hβ, b) Hα, c) He i λ5876 and d) He i λ7065 emission lines by gaussians of the LBT spectrum of J1444+4840.Very broad component in the case of Hα is shown by green solid line.The rest of the designations in the figure are the same as in Fig. 4.

Figure 6 .
Figure6.The rest-frame Hα emission line profiles in the SBS 1420+544 spectra at different epochs from collection of our observations carried out with 4m KPNO, LBT/MODS and 3.5m APO telescopes.Wavelengths are in Å and fluxes are in units of 10 −16 erg s −1 cm −2 Å−1 .

Figure 7 .
Figure7.The rest-frame Hα emission line profiles in the J1444+4840 spectra at the different epochs, observed with the LBT/MODS and 3.5m APO telescopes.The spectrum from the SDSS is also shown.Wavelengths are in Å and fluxes are in units of 10 −16 erg s −1 cm −2 Å−1 .

Figure 8
Figure 8. a) Temporal variations of Hα broad-to-narrow flux ratio in SBS 1420+540.Note that in a) the broad component means the sum of the broad and very broad emission, i.e. the whole broad bump.b) temporal variation of Hα flux in units of log F (Hα) + 16, where F (Hα) is in erg s −1 cm −2 , of different components, narrow, broad and very broad, marked by different colours.The break in the sequence for the very broad component (red curve) is explained by the disappearance of the most fastmoving gas on 2019-04-08 and its reappearance in 2023.c) The same as in b) but for full width at half maximum in Å (left scale) and in km s −1 (right scale).

Table 1 .
Observed characteristics Derived from the Kennicutt (1998) relation using the extinction-and aperture-corrected Hβ luminosity.
† Corrected for Milky Way extinction.‡ Derived from the extinction-and aperture-corrected SDSS spectrum.* Corrected for extinction.† †

Table 3 -
continued LBT extinction-corrected emission-line fluxes * Note that for bright hydrogen lines only nebular narrow emission components were measured.a Extinction coefficient, derived from the observed nebular hydrogen Balmer lines.b Observed flux of narrow Hβ in units of 10 −16 erg s −1 cm −2 .c Rest-frame equivalent width in Å.

Table 4 .
Electron temperatures, electron number densities and element abundances from LBT observations

Table 5 .
Decomposition of strongest emission lines, LBT observations 2+, Cl 2+ and Ar 2+ abundances.Since the high ionization line He ii λ4686 Å is present in the spectra, we can take into account unobservable O 3+ : −4Te(O ii), t(S iii) = 10 −4 Te(S iii) and t = 10 −4 Te(O iii).The electron number densities Ne(S ii) are derived from the singly ionized sulfur line ratio [S ii] λ6717/λ6731.We adopt Te(O iii) for O 2+ , Ne 2+ and Ar 3+ abundance calculations.Te(O ii) is used for O + , N + , S + and Fe 2+ abundance determination.Te(S iii) is adopted for the calculation of S