INT monitoring survey of Local Group dwarf galaxies: star formation history and chemical enrichment
Tahere Parto, Shahrzad Dehghani, Atefeh Javadi, Elham Saremi, Jacco Th. van Loon, Habib Khosroshahi, Mohammad Taghi Mirtorabi, Hedieh Abdollahi, Mahtab Gholami, Seyed Azim Hashemi, Mahdieh Navabi, Majedeh Noori, Sima Taefi Aghdam, Maryam Torki, Mahshid Vafaeizade
CCommunications of BAO, Vol. 67, Issue 2, 2020, pp. 232-235
INT monitoring survey of Local Group dwarf galaxies:star formation history and chemical enrichment
T. Parto ∗ , Sh. Dehghani , A. Javadi , E. Saremi , J. Th. van Loon , H. Khosroshahi , M.T. Mirtorabi , H. Abdollahi , M. Gholami , S.A. Hashemi , M. Navabi , M. Noori , S. T.Aghdam , M. Torki , and M. Vafaeizade Physics Department, Alzahra University, Vanak, 1993891176, Tehran, Iran Physik. Institut, University of Cologne, Zulpicher Str. 77, D-50937 Cologne, Germany Department of Physics, Sharif University of Technology, Tehran, 11155-9161, Iran Astrophysics Group, Lennard-Jones Laboratories, Keele University, Staffordshire ST5 5BG Physics Department, Faculty of Science, University of Zanjan, Zanjan 45371-38791, Iran
Abstract
The Local Group (LG) hosts many dwarf galaxies with diverse physical characteristics in termsof morphology, mass, star formation, and metallicity. To this end, LG can offer a unique site totackle questions about the formation and evolution of galaxies by providing detailed information.While large telescopes are often the first choices for such studies, small telescope surveys thatperform dedicated observations are still important, particularly in studying bright objects in thenearby universe. In this regard, we conducted a nine epoch survey of 55 dwarf galaxies calledthe “Local Group dwarf galaxies survey” using the 2.5m Isaac Newton Telescope (INT) in the LaPalma to identify Long-Period Variable (LPV) stars, namely Asymptotic Giant Branch (AGB)and Red Super Giant (RSG) stars. AGB stars formed at different times and studying their radialdistribution and mass-loss rate can shed light on the structure formation in galaxies. To furtherinvestigate the evolutionary path of these galaxies, we construct their star formation history (SFH)using the LPV stars, which are at the final stages of their evolution and therefore experiencebrightness fluctuations on the timescales between hundred to thousand days. In this paper, wepresent some of the results of the Local Group dwarf galaxies survey.
Keywords: stars: AGB and RSG - Stars: LPVs - Stars: dust - galaxies: evolution - galaxies:star-formation - galaxies: Local-Group - galaxies: dwarf
1. Introduction
Dwarf galaxies are the most common type of galaxies in the universe. The importance of theinternal and external processes (e.g., supernova explosion; interaction with the massive halos) on theevolution of these small galaxies are well known, though with many unanswered aspects (Weisz et al.2014; Saremi et al. 2020). The star formation history (SFH) is a robust tracer of how different internalor external mechanisms affect the evolution of a galaxy (Saremi et al. 2019b).The cool asymptotic giant branch (AGB) stars with luminosities of ∼ L (cid:12) and wide age ranges,from 100 Myr to older than 10 Gry, are well-known probes of the stellar population of galaxies in thenear-infrared (Habing et al. 2004; Javadi et al. 2011a, 2017)especially in the nearby universe. Many evolved AGBs, to be specific, thermally pulsing AGBs(TP-AGB), are long-period variables (LPVs; Marigo et al. 2017), therefore they experience brightnessfluctuations on the timescales between 100 to 1300 days due to the low surface gravity (Javadi & vanLoon 2019). TP-AGBs are responsible for a significant fraction of the integrated light of a galaxy,they also contribute meaningfully to the chemical enrichment of the interstellar medium (ISM) through ∗ [email protected], Corresponding author T. Parto et al. a r X i v : . [ a s t r o - ph . GA ] J a n NT monitoring survey of Local Group dwarf galaxies different mechanisms that drive the stellar wind. Red supergiant (RSG) stars with the look-back time10 years are another major dust producers and are examples of LPVs and their brightness variatesbetween 600 to 900 days (Javadi & van Loon 2019).This survey aims to uniformly determine the SFH of 55 dwarf galaxies of the Local Group (LG)by adopting a novel method. This method was first proposed and applied to the M33 by (Javadiet al. 2011b) and relies on the identification of LPVs. There are some other successful applicationsof this method on dwarf galaxies in LG (e.g., IC 1613 Hashemi et al. 2019; LMC and SMC Rezaeikhet al. 2014; NGC 147 and NGC 185 Hamedani Golshan et al. 2017). Using this method, We canalso monitor the amount of produced dust by LPVs, and investigate their role in the star formationand evolution of a galaxy. Surveying this large sample of LG dwarfs, enables us to determine theevolutionary dependense of dwarfs galaxies on the enviroment, such as proximity to the host galaxiesand compare it with internal effects like the stellar mass or gas content (Saremi et al. 2019a).
2. Observations and Data
The sample of dwarf galaxies studied in this survey consists of all observable Andromeda systemof satellites in the northern hemisphere, along with 20 satellites of Milky Way and some isolated andtransitional dwarfs (Saremi et al. 2017). We exclude galaxies that have been studied with Javadi’smethod before.The Observations were made in 9 epochs between 2015-2018 using the Wide Field Camera (WFC),an optical mosaic camera on the INT telescope. WFC consists of four 2048 × GAIA
Data Release 2 (Gaia Collaboration et al. 2018). The complete description ofobservations and details of the photometry procedure is available at (Saremi et al. 2020).
3. Method
To identify LPV candidates, we employed a method described in (Stetson 1996) to determine thevariability index L for each star. Then we estimated a threshold for variability, using the variabilitydistribution for the stars in different magnitude bins (Javadi et al. 2011a; Saremi et al. 2020)With insufficient observation night, we can not obtain a meaningful period for LPVs. However,there is a correlation between the amplitude and period of a variable star (Goldman et al. 2019), andlarge amplitude variables (LAV) are usually evolved AGBs at the late stage of their life.Fig. 1 shows the color-magnitude diagram (CMD) of SagDIG, one of the dwarf irregular galaxies inour sample with the overplotted PARSEC–COLIBRI isochrones (Marigo et al. 2017). LPV candidatesare shown as blue circles, with a size scaled to their amplitude in the i-band with values between0 . − . mag . The luminosity of AGB stars reaches a maximum at the final stage of life, hence can be used toestimate the birth mass of the star. For determining the mass, we construct the mass function of LPVs(for the suitable metalicity) by interpolating mass-luminosity relation using the PARSEC–COLIBRIisochrones. After estimating age of LPVs using the mass-age relation, we determine pulsation duration
T. Parto et al. NT monitoring survey of Local Group dwarf galaxies by fitting multiple Gaussian functions to the mass-pulsation values in the isochrones that show strongpulsations.The star formation rate (SFR) function ξ ( t ) introduced in (Javadi et al. 2011b) takes mass, ageand pulsation duration of LPVs and estimates the stellar mass formed per year ( M (cid:12) y − ): ξ ( t ) = dn(t) δt (cid:82) maxmin f IMF ( m ) m dm (cid:82) m ( t + dt ) m ( t ) f IMF ( m ) m dm . (1)where f IMF is the initial mass function (Kroupa 2001), dt represents different age bins, and dn isthe number of observed LPVs in each age bins. i [ m a g ] SagDIG Stars in 2 r h AGB-tipRGB-tip50% CompletenessLPV candidates
Figure 1. [i] vs. [V-i] CMD for SagDIG with the overplotted isochrones. Black dots denote the starswithin the two half-light radius of this galaxy. Blue circles are the LPV candidates with a size scaled totheir amplitude. The dotted line marks RGB-tip and AGB-tip. The 50% completeness is representedby the solid red line.
4. Results and On–going works
Here we present results of published papers for three galaxies: And I and And VII, two dwarfspheroidal and satellite of Andromeda, and IC 10, an irregular and isolated galaxy.(Saremi et al. 2020) detected 59 LPV candidates within the two half-light radii of And I, includingfive extreme AGBs (X-AGB). They also modeled the spectral energy distribution (SED) of these stars,using
DU ST Y code (Ivezic & Elitzur 1997) and mid-IR bands measurement from Spitzer (3.6 and 4.5 µm ), WISE (W1=3.4, W2=4.6, W3=11.6, and W4=22 µm ) (Cutri & et al. 2014; Wright et al. 2010),INT near-infrared i-band catalog and SDSS (u, g, r ,z) filters. They evaluated the total mass-loss rateof 3 . × − M (cid:12) yr − from five X-AGBs and thirteen dusty AGBs, which suggest low growth of stellarmass ( ∼
10 %) in AND I in the next 10 Gyr.(Navabi et al. Submitted, 2020) detected 43 LPV candidates within the half-light radii of And VII,and estimated the SFR peak of this area about 0.002 M (cid:12) yr − at Z=0.0007. Probably, And VII wasquenched by environmental impacts after infall into Andromeda’s virial radius (Navabi et al. 2020a,2020b). T. Parto et al. NT monitoring survey of Local Group dwarf galaxies (Gholami et al. 2019) detected around 10000 AGB stars in IC 10 in the area of CCD4 ( ∼ T. Parto et al. NT monitoring survey of Local Group dwarf galaxies
Acknowledgements
The observing time for this survey is provided by the Iranian National Observatory and the UK-PATT allocation of time toprograms I/2016B/09 and I/2017B/04 (PI: J. van Loon). The authors thank the Iranian National Observatory and the School ofAstronomy (IPM) for the financial support of this project.
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