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Theoretical Physics Letters
2023 ° 03(09) ° 0631-66982
https://www.wikipt.org/tphysicsletters
DOI: https://www.doi.wikipt.org/10/1490/588796tpl
OPB and RG are thankful to Science & Engineering Research Board (SERB), Department of Science & Technology (DST), Government of India for financial support (EMR/2017/003092). OPB acknowledges the help received during this work from his colleagues Anshuman Borgohain and Hritwik Bora. SP acknowledges Manipal Centre for Natural Sciences, Centre of Excellence, Manipal Academy of Higher Education (MAHE) for facilities and support. MS acknowledges the financial support by the DST, Government of India, under the Women Scientist Scheme (PH) project reference number SR/WOS-A/PM-17/2019. YS acknowledges the hospitality of the Raman Research Institute, Bengaluru, India.
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Holmberg II – a dwarf galaxy in the nearby M81 group – is a very informative source of distribution of gas and dust in the interstellar discs. High-resolution observations in the infrared (IR) allows us to distinguish isolated star-forming regions, photodissociation (PDR) and HII regions, remnants of supernovae (SNe) explosions and, as such, can provide information about more relevant physical processes. In this paper we analyse dust emission in the wavelength range 4.5 to 160 𝜇m using the data from IR space observatories at 27 different locations across the galaxy. We observe that the derived spectra can be represented by multiple dust populations with different temperatures, which are found to be independent of their locations in the galaxy. By comparing the dust temperatures with the far ultraviolet (FUV) intensities observed by the UVIT instrument onboard AstroSat, we find that for locations showing a 100 𝜇m peak, the temperature of cold (20 to 30 K) dust grains show a dependence on the FUV intensities, while such a dependence is not observed for the other locations. We believe that the approach described here can be a good tool in revealing different dust populations in other nearby galaxies with available high spatial resolution data.
Dwarfs galaxies (DG) – galaxies with baryonic mass of 𝑀𝑏∼109𝑀⊙, are commonly thought to be the building blocks for the entire hierarchy of mass distribution in the Universe, being progressed during its evolution since post-recombination epochs through merging process (White & Rees 1978). Within this concept, DG in local Universe can serve as laboratories for understanding the details of physical processes regulating formation of the very first galaxies in early Universe (see review by Tolstoy et al. 2009, and more recent discussions in Henkel et al. (2022); Annibali & Tosi (2022)). Among the most important traits of DGs are their metal-poor interstellar medium (ISM) and, correspondingly, a low dust content (Henkel et al. 2022) – the features that are also commonly expected for early Universe galaxies. Observations of bright galaxies at 𝑧 > 10 conducted first by the Hubble Space Telescope (HST), and subsequently by the James Webb Space Telescope (JWST), have indeed indicated a deficient amount of dust in a set of high-𝑧 galaxies (see discussion in Finkelstein et al. 2022; Ferrara et al. 2023). In order for local DGs to indeed provide relevant information for understanding of their more distant congeners, the comparative analysis of their global emission characteristics, along with the respective properties on smaller scales, is of a high importance (relevant discussion can be found in Izotov et al. 2021; Henkel et al. 2022). A good example of a DG in local Universe is the Holmberg II (Ho II) galaxy belonging to the nearby M81 group at a distance of ≃ 3.4 Mpc. Its modest inclination angle (≈27◦ ; Sánchez-Salcedo et al. (2014)) makes Ho II a very informative source of distribution of gas and dust in the interstellar discs. Spitzer and Herschel space telescopes with the angular resolution of ∼ 2 ′′ − 5 ′′ can probe the Ho II interstellar disc with a high scrutiny. Scanning the interstellar medium (ISM) with such resolution can allow to distinguish isolated star forming regions (SFR), photodissociation (PDR) and HII regions, supernovae remnants, and can provide information about relevant physical processes. In this Letter we analyze spectra of dust emission in the range 𝜆𝜆 = 4.5, . . . 160 𝜇m from Spitzer and Herschel archival data, focusing on a few rather small-area locations. It allows us to distinguish IR emission which cannot be attributed to the isothermal dust, thus requiring existence of at least two populations of dust with different temperatures.Since the primary source of dust heating is the absorption of UV/optical photons by the dust grains, we have also utilised the highest available resolution (1.2 ′′ −1.6 ′′) FUV observations of Ho II galaxy obtained by the UVIT instrument of India’s AstroSat mission (Singh et al. 2014) as part of this study. The data correspond to 3 epochs in 2016, 2 epochs in 2019, and 3 epochs in 2020 (Vinokurov et al. 2022). These observations are crucial in understanding the UV radiation fields, responsible for modifying the dust populations and © their thermal IR emission profiles at these locations.
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We present for the first time analysis of IR dust emission in the galaxy Holmberg II to detect spatial variations of dust parameters on small physical scales of ∼ 82 pc (corresponding to a circular area of 5′′-radius) using high angular resolution data from Spitzer and Herschel. (ii) In several locations, connected to physically distinct regions, we found spectra that can represent several – up to five, dust populations with different temperatures. Spectral characteristics are not sensitive to the HI column density, except for the cold dust component with 𝑇𝑑 = 10 − 15 K which concentrates predominantly in HI deficient regions. Preliminary inspection shows that this cold dust population does not show a dependence on FUV intensity. (iii) Similarly, the cold dust in those regions with spectra peaking at 70 𝜇m has temperatures nearly in the same range as the HI voids with little dependence on FUV intensity. However, for those locations with the peak intensity at 100 𝜇m, the temperature of the cold dust component (𝑇𝑑 = 20 − 30 K) increases with FUV intensity. (iv) The estimated dust mass manifests signs of anti-correlation with its temperature, as formerly reported for dwarf star-forming galaxies by Izotov et al. (2014).
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