NGC 3521 as the Milky Way near-twin: UV–radio-decameter spectral energy distribution

¹ Main Astronomical Observatory of the NAS of Ukraine, 27, Akademik Zabolotnyi St., Kyiv 03143, Ukraine
² Institute of Radio Astronomy of the National Academy of Sciences of Ukraine, 4, Mystetstv St., Kharkiv 61002, Ukraine
³ International Center for Astronomical, Medical, and Ecological Research, National Academy of Sciences of Ukraine, 27, Akademik Zabolotnyi St., Kyiv 03123, Ukraine
⁴ Institute of Astronomy, V.N. Karazin Kharkiv National University, 35, Sumska St., Kharkiv 61022, Ukraine
⁵ Instituto de Astrofísica de Canarias (IAC), Tenerife, Spain

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 16 December 2025
Accepted: 1 April 2026

Abstract

Context. Milky Way analogs (MWAs) are defined on the basis of structural, kinematic, and global physical parameters. Our advanced approach to searching for MWAs also includes principal features of the Milky Way (MW) such as its isolated position in the local cosmic web, absent or weak nuclear activity, and the low mass of the supermassive black hole. We highlight the suggestion that the spectral energy distribution (SED) shape and properties such as star-formation rate and stellar and dust masses should be similar within the expected parametric scatter for MW and MWAs, which are on the same co-evolutionary cosmological scale. However, constructing a reliable SED reference of MWAs remains challenging due to heterogeneous photometry and the lack of constraints across the overall spectral range. NGC 3521 is one of the closest known MWAs and an excellent candidate for extending SED-based similarity criteria.

Aims. We aim to construct a homogeneous, aperture-photometry-based SED of NGC 3521 from the UV to the radio decameter range and to assess whether the integrated SED can serve as an additional indicator for the search for and validation of MWAs.

Methods. We report the SED model for NGC 3521 based on the measurements across the UV to radio wavelength ranges, exploiting, for the first time, both the data in the decameter range and aperture photometry for GALEX, SDSS, WISE, Spitzer/MIPS, Herschel/PACS, SPIRE, and VLA images. To constrain the decameter emission and derive an upper limit in the 24–32 MHz band, we present observational data obtained in January and February, 2022 using the Ukrainian T-shaped radio telescope (UTR-2). The observed SED was modeled with CIGALE, in which we developed a radio prescription (radio_extra) module that accounts for emission and absorption effects in the radio meter and decameter domains. The nuclear activity of NGC 3521 was analyzed with ZTF and NEOWISE archival data for the 2014–2025 period.

Results. The SED measurements contain 27 photometric points. The preferred SED model from UV-to-decameter ranges yields M_★ ≃ 6.0 × 10¹⁰ M_⊙, SFR ≃ 1.65 M_⊙ yr⁻¹, M_dust ≃ 1.3 × 10⁸ M_⊙, and an effective dust temperature of ∼23 K. We found genuine variability of the NGC 3521 central region. The optical trends, measured with ZTF point-spread-function-fit photometry on a seeing-limited scale of ≲3″ (full width at half maximum), primarily trace the compact nuclear region and are consistent with a variable nuclear continuum superimposed on a relatively stable stellar component. The same behavior is observed in the mid-IR, but due to the larger effective scale of the NEOWISE point-spread-function-fit photometry (∼7.5″) it reflects a combination of nuclear variations and the contribution of warm dust in the central region.

Conclusions. Considering NGC 3521 as the MW near-twin galaxy, we present, for the first time, its SED model from UV-to-radio decameter ranges. We show that the integrated SED, especially when extended below 100 MHz, provides a complementary, physically motivated diagnostic. We demonstrate that the SED’s model of our Galaxy and NGC 3521 are similar, with the most noticeable residual offsets occurring in the far-UV and near the far-IR dust peak. The decameter constraint provides an upper limit log L_ν(28 MHz)≈30.19 of the spectral luminosity at 28 MHz, which is consistent with the extrapolated luminosity of a MW of log L_ν(28 MHz ≈ 30.17 placed at 10.7 Mpc. An exceptional consistency of SED shapes of our Galaxy and NGC 3521 allows us to conclude that SED shape may serve as an additional indicator in the search for MWAs. In turn, it helps to extrapolate how MW properties appear to an extragalactic observer.