NGC 3521 as the Milky Way near twin: spectral energy distribution from UV to radio decameter ranges

Abstract

Milky Way analogues (MWAs) are usually selected from structural and kinematic properties, but robust SED-based similarity criteria are limited by heterogeneous photometry and incomplete wavelength coverage. We present a homogeneous, aperture-photometry SED of the Milky Way near-twin NGC~3521 from the ultraviolet to the radio decameter range. Fluxes are measured within a fixed elliptical isophotal aperture using GALEX, SDSS, WISE, Spitzer/MIPS, Herschel/PACS+SPIRE, and VLA data, and supplemented by meter/decameter constraints. We report new observations obtained in Jan-Feb 2022 with the Ukrainian T-shape radio telescope and derive, for the first time, an upper limit in the 24--32~MHz band. The UV-to-decameter SED (27 points) is modelled with \textsc{CIGALE}, including a dedicated low-frequency radio prescription (\texttt{radio_extra}) that accounts for emission and absorption effects. Using ZTF and NEOWISE data (2014--2025), we detect genuine nuclear variability; optical trends at $\sim2^{\prime\prime}$ primarily trace the compact nucleus, while NEOWISE variability reflects a mix of nuclear changes and warm-dust emission within the larger aperture. The preferred fit yields $M_\star \simeq 6.0\times10^{10},M_\odot$, ${\rm SFR}\simeq1.65,M_\odot,{\rm yr}^{-1}$, $M_{\rm dust}\simeq1.3\times10^{8},M_\odot$, and an effective dust temperature of $\sim23$~K. The decameter constraint gives $S_{28,{\rm MHz}}<11.22$~Jy, consistent with expectations for a Milky Way-like system placed at 10.7~Mpc. We conclude that an integrated, homogeneous SED, especially below 100~MHz, provides a complementary diagnostic for identifying and validating MWAs and for interpreting how Milky Way properties would appear to an external observer.

Figures (10)

• Figure 1: Examples of multiwavelength images of NGC 3521, which are used for aperture photometry and SED construction. On the axes, the size of each image in pixels. Panel (a): GALEX NUV emission tracing recent star formation. Panel (b): VLA L--band radio continuum image. Panel (c): Herschel/PACS 160 $\mu$m map. Panel (d): SDSS $g$--band optical image. The red ellipse in all panels corresponds to the adopted isophotal aperture for integrated flux measurements.
• Figure 2: Upper: Map of nonthermal Galactic radio emission distribution (in terms of brightness temperatures $T_B$), obtained at UTR-2 for 20 MHz Sidorchuk2021. The horizontal black strip schematically represents the intersection of the Galaxy in the UTR-2 sky scans at DecJ = $0^\circ$. A white square indicates the position of NGC 3521. Middle: Averaged scan for January 20-22, 2022 (daily sequence of total signal power changes at antenna output), frequency band 24-32 MHz, integration time 30 s. Bottom: Averaged scan for February 3-5, 2022, frequency band 24-32 MHz, integration time 30 s.
• Figure 3: Part of the time sequence relative to the coordinates of NGC 3521 on the scan averaged over January 20–22 and February 3–5, 2022, obtained at the West-East antenna of the UTR-2 radio telescope for 24–32 MHz frequency band and integration time 30 s.
• Figure 4: Part of the time sequence relative to the back diffraction side lobe of Cassiopeia A on the scan averaged over January 20–22 and February 3–5, 2022, obtained at the West-East antenna of the UTR-2 radio telescope for 24–32 MHz frequency band and integration time 30 s.
• Figure 5: NGC3521 SED from UV to decameter range