A $z\simeq0.4$ Little Red Dot analog: An Extended Starburst with an Overmassive Black hole

TL;DR

This study presents J2048, a low-redshift analog of JWST-identified Little Red Dots, at $z=0.4332$, using Gemini-N/GMOS IFU spectroscopy and archival multi-band photometry to dissect an obscured AGN embedded in a vigorously star-forming host. The analysis combines per-pixel spectral fitting and joint spectrum+SED modeling to isolate AGN, host stellar populations, and ionized outflows, revealing a compact red AGN continuum, a broad BLR H$\\alpha$ line, and a fast, spatially extended ionized outflow with $v_ ext{out}\sim2000 m\,km\ s^{-1}$. The host shows a young starburst with $ ext{SFR}\approx400 ext{ M}_\odot ext{ yr}^{-1}$ on $\sim20$ Myr, an upper limit to an old stellar population, and a total stellar mass in the range $M_ ext{*}\sim10^{9.9}-10^{11} ext{ M}_\odot$. The inferred black hole mass is $M_ ext{BH}acksim10^{10.2} ext{ M}_\odot$, yielding $M_ ext{BH}/M_ ext{*}\sim0.6$ and indicating an extremely overmassive SMBH relative to local relations. These findings imply that LRDs may undergo rapid early BH growth with substantial outflows and thin obscuring structures, providing a valuable local laboratory to interpret the properties and evolution of high-$z$ LRDs.

Abstract

One of the most remarkable discoveries of JWST is a population of compact, red sources at z > 4, commonly referred to as Little Red Dots (LRDs). Spectroscopic identifications reported that most LRDs are active galactic nuclei (AGNs), which are preferentially found around z~6 and could imply a key phase in the formation and growth of black holes (BHs) in the early universe. Photometric surveys at lower redshift have recently been carried out to trace their evolution across cosmic time, and a small number of LRDs have been spectroscopically identified at both Cosmic Noon and in the local universe. Here we report the discovery of one of the lowest-z analogs of LRDs, J204837.26-002437.2 (hereafter J2048) at z = 0.4332, using new Gemini-N/GMOS IFU observations combined with archival multi-band photometric SED data. The GMOS data reveal extended blue emission from starburst with a star formation rate of 400 Msun yr-1, together with an extended, highly fast ionized outflow. This is the first spectroscopic confirmation of extended host emission and outflow in an LRD-like galaxy, providing a unique laboratory for understanding the nature of their high-redshift counterparts. Moreover, J2048 would host an extremely overmassive BH with a BH-to-stellar mass ratio of 0.6, with the BH mass and host stellar mass estimated to be 10^10.2 and 10^10.4 Msun, respectively. We discuss the origin and evolutionary fate of J2048, and the implications that such low-z analogs have for interpreting the properties of high-z LRDs.

Figures (9)

• Figure 1: Left: SDSS $gri$-bands composite image of J2048. The field of view of the GMOS IFU observation is over-plotted as a blue rectangular. Middle: Composite image of J2048 from the best spectral fitting results of GMOS IFU data. The red, green, and blue colors denote the AGN BLR H$\alpha$ line, narrow H$\alpha$ line, and young stellar continuum (integrated in rest frame 3500--4000 Å), respectively. The grey circle shows the PSF FWHM of 0.65. Right: Average radial profiles of young stellar continuum (blue; 3500--4000 Å), narrow H$\alpha$ line (green), outflow [O iii] line (purple; the "outflow line 1" component in Figure \ref{['fig:J2048_s1fit_line']}), AGN BLR (red solid) and disk power-law component (red dashed). The profiles are arbitrarily normalized at the center. An approximate PSF using a Gaussian profile with FWHM of 0.65 is shown in grey dotted line, which can reproduce the bulk of the unresolved AGN BLR and power-law components.
• Figure 2: Best-fit results of the pure spectral fitting (Section \ref{['subsec:method_ifu_spec']}) for the GMOS spectrum of the entire galaxy with fitting residuals (top and middle panels), as well as two zoomed-in regions around H$\beta$-[O iii] (bottom-left) and H$\alpha$-[N ii] (bottom-right), respectively. The observed spectrum is shown in grey. The total model is shown in orange, with the young stellar continuum in blue and the AGN power-law component in red dashed lines. Emission lines include the narrow component (green), two broad blueshifted outflows (purple and violet), and the BLR H$\alpha$ line (thick red). Grey hatched areas are excluded due to poor data quality. Note that in the top panel, the outflow profiles of the sum of H$\alpha$-[N ii] complex are shown for the sake of illustration.
• Figure 3: Best-fit results of the simultaneous spectrum+SED fitting (Section \ref{['subsec:method_s3fit']}) for observations of the entire galaxy in the wavelength range of the spectrum (top) and all used SED data (lower-middle; rest-frame 0.2--20 ) with their fitting residuals, respectively (upper-middle and bottom panels). Black dots and brown open circles denote the photometric data and the best-fit models, respectively. The legends of models are the same as those used in Figure \ref{['fig:J2048_s1fit_line']}. The AGN torus model is denoted in yellowgreen dashed curve. Shaded areas show the 25%–75% uncertainty ranges from Monte Carlo simulation for each component. The unconstrained old stellar population is shown as a 1$\sigma$ upper limit (brown hatched; Section \ref{['subsec:result_host']}). The best-fit ISM dust model of J2048 from Chen2020 is shown in green dotted line as a reference. Note that all the residuals in the upper-middle and bottom panels are shown in unit of $F_\lambda$ ($10^{-16}$ erg s$^{-1}$ cm$^{-2}$ Å$^{-1}$) for the sake of illustration. The scatters reflect the fitting residuals of the original and mock data of the spectra (grey curves) and photometric points (black dots). Modified errors (to reflect the calibration errors) are shown in green shadow region and dashed lines.
• Figure 4: Best-fit models of the simultaneous spectrum+SED fitting for J2048 in the unresolved ($r<2$ kpc) region of J2048. The unresolved SED (orange thick line) is obtained by subtracting the best-fit models of the extended outskirt region (Figure \ref{['fig:J2048_outskirt_comp']} and \ref{['fig:J2048_outskirt_s1fit']}) from the best-fit SED of the entire galaxy (Figure \ref{['fig:J2048_s3fit_SED']}). The best-fit power-law models for the unresolved SED are shown in purple dotted (UV, $<3500$ Å) and green dash-dotted lines (optical, 4000--6500 Å), respectively. The stacked SED of LRDs in JADES perezgonzalez2024 and COSMOS-Web surveys akins2024 are shown in grey thick curves and squares, respectively; the upper limits of LRDs in COSMOS-Web survey are shown in grey triangles. All stacked SED are normalized to 0.01 mJy at rest 3000 Å for a direct comparison of the unresolved SED of J2048 and high-$z$ LRDs.
• Figure 5: GMOS spectra integrated in the unresolved central region (red) and the extended outskirt region (blue). The extended spectrum is rescaled by 50% to match the continuum flux of the unresolved region at 3500--4500 Å. Only the unresolved region shows a red continuum and a BLR component of H$\alpha$ line, which are emitted from an obscured AGN. The integration method to create the two spectra are shown in Appendix \ref{['sec:appendix_outskirt']} with the best-fit models in Figure \ref{['fig:J2048_outskirt_s1fit']}.