Euclid: A UV-faint quasar in a highly luminous star-forming host galaxy at $z \approx 7.7$

Technical Summary: Euclid: A UV-faint quasar in a highly luminous star-forming host galaxy at z ≈ 7.7

Problem and Context
Understanding the co-evolution of supermassive black holes (SMBHs) and their host galaxies during the first billion years after the Big Bang is critical for modeling early cosmic structure formation. Prior to this study, the redshift frontier for known quasars was anchored by only three systems at $z \approx 7.5$. These sources are exceptionally UV-bright ($\langle M_{1450} \rangle \simeq -26.5$) and host SMBHs exceeding $10^9 M_\odot$, challenging theories of rapid black hole assembly. However, this small, biased sample limits the ability to constrain assembly models, particularly regarding the properties and dynamics of molecular gas in the earliest epochs. The Euclid mission has recently expanded this sample, identifying 31 new quasars at $6.6 \le z \le 7.8$, including UV-fainter candidates that may represent a different evolutionary stage or population.

Methodology
This paper presents IRAM/NOrthern Extended Millimeter Array (NOEMA) observations of the quasar EUCL J125308.55+705432.3 (hereafter EUCL J1253+7054), discovered in the Euclid Wide Survey first data release at $z \approx 7.7$. The observations were conducted in March 2025 using the 12B and 12C configurations.

• Data Reduction: Data were calibrated using the GILDAS software suite (clic pipeline). The analysis utilized both the Lower Sideband (LSB; 214.3–222.1 GHz) for the [C ii] line and the Upper Sideband (USB; 229.9–237.6 GHz) for continuum emission.
• Spectral Analysis: A 1D spectrum was extracted from a $1.5''$ aperture. The [C ii] 158 $\mu$m line profile was fitted with a single Gaussian component plus a flat dust continuum. Bayesian Information Criterion (BIC) tests ruled out the necessity of a second Gaussian component (outflow/broad wings).
• Imaging and Continuum: Continuum maps were generated from USB data, and a continuum-subtracted [C ii] moment-0 map was created from LSB data. Both image-plane and visibility-plane (uv-plane) analyses were performed to measure flux densities and source sizes.
• Physical Parameter Derivation:
  • Luminosities: Calculated from integrated line flux and continuum flux using standard cosmological parameters ($H_0=70, \Omega_m=0.3, \Omega_\Lambda=0.7$).
  • Star Formation Rate (SFR): Estimated via two methods: (1) Dust continuum luminosity ($L_{IR}$) using the Kennicutt & Evans (2012) relation, and (2) [C ii] luminosity using calibrations from De Looze et al. (2014) and Herrera-Camus et al. (2015), the latter accounting for the [C ii] deficit in luminous sources.
  • Masses: Dust mass ($M_{dust}$) was derived assuming a modified blackbody (MBB) with $T_{dust}=47$ K and $\beta=1.6$. Molecular gas mass ($M_{gas}$) was estimated using a gas-to-dust ratio of 100. Dynamical mass ($M_{dyn}$) was calculated using two approaches: a dispersion-dominated model and a rotation-dominated model, utilizing the [C ii] line width and source size derived from uv-plane fitting.
  • Black Hole Mass: A lower limit ($M_{BH, min}$) was estimated assuming Eddington-limited accretion based on the bolometric luminosity derived from the rest-frame UV magnitude ($M_{1450}$).

Key Results

• Redshift and Line Properties: The [C ii] emission line sets the systemic redshift at $z = 7.6980 \pm 0.0004$. The integrated line flux is $1.51^{+0.19}_{-0.17}$ Jy km s$^{-1}$, corresponding to a luminosity $L_{[C II]} = 2.03^{+0.26}_{-0.23} \times 10^9 L_\odot$.
• Unprecedented Brightness: Despite being nearly two magnitudes fainter in the rest-frame UV ($M_{1450} = -24.06$) than the previously known $z \approx 7.5$ quasars, EUCL J1253+7054 exhibits the brightest [C ii] emission among all known quasars at $z > 7.5$.
• Host Galaxy Properties:
  • Star Formation: The host is a vigorously star-forming galaxy with an IR-based SFR of $715 \pm 53 M_\odot$ yr$^{-1}$. The [C ii]-based SFR (corrected for the deficit) is $644^{+86}_{-76} M_\odot$ yr$^{-1}$, consistent with the IR estimate.
  • Dust and Gas: The dust mass is $1.4 \times 10^8 M_\odot$, and the molecular gas mass is estimated at $0.41 - 1.24 \times 10^{10} M_\odot$.
  • Dynamical Mass: The dynamical mass is constrained to $0.33 - 1.3 \times 10^{10} M_\odot$, depending on the kinematic model (dispersion vs. rotation). This is lower than the typical $>10^{11} M_\odot$ found in UV-bright $z \sim 7.5$ quasars, though the authors note that extended emission could increase this value.
  • [C ii] Deficit: The ratio $L_{[C II]}/L_{TIR} = 4.2 \times 10^{-4}$ indicates a significant [C ii] deficit, consistent with compact, high-surface-density star formation in IR-luminous systems.
• Black Hole to Host Ratio: Assuming a lower limit for the black hole mass of $\sim 10^8 M_\odot$ (based on Eddington limits), the $M_{BH}/M_{dyn}$ ratio falls in the range $0.007 - 0.03$. This is comparable to local scaling relations and significantly lower than the ratios observed in UV-bright high-redshift quasars.

Significance and Claims
The paper claims that EUCL J1253+7054 represents a crucial link between the most luminous quasars and the fainter broad-line AGNs detected by JWST. The primary significance of the findings is the demonstration that UV-faintness in high-redshift quasars does not necessarily imply a low-mass host galaxy.

Instead, the authors propose that the faint UV luminosity likely results from dust obscuration or sub-Eddington accretion, while the host galaxy remains massive and actively star-forming. This suggests that UV-faint quasars at $z > 6$ may reside in a different evolutionary phase compared to their UV-bright counterparts, potentially marking a transition toward the local $M_{BH}$–$M_{dyn}$ relation. The study highlights the power of combining Euclid's wide-area discovery capabilities with submillimeter follow-up (NOEMA/ALMA) to characterize early SMBH hosts across a broader range of luminosities than previously accessible, moving beyond the bias toward only the most extreme UV-bright systems.