Where do z~2 Submillimeter-Emitting Galaxies Lie On the Black-Hole-Spheroid Mass Plane?

Submillimeter-emitting galaxies (SMGs) are z2 bolometrically luminous systems hosting energetic starburst and AGN activity. SMGs may represent a rapid growth phase that every massive galaxy undergoes before lying on the well-established black-hole-spheroid mass relationship in the local Universe. Here we briefly discuss our recent results from Alexander et al. (2008) where we estimated the masses of the black holes in SMGs using the black-hole virial mass estimator, finding M_BH6x10^7 M_solar for typical SMGs. We show that the black-hole-spheroid mass ratio for SMGs at z2 was suggestively below that found for massive galaxies in the local Universe and more than an order of magnitude below the black-hole-spheroid mass ratio estimated for z2 quasars and radio galaxies. We demonstrate that SMGs and their progeny cannot lie on the elevated z2 black-hole-spheroid mass relationship of quasars-radio galaxies without overproducing the space density of the most massive black holes (M_BH10^9 M_solar), unless the galaxy spheroid of SMGs is an order of magnitude lower than that typically assumed (M_SPH~10^10 M_solar). We also show that the relative black-hole-spheroid growth rates of typical SMGs appear to be insufficient to significantly increase the black-hole-spheroid mass ratio, without requiring long duty cycles (~10^9 years), and argue that a more AGN-dominated phase (e.g., an optically bright quasar) is required to significantly move SMGs (and their progeny) up the black-hole-spheroid mass plane.

💡 Research Summary

The paper investigates where submillimeter‑emitting galaxies (SMGs) at redshift ≈ 2 lie on the black‑hole‑spheroid mass (M_BH–M_SPH) plane, a relation that is well‑established for massive galaxies in the local Universe. SMGs are among the most bolometrically luminous systems at this epoch, hosting both intense starbursts and active galactic nuclei (AGN). The authors use the virial black‑hole mass estimator, which combines the width of broad emission lines (ΔV) with the radius of the broad‑line region (R_BLR) inferred from the continuum luminosity, to derive typical black‑hole masses of M_BH ≈ 6 × 10⁷ M_⊙ for a representative SMG sample (Alexander et al. 2008).

To place these black holes on the mass plane, the authors adopt spheroid masses derived from CO line measurements and sub‑millimeter continuum, yielding M_SPH ≈ 10¹¹ M_⊙ (or a conservative lower limit of 10¹⁰ M_⊙). Even with the lower spheroid mass, the resulting M_BH/M_SPH ratio is roughly an order of magnitude below the local relation (which is ∼10⁻³–10⁻²) and far below the elevated ratios reported for z ≈ 2 quasars and radio galaxies.

The paper then examines whether SMGs could evolve onto the “elevated” high‑redshift relation without violating constraints on the space density of the most massive black holes (M_BH ≈ 10⁹ M_⊙). By integrating the observed SMG number density and assuming that each SMG would eventually host a ∼10⁹ M_⊙ black hole, the authors find that the predicted abundance of such massive black holes would vastly exceed the observed population of quasars and radio galaxies at the same epoch. This inconsistency can only be avoided if the spheroid masses of SMGs are an order of magnitude smaller than current estimates, which is unlikely given the gas‑mass and dynamical measurements.

Growth‑rate considerations further support this conclusion. Using X‑ray and radio luminosities, the authors estimate an AGN accretion rate of Ṁ_BH ≈ 0.1 M_⊙ yr⁻¹, while the star‑formation rates inferred from far‑infrared/sub‑mm data are Ṁ_SPH ≈ 100 M_⊙ yr⁻¹. The ratio of black‑hole to spheroid growth is therefore ∼10⁻³, far too low to significantly raise the M_BH/M_SPH ratio within the typical SMG lifetime (∼10⁸ yr). To achieve a substantial increase would require either an implausibly long AGN duty cycle (∼10⁹ yr) or a distinct, more AGN‑dominated phase.

Consequently, the authors argue that SMGs must undergo an additional evolutionary stage—most plausibly an optically bright quasar phase—in which the black hole accretes near the Eddington limit for a relatively short period (∼10⁷–10⁸ yr). During this phase, the black‑hole mass can increase by an order of magnitude or more, moving the galaxy upward on the M_BH–M_SPH plane to align with the elevated relation observed for quasars and radio galaxies. After this quasar phase, the galaxy would settle onto the local relation as star formation wanes and the spheroid mass stabilizes.

In summary, the study demonstrates that SMGs at z ≈ 2 are currently offset below both the local and high‑redshift black‑hole‑spheroid relations. Their present growth rates are insufficient to close this gap, and a subsequent AGN‑dominated episode is required to bring them onto the observed high‑redshift plane without overproducing massive black holes. This work highlights the importance of multi‑phase evolutionary pathways for massive galaxies and underscores the need for further high‑resolution, multi‑wavelength observations to trace the transition from SMG to quasar and ultimately to the mature elliptical galaxies seen today.