Resolved molecular gas and star-formation in massive unquenched spirals : I. UGC 8179

Abstract

Recent studies have uncovered a rare population of super-massive (M* > 1e11 Msun) star-forming spiral galaxies, Super Spiral Galaxies (SSGs), whose existence challenges classical quenching scenarios. We investigate the resolved star-forming and molecular-gas properties of UGC 8179 (z=0.052, log(M*/Msun)=11.62) and assess whether its local star formation (SF) follows the same physical processes as typical Star-Forming Main Sequence (SFMS) spirals. We combined the first NOEMA CO(1-0) interferometric observations of an SSG with pixel-by-pixel SED fitting, based on archival UV-to-mid-IR imaging. Our 3"x3" pixel maps provide resolved measurements of M*, SFR and molecular gas surface densities across its extended disc. UGC 8179 hosts a massive rotating molecular gas reservoir of M_H2 = 1.02 1e10 Msun, yielding a standard molecular gas fraction, with typical depletion time \sim 1 Gyr in the observed region, despite its extreme mass. We derived lower limits of log(fmol) > -1.61 \pm 0.06 and log(tdep) > -8.82 \pm 0.13 at the scale of the galaxy. The large spatial extent of UGC 8179 enables us to probe low surface-density regimes hardly accessible in nearby disks (Σ* < 1e7 Msun / kpc2 ; Σ_SFR < 1e-3.5 Msun/yr/kpc2). All three resolved scaling relations (rSFMS, rKS and rMGMS) are well defined. The rKS slope (0.87 \pm 0.09) is broadly consistent with unity, indicating standard local SF processes. The rSFMS shows a shallower global slope (0.80 \pm 0.02) due to a central suppression in sSFR (~ -0.5 dex). This break suggests the influence of a bulge, driving a transition to a more dynamically regulated SF regime in the inner disc. UGC 8179 provides evidence that SSGs can sustain standard local SF processes while exhibiting central dynamical regulation at high stellar surface densities.

Figures (14)

• Figure 1: CO(1-0) moment maps rom the cleaned data cube. Top: Moment-0 in log$_\text{10}$ (integrated line intensity). Insert: Integrated spectrum at 20 km/s resolution over the region where CO reaches significant SNR. Middle: Moment-1 ($V_\text{los}$, intensity-weighted velocity). Bottom: Moment-2 (line width). The dashed gray circle is NOEMA HPBW at the redshifted CO(1-0) frequency. The black ellipse at the bottom right corner is the synthesized beam ($4.9\arcsec\times2.9\arcsec$).
• Figure 2: Distribution of CO(1-0) maxima from NOEMA data on a background optical image from DESI Legacy Survey (DR9). The contour levels are [4.5, 9, 20, 40, 60] mJy beam$^{-1}$. The dashed gray circle is NOEMA HPBW size at CO(1-0) redshifted frequency, and the purple ellipse with semi-major axis of 4 $R_e$. The cyan hexagon is MaNGA IFU FoV. The white ellipse at the bottom left corner is the synthesized beam of NOEMA ($4.9\arcsec\times2.9\arcsec$).
• Figure 3: Left: log s$\text{SFR}_\text{100}$ with respect to M$_\star$. The green (purple) diamond and square are the integrated fit and the sum of the resolved fits in the PB$_\text{CO}$ (4 $R_e$) region, respectively. The SFMS from Janowiecki20 is included for comparison. The black dash-dotted line shows a constant log s$\text{SFR}_\text{100}=-10.5$ and serves as a guide to the eye. Middle: molecular gas mass fraction with respect to M$_\star$. The purple pentagon represents the large scale lower limit of $f_\text{mol}$. The SFMS is also from Janowiecki20. The green pentagon corresponds to the $f_\text{mol}$ in the PB$_\text{CO}$ region only. Right: molecular gas depletion time with respect to M$_\star$. The grey solid line is the linear fit for SMFS galaxies from SaintongeCatinella22, with a 0.2 dex scatter. The rest of the legend is identical to the middle panel, with triangles instead of pentagons.
• Figure 4: Mapped results of resolved SED fitting run at a 3$\arcsec$ resolution. White contours show the distribution of emission in SDSS $g$-band. The dashed yellow circle represents the PSF at FWHM of $WISE3$. Left: Log surface density of the Bayesian estimate of SFR averaged over the past 100 Myrs. Center: Log surface density of the Bayesian estimate of M$_\star$. Right: Specific $\text{SFR}_\text{100}$.
• Figure 5: Top: Log $f_\text{mol}$, the molecular gas mass fraction. Bottom: Log $\tau_\text{dep}$, the molecular depletion time. The hatched pixels are below a detection threshold of 4 $\sigma$, where $\sigma$ is the noise in the CO(1-0) moment 0 map. The white dashed circle is the HPBW of NOEMA.