Why Are Some Optically Red Spirals NUV-r Blue?

Abstract

To understand the complicated formation processes of disk galaxies, we carry out a comparative study for NUV-r blue and red spiral galaxies drawn from a parent sample of u-r red spirals with $M_{*} > 10^{10.5} M_{\odot}$ at 0.02 < z < 0.07, based on the optical data from the Sloan Digital Sky Survey (SDSS) and the ultraviolet (UV) data from the Galaxy Evolution Explorer (GALEX). The analyses of the images and surface brightness profiles in the NUV and optical bands show that the differences between NUV-r blue and red spirals mainly occur in the outer disks (1-3 $R_{\rm e}$), and the contrast in NUV band is much larger than that in the optical bands. Both the positions on the star formation main sequence diagram and the NUV-r color profiles suggest that NUV-r red spirals have been fully quenched, whereas NUV-r blue spirals host quenched bulges and inner disks, as well as star-forming outer disks. Particularly, the disk mass-size relations indicate that, at a given disk mass, NUV-r blue spirals possess larger optical disks than NUV-r red spirals, by a factor of $\sim 1.20$. The environments and optical morphologies are consistent with the scenario that NUV-r blue spirals obtained fresh fuel for star formation either by interacting or merging with gas-rich galaxies or through accreting surrounding HI gas.

Figures (11)

• Figure 1: Matched aperture NUV-r color vs. u-r color diagram for massive red spirals selected by the u-r colors derived from SDSS model magnitudes. Blue triangles and red stars represent the final NUV-r blue and red sample spirals. The vertical dotted line represents $u-r=2.3$. The horizontal dotted lines represent $NUV-r=4.3$ and $NUV-r=5.3$, respectively.
• Figure 2: SFR vs. stellar mass relation for optically red spirals with blue (blue triangles) and red (red stars) NUV-r colors. The contours show the number density distribution of a parent sample of galaxies with $0.02 < z < 0.07$ in the catalog of Mendel2014. The black solid line represents the main sequence (${\rm log\, SFR} = 0.56 \times {\rm log} (M_*/M_{\odot})\, -\, 5.58$) with $\sim$ 0.3 dex scatter (dashed lines) defined by the parent sample. The error bars in the bottom-right corner are the median measurement errors for NUV-r blue and red spirals, respectively. The top- and right-hand panels show the histograms of the stellar mass and SFR for each sample with dashed lines representing the corresponding median values. NUV-r red spirals are fully quenched, with SFRs 1-3 dex lower than those of MS galaxies, while NUV-r blue spirals still exhibit weak star formation, with a median sSFR of $8.09 \times 10^{-12} {\mathrm{ yr^{-1}}}$.
• Figure 3: True color SDSS images (first row), DESI images (second row), GALEXNUV-band images (third row) and SDSS fiber spectra (fourth row) for example NUV-r red (left two columns) and blue (right two columns) spirals, respectively. The physical size of each image is 80 $\times$ 80 kpc$^2$. The red, green and blue circles on the GALEX images represent 1, 2, and 3 $R_{\rm e}$. NUV-r blue spirals exhibit intense NUV emission in their outer regions.
• Figure 4: GALEXNUV-band (left panel) and SDSS r-band (right panel) SBPs for NUV-r blue (blue lines) and red (red lines) galaxies. Blue and red solid circles are the median SBPs with errors bars denoting the lower (25%) and upper (75%) quartiles for each sample. Blue spirals have brighter SBPs than the red ones in the NUV band, especially outside 1 $R_{\rm e}$. While the r-band SBPs of the NUV-r blue spirals are fainter than the NUV-r red ones, but the difference is comparable to the rms scatters.
• Figure 5: NUV-r color profiles for NUV-r blue (blue lines) and red (red lines) galaxies. Blue and red solid circles are the median color profiles with errors bars denoting the lower (25%) and upper (75%) quartiles for each sample. NUV-r red spirals exhibit a consistent red color from the centers to the outskirts, whereas NUV-r blue spirals are red within $\sim$ 0.8 $R_{\rm e}$, yet transition to the blue region ($NUV-r < 4$) beyond $\sim$ 1.5 $R_{\rm e}$.