HI content of massive red spiral galaxies observed by FAST

It has been well-established that galaxies roughly fall into two groups in the color-mass diagram: one group is dominated by red quiescent galaxies and the other mainly consists of blue star-forming galaxies (Baldry et al., 2004; Baldry et al., 2006). Morphological studies revealed that red sequence galaxies are mostly early-type galaxies and blue galaxies are late-type ones (e.g. Schawinski et al., 2014). Theoretical studies proposed several mechanisms that can cease star formation in massive galaxies (Faber et al., 2007; Fang et al., 2013; Guo et al., 2016b, a; Xu et al., 2020), during which morphological transformation from disk-dominated systems to bulge-dominated systems usually accompanies. However, massive red passive spiral galaxies were found in both clusters and field environments, in the local Universe and at redshift as high as $\sim 1$ (Bundy et al., 2010; Xu et al., 2021), indicating that these spiral galaxies can quench their star formation without experiencing morphological transformation. It is therefore important to investigate more the various properties of massive red spiral galaxies, to understand their formation and quenching in detail.

Many works have been done to study the optical properties of passive spiral galaxies (e.g. Masters et al., 2010; Robaina et al., 2012; Fraser-McKelvie et al., 2018). Recent studies of Guo et al. (2020) and Hao et al. (2019) focused on a sample of 279 massive red spiral galaxies optically selected from SDSS DR7, and investigated a subset of these galaxies with MaNGA observations. They found that both the disk and bulge components of massive red spirals formed earlier and in shorter timescales than those of massive blue spirals. Based on the same set of MaNGA data, Zhou et al. (2021) reached a similar conclusion via a Bayesian analysis on the star formation histories. However, the quenching mechanisms for the massive red spirals are still unclear.

Gas content is the key component to understand galaxy formation and quenching as it is the requisite fuel for star formation. The quenching of massive red spirals is inevitably related to the insufficient supply or collapse of cold gas, regardless of the details of various mechanisms (e.g., AGN feedback, halo gas shock heating, morphological quenching) that stop star formation (Tacconi et al., 2020). Although molecular gas is considered to be more directly linked to star formation (e.g. Bigiel et al., 2008), atomic gas is a probe of gas reservoir in general and can serve as the first step to understand the quenching processes that affect gas content. Guo et al. (2020) studied the HI content in red spirals by cross-matching with the ALFALFA catalog. The cross-identification with ALFALFA led to 166 galaxies, among which 74 have HI detections.

To obtain a complete census of the HI content of the massive red spiral galaxy sample of Guo et al. (2020), we observe the remaining 113 red spirals that are not covered by ALFALFA using FAST. This almost doubles the sample size of massive red spirals with HI observations and hence will enable us to have a better statistics of the HI content of such systems. In addition, the HI observations obtained by FAST can be used to compare FAST with Arecibo in their capabilities in detecting atomic gas of galaxies.

Guo et al. (2020) found that $\sim 45\%(74/166)$ of the red spirals within the ALFALFA coverage have HI detections, similar to the blue spirals. While these galaxies have overall red color, it is interesting to find out whether the atomic gas is related to any residual star formation and where the HI content reside in these galaxies. Unfortunately single dish observations obtained by FAST and Arecibo only provide total amount of HI fluxes in galaxies, and spatially resolved HI observations that can locate in detail the origin of atomic gas are very expensive and have been applied only to small samples (e.g. Lemonias et al., 2014). Optical blue color normally indicates the existence of young stellar populations and possibly recent star formation activities. Thus a blue component may also indicate the existence of HI content. In this work, we examine galaxy color profile in the massive red spirals, to get a clue of the location of HI content in these galaxies indirectly.

The paper is organised as follows. In section 2, we first introduce briefly the sample of massive red spiral galaxies we use, then describe in detail how we analyse the spectra observed by FAST to get HI content of galaxies. In section 3, we first show FAST observation of 6 test galaxies that have been observed by ALFALFA, and compare the detailed spectra and the derived HI masses in the two observations. Then we present results of HI detection for 113 massive red spiral galaxies observed by FAST, and compare the statistics with the galaxies covered by ALFALFA. In section 4, we study the color profiles of the massive red spirals, and show that galaxies with larger HI mass have in general bluer outer disks, where the HI gas should reside in. Conclusions and discussions are presented in the final section.

Before presenting results of our sample galaxies observed by FAST, we first show results for the 6 test galaxies that have been observed both in ALFALFA and by FAST in this work. We compare the spectrum and HI mass obtained in the two observations, to have an idea of the difference in between. After that, we show results of the HI detection of our 113 massive red spirals, and compare with the ones in the coverage of ALFALFA.

The massive red spiral galaxies of Guo et al. (2020) used in this work are selected from the red sequence galaxies in the dust-corrected u-r color – stellar mass diagram, with red color being the total color of the galaxies. While both the results from Guo et al. (2020) and from this work show that many of the red spirals contain prominent amount of HI gas, we investigate further where the HI mass comes from in these overall red galaxies.

We examined the images of these massive red spirals from the DESI Legacy Imaging Surveys (Dey et al., 2019), including Beijing-Arizona Sky Survey (Zou et al., 2017, 2019), Mayall z-band Sky Survey (Silva et al., 2016), and DECam Legacy Survey (Blum et al., 2016). These surveys provide optical imaging of g, r, and z bands over the sky area of about 14000 deg². The average depths are about 2–3 mag deeper than the SDSS. From the DESI imaging data, we found that many galaxies have yellow and/or blue disks, although the bulge/central components of the galaxies are red without exception. A first look at the images of the galaxies with and without HI detection indicated that many of the galaxies with HI detection seem to have a yellow or blue disk, while the galaxies without HI detection tend to have orange/red disks. We therefore study in detail the color profiles of the massive red spirals, to see if there indeed exists a correlation between the HI mass and the color of the disk for these galaxies.

Cutouts of the massive red spirals are obtained from Data Release 8 of the DESI Legacy Imaging Surveys²²2https://www.legacysurvey.org/dr8/, which include images of $g,r,z$ bands for each galaxy. Sigma-clipping is applied to both $g$ and $r$ band images to estimate the background and background noise. After subtracting the median background level from each pixel, galaxy images are analysed for pixels with flux above 3 times the background noise. Elliptical isophotes are fitted to the galaxy image of $r$ band, with python package $photutils.isophote$, using an iterative method as described by Jedrzejewski (1987). Then the $g$ band image is sampled at the same geometry of elliptical isophotes derived from the $r$ band image, and color $g-r$ at each isophote is calculated as the difference of magnitudes in the two bands. When fitting elliptical isophotes for each galaxy, position angle and ellipticity of the isophotes are not required to be fixed, to represent better the properties of the outer disks. We have tested that with fixed position angle and ellipticity, the resulting statistics regarding color profiles shown in the following remain qualitatively similar.

Fig. 8 gives images and color profiles of two example galaxies in the massive red spiral sample. The left panels show their $g,r,z$-bands image cutouts from the DESI Legacy Imaging Surveys, and the middle panels show their $g-r$ color map. The example galaxy shown in the upper panels has a red central component but an obvious blue outer disk component, and its HI mass detected by FAST is $10^{10.03}M_{\odot}$. The galaxy shown in the lower panels has red components from center to edge, and has no HI detection by FAST. In the right panels, color profiles of the galaxies are shown, with $g-r$ color at different elliptical isophotes as a function of the semimajor axis length of the isophotes normalized to r₅₀ of the galaxy. r₅₀ is the elliptical Petrosian half-light radius from the NASA-Sloan Atlas³³3https://data.sdss.org/datamodel/files/ATLAS_DATA/ATLAS_MAJOR_VERSION/nsa.html (Blanton et al., 2011). Both galaxies have declining color profile from center to edge. The galaxy with HI detection in the upper panels has $g-r$ color less than 0.8 at radius greater than its r₅₀, and becomes less than 0.6 at radius greater than 2 r₅₀. On the other hand, the galaxy without HI detection in the lower panels is always red in the color profile with $g-r$ color greater than 0.8.

We analyse color profiles for all galaxies in the sample of massive red spirals. One galaxy is not in the region of DESI Legacy Imaging Surveys, and 5 galaxies have no r₅₀ provided in the NASA-Sloan Atlas catalog (Blanton et al., 2011), which leave us 273 massive red spirals in total. Fig. 9 shows the results of color profiles of these galaxies. In the left two panels, the median color profiles for galaxies with and without HI detection are plotted in different radius ranges. Solid red (blue) line is for ALFALFA (FAST) galaxies without HI detection (with S/N $<4.7$). Dotted and dashed red lines give median color profiles for galaxies with 50 per cent lower and higher HI mass in the subsample covered by ALFALFA, divided by their median HI mass of $10^{10.09}M_{\odot}$. For galaxies with HI detection observed by FAST, we also divide them into two subsamples with HI mass higher and lower than $10^{10.09}M_{\odot}$, to be compared directly with the subsamples of the ALFALFA galaxies. Within radius of around 0.5 r₅₀, the left panel shows that the galaxies observed by FAST are on average a bit redder than the galaxies in ALFALFA. This is consistent with the overall redder color of the FAST galaxies than the galaxies in ALFALFA as seen in Fig. 1. Nevertheless, there is no obvious difference between subsamples with and without HI detection in both observations. When looking at the color profile at larger radius as shown in the middle panel, deviations start to show up. Galaxies without HI detection are on average redder, especially at radius greater than 1.5 r₅₀. For galaxies with HI detection, the ones with larger HI mass appear to be bluer than the ones with smaller HI mass, for galaxies both observed by FAST and in ALFALFA.

In the right panel of Fig. 9, we show the trend seen in the left and middle panels in another way, by plotting the relation between galaxy color of outer disk (represented by the color at 2 r₅₀) and galaxy HI mass detected in the two observations. Galaxies with HI detection (with S/N $>4.7$) in the ALFALFA and FAST observations are plotted by red and blue circles respectively. For these galaxies in both observations, there exits a clear correlation that galaxies with larger HI mass are bluer in color at its outer radius. The HI mass detected should therefore mainly come from the outer blue disks, although in total the galaxies appear red due to the dominant red central component. Note that in the right panel of Fig. 9, the most right blue point represents a galaxy observed by FAST, which has relatively large HI mass of $10^{10.15}M_{\odot}$, but red color of 0.94 at 2 r₅₀. We have checked that the disk is blue at even larger radius, and therefore is not conflict with the picture described above.

For galaxies without HI detection (with S/N $<4.7$) in the FAST observation, we calculate their HI mass upper limit, and plot them using light blue arrows in the right panel of Fig. 9, where the tail ends of the arrows correspond to their HI mass upper limits in the y-axis. For these galaxies, we use equation (2) to derive the S₂₁ upper limit by setting S/N=4.7. W₅₀ is adopted to be the median value for galaxies with HI detection. $\sigma_{\rm rms}$ is calculated on the two continuum regions around the potential HI signal of each galaxy as described in section 2.2. Then the HI mass upper limit is calculated according to equation (3). Their median g-r color at 2 r₅₀ is 0.75 , with 16 and 84 percentiles to be 0.69 and 0.86. For galaxies covered by ALFALFA and have no HI detection, the median g-r color at 2 times of r₅₀ is 0.75, and the 16 and 84 percentiles are 0.68 and 0.82. On average, these galaxies are red at outer radius, with old stellar populations and do not have enough cold gas to be detected.

In this work we use the FAST radio telescope to observe HI content of 113 optically selected massive red spiral galaxies. 75 of the galaxies have HI detection with S/N greater than 4.7. Compared with the red spirals in the same sample selected by Guo et al. (2020) but have been observed in the ALFALFA survey, the observation by FAST in this work has lower rms noise and higher S/N. When applying the same limit of S/N, the detection rate of observation in FAST is higher, with more galaxies of low HI mass detected. For galaxies observed either in ALFALFA or by FAST, the large fraction of galaxies with HI detection is consistent with previous studies that many optically color/SFR selected red/quiescent spirals can contain large amount of atomic gas (Zhang et al., 2019; Guo et al., 2020).

We also observe 6 massive red spirals that have been observed by ALFALFA, and compare in detail the spectra and HI mass derived in the two observations. For galaxies with spectra of S/N higher than 6.5 in ALFALFA, which can be considered as fairly safe detections (Saintonge, 2007), the flux of the HI lines is consistent in the two observations for spectra not affected by RFI. The HI mass from the two observations are consistent, not affected by the different integration time adopted in the two observations. For galaxies with spectra of lower signal-to-noise ratio, observation by FAST provides lower HI mass than that of ALFALFA, mainly due to the much larger line widths identified in ALFALFA.

To study why many of the massive red spirals contain significant amount of atomic gas, we further analyse their optical images and study their color profiles in detail. We find that the massive red spirals selected according to the galaxy total color are not always all red from center to edge. Many of the red spirals have obvious blue disks in the outer region. There exists a clear correlation between HI mass and color of the outer disk of these galaxies, where galaxies with higher HI mass have in general bluer outer disks. These results suggest the failure of the global optical color to identify truly passive galaxies, and a close association between the cold gas content and the outer disc of galaxies. Both points have been found in previous works, as discussed below.

A decade ago, Cortese (2012) pointed out that at high stellar masses, optical colours are not sufficient to distinguish between star forming and quiescent galaxies. They found that optically selected red spirals lie on the UV-optical blue sequence, and are actually star-forming systems. The study of Cortese et al. (2020) further demonstrated that claims of the existence of HI-rich, passive discs largely result from the failure to measure their outer star formation activities, and passive discs typically have more than 0.5 dex less HI than their active counterparts. In a recent work, Zhou et al. (2021) studied the massive red spiral sample of Guo et al. (2020) matched with MaNGA, and found that the optically selected red spirals are mostly green/blue in NUV-r, indicating on-going star formation in these galaxies. They found that the optically selected red spirals have on average young stellar populations in the outer region, consistent with our finding that many red spirals have blue outer disk and therefore high gas fraction.

Galaxies are multi-component systems. Previous studies have suggested that cold gas is more tightly related/determined by single component of a galaxy, and independent of other components. For example, Cook et al. (2019) found little dependence of the presence of a photometric bulge on the global HI gas content of local galaxies. Namiki et al. (2021) showed that at fixed stellar mass and SFR, the presence of small-scale structures in galaxy images is linked to the total HI gas content of galaxies. Wang et al. (2011) revealed that galaxies with higher HI fractions have bluer outer discs with more star-formation. Although we have not applied decomposition to the sample galaxies in this work or studied the relation between color of different galactic components and cold gas mass, our results also indicate that cold gas content is more related to the spatially resolved properties such as properties of disk component than the global properties of galaxies.

A missing point in this work is the identification of a direct connection between HI mass and star formation in the blue discs. Such a connection has been found at the outskirts of early-type galaxies (Yıldız et al., 2017). Furthermore, Thilker et al. (2007) and Lemonias et al. (2011) have shown that a significant fraction ($\sim 4\%-30\%$) of both late and early type galaxies could have an extended UV (XUV) disk, which is an indication of recent star formation. Galaxies with an XUV-disk are nearly twice as gas-rich as their non-XUV-disk counterparts with a similar morphological type(Thilker et al., 2007). These findings imply that a centrally quenched galaxy, given a large neutral gas reservoir, could also have recent star formation activities in the outskirt and thus a bluer outer disk. A large fraction of red spiral galaxies with HI detection in our sample could also be XUV-disk galaxies. We would defer a further study using UV photometry in a future paper.

We thank the reviewer for useful comments and constructive suggestions that helped improve the manuscript. We thank Cheng Li, Yougang Wang, Jing Wang and Marko Krco for helpful discussions and comments. This work was supported by the National Key Program for Science and Technology Research Development of China 2018YFE0202902, the National Natural Science Foundation of China (NSFC) under grants 11988101, 11733002, and K.C.Wong Education Foundation. ZZ is supported by NSFC grants No. U1931110 and 12041302, and CAS Interdisciplinary Innovation Team (JCTD-2019-05). RL acknowledges the support of NSFC (Nos 11773032,12022306), and the science research grants from the China Manned Space Project (Nos CMS-CSST-2021-B01,CMS-CSST-2021-A01). H. Zou acknowledges the support of the NSFC (grant No. 12120101003) and the supports of the China Manned Space Project (grant Nos. CMS-CSST-2021-A02 and CMS-CSST-2021-A04).

Five-hundred-meter Aperture Spherical radio Telescope (FAST) is a National Major Scientific Project built by the Chinese Academy of Sciences. Funding for the project has been provided by the National Development and Reform Commission. FAST is operated and managed by the National Astronomical Observatories, Chinese Academy of Sciences.

The Legacy Surveys consist of three individual and complementary projects: the Dark Energy Camera Legacy Survey (DECaLS; Proposal ID #2014B-0404; PIs: David Schlegel and Arjun Dey), the Beijing-Arizona Sky Survey (BASS; NOAO Prop. ID #2015A-0801; PIs: Zhou Xu and Xiaohui Fan), and the Mayall z-band Legacy Survey (MzLS; Prop. ID #2016A-0453; PI: Arjun Dey). DECaLS, BASS and MzLS together include data obtained, respectively, at the Blanco telescope, Cerro Tololo Inter-American Observatory, NSF’s NOIRLab; the Bok telescope, Steward Observatory, University of Arizona; and the Mayall telescope, Kitt Peak National Observatory, NOIRLab. The Legacy Surveys project is honored to be permitted to conduct astronomical research on Iolkam Du’ag (Kitt Peak), a mountain with particular significance to the Tohono O’odham Nation.

Table 1 that provides the results of the 113 massive red spirals observed by FAST is available online. The data from ALFALFA survey is available in the paper of Haynes et al. (2018). The images of galaxies are available from the DESI Legacy Imaging Surveys at: https://www.legacysurvey.org/dr8/. Other data produced in this article will be shared on reasonable request to the corresponding author.