Star Formation in Outer Rings of S0 galaxies. V. UGC 4599 -- an S0 with gas probably accreted from a filament

Though S0 galaxies are usually thought to be `red and dead', they often demonstrate weak star formation organised in ring structures and located in their outer disks. We try to clarify the nature of this phenomenon and its difference from star formation in spiral galaxies. The moderate-luminosity nearby S0 galaxy, UGC 4599, is studied here. By applying long-slit spectroscopy at the Russian 6m telescope, we have measured stellar kinematics for the main body of the galaxy and strong emission-line flux ratios in the ring. After inspecting the gas excitation in the ring using line ratio diagrams and having shown that it is ionized by young stars, we have determined the gas oxygen abundance by using conventional strong-line calibration methods. We have inspected the gas kinematics in the ring with Fabry-Perot interferometer data obtained at the William Herschel Telescope. The pattern and properties of the brightest star formation regions are studied with the tunable filter MaNGaL at the 2.5m telescope of the Caucasian Mountain Observatory of the Sternberg Astronomical Institute (CMO SAI MSU). The gas metallicity in the ring is certainly subsolar, [O/H]$=-0.4 \pm 0.1$~dex, that is different from the majority of the outer starforming rings in S0s studied by us which have typically nearly solar metallicity. The total stellar component of the galaxy which is old in the center is less massive than its extended gaseous disk. We conclude that probably the ring and the outer disk of UGC~4599 are a result of gas accretion from a cosmological filament.


Introduction
The morphological type S0 was initially introduced as starformation free disk galaxies (Hubble 1936), while outer rings were early recognised as common attributes of S0 galaxies (de Vaucouleurs 1959). Later a significant amount of cold gas has been found in many S0 galaxies, and in half of gas-rich S0s the gas feeds star formation organised in ring structures (Pogge & Eskridge 1993). Moreover more than the half of outer stellar rings in S0s are bright in ultraviolet (UV) so betraying recent star formation on a timescale of a few hundred Myr (Kostiuk & Sil'chenko 2015). In the frame of the current paradigm according to which evolution of disk galaxies is driven by persistent accretion of outer cold gas, this situation with S0s is quite understandable because in sparse environments the S0s may suffer the same outer gas accretion as spirals, with a possibility of star formation in accreted gas. However, the source of outer gas accretion remains still unknown. Cosmologists are sure that this source is provided by large-scale Universe structure, in particular, by large-scale filaments filled by dark matter and primordial gas (Ocvirk et al. 2008, e.g.). The observations reveal nearly solar metallicity in the outer starforming rings of S0 galaxies favouring rather gas-rich satellite merging (Sil'chenko et al. 2019;. In this Letter we examine an outer starforming ring in UGC 4599, a moderate-luminosity nearby S0 galaxy, which has indeed rather low metallicity and can therefore be fed by primordial gas from cosmological filaments. The galaxy image taken from the LegacySurvey resource (Dey et al. 2019) including the data from the DECaLS photometric survey is presented in Fig. 1. It is seen almost face-on, and with the adopted distance of 32 Mpc (cosmology-corrected luminosity distance from the NASA Extragalactic Database, NED) the radius of the ring is 7.7 kpc. Finkelman & Brosch (2011) claimed that UGC 4599 is an analog of the famous Hoag galaxy, so being a small elliptical surrounded by a detached ring. However there were also other opinions: Dowell (2010) treated UGC 4599 as a classical S0, with the de-Vaucouleurs' bulge contributing only 32% to the total luminosity, and a low-surfacebrightness (LSB) exponential disk. Gutiérrez et al. (2011) found even more disks: they classified UGC 4599 as a Type-III-d so discovering two LSB disks, with the exponential scalelengths of 6.5 kpc and 10 kpc. Indeed, the deep image in Fig. 1 allows us to trace the UGC 4599 blue disk extending well beyond the ring radius. The galaxy is extremely rich in HI: the data of the AL-FALFA survey reveal about 10 10 M ⊙ of neutral hydrogen, with the diameter of the HI disk of ∼ 100 kpc (Grossi et al. 2009). We have undertaken further investigation of the galaxy by applying long-slit spectroscopy, Fabry-Perot interferometry, and narrowband imaging in the strong emission lines. Fig. 1. The Legacy Imaging Surveys colour image of the UGC 4599 and its environments. The HI density contours are overlaid following Grossi et al. (2009). The north is up, the east is to the left.

Observations and the data involved
Our long-slit spectral observations were made with a multi-mode reducer SCORPIO-2 (Afanasiev & Moiseev 2011) at the prime focus of the Russian 6m telescope of the Special Astrophysical Observatory, Russian Academy of Sciences (SAO RAS). UGC 4599 was observed on February 25th, 2014, at PA(slit) = 115 • , with the total exposure time of 75 min, on March 26th, 2015, in the orientation through the neighbouring galaxy (designated by A in Fig. 2, the third plot) , at PA(slit) = 132 • , with the total exposure time of 60 min, and with a short exposure at PA(slit) = 49 • in February 2022. The slit orientations are shown in Fig. 2. During the observations the range of airmass was 1.2-1.5. The seeing was ∼ 1 ′′ , the VPHG1200 grism provided an intermediate spectral resolution FWHM ≈ 5 Å in the wavelength region from 4000 Å to 7200 Å. This spectral range includes a set of strong absorption and emission lines making it suitable to analyse both stellar and gaseous kinematics of the galaxy as well as the gas excitation and chemistry. The slit is 1 ′′ in width and 6 ′ in length allowing us to use the edge spectra to subtract the sky background. The CCD E2V 42-90, with a format of 2048 × 4600, used in the 1 × 2 binning mode provided a spatial scale of 0.357 ′′ per px and a spectral sampling of 0.86 Å/px. The data reduction as well as the derivation of the characteristics of the gaseous kinematics were standard for our SCORPIO-2 data -see for example Proshina et al. (2020). The profiles of the line-of-sight stellar velocity have been calculated by cross-correlation of galactic spectra binned along the slit with the best-matched template spectra of K-type stars from the library MILES (Sánchez-Blázquez et al. 2006).
To study the kinematics in the Hα emission we obtained the data using the Fabry-Perot Interferometer GHαFaS (Hernandez et al. 2008) on the William Herschel Telescope (WHT) at the Roque de los Muchachos Observatory, La Palma. GHαFaS has a circular field of view of 3.4 arcmin, free spectral range of 8 Å which corresponds to 390 km s −1 with a velocity resolution of 8 km s −1 , with spatial sampling of 0.2 ′′ . The observations were undertaken on March 14th, 2016, with the total exposure of 160 s in each of 40 spectral channels. The seeing was 1.2 ′′ .
We also carried out observations at the 2.5m CMO SAI MSU telescope with a narrow-band tuned photometer MaNGaL ). The observations were performed in two redshifted emission lines, [OIII]λ5007 and [NII]λ6583, within the band of 13 Å, to study the surface brightness distributions in these lines characterising the ionised gas in the ring. The detector, CCD iKon-M934 with the format of 1024 × 1024, provided the field of view of 5.4 arcmin and the sampling of 0.33 ′′ per pixel. The observations were undertaken on November 14th, 2020, for [NII]λ6583, with an exposure time of 75 min, and on November 17th, 2020, for [OIII]λ5007, with an exposure time of 60 min. The seeing was 1.3 ′′ on the first date and 1.9 ′′ on the second date.
To study the large-scale structure of the galaxy, we have used the gr-band images from the LegacySurvey (the DECaLS data).

Emission lines in the ring of UGC 4599
The ring of the galaxy at R ∼ 50 ′′ prominent in the optical continuum ( Fig. 1) and in the UV (Fig. 2), is also well traced by the emission-line regions in the [OIII]λ5007 line (Fig. 2, left): we can notice more than a dozen compact emission-line sources in the Fig. 2 (left). Only a few of these regions are also seen in the [NII]λ6583 emission line (Fig. 2), and these detections O. Sil'chenko et al.: Star formation in outer rings of S0 galaxies. V.  2020) searched for compact starforming galaxies by using deep narrow-band photometry in the Hα line and found four HII-regions in the ring of UGC 4599 (their no.103, 104, 105, 106); for these 'HaDots' the full-range spectra were also obtained at the Hobby-Eberly 9.2m telescope (HET). We use their emission-line ratios together with our results to inspect the gas excitation (Fig. 3).
Our long slit at PA(slit) = 115 • crossed the eastern emissionline region, the brightest one in the ring, and our long slit at PA(slit) = 132 • passed through a small galaxy to the north-west from UGC 4599 which is designated by 'A' in Fig. 2. In fact, when we looked at the spectrum we saw a lot of emission-line objects to the north-west from the centre of UGC 4599 (Fig.3). Two of them belong to the ring of the galaxy which is split into two arms to the west; and three of them are background galaxies with redshifts of 0.08 (A), 0.360 (B), and 0.324 (C).
We use the strong emission-line flux ratios to check the gas excitation by inspecting the so called Baldwin-Phillips-Terlevich(BPT)-diagram (Baldwin et al. 1981) in Fig. 3. All the HII-regions with full-range spectra -four in the eastern part of the ring measured by Salzer et al. (2020) and our data for the brightest eastern region, and two our cross-sections of the ring to the north-west from the centre -indeed show HIItype excitation, as they are located to the left of the dividing lines prescribed by Kewley et al. (2001) and Kauffmann et al. (2003). Then we can determine the ionised-gas oxygen abundance by using the strong-line calibrations. We have involved the widely used calibrations from Perez-Montero (2014) and Marino et al. (2013). By averaging the oxygen abundances derived from our measurements of N2 and O3N2 for three more bright HII-regions, we have obtained 12 + log (O/H) = 8.23 ± 0.05 dex from the Marino et al. (2013)

The central stellar spin and the gas kinematics in the ring of UGC 4599
For the main optical emission line of starforming regions, Hα, we have observed UGC 4599 with the scanning Fabry-Perot in-A&A proofs: manuscript no. text terferometre at the 4.2m WHT to derive not only the Hα map, but also the line-of-sight (LOS) velocity map reflecting the projection of the ionised-gas rotation in the ring. The results are presented in Fig. 2, right. Under the assumption of planar circular gas rotation we can restrict the gaseous disk orientation by using the two-dimensional LOS velocity distribution: at a fixed radius the projection of the tangential rotation velocity would be maximal at the line of nodes of the gaseous disk and would be zero in the orthogonal direction. Using the tilted-ring model of circular rotation as described in Sil'chenko et al. (2019) we obtained the orientation of the line of nodes of the gaseous disk at the radius of the ring: PA 0 = 271 • ± 9 • . As for the inclination, we can estimate it by plotting UGC 4599, with its HI line width W 50 = 148 km s −1 (Grossi et al. 2009) and its baryonic mass, stellar plus gaseous, log M b = 10.18 (Huang et al. 2014), onto the baryonic Tully-Fisher relation from e.g. Lelli et al. (2019), obtaining i g ≈ 32 • . The orientation of the stellar rotation plane can be estimated from the long-slit data. The photometric inclination of the stellar disk was given by Gutiérrez et al. (2011) as i * = 24 • , under the assumption for the disk relative thickness of q 0 = 0.2. Under the more realistic assumption of the relative typical thickness q 0 = 0.4 found by us for lenticular galaxies in sparse environments , this estimate transforms into i * = 26 • . The model line-of-sight velocity profiles calculated with this assumed inclination for three different stellar-disk line-of-nodes orientations are superposed onto our long-slit measurements in Fig. 4. It can be seen that the lineof-nodes position angle of the gaseous disk, PA 0 = 271 • ± 9 • , can be excluded for the stellar disk due to zero velocity gradient along the PA = 49 • . By fitting all three long-slit cross-sections (Fig. 4), the best estimate of the line-of-nodes position angle for the central stellar disk would be PA 0 = 324 • ± 20 • . Evidently, the plane of the gaseous disk is inclined with respect to the central stellar disk. This configuration is dynamically unstable and gives evidence for recent gas accretion. We have inspected the rather deep new gr-images of UGC 4599 provided by the DECaLS survey. We present our results of isophote analysis and the azimuthally averaged surface brightness profile of UGC 4599 in Fig. 5. Indeed, we do not see any gap between the central body (including perhaps the exponential pseudobulge) and the ring: the ring in the radius range of 45 ′′ -65 ′′ is superposed onto the extended exponential disk which can be fitted over R = 35 ′′ − 100 ′′ range by a model profile µ r = 23.6 + 1.086R/49.1 ′′ . With respect to the measurements by Gutiérrez et al. (2011), we have obtained slightly larger exponential scalelength for the inner portion of the UGC 4599 disk because we have excluded the ring from our fitting. In any case, with its scalelength of 7.5 kpc and its central surface brightness of µ 0 (r) = 23.6 the galaxy can be classified as an LSB disk galaxy. Moreover, when we estimate the integrated characteristics of the disk in Fig. 5 (right), M V = −19.8 and r e f f = 12.6 kpc, and compare them with the data in Fig. 12 by Greco et al. (2018) and with the data in Fig. 1 by Saburova et al. (2021), we ascertain that UGC 4599 belongs to the class of giant LSB disk galaxies and resembles such objects considered by Saburova et al. (2021) as UGC 1378 or UGC 1382.

The origin of the gaseous disk in UGC 4599
As was found by Saburova et al. (2021), the most frequent scenario of giant LSB galaxy formation is accretion of highmomentum gas from outside. The second most favoured scenario, coplanar merging of two large spiral galaxies (that was also suggested by Finkelman & Brosch (2011)), can be excluded for UGC 4599 by our data because the low oxygen abundance of the ionised gas in the starforming ring contradicts the typically solar abundance of gas in non-dwarf spiral galaxies (Tremonti et al. 2004;Pilyugin et al. 2004). The outer gas accretion scenario is more suitable also because of the inclined orientation of the gaseous ring spin vector with respect to the collective stellar angular momentum. But what can be a source of the outer gas?
The environment of UGC 4599 is rather sparse though the galaxy has been included into galaxy groups by several catalogues: in USGC 191 by Ramella et al. (2002) and in the compact triplet UZC-CG 79 by Focardi & Kelm (2002). Indeed, the group contains three galaxies of comparable luminosities, UGC 4599, UGC 4590, and UGC 4550. The tightest pair separation, between UGC 4599 and UGC 4590, is 179 kpc, and the third galaxy, UGC 4550, is classified as completely isolated in the 2MIG catalog (Karachentseva et al. 2010). It is important to note that UGC 4590 is devoid of neutral hydrogen though demonstrating a starforming nucleus (Grossi et al. 2009). In the latter paper, the authors proposed a hypothesis of gas flow to UGC 4599 from the nearby dwarf PGC 24666 (CGCG 061-011) because several HI clumps were detected between UGC 4599 and CGCG 061-011. But CGCG 061-011 is a very small galaxy, with a stellar mass of 3 · 10 8 M ⊙ (Alatalo et al. 2016) and HI mass of 4 · 10 8 M ⊙ (Haynes et al. 2018); it seems improbable that CGCG 061-011 may provide a two order larger mass of neutral hydrogen for UGC 4599. Moreover, the metallicity of the ionised gas may be higher in the dwarf CGCG 061-011 than in UGC 4599. For the latter we have measured log [NII]λ6583/Hα = −0.95 ± 0.01 by averaging our emission-line measurements for five HII-regions (it corresponds to 12 + log (O/H) = 8.23 dex (Marino et al. 2013)). For CGCG 061-011 we have obtained a long-slit spectrum with the SCORPIO-2 on October 31, 2022. After we have excluded the more metal-rich galaxy core, the remaining log [NII]λ6583/Hα profile up to r = 13 ′′ has appeared to be flat, and it reveals the mean nitrogen-to-hydrogen line ratio of −0.809 ± 0.005 corresponding to 12 + log (O/H) = 8.30 ± 0.09 dex (Marino et al. 2013). So the ionised gas in the starforming ring of UGC 4599 is poorer by oxygen than the gas in the dwarf galaxy CGCG 061-011: it cannot be a donor. The completed merger of a small gas-rich satellite is not a good perspective to explain the large HI disk of UGC 4599 too because of its very large mass of accreted neutral hydrogen, 1.1 · 10 10 M ⊙ (Haynes et al. 2018); the total stellar mass of UGC 4599 is less than half of this, M * = 4 · 10 9 M ⊙ (Huang et al. 2014). Therefore we cannot identify a suitable galaxy to play the role of gas donor in the vicinity of UGC 4599.
The metallicity of the ionised gas in the ring of UGC 4599 appears to be unusually low, -0.4 dex. Up to now we have studied a dozen outer starforming rings in lenticular galaxies, and the gas metallicities in these rings are very homogeneous, −0.15 dex independently of the galaxy luminosity or ring radius (Sil'chenko et al. 2019;Proshina et al. 2020). By confronting the high relative mass of HI and the low metallicity of the ionised gas, we reach the conclusion that this is perhaps the first clear case of gas accretion to a ring of S0 galaxy from a cosmological filament. Then the chain of HI clumps in the direction from UGC 4599 to PGC 24666 (Grossi et al. 2009) may trace this filament. A quite similar structure has been observed in 21 cm near the Hoag object which is a recognised case of filamentary gas accretion onto an elliptical galaxy (Brosch et al. 2013).  and the azimuthally averaged surface brightness profile with pseudobulge and inner disk fitted by exponential laws (the right plot). The two fitted exponential relations are: µ r = 20.5 + 1.086R/8.8 ′′ (blue dashed line) and µ r = 23.6 + 1.086R/49.1 ′′ (red dashed line).