SUPERLUMINOUS SPIRAL GALAXIES

The most massive galaxies in the universe are thought to form from the largest density peaks in the primordial matter distribution. Galaxy mergers change the initial galaxy mass function, forming more massive galaxies by combining less massive ones. The result is the galaxy mass distribution we see in the local universe, empirically described by the Schechter (1976) luminosity function, together with a morphology-dependent mass-to-light ratio. The luminosity function also depends upon the star formation history of galaxies, regulated by gas content, gas accretion, stellar feedback, and active galactic nucleus (AGN) feedback. Galaxy mergers play an important role here too, since tidal torques in merging systems force gas into the galaxy centers, leading to starburst activity that grows the stellar bulge and AGN activity that grows the supermassive black hole (Toomre & Toomre 1972; Barnes & Hernquist 1991; Hopkins et al. 2009).

Galaxies segregate into two major classes based on color and morphology (Strateva et al. 2001; Lintott et al. 2008). Blue, star-forming disks (late-type galaxies, LTGs) lie in one region of color-space called the blue-sequence. Red-and-dead spheroids (early-type galaxies, ETGs) lie in a different region of color space called the red sequence. LTGs demonstrate a correlation between star formation rate (SFR) and stellar mass (M_stars) called the star-forming main sequence (SFMS: Brinchmann et al. 2004; Elbaz et al. 2007; Wuyts et al. 2011). The SFMS may be a consequence of an equilibrium between inflowing gas and star-formation driven outflows, with the specific star formation rate (SSFR) regulated by the halo mass growth rate (Lilly et al. 2013). Most ETGs on the other hand have much lower SFRs because they lack the cold gas needed to sustain star formation.

It appears that there is a limit to the mass of star-forming disk galaxies of roughly 3 × 10¹⁰M_⊙, with the most massive disk galaxies transitioning away from the main sequence as their SSFR declines. This decline appears to be a gradual process, occurring over a period longer than 1 Gyr after the gas supply to the galaxy disk has been interrupted (Schawinski et al. 2014). Rapid quenching does not appear to occur for most galaxies that remain disk galaxies, contrary to early attempts to explain the apparently bimodal distribution of galaxy colors.

A number of mechanisms have been suggested to explain why the gas supply is interrupted for the most massive disk galaxies. Major galaxy mergers may disrupt merging disk galaxies and transform them rapidly into elliptical galaxies (Baldry et al. 2004), though this does not explain the transformation of galaxies that remain disks. The accretion of cold gas onto a galaxy may be stopped when the galaxy halo becomes massive enough that accretion shocks develop, interrupting the cold streams of gas needed to replenish the disk (Dekel & Birnboim 2006). Increasing AGN feedback from a growing supermassive black hole may shock or eject gas from the galaxy disk, reducing its capacity to form stars (Hopkins et al. 2006; Ogle et al. 2014). Ram-pressure stripping of the interstellar medium by the intercluster medium of a galaxy cluster can also remove cold gas (Sivanandam et al. 2014).

Studying the most massive spiral galaxies can give us clues as to which of the above evolutionary processes are primarily responsible for converting star-forming disk galaxies into red-and-dead lenticulars or ellipticals. The existence of rapidly star-forming, massive spirals with M_stars > 10¹¹M_⊙ indicates that disk galaxies can postpone this fate under special circumstances. We present here the most optically luminous and biggest spiral galaxies at redshift z < 0.3, found by mining the NASA/IPAC Extragalactic Database (NED). We assume a cosmology with H₀ = 70, Ω_m = 0.3, and Ω_Λ = 0.7 for computing all linear sizes and luminosities.

This project is an offshoot of our work to determine the completeness of NED and explore its potential for systematic studies of galaxy populations (P. Ogle et al. 2016, in preparation). NED provides a unique fusion of multi-wavelength photometry from Galaxy Evolution Explorer (GALEX), Sloan Digital Sky Survey (SDSS), and the 2-Micron All-Sky Survey (2MASS), among others, which we augment by Wide-field Infrared Survey Explorer (WISE) photometry, that allows us to estimate stellar masses and SFRs. We compared the redshift distribution of galaxies in NED at z < 0.3 to a model redshift distribution for the universe derived using a redshift-independent luminosity function, in order to estimate the spectroscopic completeness of NED. We used the Schechter (1976) luminosity function fits of Blanton et al. (2003), which are based on ∼150,000 SDSS galaxies as our benchmark. We adopt their characteristic absolute magnitude value of ${M}^{*}-5{\mathrm{log}}_{10}h\ =-20.44\pm 0.01$ (L* = 5.41 × 10⁴³ erg s⁻¹ at 6200 Å) for the SDSS r-band luminosity function. The redshift limit was made large enough to capture the rarest, most luminous galaxies, but not so large as to require consideration of redshift evolution in the luminosity function.

2.1. SDSS r-band Selection of the Most Optically Luminous Galaxies

2.2. UV Selection Method for Super Spiral Galaxies

We make a further selection for UV emission because we are interested in finding the most massive, actively star-forming disk galaxies. We recently matched and integrated the GALEX All-Sky Survey Catalog (GASC) and GALEX Medium Sky Survey Catalog (GMSC) with NED, using an automated, statistical algorithm (Ogle et al. 2015). We inspected the SDSS images of all 196 galaxies from the OGC that are detected in the GALEX NUV band (the OGC-UV subsample). Of these, we find 46 NUV-detected, L_r > 8L* galaxies with spiral morphology (Table 1). The remaining NUV sources include 118 ellipticals, 11 galaxies with E+A spectra, 2 quasi-stellar objects (QSOs) with extended emission, and 19 galaxies with erroneous redshifts or magnitudes. The most luminous elliptical galaxy in OGC-UV is a 16L* BCG, while the most luminous spiral galaxy has L_r = 14L*.

Table 1.  OGC Super Spiral Sample

SS	OGC	NED Name	Lr(L*)	Da	Mstarsb	SFRc	Redshiftd	NUV	r	$u-r$	Notes
01	0065	2MASX J10301576–0106068	13.9	81.3	11.25	1.54	0.28228	21.02e	16.92	2.54	bar
02	0073	2MASX J10405643–0103584	13.4	82.2	11.39	0.97	0.25024	21.65	16.64	2.16	BCG
03	0139	2MASX J16394598+4609058	12.0	134.	11.05	1.48	0.24713	19.85	16.63	2.37	edge-on
04	0170	2MASX J10100707+3253295	11.6	87.1	11.27	1.40	0.28990	20.14	17.10	2.68	BCG, bar
05	0217	2MASX J13275756+3345291	11.2	68.8	11.05	1.81	0.24892	19.44	16.72	2.02	starburst, bar
06	0256	2MASX J11593546+1257080	10.9	87.2	10.89	1.26	0.26353	20.04	16.95	1.79	⋯
07	0265	SDSS J115052.98+460448.1	10.8	88.1	10.94	<0.74	0.28946	>21.51e	17.19	3.25	faint spiral
08	0290	2MASX J12343099+5156295	10.6	62.4	11.13	1.71	0.29592	19.57	17.25	1.67	Sy1, asymm.
09	0299	2MASX J09094480+2226078	10.5	83.1	11.26	<1.15	0.28539	21.40	17.25	3.73	BCG, shells?
10	0302	2MASX J15430777+1937522	10.5	65.5	11.37	2.45f	0.22941	⋯	17.07	0.40	QSO, tidal arm
11	0306	SDSS J122100.48+482729.1	10.5	75.0	10.82	1.02	0.29966	20.15	17.29	1.69	⋯
12	0345	2MASX J09260805+2405242	10.3	81.2	11.27	1.38	0.22239	19.61	16.57	3.38	BCG, face-on
13	0388	2MASX J17340613+6029190	10.1	63.5	11.20	1.27	0.27596	20.51	17.19	2.71	BGG
14	0441	SDSS J095727.02+083501.7	9.9	87.6	11.53	1.13	0.25652	20.88	16.99	2.19	⋯
15	0454	2MASXi J1003568+382901	9.9	56.4	10.82	1.65	0.25860	19.79	16.97	1.70	starburst
16	0543	2MASX J09470010+2540462	9.6	99.3	11.07	1.13	0.10904	17.74	14.83	2.57	bar, Sy1?
17	0586	2MASX J11535621+4923562	9.5	90.2	11.11	1.43	0.16673	19.92	15.90	2.64	BCG, Sy2
18	0595	2MASX J07550424+1353261	9.5	76.6	11.12	1.30	0.22264	19.71	16.67	2.47	bar
19	0696	SDSS J102154.85+072415.5	9.2	69.7	<11.57	1.35	0.29061	19.89	17.37	2.01	⋯
20	0713	2MASX J08265512+1811476	9.2	81.9	11.27	1.32	0.26545	21.01	17.16	3.26	bar
21	0755	SDSS J113800.88+521303.9	9.1	63.9	10.76	1.14	0.29593	20.94	17.41	2.12	ring
22	0789	2MASX J08542169+0449308	9.0	86.0	10.96	1.30	0.15679	18.68	15.83	2.49	2 spirals, bar
23	0799	2MASX J10472505+2309174	9.0	72.2	11.12	1.20	0.18256	20.61	16.19	2.68	bar
24	0800	2MASX J11191739+1419465	9.0	70.8	10.93	1.15	0.14377	18.75	15.57	2.39	⋯
25	0804	SDSS J135546.07+025455.8	9.0	84.2	<11.35	1.01	0.23884	19.77	16.87	1.74	⋯
26	0830	SDSS J141754.96+270434.4	9.0	68.6	10.70	1.11	0.15753	19.74	15.79	2.86	⋯
27	0926	2MASX J10304263+0418219	8.8	72.7	11.19	1.60	0.16902	19.08	15.93	2.16	⋯
28	0928	2MASX J12374668+4812273	8.8	66.0	11.01	1.57	0.27245	19.79	17.24	2.10	⋯
29	0975	2MASX J11410001+3848078	8.7	72.1	11.08	1.38	0.26770	20.79	17.21	2.15	⋯
30	0983	SDSS J153618.97+452246.8	8.7	80.2	10.48	1.07	0.23618	20.15	16.85	2.13	⋯
31	1046	2MASX J09362208+3906291	8.6	69.6	10.99	1.37	0.28293	20.09	17.36	1.78	⋯
32	1088	SDSS J140138.37+263527.6	8.5	78.2	<11.50	1.24	0.28396	19.99	17.38	2.08	⋯
33	1196	SDSS J154950.91+234444.1	8.4	69.3	<11.35	1.30	0.26208	20.48	17.27	2.02	⋯
34	1250	2MASX J12321515+1021195	8.3	71.4	10.95	1.06	0.16588	19.69	16.04	2.76	2 bulges?
35	1268	2MASX J12005393+4800076	8.3	62.7	11.10	1.45	0.27841	20.04	17.37	2.05	BCG
36	1273	2MASX J07380615+2823592	8.3	76.6	11.01	1.28	0.23091	20.13	16.92	2.34	⋯
37	1304	2MASX J16014061+2718161	8.3	82.3	11.03	1.17	0.16440	17.60	16.06	1.60	BCG, 2 spirals
38	1312	SDSS J143447.86+020228.6	8.2	75.4	10.67	1.48	0.27991	20.43	17.42	2.24	⋯
39	1323	SDSS J112928.74+025549.9	8.2	69.7	10.63	1.23	0.23960	19.56	17.01	2.00	⋯
40	1352	SDSS J101603.97+303747.9	8.2	68.8	10.73	⋯g	0.25191	21.16	17.13	2.94	⋯
41	1375	2MASX J00155012–1002427	8.2	68.4	10.94	0.91	0.17601	⋯	16.23	2.09	⋯
42	1395	2MASX J13103930+2235023	8.1	65.6	11.08	1.15	0.23123	19.91	16.87	2.59	⋯
43	1420	2MASX J13475962+3227100	8.1	87.5	10.94	1.23	0.22306	20.25	16.79	2.37	BGG
44	1464	2MASX J10041606+2958441	8.1	57.4	11.04	1.81	0.29844	20.64	17.59	2.06	starburst
45	1500	2MASX J10095635+2611324	8.1	63.7	10.98	1.33	0.24089	19.99	17.03	2.19	⋯
46	1501	2MASX J09334777+2114362	8.1	63.6	11.00	1.69	0.17219	17.84	16.17	1.60	QSO, 2 nuclei
47	1544	2MASX J14472834+5908314	8.0	68.4	11.13	1.13	0.24551	20.22	17.04	2.15	⋯
48	1546	2MASX J13435549+2440484	8.0	60.7	11.05	0.89	0.13725	19.54	15.56	2.50	⋯
49	1554	2MASX J13422833+1157345	8.0	57.1	11.08	1.43	0.27873	21.66	17.43	2.20	⋯
50	1559	CGCG 122–067	8.0	81.4	11.13	1.00	0.08902	18.27	14.56	3.13	BCG, 2 bulges
51	1606	SDSS J121644.34+122450.5	8.0	77.9	<11.31	1.13	0.25694	20.12	17.22	1.76	bar, Sy1
52	1608	SDSS J040422.91–054134.9	8.0	79.5	10.56	1.07	0.25055	20.38	17.27	2.37	flocculent
53	1611	2MASX J00380781–0109365	8.0	83.9	11.31	0.91	0.20828	21.33	16.65	4.38	E with shells?

Notes.

^aIsophotal diameter (kpc) at r = 25.0 mag arcsec⁻². ^b ${\mathrm{log}}_{10}\;{M}_{\mathrm{stars}}$ (M_⊙) or 3σ upper limit, based on 2MASS Ks luminosity and $u-r$ color. ^clog₁₀ SFR (M_⊙ yr⁻¹) or 95% confidence upper limit, based on WISE 12 μm luminosity. ^dSDSS DR9 redshift. ^eGALEX NUV-band photometry measured in 14'' aperture. Source not in GASC or GMSC. ^fThis SFR may be overestimated by a large factor because of the QSO nucleus. ^gWISE data compromised by nearby IR-bright star.

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2.3. Morphological Selection Method for Super Spiral Galaxies

We conduct our investigation of super spirals primarily with photometry compiled by NED. We use SDSS DR6 u, g, r, i, z photometry measured with the CModel method, which combines exponential plus deVaucouleurs model fitting. GALEX FUV and NUV photometry is taken from the GASC and GMSC, measured within a Kron elliptical aperture. We use 2MASS J, H, Ks total magnitudes from the 2MASS Extended Source Catalog (2MASX). NED objects are matched to All WISE using the Gator tool in the Infrared Science Archive (IRSA). We use AllWISE 4.6 and 12 μm photometry within the largest available fixed-radius aperture of 2475, which is well-matched to the largest galaxy in our sample, with semimajor axis a = 245.

4.1. Optical and Mid-IR Colors

The SDSS and WISE colors of super spirals lie along the blue sequence, similarly to less luminous star-forming disk galaxies (Figure 1(a)). The SDSS comparison sample is adopted from Alatalo et al. (2014), who show that LTGs and ETGs classified by Galaxy Zoo (GZ; Lintott et al. 2008) are well-separated in WISE [4.6]–[12] versus SDSS $u-r$ color space. The WISE [4.6]–[12] color ranges from 2.0 to 4.2, typical of polycyclic aromatic hydrocarbon (PAH) and warm dust emission from gas-rich, actively star-forming galaxies. The $u-r$ color ranges from 1.4 to 4.4, indicating star-forming disks with a range of SSFR or dust extinction. We estimate differential K-corrections of ${\rm{\Delta }}(u-r)\lt 0.2$ mag in the redshift range z = 0.1–0.3, by convolving several spectral energy distribution (SED) models (e.g., those in the Appendix) with the SDSS filter curves. These corrections are not large enough to explain the additional scatter in the observed $u-r$ colors of super spirals.

There is a shift in the locus of super spiral colors compared to less-massive blue sequence galaxies. Super spirals tend to have redder $u-r$ and bluer [4.6]–[12] colors compared to the SDSS distribution. This could in principle indicate either lower SSFR or increased extinction. However, the high SSFR of our sample (Figure 1(b)) runs contrary to the first explanation. Six super spirals have $u-r$ > 3.0, a value not attained by less luminous SDSS LTGs. The two reddest galaxies (SS 53 and SS 09) may be misclassified peculiar elliptical galaxies with prominent shells. CGCG 122-067 (SS 50) may be redder because of its double bulge. The other 4 are clearly spirals, and require further investigation and custom photometry to determine the cause of their unusually red $u-r$ colors.

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4.2. Stellar Mass and SFR

4.3. Active Galactic Nuclei

4.4. Size, SFR Surface Density, and Morphology

The sizes of super spirals range from 57 to 134 kpc, with a median size of 72 kpc, using the SDSS DR6 r-band isophotal diameter at 25.0 mag arcsec⁻² (Table 1 and Figure 2). Their deprojected SFR surface densities range from 1.5 × 10⁻³ to 2.0 × 10⁻²M_⊙ yr⁻¹ kpc⁻². A plot of SFR versus diameter shows considerable scatter (Figure 2). However, the five most rapidly star-forming galaxies, with log(SFR) > 1.6, all have diameters D < 70 kpc. The most MIR-luminous super spiral (SDSS J094700.08+254045.7, see the Appendix), also has the largest deprojected SFR surface density. The largest super spiral, 2MASX J16394598+4609058 (SS 03, Figure 3), has a diameter of 134 kpc and a relatively low SFR surface density of 2.0 × 10⁻³M_⊙ yr⁻¹ kpc⁻².

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Super spirals display a range of morphologies, from flocculent to grand-design spiral patterns (Figure 4). At least 9 super spirals have prominent stellar bars visible in the SDSS images (Table 1: Notes). There are morphological peculiarities in several cases, including one-arm spirals, multi-arm spirals, rings, and asymmetric spiral structure (Figure 4 and Table 1). These types of features may indicate past or ongoing galaxy mergers or collisions.

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4.5. Bulge-disk Decomposition

We make use of the bulge-disk decompositions of Simard et al. (2011) to quantify the relative contributions of the bulge and disk to the luminosity of super spirals (Table 2). The galaxy g and r band SDSS images are jointly fit by a de Vaucouleurs profile for the bulge (Sérsic index n_b = 4), plus an exponential disk. We compare super spirals to a representative subsample of 4686 spiral galaxies with z > 0.09 classified by Galaxy Zoo (GZ, Figure 5), with bulge-disk decompositions also by Simard et al. (2011). We find a much narrower distribution of r-band bulge to total luminosity (B/T) for super spirals, with a median value of B/T = 0.17, and a deficit of B/T values <0.1. A Kolmogorov–Smirnoff (K–S) test shows that the distributions differ significantly, with a probability of only 0.0027 that super spirals are drawn from the same population as GZ spirals. The lack of super spirals with B/T < 0.1 may be consistent with a past history of significant merger activity. The bulge ellipticity distribution of super spirals is not significantly different from that of GZ spirals (Figure 5(c)). We note that since the profile fits do not include a bar component, the ellipticities may be augmented by the presence of a bar or double bulge.

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Table 2.  Bulge-disk Decomposition (Simard et al. 2011)

SS	NED Name	Scalea	B/Tb	ec	Rdd	ie	PAf	S2g
01	2MASX J10301576–0106068	4.268	0.13	0.67	14.39	37	33	0.07
02	2MASX J10405643–0103584	3.914	0.66	0.25	12.43	2	74	0.04
03	2MASX J16394598+4609058	3.879	0.13	0.01	32.98	76	28	0.04
04	2MASX J10100707+3253295	4.349	0.12	0.22	15.31	31	57	0.02
05	2MASX J13275756+3345291	3.898	0.29	0.70	11.34	40	77	0.17
06	2MASX J11593546+1257080	4.063	0.03	0.03	16.24	35	114	0.04
07	SDSS J115052.98+460448.1	4.345	0.46	0.64	12.17	62	125	0.04
08	2MASX J12343099+5156295	4.412	0.26	0.56	11.52	27	−17	0.24
09	2MASX J09094480+2226078	4.302	0.41	0.48	19.04	46	153	0.06
10	2MASX J15430777+1937522	3.667	⋯	⋯	⋯	⋯	⋯	⋯
11	SDSS J122100.48+482729.1	4.452	0.19	0.01	12.14	27	13	0.03
12	2MASX J09260805+2405242	3.583	0.15	0.53	17.45	29	96	0.02
13	2MASX J17340613+6029190	4.200	0.29	0.05	9.06	20	13	0.03
14	SDSS J095727.02+083501.7	3.986	0.13	0.42	16.97	39	4	0.06
15	2MASXi J1003568+382901	4.009	0.11	0.24	6.25	39	50	0.06
16	2MASX J09470010+2540462	1.991	0.21	0.51	17.31	43	150	0.08
17	2MASX J11535621+4923562	2.855	0.17	0.38	15.67	63	144	0.13
18	2MASX J07550424+1353261	3.585	0.07	0.69	12.91	24	87	0.07
19	SDSS J102154.85+072415.5	4.357	0.16	0.69	13.24	42	54	0.11
20	2MASX J08265512+1811476	4.086	0.21	0.32	13.45	57	5	0.09
21	SDSS J113800.88+521303.9	4.414	0.23	0.50	12.83	36	103	0.11
22	2MASX J08542169+0449308	2.713	0.08	0.68	12.14	36	58	0.10
23	2MASX J10472505+2309174	3.071	0.18	0.70	8.84	45	156	0.17
24	2MASX J11191739+1419465	2.523	0.46	0.11	10.22	47	153	0.07
25	SDSS J135546.07+025455.8	3.779	0.07	0.29	16.28	31	140	0.04
26	SDSS J141754.96+270434.4	2.723	⋯	⋯	⋯	⋯	⋯	⋯
27	2MASX J10304263+0418219	2.774	0.19	0.47	8.67	48	25	0.14
28	2MASX J12374668+4812273	4.164	0.12	0.66	10.39	31	192	0.11
29	2MASX J11410001+3848078	4.108	0.12	0.55	10.62	37	3	0.04
30	SDSS J153618.97+452246.8	3.749	0.11	0.46	16.36	44	81	0.08
31	2MASX J09362208+3906291	4.276	0.07	0.50	11.43	30	−4	0.05
32	SDSS J140138.37+263527.6	4.287	0.09	0.13	14.41	40	11	0.04
33	SDSS J154950.91+234444.1	4.049	0.18	0.19	12.20	36	168	0.06
34	2MASX J12321515+1021195	2.841	0.16	0.02	13.18	42	10	0.10
35	2MASX J12005393+4800076	4.226	0.18	0.50	8.51	36	145	0.05
36	2MASX J07380615+2823592	3.687	0.13	0.66	12.61	45	31	0.06
37	2MASX J16014061+2718161	2.823	0.04	0.70	11.43	59	180	0.23
38	SDSS J143447.86+020228.6	4.244	0.24	0.10	11.61	63	10	0.09
39	SDSS J112928.74+025549.9	3.788	0.09	0.39	14.45	13	221	0.05
40	SDSS J101603.97+303747.9	3.934	0.17	0.68	10.03	12	107	0.04
41	2MASX J00155012–1002427	2.982	0.36	0.36	9.81	14	38	0.08
42	2MASX J13103930+2235023	3.690	0.13	0.69	9.73	36	30	0.06
43	2MASX J13475962+3227100	3.590	0.14	0.16	14.17	37	97	0.05
44	2MASX J10041606+2958441	4.439	0.17	0.30	6.74	48	137	0.09
45	2MASX J10095635+2611324	3.804	0.08	0.70	11.82	1	105	0.11
46	2MASX J09334777+2114362	2.929	0.22	0.38	5.48	6	140	0.06
47	2MASX J14472834+5908314	3.857	0.14	0.69	11.70	21	48	0.08
48	2MASX J13435549+2440484	2.427	0.55	0.00	11.14	33	159	0.05
49	2MASX J13422833+1157345	4.234	0.40	0.18	8.76	2	53	0.06
50	CGCG 122–067	1.663	0.34	0.41	9.01	38	84	⋯
51	SDSS J121644.34+122450.5	3.992	0.06	0.70	12.23	21	125	0.10
52	SDSS J040422.91–054134.9	3.917	0.31	0.30	14.75	39	100	0.03
53	2MASX J00380781–0109365	3.409	0.39	0.43	13.60	58	76	0.07

Notes.

^aScale [kpc/'']. ^bBulge fraction in SDSS DR7 r-band image. ^cBulge eccentricity. ^dDisk exponential scale length. ^eDisk inclination (°). ^fDisk PA (°). ^gSmoothness in r band.

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The disk inclination distribution of super spirals also differs significantly from that of GZ spirals, and from the expected $\mathrm{sin}(i)$ dependence (Figure 5(b)). A K–S test gives a probability of ≪0.001 that super spirals and GZ spirals are drawn from the same inclination distribution. Only 5 (9%) of super spirals have inclinations of i > 60°, compared to the expectation of 50% for randomly oriented disks. This indicates that we are missing roughly 45% of the super spirals in our luminosity range, possibly because of internal extinction at the NUV selection wavelength. The GZ spiral inclination distribution also differs from the expectation for randomly oriented disks, with an excess at inclinations >60° that may reveal a bias for GZ to classify edge on disks as spirals or to misclassify edge-on lenticulars as spirals.

The median disk exponential scale length of super spirals is 12.2 kpc, 2.3 times as large as the 5.3 kpc median for GZ spirals, confirming the giant disk sizes of super spirals (Figure 5(d)). A K–S test gives a probability of ≪0.001 that super spirals and GZ spirals are drawn from the same size distribution. The galaxy smoothness parameter (Simard et al. 2009), which quantifies the fractional residuals to the model fit inside two half-light radii, is S2 = 0.02–0.24 in r band. The B/T and S2 parameters of bulge-disk decompositions have been used by others to quantitatively select early-type galaxies, with B/T > 0.35 and S2 < 0.075 as criteria (Simard et al. 2009). Several super spirals in our sample meet these criteria, but we are nevertheless confident of the detection of a significant spiral disk in most of these cases.

We find four super spiral merger candidates with apparent double stellar bulges or double nuclei (Figure 6). The SDSS spectra only cover the dominant or central bulge or nucleus of each system. Spectroscopy of the secondary bulges or nuclei will be necessary to confirm or rule out these merger candidates as true physical pairs or multiples.

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The merger candidate 2MASX J08542169+0449308 (SS 22) appears to be a nearly equal mass major spiral pre-merger. The arms of both spirals are wound in the same direction, and the disks appear to be overlapping in the plane of the sky. The stretched out spiral arms of both spiral galaxy components, together with an apparent tidal arm at PA = 0° (measured counterclockwise from North) suggest an ongoing tidal interaction.

The merger candidate 2MASX J16014061+2718161 (SS 37) is a BCG, surrounded by several other disk galaxy companions (Figures 6 and 7). The host cluster is identified as GMBCG J240.41924+27.30444, with a photometric redshift of 0.193 (Table 3). There are clear distortions to the spiral structures of both spiral galaxy components that appear to be involved in this merger.

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The merger candidate 2MASX J09334777+2114362 (SS 46) appears to be a double AGN system. The primary, central nucleus is identified as an SDSS QSO. The secondary nucleus has similar flux and color to the primary nucleus, but it does not have an SDSS spectrum to confirm that it is a true physical double AGN. The galaxy disk has high surface brightness, suggestive of starburst activity. Faint outer spiral arms are also suggestive of a recent galaxy interaction.

The merger candidate and BCG CGCG 122–067 (SS 50) appears to be a late stage ∼2:1 major merger. The double bulge is surrounded by a common inner disk. Two giant spiral arms emerge from this central disk, one from each bulge, making a complete circuit around the disk. A large gap is seen between the arms at PA = 0°–90°. There are three other possible merging nuclei, including a bright green point source at PA = 270°, that raise the possibility that this is a five-component multiple merger system. Such multiple mergers are reminiscent of the elliptical nest galaxies that are sometimes found at the centers of galaxy clusters.

We checked NED for known galaxy clusters and groups within 1' of each super spiral (Table 3). Seven of the super spirals are candidate BCGs, within 08 of a galaxy cluster. Two are candidate brightest group galaxies (BGGs), within 1' of a compact galaxy group. Most of the clusters only have photometric redshifts and have yet to be verified spectroscopically. However, the photometric redshifts are all within Δz = 0.04 of the super spiral spectroscopic redshift, which suggests a true physical association. The two associations of super spirals with compact groups are only based on their small angular separation, with no independent redshift available for the groups.

We used NED's Environment Tool to further explore the environments of the super spiral BCG and BGG candidates. This tool performs a redshift-constrained cone search for galaxies and galaxy clusters within a sphere of comoving radius 10 Mpc. Because of the high redshifts of the super spirals, only the most luminous galaxies in their neighborhoods will tend to have measured spectroscopic redshifts in NED. We tabulate the number of galaxies (N1) with spectroscopic redshifts that are within 1 Mpc and 500 km s⁻¹, and the number (N10) within 10 projected Mpc and 5000 km s⁻¹. The MSPM 05544 galaxy cluster, which appears to host the super spiral CGCG 122–067 (SS 50) has the largest number of cluster members with spectroscopic redshifts in NED (302), while the SDSSCGB 59704 galaxy group has the smallest number (2). These numbers should be taken as lower limits to the cluster membership, depending primarily on the SDSS spectroscopic selection limit and redshift.

There are likely more clusters to be discovered in the vicinity of super spirals. For example, a clear overdensity of galaxies is seen to the SE of 2MASX J11535621+4923562 (SS 17, Figure 8). We verify a concentration of 69 galaxies within 10 Mpc and 5000 km s⁻¹ (Table 3: OGC 0586 CLUSTER), using NED's Environment tool. We estimate the mean redshift of OGC 0586 CLUSTER to be z = 0.166187, from 12 galaxies with spectroscopic redshifts that are within 5 projected Mpc of SS 17.

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While super spirals have similar structure to less luminous spiral galaxies, they are impressive in the vastness of their scale. A sense of how truly enormous these galaxies are can be gained by comparison to other galaxies in the same cluster (Figure 8: OGC 0586 CLUSTER). The 2MASX J11535621+4923562 (SS 17) super spiral at z = 0.16673, with a luminosity of L_r = 9.5L* and a diameter of 90 kpc, can be compared to a more common, less luminous spiral galaxy which has L_r = 2.8L* and a diameter of 39 kpc (137), at about the same redshift (z = 0.16721).

7.1. Analogs

7.2. Formation and Survival

7.3. Connection to Quenched Disk Galaxies

We report the discovery of a large sample of the most optically luminous (L_r > 8L*), biggest, and most massive spiral galaxies in the universe, which we call super spirals. These galaxies are very rare (∼60 Gpc⁻³ ), but are easily observed out to z = 0.3 because of their high luminosities and gigantic sizes. Super spirals are forming stars at 5–65 M_⊙ yr⁻¹, a rate greater than their mean SFR over the age of the universe. Bulge-disk decompositions confirm the presence of giant stellar disks, with a median exponential scale length of 12.2 kpc, 2.3 times the median scale length of less luminous spirals at the same redshift. The bulge-to-total optical luminosity distribution is also significantly different for super spirals, showing a deficit of galaxies with B/T < 0.1, and a concentration of galaxies with B/T = 0.1–0.2. Roughly 11% of super spirals have Seyfert or QSO nuclei, suggesting that they are still actively adding mass to their supermassive black holes. We find evidence that several super spirals are undergoing major mergers, but manage to keep their star-forming disks intact, and avoid being transformed in to red-and-dead elliptical galaxies. Some super spirals are brightest cluster galaxies, while others appear to be isolated in the field. We suggest that super spirals may be the progenitors of red and dead lenticular galaxies of similar mass.

This work was made possible by the NASA/IPAC Extragalactic Database and the NASA/ IPAC Infrared Science Archive, which are both operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. We thank Joe Mazzarella, Ben Chan, Marion Schmitz, and the rest of the NED team for useful discussions and their support of this work. This publication makes use of data from the Galaxy Evolution Explorer, retrieved from the Mikulski Archive for Space Telescopes (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. Support for MAST for non-HST data is provided by the NASA Office of Space Science via grant NNX09AF08G and by other grants and contracts. Funding for the Sloan Digital Sky Survey IV has been provided by the Alfred P. Sloan Foundation, the U.S. Department of Energy Office of Science, and the Participating Institutions. SDSS-IV acknowledges support and resources from the Center for High-Performance Computing at the University of Utah. The SDSS web site is www.sdss.org. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. This work is based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. We also make use of data from Herschel, an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. We thank Katey Alatalo for providing the SDSS-WISE comparison data in Figure 1(a), which is adapted from Alatalo et al. (2014). We thank Phil Hopkins and Ski Antonucci for insightful discussions that contributed to the manuscript. Finally, we thank the anonymous referee for suggesting that we analyze available bulge-disk decompositions by Simard et al. (2009), strengthening our results.

In order to validate our stellar mass and SFR estimates, which are based on Ks, u, r, and WISE 12 μm photometry, we make a more detailed analysis of two representative examples from our super spiral sample. We remeasure their photometry in matched apertures, rather than relying on catalog photometry. Then we fit their SEDs to make full use of the available multi-band photometry to estimate more accurate M_stars and SFR. We chose SDSS J094700.08+254045.7 (SS 16) for this analysis because it is one of the brightest super spirals in our sample, with good photometry in many bands, and typical colors. The SDSS spectrum of its bulge is also typical of most super spirals, being dominated by an old stellar population (Figure 9). We also make a detailed study of 2MASX J13275756+3345291 (SS 05), which is the most luminous (non-QSO) mid-IR source in our sample and has an SDSS nuclear spectrum with strong young stellar component and high-equivalent width Hα emission (Figure 10), characteristic of starburst activity.

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We remeasured GALEX (FUV, NUV), SDSS (u, g, r, i, z), 2MASS (J, H, Ks) and WISE band 1–4 photometry for SS 16 using the SAOImager ds9 (Joye & Mandel 2003) on images retrieved from MAST, SDSS, and IRSA (Figure 11). Aperture and color corrections were applied as necessary and the GALEX and SDSS photometry was corrected for foreground extinction due to the Milky Way dust (Stoughton et al. 2002; Wyder et al. 2005). The Galactic extinction is a modest A_V = 0.063 mag (NED). We used an elliptical aperture with semimajor and semiminor axes of 315 and 255, respectively, in order to capture the full flux of the spiral disk in all bands. This corresponds to major and minor diameters of 125 kpc and 102 kpc. We also compute 3σ IRAS upper limits based on the rms uncertainty measured by IRSA's Scan Processing and Integration tool (SCANPI) to constrain the FIR luminosity.

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We present the SED of SS 16 in Figure 12. The galaxy is detected in all GALEX, SDSS, 2MASS, and WISE bands, but is undetected by IRAS. The UV through near-IR data points reveal a massive old stellar population plus a young stellar population. Mid-IR emission appears to be dominated by PAHs and warm dust from star formation. We fit the SED using magphys template fitting (da Cunha et al. 2008). This gives a total stellar mass of ${1.8}_{-0.2}^{+0.3}\times {10}^{11}{M}_{\odot }$ and a SFR of ${9.9}_{-0.3}^{+1.6}{M}_{\odot }$ yr⁻¹. We get a consistent estimate of 1.2 ± 0.1 × 10¹¹M_⊙ for the stellar mass from the $u-r$ color and Ks band luminosity, applying the Bell et al. (2003) prescription for color-dependent mass-to-light ratio (Table 1). The WISE band 3 luminosity gives a consistent SFR of 13.5 ± 0.2 M_⊙ yr⁻¹, using the prescription of Chang et al. (2015). Lacking FIR detections, we do not have a good handle on the total dust mass, however, the SED fit formally yields a dust mass of ∼10⁸M_⊙, based on the PAH emission and FIR upper limits. This corresponds to roughly ∼10¹⁰M_⊙ of gas, assuming a standard gas/dust ratio of 100.

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We remeasured GALEX (FUV, NUV), SDSS (u, g, r, i, z), 2MASS (J, H, Ks) and WISE band 1–4 photometry for SS 05 (Figure 13), using a similar procedure. We also retrieved Spitzer IRAC and MIPS, and Herschel PACS and SPIRE images from the respective IRSA and ESA archives to measure the IR fluxes. We used a circular aperture with 200 (156 kpc) radius for most bands. However, at SPIRE wavelengths, we used the larger point source apertures of 22'', 30'', and 42'', in order to contain the broader point-spread function. The Galactic extinction is only A_V = 0.034 mag (NED).

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We present the SED of SS 05 in Figure 14. The galaxy is detected in all measured bands except the SPIRE 500 μm band. In contrast to SS 16, there is a stronger component of emission from young stars, and much more luminous IR emission from star formation activity. We fit the SED using magphys, yielding a total stellar mass of ${2.04}_{-0.09}^{+0.05}\times {10}^{11}{M}_{\odot }$ and SFR of ${40.5}_{-0.5}^{+6.5}{M}_{\odot }$ yr⁻¹. The stellar mass is consistent with the value of 1.6 ± 0.3 × 10¹¹M_⊙ that we obtain from the $u-r$ color and Ks band luminosity (Table 1). The WISE [12] luminosity gives a somewhat larger SFR of 65 ± 4 M_⊙ yr⁻¹, using the conversion factor of Chang et al. (2015). The Herschel FIR measurements yield a secure estimate of total dust mass from the SED fit of ${7}_{-1}^{+3}\times {10}^{8}{M}_{\odot }$, corresponding to 7 × 10¹⁰M_⊙ of gas, assuming a standard gas/dust ratio of 100.

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