'Scraggy' dark halos around bulge-less spiral galaxies
aa r X i v : . [ a s t r o - ph . GA ] A p r MNRAS , 1– ?? (XXX) Preprint 1 May 2019 Compiled using MNRAS L A TEX style file v3.0 ”Scraggy” dark halos around bulge-less spiral galaxies
I. D. Karachentsev ⋆ , V. E. Karachentseva Special Astrophysical Observatory, Russian Academy of Sciences, N.Arkhyz, 369167 Russia Main Astronomical Observatory of National Academy of Sciences of Ukraine, Kiev, 03143 Ukraine
Accepted XXX. Received XXX; in original form XXX
ABSTRACT
We use a sample of 220 face-on bulge-less galaxies situated in the low densityenvironment to estimate their total mass via orbital motions of supposed rare satellites.Our inspection reveals 43 dwarf companions having the mean projected separation of130 kpc and the mean-square velocity difference of 96 km/s. For them, we obtain themean orbital-mass-to-K-band luminosity ratio of 20 ±
3. Seven bulge-less spirals inthe Local Volume are also characterized by the low mean ratio, M orb /L K = 22 ± Key words: galaxies: bulges – galaxies: spiral – galaxies
Among the galaxies with stellar masses of more than, 10 M ⊙ the flat disc-like objects without the evidence of the centralbulge constitute about 10%. Almost all of them are classified as late-type spirals: Sc, Scd, and Sd according to de Vaucouleurset al. (1976). Some of them demonstrate small pseudo-bulges formed during the process of the secular disc evolution. Unlikeclassical bulges, the pseudo-bulges show a nearly exponent stellar density profile and the evidences of current star formation(Kormendy & Kennicutt 2004). The observed abundance of the spiral discs without spheroidal stellar component imposes achallenge to models of the hierarchical clustering of galaxies via their consecutive merging (Kormendy et al. 2010).Bulge-less galaxies are the most distinguishable when they are seen strictly edge-on. A Reference Flat Galaxy Catalog= RFGC (Karachentsev et al. 1999) lists 4236 ”flat” galaxies covering the all sky. Karachentseva et al. (2016) have selectedfrom the RFGC catalog 817 ultra-flat galaxies (UFG) with an apparent axial ratio a/b >
10. The environment of the UFGsis characterized by low spatial density, a deficit of massive satellites and almost total absence of dwarf spheroidal satelliteswith old stellar population.Inner structure of the UFGs is practically unseen due to the disc inclination. To determine the bulge-less galaxy structuralfeatures, Karachentsev & Karachentseva (2019) collected 220 galaxies of the Sc, Scd, and Sd types, seen almost face-on.According to statistical analysis, about half of the galaxy sample have bar-like substructures, in most part of the discs theunresolved (star-like) nuclei are seen, and a considerable part (27–50)% of the face-on galaxies have peripheral distortion oftheir spiral pattern.There is an idea that secular stability of very thin stellar discs requires the presence a very massive dark halo around them(Banerjee & Jog 2013). The shape of this halos is supposed to be nearly spherically symmetric. Unfortunately, the rotationcurves obtained for the UFGs, extend not so far from the center (Uson & Matthews 2003, Makarov et al. 2001), which makes itimpossible to evaluate the true dimension and mass of the dark halo. Determining the total dark halo mass by radial velocitiesand projected separations of the satellites is more promising method. However, the difficulties arise even with such approach.The search for satellites with measured radial velocities around the UFGs shows that ∼
60% of ultra-flat galaxies have notany detected physical satellites inside the virial radius R vir ≃
250 kpc, about 30% of the UFGs enter together with a ratherbright neighbors in scattered associations (filaments, walls), and only ∼
10% of ultra-flat galaxies are the dominant objects inphysical multiple systems (Karachentsev et al. 2016). In the following we present the results of searches for satellites around220 late type spiral galaxies seen face-on as well as the estimates of the total mass of the bulge-less galaxies. ⋆ E-mail: [email protected] (cid:13)
XXX The Authors
I. D. Karachentsev, V. E. Karachentseva
Table 1.
Face-on bulge-less galaxies with orbital mass estimates.Galaxy B t V LG mod D T log L K ∆ V R p log M orb M orb /L K mag km/s mag L ⊙ km/s kpc M ⊙ (1) (2) (3) (4) (5) (6) (7) (8) (9) (10)IC1562 13.60 3771 33.56 c 10.69 122 60 12.02 21NGC0255 12.41 1694 31.70 c 10.45 37 173 11.45 10-72 243 12.17 52ESO542-004 15.00 5657 34.50 d 10.29 -66 199 12.01 52UGC02043 14.96 5316 34.40 c 10.67 -18 128 10.69 1ESO479-022 15.39 7266 35.07 c 10.61 45 168 11.60 10UGC02323 15.66 8139 35.33 c 10.99 -281 41 12.58 39UGC02692 14.07 6337 34.77 c 11.09 -42 92 11.28 2114 233 12.55 29NGC1376 12.85 4137 33.82 c 11.14 -115 134 12.32 15-100 157 12.27 14NGC1599 14.10 3947 33.73 c 10.60 100 155 12.26 46UGC03703 15.35 7260 35.14 c 10.72 -96 189 12.31 39UGC04380 15.05 7554 35.24 c 10.86 134 208 12.64 60UGC05169 15.44 7699 35.29 d 10.45 -81 41 11.50 11NGC2967 12.28 1679 32.16 c 10.80 101 191 12.36 36UGC05474 14.89 5847 34.71 cd 10.54 42 80 11.22 5-117 154 12.39 71-67 161 11.93 24UGC05483 15.13 6014 34.77 c 10.62 -21 46 10.38 1NGC3506 13.16 6252 34.87 c 11.42 1 135 8.20 0PGC034006 14.13 7543 35.26 c 11.25 168 190 12.80 35NGC3596 11.79 1062 31.32 cd 10.42 -101 25 11.48 12-60 60 11.41 10NGC3938 10.87 841 30.87 c 11.03 -179 156 12.77 55-82 166 12.12 12-191 168 12.86 6835 223 11.51 327 251 11.33 2IC3271 14.57 7083 35.13 c 10.97 63 103 11.68 5NGC4653 12.77 2471 33.08 c 10.85 37 68 11.04 213 96 10.45 0NGC5434 13.94 4587 34.21 c 10.84 115 41 11.81 9NGC5468 12.95 2734 33.40 c 10.90 68 55 11.48 4PGC058201 15.69 8562 35.51 c 10.70 43 54 11.07 2IC1221 14.59 5706 34.63 cd 10.64 75 96 11.80 14NGC6821 13.62 1680 31.86 d 10.33 -28 127 11.07 6NGC7137 13.05 1977 32.16 c 10.57 152 77 12.32 56NGC7495 13.76 5133 34.28 c 11.06 -39 73 11.12 1NGC7535 14.28 4884 34.17 cd 10.63 8 47 9.55 0-100 174 12.31 48-15 182 10.68 1ESO605-016 13.23 7999 35.28 c 11.55 -68 142 11.89 2(1) galaxy name; (2) apparent B -magnitude from HyperLEDA (Makarov et al. 2014);(3) radial velocity with respect to the Local Group center;(4,5) distance modulus and morphological type from HyperLEDA;(6) logarithm of K-band luminosity expressed in the solar units;(7) radial velocity difference of satellite relative to the central galaxy;(8) a satellite projected separation;(9) logarithm of orbital mass; (10) orbital mass-to-luminosity ratio. To reveal the physical satellites, we have used the option ”To Search for Nearby Objects” proposed in the NASA Extra-calactic Database (=NED, http://ned.ipac.caltech.edu/). Around each bright enough galaxy from the list by Karachentsev &Karachentseva (2019) with absolute magnitude M B < − . m | ∆ V | <
300 km/s ranging in the projected separation R p = 250 kpc. In the next stage, we left only those conditionallyisolated cases where other neighbors brighter than the central object were absent within R p = 750 kpc around the face-ongalaxy under study. In the process, we excluded a lot of neighboring objects that turned out to be stars or parts of a normal MNRAS , 1– ????
300 km/s ranging in the projected separation R p = 250 kpc. In the next stage, we left only those conditionallyisolated cases where other neighbors brighter than the central object were absent within R p = 750 kpc around the face-ongalaxy under study. In the process, we excluded a lot of neighboring objects that turned out to be stars or parts of a normal MNRAS , 1– ???? (XXX) Scraggy” dark halos around bulge-less spiral galaxies R p , kpc V , k m s - Figure 1.
Distribution of the face-on bulge-less galaxy satellites on their radial velocity difference and projected separation. galaxy. Altogether, we detected 43 satellites around 30 face-on galaxies. The remaining galaxies from our list do not havesatellites in the specified limits of | ∆ V | and R p , or they have as neighbors other bright galaxies violating the isolationcondition.In Table 1 data on the face-on bulge-less galaxies and their satellites are given. Before presenting the data we have to dosome explanations. The total B -magnitude is corrected for Galactic extinction; the inner extinction for face-on galaxies weprove to be zero. The K - band magnitude is calculated via the corrected B - magnitude and morphological type (Melnyk et al.2017). Calculating the satellite projected separation R p , we use the galaxy distance D = V LG /H with the Hubble parameter H M orb = (16 /πG ) × ∆ V × R p , is estimated assuming arandom orientation of satellite orbits with the mean orbital eccentricity of h e i = 1 / G is the gravitation constant. Notes to columns of Table 1 are given below the Table. The distribution of 43 satellites aroundtheir host face-on bulg-less galaxies on the modulus of radial velocity difference and the projected separation is presented inFig.1. Here the mean-square of radial velocity difference is h ∆ V i / = 96 km/s, and the mean projected separation is equalto h R p i = 130 kpc.As it is seen from Table 1, the majority of face-on galaxies with detected satellites belong to the Sc morphological type.The median of their luminosity, 10.71 dex, is comparable with the Milky Way luminosity. (This high value is caused by thefact that in searching for satellites we donated the face-on low luminosity galaxies with their shallow potential well.) Theorbital mass values show a pronounced scatter that depends on prevailing character of satellites motions. At fixed value ofthe orbital eccentricity, the ensemble mean of M orb is the unbiased estimate of the mass of central galaxy. In the case of anarbitrary eccentricity e , the orbital Keplerian mass estimate is expressed as M orb = (32 / π )(1 − e / − × G − × h ∆ V × R p i . This value grows in three times from severely round motions, e = 0, to the pure radial ones, e = 1. We assumed the meanvalue of h e i = 1 /
2, corresponding the expected average obtained in the N-body simulations (Barber et al. 2014).The mean value of the orbital-mass-to-K-band luminosity ratio for face-on bulge-less galaxies is h M orb /L K i = 20 ± ± The Local Volume limited by the radius of 11 Mpc, is unique in having an abundance of galaxies which distances are measuredat the Hubble Space Telescope with accuracy of about 5%. The summary sample on the galaxy distances is presented in the
MNRAS , 1– ?? (XXX) I. D. Karachentsev, V. E. Karachentseva R p , kpc V , k m s - Figure 2.
Distribution of satellites around the nearby face-on bulge-less galaxies: IC 342, M 101, NGC 6946, NGC 628, and NGC 3184on their radial velocity difference and projected separation. B -magnitude, (3) the projected separation in kpc, (4) the difference between the radial velocities of the satellite and the hostgalaxy, in km/s.The distribution of 38 satellites on their modulus velocity difference and projected separation is presented in Fig.2. Asone can see, the satellite velocity differences do not exceed 150 km/s, justifying the above selection limit of | ∆ V | < ∼
250 kpc. Nethertheless, they all are inside the ”zero velocity sphere” ( R ∼ R p > R vir are lost among the general field galaxies. Table 3 contains the basic parameters of the nearby groups. As seenfrom the data, even the brightest satellites appear to be fainter of the central galaxy at 3–5 magnitudes. Thus, the applicationof the model of test particles moving around central massive body is quite correct in evaluating the total mass of the centralgalaxy.In the Local Volume there are two another groups, NGC 253 and NGC 5236, where the dominated member is a spiralbulge-less galaxy but oriented arbitrary. We give their data in the bottom of Table 3. The K-band luminosities and orbitalmass estimates for these nearby groups are rather similar to ones from Table 1 obtained for more distant systems. Particularly,the mean total mass-to-luminosity ratio for the Local Volume bulge-less galaxies, h M orb /L K i = 22 ±
5, is practically the sameas the value (20 ±
3) derived for the remote Sc-Sd galaxies.It should be noted that according to the data by Karachentsev & Kudrya (2014), the Local Volume contains five groups:M 31, M 81, NGC 4258, NGC 4736, and NGC 3627, where the bulgy Sab–Sbc galaxies dominate, and also three groups:NGC 5128, NGC 4594, NGC 3115, where the objects are grouping around the E, S0, Sa galaxies. These groups are characterizedof the mean values of total mass-to-luminosity ratio h M orb /L K i = 41 ± h M orb /L K i = 88 ±
30, respectively. In spiteof poor statistics, these results can indicate that the dark mass-to-luminous mass ratio grows when the fraction of galaxyspheroidal stellar sub-system increases.
MNRAS , 1– ????
MNRAS , 1– ???? (XXX) Scraggy” dark halos around bulge-less spiral galaxies Table 2.
Companions of face-on bulge-less galaxies in the Local Volume.Galaxy B t R p ∆ V mag kpc km/s IC 342
M 101
NGC6946
NGC 628
NGC 3184
As we noticed above, the thin spiral bulge-less galaxies reside in the low density regions, i.e. have a poor environment.The rare satellites of the Sc–Scd–Sd galaxies are, as a rule, dwarf galaxies containing gas and young stars. Our statistics ofsatellites around 220 face-on bulge-less galaxies shows that they have the average projected separation of h R p i = 130 kpc andmean-square velocity difference of 96 km/s. The estimate of the total mass of bulge-less galaxies via orbital motions of theirsatellites yields the value of h M orb i = 12 .
14 dex and h M orb /L K i = 20 ±
3. A similar value, h M orb /L K i = 22 ±
5, is obtainedby us for the Local Volume Sc–Sd galaxies, where the separation of physical satellites and field galaxies is more reliable due
MNRAS , 1– ?? (XXX) I. D. Karachentsev, V. E. Karachentseva
Table 3.
Suites of companions around the Local Volume bulge-less spirals.Galaxy D n ∆ m h R p i σ V log L K log M orb M orb /L K Mpc mag kpc km/sIC 342 3.28 8 3.0 319 76 10.60 12.28 48M 101 6.95 9 3.2 204 64 10.79 11.93 14NGC 6946 7.73 8 5.8 323 68 10.99 12.05 11NGC 628 10.19 9 4.6 309 68 10.60 12.11 32NGC 3184 11.12 5 3.3 454 49 10.52 11.87 22NGC 253 3.94 7 2.0 500 60 11.04 12.18 14NGC 5236 4.92 10 4.4 294 61 10.86 12.03 15Mean - 8 3.8 343 65 10.77 12.06 22 to abundance of data on the galaxy distances. For comparison, the Sab–Sbc spirals in the Local Volume with much moredeveloped bulges have the ratio h M orb /L K i = 41 ±
9, and a few in number galaxies of E, S0, Sa types are characterised by theratio of h M orb /L K i = 88 ±
30. It seems unlikely that this difference would be caused by different manner of transition fromthe B-luminosity of a galaxy to its K-luminosity, or the different nature of the orbital motions of the satellites. Obviously, theconclusion about the reduced amount of dark matter per unit of K-band luminosity in the Sc–Sd galaxies needs confirmationon a richer observational material. It is quite possible that the presence of ”scraggy” halos in spiral bulge-less galaxies will beexplained within the framework of the standard paradigm of the hierarchy galaxy clustering.
Acknowledgements.
We thank the referee for his/her valuable comments. The work is supported by the Russian Science Foundation grant19-12-00145. The paper makes use of the data from NASA Extragalactic Database and from the HyperLEDA.
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