Look-alike galaxies: - HI observations for look-alike galaxies of four calibrators

Astronomy and Astrophysics Supplement Series, Jul 2018

We present a programme aiming at applying the Tully-Fisher relation for galaxies with the same morphological type and the same inclination (look-alike galaxies or sosie galaxies) as calibrating galaxies. The advantage of sosie galaxies is discussed. In particular, it is shown that using sosies of bright calibrators will allow us to explore the universe deeper and more efficiently than the classical TF method applied to different morphological types and different inclinations. As a preliminary part, we report in this paper new HI observations performed with the radiotelescope of Nançay (France) for sosies of four calibrators NGC 224, NGC 3031, NGC 253 and NGC 5457. 82 galaxies were detected.

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Look-alike galaxies: - HI observations for look-alike galaxies of four calibrators

Astron. Astrophys. Suppl. Ser. Look-alike galaxies: L. Bottinelli 1 2 L. Gouguenheim 1 2 G. Theureau 0 N. Coudreau 1 G. Paturel 3 0 Osservatorio di Capodimonte , via Moiariello 16, I-80131 Napoli , Italy 1 Observatoire de Paris-Meudon , URA 1757, F-92195 Meudon Principal Cedex , France 2 Universite Paris-Sud , F-91405 Orsay , France 3 CRAL-Observatoire de Lyon, UMR 5574 , F-69230 Saint-Genis Laval , France We present a programme aiming at applying the Tully-Fisher relation for galaxies with the same morphological type and the same inclination (look-alike galaxies or sosie galaxies) as calibrating galaxies. The advantage of sosie galaxies is discussed. In particular, it is shown that using sosies of bright calibrators will allow us to explore the universe deeper and more e ciently than the classical TF method applied to di erent morphological types and di erent inclinations. As a preliminary part, we report in this paper new HI observations performed with the radiotelescope of Nancay (France) for sosies of four calibrators NGC 224, NGC 3031, NGC 253 and NGC 5457. 82 galaxies were detected. galaxies; general | radio lines; galaxies | astronomical data bases; miscellaneous | cosmology; distance scale 1. Introduction Most of the methods used for extragalactic distance determination are based on a linear relationship between the absolute magnitude M and a given observable parameter P . M = a log P + b: (1) Send o print requests to: G. Paturel ? These observations made use of Nancay radiotelescope. The Nancay Radio Observatory is the Unite scienti que de Nancay of the Observatoire de Paris,associated as Unite de Service et de Recherche (USR) No. B704 to the French Centre National de la Recherche Scienti que (CNRS). The Nancay Observatory also acknowledges the nancial support of the Conseil Regional of the Region Centre in France. The distance modulus is thus derived from the classical relation = mc − M (2) where mc is the apparent magnitude corrected for perturbing e ects (inclination, redshift, galactic extinction...) Similarly, the parameter P must also be corrected for such e ects. Without considering the galactic extinction, these corrections depend essentially on axis ratio and morphological type. For spiral galaxies, one of the best parameters P is the 21-cm line width (Tully & Fisher 1977) . The principle of the method of sosie galaxies1 (Paturel 1981; Sandage 1996) consists in selecting galaxies having similar observational properties as calibrating galaxies. For instance, one can select galaxies having the same morphological type, the same axis ratio and the same 21-cm line width, as a given calibrating galaxy (e.g., M 31). From Rel. 1, these galaxies have the same absolute magnitude as the considered calibrating galaxy. This method has revealed to be a powerful tool in obtaining accurate galaxy distances because it does not assume anything about the expression of, e.g. the Tully-Fisher relationship. For instance, the value of a or b does not intervene in the nal result. Any morphological type dependences are removed etc. We started a new observational program to search for new sosie candidates. This means that we are observing galaxies with at least the same morphological type and the same axis ratio as calibrators, but, obviously not necessarily the same 21-cm line width. Among detected galaxies, some will be pure sosies of the calibrators (i.e. galaxies with the same 21-cm line width), others not. This last class will be called \sosie candidates". Nevertheless, sosie candidates can also be used through the conventional TF relation with the still valid advantage that the result does not depend on morphological type and inclination e ects. 1 Look-alike in English. Only, uncertainty about the value of the slope a may a ect the nal result which anyway will be more secure. Another problem still plagues the use of any distance determination. It has been shown that Malmquist bias appears at very small distances. For instance, for galaxies as luminous as M 31, the bias starts for radial velocities as small as 2000 km s−1. To push this limit deeper we will enlarge our sample and observe galaxies up to fainter apparent magnitude. In order to check the e ect of the bias it is compulsory to work with a sample complete up to a well determined apparent magnitude. We present in Fig. 1 the completeness curve for all sosie candidates of M 31 extracted from the LEDA database. This sample contains 1434 galaxies. The completeness curve ( lled circles) is drawn. On the same gure the completeness curve is given for those 351 galaxies of the sample having known 21-cm line width (open circles). This shows that more than 1083 galaxies have still to be observed to get all sosie candidates of M 31 (hence, all sosies of M 31) up to an apparent B-magnitude 15. Fortunately, among these 1083 galaxies, 367 already have an optical radial velocity and will be easy to observe in HI without having to use the search mode. They will constitute our rst priority target. In order to show the bene t of using a deeper sample, we plot in Fig. 2 the apparent Hubble constant H0 vs. the radial velocity for M 31 sosie galaxies and four di erent completeness limits (Blim = 12, 13, 14 and 15). These curves show the e ect of the Malmquist bias under di erent conditions. They are calculated according to Teerikorpi (1975 , Eq. (5)) using a dispersion of the Tully-Fisher relation of = 0:6 and a Hubble constant of 50 km s−1 Mpc−1. It appears clearly that the bias would be negligible up to a radial velocity of 6000 km s−1 if a completeness limit of Blim = 15 is reached. The Hubble constant derived in this way would not be influenced very much by large scale flows, because these flows are generally assumed to be smaller than about 600 km s−1. Note that, the bias e ect would be much stronger if sosies of a less luminous calibrator were used. For instance, we compare the bias for M 33 and M 31 (Fig. 3) using the same bias curve as above but a completeness limit of Blim = 13. It is obvious that it is better to use a luminous calibrator. A preliminary study from pure sosies of M 31 and M 81 is presented in a companion paper (Paturel et al. 1998) where distances to the calibrators come from Cepheid Period-Luminosity relation calibrated with geometrical parallaxes. Another study is in preparation with all available sosie candidates. This program will be pursued by systematic HI observations of sosie candidates of di erent calibrators. In this paper we present a rst step of observations obtained for this program. We started it by searching sosie candidates of two intrinsically luminous galaxies (M 31 and M 81), one low luminosity galaxy (NGC 253) and one face-on galaxy (NGC 5457). The look-alike galaxies are far more distant than the calibrating galaxies. Hence morphological classi cation and axis ratio measurements are less certain. Further, the morphological types and axis ratios of galaxies of our program are revised from time to time due to inclusion of new data. The morphological type is coded numerically with T according to de Vaucouleurs et al. (1976 ). The axis ratio is noted R25 and corresponds to the ratio of the major to the minor axis of the external isophote at the limiting surface brightness of 25 Bmag arcsec−2. In Figs. 4 and 5 we present the histograms of T = T (look− alike) − T (calibrator) and log R25 = log R25(look − alike) − log R25(calibrator) in order to show the actual uncertainty in the de nition of a look-alike candidate. The standart deviations of these quantities, ( T ) = 1:0 and ( log R25) = 0:07, correspond to the typical error of T and log R25, respectively but two galaxies are rejected afterwards (PGC 06289 and PGC 68360). 2. HI observations 2.1. HI velocity measurements The technical procedure has been described in Fouque et al. (1990) . The main characteristics of the Nancay radiotelescope receiver are given in Table 1. The Nancay radiotelescope has a non circular HalfPower-Beam-Width (HPBW) of roughly 220 NS 40 EW which actually depends on the elevation along the NorthSouth direction. The variation is presented in Fig. 6 (E. Gerard, private communication). The change of the HPWB intervenes for estimating the confusion of a measurement and for the beam lling correction. In this paper, only raw fluxes are given (i.e. uncorrected for beam- lling e ect). It is worth noting that, the beam- lling correction also depends on the position angle of the galaxy as described in Bottinelli et al. (1990) . Annexe: 21-cm line pro les for sosie candidates of NGC 224 (Fig. 7), NGC 3031 (Fig. 8), NGC 253 (Fig. 10) and NGC 5457 (Fig. 9). The x-axis gives the heliocentric radial velocity c = in km s−1. The y-axis gives the flux density in mJy Fig. 7. 21-cm line pro les for sosie candidates of NGC 224 The 21-cm line widths are measured using an interactive method. The smoothed 21-cm line is displayed on a screen; the user estimates visually the maximum of the pro le; the program calculates the width at 20% and 50% of this maximum by considering the points where both levels intercept the pro le. This procedure is impossible when the S/N ratio is poor (less than 3 like for NGC 4675, NGC 5267 and ESO 404-15). Nevertheless, the user has full control to obtain the most secure result, for instance by smoothing the pro le in noisy regions. The most critical part is the proper estimation of the maximum. When the pro le has two maxima at di erent levels, the adopted maximum is the mean of both if it is compatible with the noise (otherwise, the higher level is chosen, but it is reduced by the standard deviation of the noise). 2.2. Results All new measurements are presented in Table 2. Mean errors and S/N ratio are calculated according to Fouque et al. (1990) . Columns are arranged as it follows: { Column 1: Identi cation with the PGC name (According to Paturel et al. 1989a,b) . { Column 2: Identi cation with an alternative name according to the following hierarchy: NGC/IC, UGC, ESO, MCG (Paturel 1989a) . { Column 3: Measured heliocentric velocity measured at the middle of the 20% level in km s−1. { Column 4: Measured heliocentric velocity measured at the middle of the 50% level in km s−1. { Column 5: Internal mean error on both V 20 and V 50. { Column 6: 21-cm line width W 20 at 20% of the peak value in km s−1. { Column 7: Internal mean error on W 20. { Column 8: 21-cm line width W 50 at 50% of the peak value in km s−1. { Column 9: Internal mean error on W 50. { Column 10: 21-cm flux FH (area de ned by the 21-cm line pro le, in Jy km s−1). { Column 11: Internal mean error on FH. { Column 12: S/N ratio. The HI-pro les are given in Figs. 7 to 9 of the Annexe for sosies candidates of NGC 224, NGC 3031, NGC 253 and NGC 5457, respectively. For each spectrum the galaxy name is given above the frame. The PGC name is given in the upper-right corner. The x-axis gives the heliocentric velocity expressed in km s−1 with the optical convention ( = ). The y-axis gives the flux in mJy. 2.3. Discussion The confusion of HI measurements is an old problem in astronomy. For each galaxy a visual inspection of the eld has been made using the charts produced with the LEDA database. The galaxies listed below are confused or possibly confused. For each of them we calculate two Cp = 1 − + x2 a2 y2 b2 coe cients in order to quantify the degree of confusion. The rst coe cient gives the degree of confusion in position. It is de ned as: r Cv = 1 − where x and y are the angular distances (in right ascension and declination) between the observed galaxy and the galaxies of the eld and where a and b are the HPBW in the corresponding directions. The HPBW in the N − S direction is calculated according to Fig. 6. The second coe cient gives the degree of confusion in velocity. It is de ned as: j V j W20 where V is the velocity di erence between the observed galaxy and the galaxies of the eld and where W20 is the 21-cm line width for the observed galaxy. A confusion exists when Cp and Cv are both larger than zero. For each confused galaxy we give the maximum value of Cp and Cv. (3) (4) { PGC 02791: Confused by PGC 2794 = ESO 540-24 (V = 6259 km s−1) and/or PGC 2796 = ESO 540-25 (V = 6362 km s−1); Cp = 0:83 and Cv = 0:96. { PGC 10595: Confused by PGC 10597 = NGC 1103 (V = 4156 km s−1). { PGC 42708: Confused by PGC 42725 = MCG 9-21-32 (V = 4913 km s−1); Cp = 0:79 and Cv = 0:97. { PGC 08351: Confused by PGC 08352 = NGC 834 (V = 4594 km s−1); Cp = 0:59 and Cv = 0:58. { PGC 02305: The HI spectrum belongs to PGC 2308 = IC 1562 (face on spiral at V = 3671 km s−1). { PGC 31500: The HI spectrum belongs to PGC 31510 (V = 3799 km s−1). { PGC 51456: Possibly confused by PGC 51445 = NGC 5595 (V = 2697 km s−1); Cp = 0:15 and Cv = 0:88. { PGC 70130: Confused by PGC 70127 = MCG-1-589 and PGC 70133 = MCG-1-58-11 (V = 3666 and 3898 km s−1, respectively); Cp = 0:77 and Cv = 0:99. Further, galaxies PGC 04353, PGC 42998, PGC 44589, PGC 05841, PGC 38712, PGC 44294, PGC 53965, PGC 28388, PGC 37719 may be confused by galaxies with unknown radial velocity and/or morphological type. Acknowledgements. We express our gratitude to the observers of the Nancay radioastronomical observatory and also to N. Hallet and M. Loulergue for their helpful contribution. We would like to thank also Dr. W.K. Huchtmeier for his helpfull comments on the manuscript. m.e. (5) 17. 13. 9. 24. 10. 8. 14. 8. 11. 12. 9. 3. 7. 9. 9. 12. 8. 10. 12. 9. 14. 11. 17. 10. 12. 6. 6. 6. 10. 17. 15. 10. 7. 10. 16. m.e. (5) 6. 8. 8. 8. 19. 8. 8. 6. 5. 3. Bottinelli L. , Gouguenheim L. , Fouque P. , Paturel G. , 1990 , A &AS 82 , 391 de Vaucouleurs G ., de Vaucouleurs A. , Corwin H.G. , 1976 , Second Reference Catalogue of Bright Galaxies. University of Texas Press, Austin (RC2) Fouque P. , Bottinelli L. , Durand N. , Gouguenheim L. , Paturel G. , 1990 , A &AS 86 , 473 Paturel G. , Fouque P. , Bottinelli L. , Gouguenheim L. , 1989a, Monographies de la base de donnees extragalactiques No. 1, Vols. I, II and III , ISBN 2.908288.00.1 Paturel G. , 1981 , ApJ 282 , 382 Paturel G. , Lanoix P. , Teerikorpi P. , et al., 1998 , A& A (in press) Sandage A. , 1996 , AJ 111 , 18 Teerikorpi P. , 1975 , A &A 45 , 117 Tully R.B ., Fisher J.R. , 1977 , A &A 54 , 661


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L. Bottinelli, L. Gouguenheim, G. Theureau, N. Coudreau, G. Paturel. Look-alike galaxies: - HI observations for look-alike galaxies of four calibrators, Astronomy and Astrophysics Supplement Series, 429-436, DOI: 10.1051/aas:1999451