The Astronomical Journal, 132:1035Y1045, 2006 September # 2006. The American Astronomical Society. All rights reserved. Printed in U.S.A.
PROPERTIES OF THE DWARF IRREGULAR GALAXY DDO 169 Mansie G. Iyer1 and Caroline E. Simpson Department of Physics, Florida International University, CP 204, 11200 Southwest 8th Street, Miami, FL 33199;
[email protected],
[email protected] Received 2005 September 17; accepted 2006 May 21
ABSTRACT This paper investigates the H i, optical, and dark matter properties of the dwarf irregular galaxy DDO 169. The galaxy shows signs of being tidally disrupted. Evidence of this can easily be seen in the galaxy’s velocity field. We also show that DDO 169, which was thought to have a companion galaxy, does not. Key words: dark matter — galaxies: dwarf — galaxies: interactions — galaxies: irregular — galaxies: ISM
1. INTRODUCTION
On obtaining a satisfactory result with the edited calibrator data, the gain, phase, and bandpass calibrations were determined. The calibration was then applied to the channel-zero data set and was checked for problems. The few that were identified were edited out, and the calibration was then applied to the line data set. The (u, v) data were examined very carefully to see whether any further editing was necessary before being transformed into images. The C-configuration source line data showed signs of solar contamination. Hence, all flux values above 1 Jy were edited from the C-configuration source data. After each configuration data set for the galaxy was individually calibrated and edited, the continuum emission was subtracted. This was done by making linear fits to the (u, v) data set in the H i lineYfree channels. A fast Fourier transform was then applied to the individual data sets to produce a cube (, , v) of images. During the imaging process, each channel in the cube was CLEANed (Ho¨gbom 1974; Clark 1980) to reduce the effects of sidelobes produced by non-Gaussian features of the synthesized beam (the ‘‘dirty beam’’). Various weighting schemes (ROBUST parameter in the imagr task in AIPS) were tested to increase the resolution. However, the increase in resolution was not worth the corresponding decrease in sensitivity. Thus, in order to maximize the sensitivity to low surface brightness features, the natural weighting scheme was used in all imaging. The cube for each individual data set was CLEANed down to the rms values shown in Table 1. These fluxes are 1 as determined by the statistical analysis of a signal-free channel map. The cubes were then integrated using a cutoff of 2.5 to produce moment maps. Once the individual array moment maps were checked and determined to be free of problems, the individual data sets were combined in the (u, v) plane to produce sensitive and better resolved cubes for DDO 169. The C and D configurations were coalesced using the task dbcon, and the resulting cube was cleaned down to 1 ¼ 2:44 mJy beam1. The combined array cube was then integrated with a 2.5 cutoff to produce moment maps representing the H i integrated column density, the temperature-weighted mean velocity, and the velocity dispersion. Single-dish H i observations of DDO 169 found a flux value of approximately 14.49 Jy km1 s1 (Huchtmeier & Richter 1989). We have detected 11.21 Jy km1 s1. Figure 1 shows the integrated spectrum for the galaxy. If the single-dish observations for DDO 169 are correct, we have detected 77% of the total H i flux. There are two possible explanations for this discrepancy in the flux values. The amount of emission that the D configuration can
DDO 169 is part of a large study of star formation in dwarf galaxies ( Hunter & Elmegreen 2004). In a previous H i study, Simpson & Gottesman (2000) identified a northwest ‘‘companion’’ to DDO 169. They found that this companion had no known cataloged position attributed to it and was hence considered to be a new detection. In this paper we present a follow-up study of DDO 169, using new, higher sensitivity H i data along with optical data. The H i data are used to investigate the nature of the companion and to search the extended gas surrounding DDO 169 to see if there are any peculiarities. The optical data are used to examine the stellar distribution and star formation regions (SFRs). This paper is divided into the following sections. In x 2 we present the H i and optical observations. Section 3 discusses the results, and in x 4 the conclusions on DDO 169 are presented. 2. OBSERVATIONS 2.1. H i Observations and Imaging Presented here are 21 cm C- and D-configuration observations of DDO 169 using the Very Large Array 2 ( VLA). DDO 169 was observed for 10 minutes in the C configuration in 1997 November and for 20 minutes in 1993 July in the D configuration. Both configurations used a 128 channel spectrometer with a total bandwidth of 1.56 MHz. A channel separation of 12.2 kHz was used, and the observations were made at a central velocity of 260 km s1. The observing parameters are shown in Table 1. 2.2. Data Reduction Standard calibration and editing procedures were performed on each individual configuration data set, using the Astronomical Image Processing System (AIPS; Napier et al. 1983) data reduction package available from the NRAO. We used 1331+305 (J2000.0) and 1219+484 (J2000.0) as flux and phase calibrators, respectively, for both the C and D configurations. There were no obvious issues such as solar or Galactic contamination associated with the calibrator data. 1
Postdoctoral Fellow at Indiana University;
[email protected]. The Very Large Array is operated by the National Radio Astronomy Observatory, which is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. 2
1035
1036
IYER & SIMPSON TABLE 1 VLA Observations for DDO 169 Array Configuration Parameter
C
D
C+D
Time on sources (minutes) ........................................ Bandwidth ( MHz) ..................................................... No. of channels.......................................................... Channel separation ( kHz).......................................... Channel separation (km s1) ..................................... Heliocentric velocity ( km s1).................................. Beam ( FWHM ) (arcsec) ........................................... rms noise in one channel (mJy beam1)................... Equivalent brightness temperature ( K) .....................
10 1.56 127 12.2 2.6 260 30:4 ; 17:9 5.9 17.7
20 1.56 127 12.2 2.6 260 54:4 ; 51:9 2.7 0.81
... ... ... ... ... ... 47:5 ; 43:1 2.44 ...
detect is restricted by the signal-to-noise ratio and the minimum baseline length. The D configuration can detect emission only from structures less than 150 in angular extent. Thus, any extended emission beyond this value would not be detected. The other limitation is the low signal-to-noise ratio, which is, in all likelihood, the main culprit in this case. 2.3. Optical Imaging The optical data and photometry were kindly supplied by D. Hunter. Details of the observations and data reduction are given in Hunter & Elmegreen (2005). In brief, 3000 s B, V, and U exposures were taken using the Lowell Observatory 1.8 m Perkins telescope. The pixel scale was 0B5, and the seeing was 1B2. In addition, H images (1800 s exposure times) were taken; the pixel scale was 0B49, and the seeing was 2B3. The H ii region calibration was in good agreement with the spectrophotometric standard star calibration up to 4%. 3. RESULTS: DDO 169 DDO 169 (UGC 08331) is located at ¼ 13h 15m 30:s3 and ¼ þ47 29 0 56 00 (J2000.0) and has a heliocentric velocity of 260 km s1. A distance of 5.3 Mpc has been adopted (Youngblood & Hunter 1999).
Fig. 1.—Velocity profile for DDO 169.
3.1. Optical Results 3.1.1. General Morphology
Figure 2 represents the V- and U-band optical images of DDO 169. DDO 169 is fairly well resolved in the V band and appears to have a central concentration associated with the main body of the galaxy and extended emission in both the southeast and northwest directions. The central concentration, which extends along the northwest-southeast direction, appears to have three distinct knots embedded in luminous material. In the U-band image one can again see the main body of the galaxy with extensions in both the northwest and southeast directions. The extension to the northwest is more clearly seen in this image and contains numerous bright knots of emission. 3.1.2. Surface Photometry
The values of the surface brightnesses and colors provided by D. Hunter are listed in Table 2; R is the radius measured in arcseconds, and the surface brightnesses are measured in magnitudes per square arcsecond. B- and V-band luminosity profiles and UBV colors are shown in Figure 3. Elliptical annuli increasing in radius from 5B7 to 107B7 in steps of 11B3 were used. All the photometry has been corrected for reddening using a total E(B V )t ¼ E(B V ) f þ E(B V )i , where E(B V )f is zero (Burstein & Heiles 1984); E(B V )i refers to the internal extinction in the galaxy itself, and for dwarf irregular galaxies (dIrrs) like DDO 169 it is taken to be 0.05. Thus, a total E(B V ) of 0.05 has been used to correct the photometry. From the B- and V-band luminosity profiles, it can be seen that even though DDO 169 has a fairly irregular optical appearance, the profiles are fairly regular and are comparable to other dIrrs. Bremnes et al. (1999) reported a slight drop in the B surface brightness in the central region, but that is not seen here. The color profiles for DDO 169 appear to be fairly flat with a radius that is normal for dwarf galaxies (Hunter & Gallagher 1986). From the B-band surface photometry, R25 was measured to be 41B35 (=1.06 kpc). For an average (B V ) of 0.287, the Holmberg radius was determined to be 26.63 mag arcsec2, and its value RH was determined to be 80B5 (0.2 kpc). These values are listed in Table 3. In order to determine the central surface brightness of DDO 169, exponential fits were made to the B and V luminosity profiles. Radial intensity profiles of dwarf galaxies can generally be fitted well by an exponential function (de Vaucouleurs 1959). Fitting the V-band luminosity profile gives V (0) ¼ 23:18 mag arcsec2 and a scale length of 25B64. By making such a fit to the B surface brightness profile, a central surface brightness B (0) ¼ 23:47 mag arcsec2 and a scale length of 25B64 were determined.
Fig. 2.—V-band (left) and U-band (right) images of DDO 169.
TABLE 2 Surface Photometry for DDO 169 Radius (arcsec)
V (mag arcsec2)
B (mag arcsec2)
U (mag arcsec2)
(B V )0
(U B)0
5.7............................... 17.0............................. 28.3............................. 39.7............................. 51.0............................. 62.4............................. 73.7............................. 85.0............................. 96.4............................. 107.7...........................
23.618 23.859 24.221 24.631 25.118 25.596 26.003 26.556 27.116 27.444
23.901 24.115 24.497 24.926 25.434 25.912 26.311 26.842 27.348 27.712
23.44 23.622 24.152 24.6129 25.163 25.632 25.969 26.507 26.961 27.203
0:283 0:009 0:256 0:009 0:276 0:012 0:295 0:017 0:316 0:027 0:316 0:041 0:308 0:060 0:286 0:096 0:232 0:156 0:268 0:212
0:461 0:016 0:493 0:014 0:345 0:022 0:307 0:031 0:271 0:051 0:280 0:078 0:342 0:106 0:335 0:171 0:387 0:260 0:509 0:331
1038
IYER & SIMPSON
Vol. 132
Using the reddening corrected B0 of 14.33, the absolute blue magnitude (MB ) was calculated to be 14.29, and by using an integrated, corrected V magnitude of 14.04, an absolute V magnitude (MV ) of 14.58 was determined. The uncorrected integrated (U B)T is 0:324 0:036, and (B V )T is 0:338 0:021. The value of (B V )T compares well with that given by de Vaucoulers of 0:42 0:10; their value for (U B)T , 0:16 0:13, is different but agrees to within the estimated errors. With reddening-corrected integrated colors of (U B)0 ¼ 0:361 and (B V )0 ¼ 0:287, DDO 169 is bluer compared to most dIrrs (Hunter & Elmegreen 2004, Fig. 1). 3.1.3. Star-forming Regions
Fig. 3.— Luminosity profiles for the V and B bands of DDO 169. The dashed and the dotted lines represent the V and B bands, respectively. The bottom two panels represent the radial color distributions for the galaxy.
Bremnes et al. (1999) derive a B scale length of 14B74 and a B (0) of 22.15 mag arcsec2. The values derived by Bremnes et al. (1999) for their B band are significantly different from those determined here, perhaps because Bremnes et al. (1999) have fitted their luminosity profile by using only the mid and outer values. This work considers the luminosity profile of the entire galaxy. An uncorrected integrated total apparent B magnitude (BT ) of 14:51 0:04 at a radius of 113B4 was measured and compares well with the uncorrected BT ¼ 14:60 0:2 given by de Vaucouleurs et al. (1991). From their photometry, Bremnes et al. (1999) derive an integrated (uncorrected) B magnitude of 14.27 at a radius of 11000 , which is brighter than the apparent magnitude determined here. This is not surprising, since their value was integrated from a steeper exponential fit.
H has been detected in DDO 169. Since H acts as a tracer of star formation via the ionization of the progenitor cloud by short-lived massive O and B stars, it can be said that DDO 169 has undergone recent star formation. Figure 4 shows the H image of DDO 169 on a broadband V image. Most of the H appears to be concentrated at the center of the image and coincides with the central optical knot, with a second area of H emission in the region of the northern optical knot. The integrated H luminosity and inferred SFR are given in Table 3. The H fluxes were corrected for foreground reddening by Youngblood & Hunter (1999). We have calculated the total H luminosity of DDO 169 to be 7:29 ; 1037 ergs s1. The SFR is calculated from the H luminosity by using the formula SFR ¼ 5:96 ; 1042 L(H) M yr1 ;
ð1Þ
given by Hunter & Elmegreen (2004). This formula assumes a Salpeter (1955) stellar initial mass function from 0.1 to 100 M . In order to compare the SFR of DDO 169 to that of other galaxies, the SFR is normalized to the size of the galaxy, defined as R25. The normalized SFR shows no major deviations as compared to other dIrrs (see Hunter & Elmegreen 2004, Fig. 7), although it seems to fall at the low end of the distribution. The timescale required to use up the total prevalent gas content in
TABLE 3 Optical Parameters of DDO 169 Parameter
Value
Distance ( Mpc) .................................................... R25 (arcsec) .......................................................... Holmberg radius (arcsec)..................................... B (0) (mag arcsec2) ........................................... Scale length for the B band (arcsec) ................... (0) (mag arcsec2) ............................................. Scale length for the V band (arcsec) ................... Total apparent blue magnitude ............................ Total absolute blue magnitude............................. Total blue luminosity (LB)................................. Total apparent V magnitude................................. Total absolute V magnitude ................................. Total V luminosity (LV) ..................................... (U B)0 ............................................................... (B V )0 ............................................................... log H (ergs s1)................................................. SFR (M yr1)..................................................... log SFR (M yr1)............................................... log SFR /area (M yr1 kpc2) ............................
5.3 41.35 80.5 23.47 25.64 23.18 25.64 14.329 14.29 7:46 ; 107 14.042 14.58 5:65 ; 107 0.361 0.287 37.863 4:35 ; 104 3.36 3.38
Fig. 4.— H overlaid on the V-band image. The contours levels are 20%, 30%, 40%, and 50% of the peak value.
No. 3, 2006
DWARF IRREGULAR GALAXY DDO 169
1039
TABLE 4 Derived H i Properties Parameter
Value
MH i (M) ............................................................. Peak column density (atoms cm2)..................... R S dv (Jy km1 s1) ........................................... V20 ( km s1)..................................................... V50 ( km s1)..................................................... Systemic velocity ( km s1).................................
7:42 ; 107 1:12 ; 1021 11.21 68 45 255
the galaxy at its current SFR is 1:41 ; 1011 yr. This value is at the high end of the range for dIrrs ( Hunter & Elmegreen 2004, Fig. 9). However, the timescale to exhaust gas becomes much larger if the recycling of gas from stars in their final evolutionary stages is also considered ( Kennicutt et al. 1994). 3.2. H i Results 3.2.1. H i Morphology
The measured parameters from the H i observations for DDO 169 are given in Table 4. Figure 5 is a gray-scale image of the H i integrated flux density. The H i distribution has a central concentration of H i associated with the main optical body of the galaxy, with a small extension toward the southeast. Simpson & Gottesman (2000) reported a northwest ‘‘companion’’ to DDO 169 at a distance of 20 from it. They, however, did not have the requisite sensitivity to detect the connecting H i emission that can be seen in Figure 5. These observations show that the H i ‘‘companion’’ is not separate but is connected by an extension of H i to the main body of DDO 169. This extension appears to have H i lumps in it. In addition, there is a small extension to the southeast. The total H i mass and peak column density of DDO 169, including the extension, are 7:42 ; 107 M and 1:21 ; 1021 atoms cm2, respectively.
Fig. 5.— Integrated H i for DDO 169. The contour levels are (2, 9, 16, 23, 30, 37, 44, 51, 58, 65, 72, and 79) ; 1019 atoms cm2, in which 1019 atoms cm2 = 18.85 Jy beam1 m s1.
Fig. 6.— U-band image of DDO 169 with integrated H i flux density contours. The contour levels are those used in Fig. 5.
Figure 6 represents the U-band optical image overlaid with H i contours. The spread of the H i beyond the optical regions can be seen easily in both these images. An H i extent of 420 00 ; 221 00 (10:8 kpc ; 5:7 kpc) (not deconvolved) is measured out to 2 ; 1019 atoms cm2. The highest concentration of the H i coincides with the three-knot optical system. Figure 7 verifies that the bright knots in the optical tail coincide with the H i extension. The faint outer optical knots can only clearly be seen in the U-band image and are therefore very blue, and so most likely are regions of recent star formation. However, the H map does not extend out to these knots to confirm this. From Figure 7, which
Fig. 7.— H image of DDO 169 with integrated H i flux density contours. The contour levels are those used in Fig. 5.
Fig. 8.—H i channel maps of DDO 169 contoured onto the V optical image. The heliocentric velocity for each channel is listed in the top right corner.
1040
Fig. 8.—Continued
1041
Fig. 8.—Continued
1042
Fig. 8.—Continued
1043
1044
IYER & SIMPSON
Fig. 9.—Position-velocity diagram for DDO 169. Right ascension relative to the center of the field vs. velocity relative to the systemic velocity is plotted. The contours are percentages of the peak flux.
shows the H map overlaid with H i contours, it can be seen that the H only coincides with the central concentration of H i in DDO 169. 3.2.2. Kinematics
Channel maps for DDO 169 are presented in Figure 8. From these maps it can be seen that the H i ‘‘companion’’ is part of
Vol. 132
Fig. 11.—Velocity dispersion map of DDO 169.
an H i distribution connected to the main body of the galaxy. This extension appears in channels with velocities from 234.2 to 260 km s1. This is a shallow velocity gradient, which is common in tidal tails (Hibbard 1995). The presence of this tail can also be seen in a position-velocity (Fig. 9) diagram at a relative velocity of 10 km s1 and a relative right ascension of 30000 . The positionvelocity diagram was made by rotating the cube 40 , so the long axis (which very roughly corresponds to the kinematic major axis) was along the x-direction (rotated right ascension). Then xsum was used to sum along the y-axis (rotated declination). As noted earlier, Figure 1 represents the integrated profile for DDO 169. Its shape is mostly Gaussian, which is not surprising considering the central concentration of H i in DDO 169. It should be noted that because of the spatial resolution of the data, any structure in this ‘‘central concentration’’ would be smoothed out by the beam; i.e., if there is structure present in this region, it is not resolved. The velocity width at the 50% level of peak emission (V50 ) is found to be 45 km s1, and that at the 20% (V20 ) level of peak emission is 68 km s1. Both have been corrected for instrumental broadening but not inclination. Although the intensity-weighted velocity map (Fig. 10) of DDO 169 shows little ordered rotation, the position-velocity diagram is agreeable with solid-body rotation, disparate velocities in the H i extension, and a fairly large velocity dispersion at the central H i maximum. Furthermore, there appears to be a turnover in the extension that might imply that the outer part is falling back toward the galaxy. Figure 11 shows the dispersion map of DDO 169, in which one can see the differences in the dispersion between the main body of the galaxy (14 km s1) and the tail (6 km s1). The high dispersion velocity associated with the galaxy is indicative of turbulent motions in this part of DDO 169. 4. CONCLUSION
Fig. 10.— Temperature-weighted mean velocity map of DDO 169. Contours are plotted every 5 km s1 from 225 to 285 km s1.
In an earlier study of the H i Simpson & Gottesman (2000) reported the presence of faint optical emission connecting DDO 169 to a northwest H i companion. Current observations instead
No. 3, 2006
DWARF IRREGULAR GALAXY DDO 169
show that DDO 169 does not have a separate companion but that the H i emission is associated with an extension that is connected to the main body of the galaxy. The morphology and kinematics, particularly the disturbed velocity field of DDO 169, indicate that the galaxy has probably experienced a tidal interaction in the past, in which case the H i extension is most likely a tidal tail. The presence of the H i tail in the northwest direction and what could be a countertail in the southeast direction are very consistent with tidally disturbed systems. One of the consequences of a tidal interaction is that it causes gas close to the galaxy center to lose angular momentum and collapse into the core, triggering star formation. This could
1045
explain the optical knots and the H ii regions located near the center of the galaxy. Future work on this galaxy would require more spatially resolved H i data so that a rotation curve might be derived. Detailed photometry and spectra on the optical knots and H ii regions would give better information on the star formation and its history in this unusual system. The authors are grateful to Deidre Hunter for providing us with the optical data for DDO 169 and to the anonymous referee for his/ her comments on this paper.
REFERENCES Bremnes, T., Bingelli, B., & Prugniel, P. 1999, A&AS, 137, 337 Hunter, D. A., & Elmegreen, B. G. 2004, AJ, 128, 2170 Burstein, D., & Heiles, C. 1984, ApJS, 54, 33 ———. 2005, AJ, submitted Clark, B. G. 1980, A&A, 89, 377 Hunter, D. A., & Gallagher, J. S. 1986, PASP, 98, 5 de Vaucouleurs, G. 1959, Handb. Phys., 53, 275 Kennicutt, R. C., Tamblyn, T., & Congdon, C. E. 1994, ApJ, 435, 22 de Vaucouleurs, G., de Vaucouleurs, A., Corwin, H., Buta, R., Paturel, G., & Napier, P. J., Thompson, A. R., & Ekers, R. D. 1983, Proc. IEEE, 71, 1295 Fouque´, P. 1991, Third Reference Catalogue of Bright Galaxies ( New York: Salpeter, E. E. 1955, ApJ, 121, 161 Springer) Simpson, C. E., & Gottesman, S. T. 2000, AJ, 120, 2975 Hibbard, J. H. 1995, Ph.D. thesis, Columbia Univ. Youngblood, A. J., & Hunter, D. A. 1999, ApJ, 519, 55 Ho¨gbom, J. A. 1974, A&AS, 15, 417 Huchtmeier, W. K, & Richter, O. G. 1989, A General Catalog of H i Observations of Galaxies. The Reference Catalog, XIX ( Berlin: Springer)
