Extragalactic Reference Targets for PRIMA

Transcription

1 Extragalactic Reference Targets for PRIMA Gerard T. van Belle a, Roberto Abuter a, Judith Ngoumou a,b, Francoise Delplancke a, Johannes Sahlmann a,c a European Southern Observatory, Karl-Schwarzschild-Strasse 2, Garching b. München, Germany; b Ludwig-Maximilians-Universitt München, Geschwister-Scholl-Platz 1, München, Germany; c Observatoire Astronomique de l Université degenève, 51, chemin des Maillettes, CH-1290 Sauverny, Switzerland; ABSTRACT The VLTI s PRIMA (Phase-Referenced Imaging and Microarcsecond Astrometry) instrument is designed to provide tens of microarcseond astrometry and faint-object imaging for the interferometer facility. Astrometry is to be enabled by PRIMA between object pairs that are separated on the sky by one isoplanatic patch (roughly 60 seconds of arc at Cerro Paranal in the K band), with at least one of the two objects being bright (K <10), and a second fainter object ( K <7) that is nominally a stable astrometric reference. The original expectation was for star-star pairs to be observed by PRIMA in its astrometric mode; however, we are exploring the possibility of also utilizing background galaxies in this role. Advantages of such source selection is eliminating the need to solve for dim object parallax and proper motion before obtaining similar values for the bright foreground star. Additionally, data from the galaxy may be of scientific interest as well, potentially leading to characterizations of object morphology at milliarcsecond scales. Towards that end, we have begun observations with ESO s NTT to explore the suitability of qualifying star-galaxy asterisms as potential PRIMA targets. Commissioning observations for PRIMA are slated to begin in Fall of Keywords: VLTI:PRIMA, astrometry, interferometry:near-infrared 1. INTRODUCTION The Phase-Referenced Imaging and Microarcsecond Astrometry (PRIMA) instrument of the Very Large Telescope Interferometer (VLTI) will be the last of the first generation of VLTI instruments to be commissioned, with an expectation of first light into the system late in , 2 The intent of PRIMA is to take advantage of the performance advantages possible with narrow-field dual-beam interferometric observations, with both astrometry 3 and imaging. 4 Given the dual-beam design, PRIMA requires source pairs for feeding into the system during a single observation. For the specific case of astrometry, there is a need for two (or more) sources upon the sky, with at least one of them serving in some capacity as an astrometric fiducial. Given that fringe tracking on galactic cores has been demonstrated as possible, 5 these objects bear consideration as reasonable astrometric references. Effects that will need to be carefully considered when using stars as astrometric references include relative proper motion and relative parallax, in much the same way these effects are carefully accounted for when using a Hubble Space Telescope (HST) Fine Guidance Sensor (FGS) for astrometry. 6 However, in the case of extragalactic cores employed as astrometric references, these effects can effectively be considered to be zero, even in the limit of 10 µas measures of astrometric separation vectors between reference and nearby target. As such, these cores can be considered to be absolute astrometric references, and are appealing particularly for system commissioning. Further author information: (Send correspondence to G.v.B.) G.v.B.: gerard.van.belle@eso.org, Telephone: +49 (0) Optical and Infrared Interferometry, edited by Markus Schöller, William C. Danchi, Françoise Delplancke Proc. of SPIE Vol. 7013, 70133W, (2008) X/08/$18 doi: / SPIE Digital Library -- Subscriber Archive Copy Proc. of SPIE Vol W-1

2 Selection of star-galaxy pairs is done in a manner that is often employed for other investigations in nearinfrared interferometry: given the limitations (sensitivity, angular resolution, etc.) of the instrument, the calibration or reference target is selected first, with a following inspection for bright foreground targets of interest that satisfy the instrumental observing criteria. That selection is described in detail in 2, observations that followed with NTT-SOFI being discussed in 3, and results are presented in TARGET LIST The 2MASS point source catalog 7 was scanned for all targets between K s =5.0 and 9.0 (presumably stellar in this brightness range). These targets were cross-referenced against the Third Reference Cat. of Bright Galaxies (RC3) 8 for targets within 75 arcseconds. Positive cross-reference hits were then back-referenced against 2MASS for K s -band brightness values for the galactic cores. From this all-sky list of 309 asterisms, targets with a zenith angle from Paranal greater than 45 o were cut: 113 targets were too far north, 10 were too far south. From the remaining 186, a cut was made for targets available late in October (RA=22 h to 6 h ), leaving 46 targets. These targets were selected for a observing run that took place in October 2007 with the European Southern Observatory (ESO) New Technology Telescope (NTT), using the SOFI imager. 9 SOFI has a 0.29 plate scale for K-band observations, a good match for attempting to characterize the galactic cores at the 1.0 level. 3. OBSERVATIONS Observations were successfully obtained on 26, 27, 30 Oct 2007, with the intervening nights being weathered out, and the open-shutter nights being somewhat poor in quality as well. Observations at K s were obtained for a majority of the targets found in Table 1 (for targets from RA=0 h to 6 h ) and Table 1 (for targets from RA=22 h to 0 h ). Observations were attempted as well in the J-band and a narrow band Brγ filter but were incomplete, being preempted by weather; as such, they will not be considered herein. Example K-band images for two of the targets observed are seen in Figures 1 and 2. The observing results are presented in Table 3. An initial qualitative assessment of the image quality was made of the images and can be found in the 4th column of that table. The separation vector of the galaxy and the bright foreground star as predicted from the 2MASS data can be seen in the second column, with the value as measured in our NTT-SOFI images presented in the third. Targets deemed to be of poor initial quality generally did not have this measurement made (in many cases because such a measurement was not possible). Some targets have slight variations in the predicted and measured numbers; this is expected to be the result of the proper motion associated with the foreground object, many of which are high proper motion stars. Large variations in the predicted and measured separation vectors are possibly attributed to misidentification of one or both of the objects. Associated qualitative notes are found in the 5th column of Table 3. Objects found to be of at least moderate quality or better were examined to characterize the peakedness of the galaxy of reference. For use as an interferometric reference object, the background galaxy will need to have a sufficient concentration of flux in its central 0.1 to produce interferometric fringes of sufficient contrast for detection and tracking, which ultimately result in an astrometric separation vector. If the galaxy is too diffuse, fringes originating from points across the image appear on the detector in an incoherent sense and blur each other out. Intensity profile cuts across the surface of the galaxy were extracted from the image, with Gaussians fit to the profiles; an identical fit was done for the bright foreground stars. A sample galaxy profile cut can be seen in Figure 3. The full-width-half-max (FWHM) for the galaxies and stars are seen in Table 3, columns 5 and 6, with the ratio of these two values seen in column RESULTS From the initial list of 46 sufficiently bright star-galaxy pairs, 20 fields were observed to be qualitatively reasonable for further characterization with Gaussian FWHMs of both the star and galaxy. The remaining 26 were discarded due to poor image quality, either due to weather or intrinsic source qualities. The galaxy-to-star FWHM ratios of the 20 retained images ranged from 0.80 to 3.94, with 4 point-like sources in the range <1.5. Proc. of SPIE Vol W-2

3 Figure 1. Extragalactic source ESO , a Seyfert 1, and nearby star HD at a distance of 35.4 from the galaxy. FWHM of the galaxy is 2.2 ; for the star, 2.7. The very best candidate, ESO / HD 23134, consists of a K s =6.0 K0IV star 33.3 away from a galaxy classified as a Seyfert 1. It is perhaps no coincidence that the Swain et al. (2003) study 5 that was able to track fringes on an extragalactic source was also observing a Seyfert 1 source: this class of object is thought to be host to an active galactic nuclei viewed pole-on. This viewing perspective results in a core that is unshielded by the surrounding dust torus and is most like to present a compact point-like source at the center of the galactic image. The other three most point-like sources have no noteworthy typing in either RC3 or SIMBAD, although this could be a subjective selection effect. 5. CONCLUSION We have imaged possible star-galaxy pairs with NTT-SOFI as possible object pairs for PRIMA commissioning observations, and found at least a few (4 or more) that appear to be attractive candidates, including pair containing a Seyfert 1 galaxy. A second, complementary, season of observing is needed for those sources in the RA=6 h to 22 h, which would probably result in a similar additional number of targets for PRIMA. Although the motivation for this study was to produce targets suitable for commissioning observations of PRIMA, we note that these candidates are also very attractive for their scientific value as well. Specifically, for stellar astrophysics, the ability to characterize the proper motion and especially parallax of the bright foreground object without having to disentangle the like quantities of the background astrometric fiducial present an opportunity measure absolute stellar distance at an unprecedented level of accuracy. Derived quantities, most notably luminosity, would also benefit from that accuracy. Targets pairs that appear to be unsuitable for use as commissioning targets, due to non-compact nature of the galactic cores, are also still of scientific interest. Using the foreground star as an interferometric phase reference, it will be possible to do tomographic mapping of the inner core of the background galaxy, even in the case of a more diffuse intensity distribution of that core. Proc. of SPIE Vol W-3

4 (pixels) Figure 2. Extragalactic source UGC and a nearby star that only as a 2MASS identifier (J ) at a distance of FWHM of the galaxy is 8.11 ; for the star, 3.3. Structure in the image is due to the 2 2SOFIdither pattern. Separation vector fit has been sketched onto the image, along with a cross-sectional cut (illustrated in the next figure). TexTd ) XPUT (TeXTd U US OUT on UDa U Ug U9 UUT sunos Figure 3. Cross section for UGC12305 as seen in the previous figure. At a plate scale of 0.29, the galaxy s FWHM is 8.11 across. Proc. of SPIE Vol W-4

5 Table 1. Source list of star-galaxy target pairs, from RA=0 h to 6 h. Ks type type sep B V Ks Galaxy RA2000 DE2000 (mag) (RC3) (SIMBAD) (asec) starid RA2000 DE2000 (mag) (mag) (mag) Sptype MCG SXS8*. G 38.6 HD G0 MCG S?... G 50.2 HD K2 UGC S..8*. G 18.2 BD G5 UGC I..9*. LSB 70.7 StKM K5 ESO S?... H2G 21.3 TYC UGC SAS5.. GiG 62.2 HD F8V MCG S?... GiG 36.2 BD K2 UGC SB.9.. G 64.9 TYC ESO LA.+P/ rg 65.2 CD UGC S..0*/ Sy2 7.6 BD G5 MCG SB?... G 62.1 HD G5 UGC E.6.$. G 56.3 BD G0 MCG PSBR2.. G 58.2 BD K2 UGC S..6*. IG UGC IXS9*. LSB 70.8 TYC PGC G 14.7 BD K0 UGC S..1.. G 33.5 HD A5 PGC LA.0P? G 58.5 BD ESO SBT1P? Sy HD K0IV UGC SXS8.. LSB 20.4 RX J K3e... UGC S?... G 25.9 HD F0 PGC LA UGC S..6*. G UGC SB.6*. G 72.5 BD G5 ESO SBR1*P G 56.3 CD MCG SBS5P* G 72.6 BD K5 UGC I..9?. G 70.3 TYC UGC S?... G 73.9 BD F5 MCG S..3P* G UGC S..1*. G 56.3 TYC UGC SB.3?. Sy ESO SXS5*. LSB 68.5 CD ESO IBS9.. G ESO S..3?/ G 73.7 HD A8IV/V Proc. of SPIE Vol W-5

6 Table 2. Source list of star-galaxy target pairs, from RA=22 h to 0 h. Ks type type sep B V Ks Galaxy RA2000 DE2000 (mag) (RC3) (SIMBAD) (asec) starid RA2000 DE2000 (mag) (mag) (mag) Sptype ESO SBS5*. G 58.8 CD ESO IAS9.. G 69.4 HD A8V ESO SBR4.. G 42 HD F3V ESO S?... G 47.1 CD ESO SBS9*P G 69.2 HD G2V MCG SXS8?. G MCG SXS8?. G ESO P... G 59.6 V* SZ Aqr KIab/Ibpe UGC E.4... GiG ESO E... G 49.5 HD F5V MCG L..+$. G 66.5 HD G0 UGC SX.8.. G 9.4 TYC Proc. of SPIE Vol W-6

7 ACKNOWLEDGMENTS This research has made use of the SIMBAD literature database, operated at CDS, Strasbourg, France, the FUTDI database at AMNH, and of NASA s Astrophysics Data System. REFERENCES [1] Glindemann, A. and Lévêque, S., PRIMA The VLTI Dual Feed System, in [From Extrasolar Planets to Cosmology: The VLT Opening Symposium], Bergeron, J. and Renzini, A., eds., (2000). [2] Delplancke, F., Derie, F., Paresce, F., Glindemann, A., Lévy, F., Lévêque, S., and Ménardi, S., PRIMA for the VLTI - Science, Astrophysics and Space Science 286, (2003). [3] Shao, M. and Colavita, M. M., Potential of long-baseline infrared interferometry for narrow-angle astrometry, A&A 262, (Aug. 1992). [4] Lane, B. F. and Colavita, M. M., Phase-referenced Stellar Interferometry at the Palomar Testbed Interferometer, AJ 125, (Mar. 2003). [5] Swain, M., Vasisht, G., Akeson, R., Monnier, J., Millan-Gabet, R., Serabyn, E., Creech-Eakman, M., van Belle, G., Beletic, J., Beichman, C., Boden, A., Booth, A., Colavita, M., Gathright, J., Hrynevych, M., Koresko, C., Le Mignant, D., Ligon, R., Mennesson, B., Neyman, C., Sargent, A., Shao, M., Thompson, R., Unwin, S., and Wizinowich, P., Interferometer Observations of Subparsec-Scale Infrared Emission in the Nucleus of NGC 4151, ApJL 596, L163 L166 (Oct. 2003). [6] Benedict,G.F.,McArthur,B.E.,Feast,M.W.,Barnes,T.G.,Harrison,T.E.,Patterson,R.J.,Menzies, J. W., Bean, J. L., and Freedman, W. L., Hubble Space Telescope Fine Guidance Sensor Parallaxes of Galactic Cepheid Variable Stars: Period-Luminosity Relations, AJ 133, (Apr. 2007). [7] Cutri, R. M., Skrutskie, M. F., van Dyk, S., Beichman, C. A., Carpenter, J. M., Chester, T., Cambresy, L., Evans, T., Fowler, J., Gizis, J., Howard, E., Huchra, J., Jarrett, T., Kopan, E. L., Kirkpatrick, J. D., Light, R. M., Marsh, K. A., McCallon, H., Schneider, S., Stiening, R., Sykes, M., Weinberg, M., Wheaton, W. A., Wheelock, S., and Zacarias, N., [2MASS All Sky Catalog of point sources.], The IRSA 2MASS All-Sky Point Source Catalog, NASA/IPAC Infrared Science Archive. (June 2003). [8] de Vaucouleurs, G., de Vaucouleurs, A., Corwin, Jr., H. G., Buta, R. J., Paturel, G., and Fouque, P., [Third Reference Catalogue of Bright Galaxies], Volume 1-3, XII, 2069 pp. 7 figs.. Springer-Verlag Berlin Heidelberg New York (1991). [9] Moorwood, A., Cuby, J.-G., and Lidman, C., SOFI sees first light at the NTT., The Messenger 91, 9 13 (Mar. 1998). Proc. of SPIE Vol W-7

8 Table 3. Results of the observations as discussed in 3, sorted in increasing RA as Tables 1 and 2. 2MASS Measured Galaxy Star Separation Separation Subjective image FWHM FWHM Galaxy (arcsec) (arcsec) Subjective reduction notes (arcsec) (arcsec) Ratio MCG MCG Bad UGC Bad Target found but not good UGC Bad ESO Ok? Also a double star 130 away UGC Bad Galaxy too dim MCG Bad Empty field - weather? UGC Bad Empty field - weather? ESO UGC Bad No reference star? MCG Ok? Dim but compact core UGC Good Too extended? Maybe not MCG Good UGC UGC Bad Galaxy too dim PGC Bad Empty field UGC Good Too extended? Probably PGC Good ESO Good triple system? UGC Poor Galaxy too dim UGC Very good Triple system PGC Good Triple system UGC Good Too extended? Probably UGC Poor Double star at 99 from galaxy; too dim ESO Very good Second phase ref at 93 from galaxy MCG Good UGC Bad Galaxy too dim UGC Very good Additional stars(?) at 120 from galaxy MCG Good Too extended? Probably UGC UGC Excellent Very bright source at 134 from galaxy ESO Good Galaxy too dim? ESO Bad Galaxy too dim ESO Good ESO Poor? Galaxy too dim ESO Poor ESO Ok? Galaxy pretty dim ESO Very good ESO Poor Galaxy too dim MCG Poor MCG Poor ESO Bad UGC Very good ESO Excellent Well peaked MCG Good UGC Bad Galaxy too dim extended Proc. of SPIE Vol W-8