The Astrophysical Journal, 718:683–694, 2010 August 1 doi:10.1088/0004-637X/718/2/683 C 2010. The American Astronomical Society. All rights reserved. Printed in the U.S.A. THE EDGE OF THE YOUNG GALACTIC DISK Giovanni Carraro1,6, Ruben A. Vazquez´ 2, Edgardo Costa3, Gabriel Perren4,7,andAndre´ Moitinho5 1 ESO, Alonso de Cordova 3107, 19100 Santiago de Chile, Chile 2 Facultad de Ciencias Astronomicas´ y Geof´ısicas (UNLP), Instituto de Astrof´ısica de La Plata (CONICET, UNLP), Paseo del Bosque s/n, La Plata, Argentina 3 Departamento de Astronom´ıa, Universidad de Chile, Casilla 36-D, Santiago, Chile 4 Instituto de F´ısica de Rosario, IFIR (CONICET-UNR), Parque Urquiza, 2000 Rosario, Argentina 5 SIM/IDL, Faculdade de Ciencias´ da Universidade de Lisboa, Ed. C8, Campo Grande, 1749-016 Lisboa, Portugal Received 2009 August 17; accepted 2010 June 2; published 2010 July 6 ABSTRACT In this work, we report and discuss the detection of two distant diffuse stellar groups in the third Galactic quadrant. They are composed of young stars, with spectral types ranging from late O to late B, and lie at galactocentric distances between 15 and 20 kpc. These groups are located in the area of two cataloged open clusters (VdB–Hagen 04 and Ruprecht 30), projected toward the Vela-Puppis constellations, and within the core of the Canis Major overdensity. Their reddening and distances have been estimated by analyzing their color–color and color–magnitude diagrams, derived from deep UBV photometry. The existence of young star aggregates at such extreme distances from the Galactic center challenges the commonly accepted scenario in which the Galactic disk has a sharp cutoff at about 14 kpc from the Galactic center and indicates that it extends to much greater distances (as also supported by the recent detection of CO molecular complexes well beyond this distance). While the groups we find in the area of Ruprecht 30 are compatible with the Orion and Norma-Cygnus spiral arms, respectively, the distant group we identify in the region of VdB–Hagen 04 lies in the external regions of the Norma-Cygnus arm, at a galactocentric distance (∼20 kpc) where no young stars have been detected so far in the optical. Key words: Galaxy: disk – Galaxy: structure – open clusters and associations: general Online-only material: color figures 1. INTRODUCTION et al. 2002; see Table 1), projected toward the Vela-Puppis constellations, well within the core of the CMa overdensity. Although the existence of a conspicuous extinction window This is the first time that such a young population is detected in the direction of the third Galactic quadrant (3GQ) has been in the very outer disk in optical. H i in our Galaxy extends out known for decades (see, e.g., Fitzgerald 1968;Moffatetal.1979; to galactocentric distances of 25 kpc, and CO clouds have been Janes 1991; Moitinho 2001), it has only recently been fully found out to 20 kpc (Brand & Wouterloot 2007). In the infrared exploited to probe the structure of the outer Galactic disk in the (IR), compact and well-confined regions of star formation have optical domain. Major results from this observational effort have been recently detected by various groups (Snell et al. 2002;Yun been the detection of (1) previously unknown spiral features et al. 2007, 2009; Brand & Wouterloot 2007), both in the second (Carraro et al. 2005; Moitinho et al. 2006;Vazquez´ et al. 2008) and in the third Galactic quadrant. and (2) stellar overdensities: the Canis Major, CMa overdensity The existence of young star aggregates at such extreme (Martin et al. 2004), and the Monoceros Ring (Newberg et al. distances from the Galactic center defies the commonly accepted 2002), both believed to be signatures of past accretion events. scenario in which the Galactic disk has a sharp cutoff at about Our research group has contributed substantially to the 14 kpc from the Galactic center (Robin & Crez´ e´ 1986a, 1986b; subject, providing a new picture of the outer disk spiral structure. Robin et al. 1992) and indicates that it extends to much greater Briefly, the most important findings are: (1) the outer (Norma- distances. The meaning of this cutoff has been questioned by Cygnus) arm has been found to be a grand design spiral feature Momany et al. (2006),asfarasold/intermediate age populations defined by young stars, (2) the region closer to the Sun, at are concerned. We also recall that the absence of structures galactocentric distances smaller than 9 kpc, is dominated by a beyond this distance in model color–magnitude diagrams has conspicuous inter-arm structure, the Local (Orion) spiral arm, been erroneously used as evidence to support the existence of at l ∼ 245◦, and (3) the Perseus arm seems to be defined in the the CMa dwarf Galaxy (Martin et al. 2004). Quite recently, Sale 3GQ by gas and dust and does not appear to be traced by an et al. (2010) made use of the IPHAS surveys to address the same evident (optical) young stellar population. point in a statistical way. By using over 40,000 A-type stars they Here, we report the detection of two diffuse stellar groups argue that they do not see any abrupt truncation of the stellar in the 3GQ, containing young stars with spectral types ranging density profile. from late O to A0, which lie at galactocentric distances between Along the same vein, but with greater detail provided by the 15 and 20 kpc, beyond the widely accepted radius of the Galactic use of multicolor photometry, we study in this work the sparse disk (14 kpc). The groups are located in the direction of two young population in the (far-) outer disk along two lines of cataloged open clusters (VdB–Hagen 04 and Ruprecht 30; Dias sight and show that the stellar density profile of young stars (earlier than A0) does not drop suddenly at 12–14 kpc from the Galactic center, but smoothly declines out to 20 kpc. Together 6 On leave from Dipartimento di Astronomia, Universita´ di Padova, with the young, sparse population, we confirm the existence Vicolo Osservatorio 5, I-35122 Padova, Italy. and properties of a compact, extremely distant star cluster, 7 Fellow of the CONICET, Argentina. 683 684 CARRARO ET AL. Vol. 718 Figure 1. V 600 s frame in the region of VdB–Hagen 04 (left) and Ruprecht 30 (right). The FOV is 20on a side. North is down and east is to the right. Table 1 Basic Properties of the Two Fields Studied in This Work Field Designation α(2000.0) δ(2000.0) lbE(B − V )FIRB Constellation (deg) (deg) (mag) Field 1 VdB–Hagen 04 07:37:44 −36:04:00 249.98 −07.13 0.49 Puppis Field 2 Ruprecht 30 07:42:25 −31:28:00 246.42 −04.05 0.68 Puppis VdB–Hagen 04, located at more than 20 kpc from the Galactic Table 2 center. Log of BVRI Photometric Observations in the Field of VdB–Hagen 04 and Ruprecht 30 2. OBSERVATIONS AND DATA REDUCTION Target Date Filter Exposure (s) Airmass U − 2.1. Observations VdB–Hagen 04 2008 Jan 29 10, 20, 100, 200, 900, 1800 1.02 1.04 B 5, 20, 100, 200, 600, 1800 1.01−1.02 The regions of interest (see Figure 1) were observed with V 5, 10, 60, 120, 600, 1200 1.01−1.03 × − the Y4KCAM camera attached to the Cerro Tololo Inter- SA 98 2008 Jan 29 U 2 10, 200, 300, 400 1.17 1.96 B 2 × 10, 100, 2 × 200 1.17−1.89 American Observatory (CTIO) 1.0 m telescope, operated by × × − 8 V 2 10, 100, 2 200 1.14 2.12 the SMARTS consortium. This camera is equipped with an × − × VdB–Hagen 04 2008 Feb 3 U 2 10, 60, 200 1.02 1.04 STA 4064 4064 CCD with 15 μm pixels, yielding a scale of B 2 × 10, 30, 60 1.01−1.02 −1 0.289 pixel and a field of view (FOV) of 20 × 20 at the V 2 × 10, 30, 60 1.01−1.03 Cassegrain focus of the CTIO 1.0 m telescope. The CCD was SA 98 2008 Feb 3 U 2 × 10, 200, 300, 400 1.07−1.96 − operated without binning, at a nominal gain of 1.44 e /ADU, B 2 × 10, 100, 2 × 200 1.07−1.89 implying a readout noise of 7 e− per quadrant (this detec- V 2 × 10, 100, 2 × 200 1.14−2.12 tor is read by means of four different amplifiers). Quantum V 2 × 10, 100, 2 × 200 1.06−2.02 efficiency and other detector characteristics can be found at Ruprecht 30 2008 Jan 31 U 5, 20, 100, 200 1.10−1.12 − http://www.astronomy.ohio-state.edu/Y4KCam/detector.html. B 5, 10, 60, 120 1.13 1.15 V − UBV 30, 120, 1200 1.13 1.16 In Table 2, we present the log of the observations. SA 98 2008 Jan 31 U 2 × 10, 200, 300, 400 1.15−2.20 All observations were carried out in photometric, good seeing B 2 × 10, 100, 2 × 200 1.05−1.79 conditions. Typical values for the seeing were 1.1, 1.4, 1.5, V 2 × 10, 100, 2 × 200 1.05−1.79 and 1.0 for 2008 January 29, January 31, February 1, and Ruprecht 30 2008 Feb 1 U 30, 600, 1500 1.00−1.00 February 3, respectively. Our UBV instrumental photometric B 30, 600, 1500 1.03−1.04 system was defined by the use of a standard broadband Kitt Peak V 30, 600, 1200 1.01−1.02 UBV set.