Kuiper Widefield Infrared Camera Far‐Infrared Imaging of the Galactic Center: The Circumnuclear Disk Revealed

Abstract

We present images of the Galactic center in the dust continuum at 31.5 and 37.7 km obtained with the Kuiper WideÐeld Infrared Camera on the Kuiper Airborne Observatory. The raw images have a spatial resolution of and cover a region roughly 10@ (R.A.) by 24@ (decl.) in extent. Here we discuss the D8A .5 innermost (6.9 ] 9.1 pc) regions roughly centered on Sagittarius A*, where the high signal-to-2@ .8 ] 3@ .6 noise ratio permits image restoration to spatial resolution at 31.5 and 37.7 km, respectively. D3A .6È5A .7 These images clearly delineate the warm dust in the "" minispiral ÏÏ but also show an elliptical structure centered on Sgr A* and intersecting the minispiral at the western arc that we identify as the photodissociated inner rim of the circumnuclear disk (CND) or ring. This is the Ðrst image of the complete disk and the Ðrst image of both the minispiral and CND in a single tracer, thereby permitting detailed studies of the intimate association between these two structures. The 31.5/37.7 km color temperature map indicates that most of the far-UV Ñux required to heat the dust grains must come from centrally located sources. The cluster of He I emission-line stars recently identiÐed near Sgr A* is sufficient to provide half the heating for the far-IR ring and minispiral structures, the balance likely being provided by O and B stars associated with the cluster. We also Ðnd D16 color temperature peaks distributed within the CND that we suggest are the locations of early type (B4 to O9) main-sequence stars. To match the observed far-IR Ñuxes from the ring, we require an unusually high 30È40 km dust grain emissivity and/or an usually high UV albedo.The far-IR morphology is reproduced by a simple model : an inclined (i D 65¡), slightly elliptical (e D 0.06) torus with one focus at Sgr A* and two streamers on parabolic orbits with foci at Sgr A*. The torus has an inner radius of 1.58 pc, is D0.4 pc thick, and consists of clumpy cloudlets with characteristic sizes less than 0.15 pc. The northern streamer is identiÐed with the northern arm, and the east-west (EW) streamer is identiÐed with the bar and eastern arm structures seen in both their [Ne II] Ðne-structure line emission and the radio continuum. The northern arm is traced in the far-IR continuum from regions D1.4 pc outside of the far-IR ring to its apparent intersection with the EW streamer. The eastern CND is not detected in the radio continuum because of extinction by the intervening northern arm, which therefore must be nearly in the plane of the CND. Extensions of the EW streamer outside of the CND are apparent to the east and especially to the northwest. The EW streamer lies D85¡ out of the plane of the CND and is on a parobolic orbit focused on Sgr A* at a distance of 0.33 pc, with its apex well in front of or behind Sgr A*.

Interstellar gas within ~10 pc of Sagittarius A∗

“…By using their 63 μm [O i] data toward the northern peak together with previous 146 μm [O i] and 158 μm [C ii] data (from Genzel et al 1985;Poglitsch et al 1991) as input to model calculations of collisional excitation and radiative transport, Jackson et al (1993) estimated that neutral (presumably atomic) gas inside the central cavity has a temperature ≈(170 ± 70) K, a true hydrogen density ≈3 × 10 5 cm −3 , a space-averaged hydrogen density ≈1.6 × 10 3 cm −3 (beam-averaged hydrogen column density in a 55 beam divided by an assumed line-of-sight pathlength of 1 pc; note, however, that the value reported in their Table 2 is too low by a factor of 2) and a total hydrogen mass (associated with the Northern and Eastern Arms) ≈300 M . Latvakoski et al (1999) found that the three arms of the Minispiral seen in thermal radio continuum emission have counterparts in dust far-infrared continuum emission, which tend to lie ≈1 −3 farther from Sgr A * . This configuration, they explained, is consistent with the radio and far-infrared features being photo-ionized and photo-dissociated, respectively, by UV sources very close to Sgr A * .…”

“…On the other hand, little molecular gas seems to be present, except possibly for a hot and dense molecular component detected in NH 3 (6, 6) emission very near Sgr A * (Herrnstein & Ho 2002). The general lack of molecular gas can be understood if any initially molecular cloud inside the central cavity has been largely photo-dissociated by the intense ambient UV radiation (Jackson et al 1993;Latvakoski et al 1999;Shukla et al 2004).…”

Approaching hell’s kitchen: Molecular daredevil clouds in the vicinity of Sagittarius A*

“…However, the assumption of virialization yields unplausible masses of several 10 3 M per clump and few 10 5 M for the whole CA. The masses of atomic hydrogen inside the cavity (dominated by the minispiral NA) range between 50 -300 M (from FIR and O i observations; Telesco et al 1996;Latvakoski et al 1999;Jackson et al 1993), while the molecular gas mass is most likely only a fraction of this.…”

Surviving the hole

“…The inner edge of the CND in the southwest lobe appears to be tightly outlined by the western ionized arc. This correlation has led to the idea that the western arc might be the ionized boundary of the CND where the strong pervasive UV field from the central cluster is responsible for the heating of the CND Amo-Baladrón et al 2011) and the mini-spiral structure (Latvakoski et al 1999). This association was kinematically confirmed by Christopher et al (2005).…”

“…Though this arm appears to be crossing the eastern extension of the CND, there is no evidence of perturbation of the molecular material, which indicates that both features are not in the same plane. Dust emission modeling (Latvakoski et al 1999) and H92α kinematics (Zhao et al 2009) showed that the eastern arm must be significantly inclined with respect to the eastern CND. Fig.…”