High‐Density, Compact HiiRegions in the Starburst Galaxies NGC 3628 and IC 694: High‐Resolution VLA Observations of the H92α Radio Recombination Line

Abstract

Using the Very Large Array (VLA), we have observed the H92a line in the nuclear regions of NGC 1 3628 and Arp 299 (IC 694 ] NGC 3690) with an angular resolution of 1A. The radio recombination lines could provide quite important constraints to the physical conditions for the ionized gas in the nuclear regions. For the central nuclear components in NGC 3628, IC 694, and NGC 3690, we Ðnd that the electron density of the ionized gas is in the range 5 ] 102 to 5 ] 104 cm~3, with corresponding geometrical sizes ranging from 25 to 0.01 pc. For the regions with a large line-to-continuum ratio, such as the anomalous kinematic components in IC 694 and NGC 3628, the electron density of the H II regions is well constrained. The derived parameters suggest the existence of D100 high-density, compact H II regions cm~3 and l D 1È2 pc) in these two components. The large number of high-density, (n e D 104 compact H II regions deduced for these components can be regarded as evidence for enhanced massive star formation in anomalous kinematic structures that are often produced in interacting systems.Our new H92a line observations reveal the presence of rotating circumnuclear ionized gas disks in both NGC 3628 and IC 694. From the analysis of the nuclear kinematics, we Ðnd that a total dynamical mass of 3 ] 108 exists within a radius R \ 120 pc in NGC 3628 and 7 ] 108 within R \ 200 pc M _ M _ in IC 694. The kinematics of the ionized gas and the models with a collection of H II regions infer that the ratio of H II to dynamical mass in the starburst nuclei varies from 1 ] 10~4 (for the high-density, compact H II region model) to 1 ] 10~2 (for the low-density, large H II region model).

Chandra Observations of the X-Ray-luminous Star-forming Galaxy Merger Arp 299

“…Neff et al 2002, private communication) that is also associated with an NIR source (not identified by AH00). Its relatively soft X-ray colors suggest that it may be associated with a radio supernova, as was originally proposed by Huang et al (1990), rather than a background QSO (Zhao et al 1997). …”

“…We also find an X-ray counterpart (source 8) for a radio source in the east of CЈ, which we will refer to as CЉ ( , ; h m s a p 11 28 31.7 d p ϩ58Њ33 49Љ .5 J2000). In addition, we have detected an X-ray counterpart (source 10) of the radio source IC 694ϩ1990 (which lies 5Љ southwest of component A; Huang et al 1990;Zhao et al 1997;S. Neff et al 2002, private communication) that is also associated with an NIR source (not identified by AH00).…”

“…Given the large young stellar population, this region may be expected to contain a significant number of high-mass X-ray binaries (HMXBs). From the derived Lyman continuum flux of this region (Zhao et al 1997; AH00) and assuming a mean Lyman continuum flux of ∼10 49 s Ϫ1 per Otype star, we estimate a population of ∼ O-type stars. 3.5 # 10 which is in good agreement with the intrinsic X-ray luminosity from this region.…”

“…We have compared the X-ray data with observations made in the radio (Zhao et al 1997;S. Neff et al 2002, private communication) and NIR wavelengths (AH00).…”

“…and typical ages of ∼10 Myr (Alonso-Herrero et al 2000, hereafter AH00; Zhao et al 1997). Mid-IR observations indicate that the major part of this activity is obscured by large amounts of dust (Charmandaris, Stacey, & Gull 2002, hereafter C02).…”