1 Introduction

  The present work is part of a larger project to study the galactic plane (ISOGAL, Omont & Blommaert 1997; Pérault et al. 1996). During the ISO mission, the ISOGAL consortium observed with ISOCAM at 15 and 7 $\mu$m selected parts of the galactic plane (about 18 sq. deg. distributed along the inner galactic disk) in order to study the stellar populations in the inner galaxy, with a sensitivity and resolution two orders of magnitude better than IRAS. The main scientific goal of the ISOGAL project was the study of the distribution and properties of the AGB stars. However, the survey is unbiased, with the only exception of excluding from the surveyed area strong IRAS sources (with 12 $\mu$m flux densities greater than 6-10 Jy) in order to avoid saturation effects. Thus the survey data can be used to study any other type of mid-IR source present in the galactic plane, as for instance the less numerous HII regions associated to young massive stars.

For a proper identification of source types, the ISOGAL results need to be compared with observations at other wavelengths. In particular, for the study of AGB stars comparisons with near IR observations, taken primarily with DENIS (Epchtein 1998), are useful. For the study of HII regions comparisons with radio continuum surveys are more appropriate.

A large fraction of the northern sky galactic fields covered by ISOGAL have already been observed at 6 cm (5 GHz) with the VLA (see Becker et al. 1994 and references therein), and a comparison of the two surveys is underway. However, these radio observations terminate at $l=+40^{\circ}$and there were no high frequency (e.g. $\ge$5 GHz) radio continuum observations for the ISOGAL field at $l=+45^{\circ}$. Observations at lower frequencies, such as the 1.4 GHz NRAO-VLA Sky Survey (NVSS - Condon et al. 1998), are inadequate to detect the younger and more dense compact HII regions, which may be optically thick at 1.4 GHz.

Given our interest, within the ISOGAL team, to study the young massive stars, we decided to observe the $l=+45^{\circ}$ field at high frequencies with the VLA, to provide a data base comparable to that of Becker et al. (1994). In order to obtain radio spectral index information we covered at 6 and 3.6 cm an area slightly larger than the $l=+45^{\circ}$ ISOGAL field.

The selection of the ISOGAL galactic plane fields does not follow any ad hoc criterion, but is based on symmetrically spaced samples on both sides of the Galactic Center, with the spacing increasing with distance from the Galactic Center. The $l=+45^{\circ}$ field happens to be located tangent to a spiral arm of our Galaxy, the Scutum arm (see e.g. Kurtz et al. 1994). Inspection of the 4.875 GHz galactic plane survey of Altenhoff et al. (1978) shows that there is very weak diffuse galactic background emission in this direction. Only 7 sources of the Altenhoff et al. catalogue fall in our surveyed area or at its borders (see Table 4). One of these (44.786 - 0.490) is partly outside our surveyed area. Most of these sources are associated with bright IRAS point sources and have not been covered by the ISOCAM observations except for 45.202-0.441 and 45.341-0.370.

In this work we present the radio observations and discuss the comparison with other radio surveys and with IRAS data. Comparison with ISOGAL data, as well as with dedicated J, H, K observations of the same field taken with TIRGO will be the subject of following works.