Galactic Spirals Structure

Team responsible: Maria Messineo (ESO)
Team members: H. Habing (Leiden), Karl Menten (MPIfR Bonn), Monika Petr-Gotzens (ESO)

Project description, including finding charts and observation strategy

To obtain a better characterisation of Galactic spiral structure and its connection to the Galactic bar, we propose SPIFFI observations in H and K band of selected candidate ionising stars in HII regions. The simultaneous classification in spectral subclasses of early type stars and the determination of extinction correction will enable us to obtain spectrophotometric distances, which are independent and complementary to kinematic distances.

Target list
NameRA(2000)DEC(2000)Plate-Scale(s)Bands(s)Exp.time(on source)
dutra08_107:35:39.12-18:48:51.00250H,K1200, 400 s
dutra08_207:35:38.89-18:48:57.00250H,K1200, 600 s
dutra33_109:01:54.69-47:43:44.87250H,K720, 90 s
dutra33_209:01:54.43-47:43:36.74250H,K150, 90 s
dutra33_309:01:55.07-47:43:50.23250H,K150, 90 s

Scientific rationale:
It is a well known fact that the Galaxy is a spiral barred galaxy. New models of the Galactic potential have been obtained based on the COBE maps (Bissantz et al. 2003), models which include a bar and spiral arms. However, many issues remain still to be investigated. How the bar and the spiral arms are related to each other? Do spiral arms go through corotation? does a ring structure exist and connect the spiral arms to the bar?

The overall shape of our spiral structure was first determined by Georgelin & Georgelin (1976) using spectrophotometric and kinematic distances of HII regions. The spectrophotometric distances were based on visual observation of OB stars (Balona & Crampton 1974). Unfortunately, star forming regions situated at large distance from the Sun are not optically observable because of interstellar extinction. This is the case toward the central regions of the Galaxy.

However, new radio and infrared surveys are now available and detailed studies of specific HII regions have been conducted revealing the ionising stars. This allows, in principle, solving the ambiguity of the kinematic distance. Furthermore the combination of radio continuum data with infrared data from 2MASS, ISOGAL and MSX enable us to identify new regions of star formation in the inner Galaxy (e.g. Felli 2002). A new and more complete census of HII regions is therefore possible and we are currently compiling an updated catalogue of HII regions.

It is therefore compelling and possible a re-examine of the spatial distribution of Galactic HII regions.

To obtain distances to the HII regions we propose a program of near-infrared spectroscopy. The Hydrogen recombination lines will yields estimates of the interstellar extinction, while the detection of Hydrogen and Helium lines from the candidate ionising star will enable the determination of the spectral-type of the star (Blum et al. 1991, Hanson et al. 1998) and consequently of its distance (e.g. Panagia 1973).

We propose to observe inner Galactic HII regions using SPIFFI, the near-infrared integral-field spectrometer for the VLT. SPIFFI using a field of view of 8arcsec x 8arcsec, would be ideal to simultaneously detect the HI recombination lines from the HII region itself and the spectrum of the candidate ionising star in the H and K band.

Observing strategy
See postscript document