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ASTRONOMY and ASTROPHYSICS the Nature of Near-Infrared Emission from Spiral Galaxies

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ASTRONOMY and ASTROPHYSICS the Nature of Near-Infrared Emission from Spiral Galaxies A&A manuscript no. ASTRONOMY (will be inserted by hand later) AND Your thesaurus codes are: 06 (11.09.1 M74; 11.09.1 NGC613; 11.09.1 NGC7741; 11.19.2; 11.19.5) ASTROPHYSICS The nature of near-infrared emission from spiral galaxies P.A. James1 and M.S. Seigar2 1 Astrophysics Research Institute, Liverpool John Moores University, Twelve Quays House, Egerton Wharf, Birkenhead CH41 1LD, UK email: [email protected] 2 Department of Mathematical Physics & Astronomy, Universiteit Gent, Krijgslaan 281 (S9), B-9000 Gent, Belgium email: [email protected] Abstract. We present K-band spectroscopy for several ever, by ∼107 years after the onset of a burst of star regions of three nearby spiral galaxies, NGC 613, NGC 628 formation, the most massive stars will have evolved to and NGC 7741. Analysis of the depth of the 2.293µm CO the red supergiant phase and will be contributing signif- absorption feature in these spectra reveals that some re- icantly to the near-IR light output (Persson et al. 1983). gions have deep absorptions, indicative of recent star for- In addition, hot (600–1000K) dust associated with OB mation, while others have anomalously shallow absorp- associations gives rise to mid-infrared emission with a tions. We interpret the latter as evidence for a signifi- short-wavelength ‘tail’ which extends into the K window cant but localised contribution to the 2.3 µm light from (Doyon, Joseph & Wright 1994). For extreme ‘starburst- hot dust with an effective temperature of ∼1000 K, which ing’ objects, the supergiant plus dust emission can exceed could have a significant effect on the K-band morphologies that of the old stellar population in the K-band (e.g. Ridg- of star-forming galaxies. way, Wynn-Williams & Becklin 1994, Doyon et al. 1994, Puxley, Doyon & Ward 1997). For normal disk galaxies, Key words: galaxies: spiral - galaxies: stellar content the effects will be more modest, but still significant. Rhoads (1998) has addressed the question of the su- 1. Introduction pergiant contribution to K-band disk light by mapping the depth of the 2.3 µm CO photospheric absorption over The stars which dominate the stellar mass in galaxies are the disks of three nearby spiral galaxies. This feature is effectively invisible. Most of the optical light in the so- stronger in supergiant stars than in the old stellar popu- lar neighbourhood comes from A dwarfs and K & M gi- lation (Frogel et al. 1978, Doyon et al. 1994), and extends ants, whereas the local stellar mass is dominated by G, K over a sufficiently broad wavelength range to be easily and (particularly) M dwarfs, which contribute a negligi- arXiv:astro-ph/9909200v1 13 Sep 1999 quantified as a colour change in narrow band filters. Using ble fraction of the visible light (Kroupa, Tout & Gilmore this technique, Rhoads (1998) determines the supergiant 1990, Binney & Merrifield 1998). In actively star-forming contribution to the three galaxies (NGC 278, NGC 2649 systems the disparity between the luminous and mass- and NGC 5713) to be only ∼3% of the overall near-IR bearing stellar components is even greater. For this reason, flux, but rising locally in regions of strong star formation and because of the diminished effects of dust obscuration, to as much as 33%. This is an important result, if gen- near-infrared (near-IR) imaging has become a key obser- erally true, since this is a significant fraction of the arm- vational technique for determining the morphologies and interarm contrast in near-IR images of normal spirals (Rix stellar contents of galaxies. For example, near-IR imaging & Zaritsky 1995, Seigar & James 1998b), making it very has been widely used to determine the underlying struc- uncertain whether near-IR features reflect changes in the ture of spiral arms in disk galaxies (e.g. Rix & Rieke 1993, overall stellar mass density, or in the location of the young Block et al. 1994, Rix & Zaritsky 1995, Seigar & James supergiant population. Given an age of 1–2×107 years, and 1998b), since arms are both extremely dusty and associ- assuming a formation velocity dispersion of ∼10 kms−1 ated with star formation. In this paper we will look in for Population I stars, the supergiants will have typically more detail at the assumption that near-IR light is a good moved only 0.1–0.2 kpc from their birthplaces in the disk, tracer of the stellar mass in galaxies. whereas the late K or early M giants which dominate the The hot OB stars which can dominate the blue light red light from the older stellar population (Frogel et al. of a star-forming galaxy have relatively little impact on 1978, James & Mobasher 1999) will be well mixed with the K-band (2.2 µm) emission (Persson et al. 1983). How- other stellar types. Thus it is important to quantify the Send offprint requests to: P. James relative contributions from giant and supergiant stars. 2 P.A. James & M.S. Seigar: Near-IR emission from spiral galaxies Doyon et al. (1994) point out that extreme caution 0.00258µm/pixel. We used the 2–pixel wide slit, oriented must be used in interpreting filter CO measurements of N–S for all observations, and moved the array by one pixel the type used by Rhoads (1998) in regions of strong star between sub-integrations to give automatic bad pixel re- formation, due to hot dust, non-thermal emission and ex- placement, and Nyquist sampling over the resolution per- tinction, which can significantly affect the shape of the un- mitted by the slit. The slit is 90′′ long. For NGC 7741 derlying galaxy continuum. More reliable measurements this was large enough for us to keep the galaxy bar on the can be obtained using long slit spectroscopy, with the slit in both beamswitch positions, but the larger sizes of wavelength range covering both the CO absorption and NGC 613 and NGC 628 required the observation of com- the continuum emission at shorter wavelengths, which en- pletely offset positions for sky monitoring. This was done ables the continuum slope to be determined explicitly. by alternating one-minute integrations at the main and This is the strategy we have adopted for the present paper. sky positions. We have obtained low-resolution, long-slit spectroscopy At regular intervals, A-type stars were observed to per- covering the full K-band for bulge and disk locations in mit variations in atmospheric transparency to be ratioed NGC 613 and NGC 628, and for the bar of NGC 7741. We out of the galaxy spectra. A-stars were chosen as they are present spectroscopic CO indices for all these regions, and much too hot to have molecules in their photospheres, and use them to determine the fraction of light contributed by thus they have no CO absorptions (and indeed no other supergiant stars. We also compare the spectroscopic prop- strong features around the wavelength range of interest erties of bulges with those of inner disk regions, and with here). Care was taken to observe stars at the same air- elliptical galaxies. mass as the corresponding galaxy. It should be noted that Oliva et al. 1995 conclude that CO absorption strength is not a clear indicator of recent 2.2. Observed galaxies star formation in stellar populations. They find that an old population of high-metallicity giants can have the same NGC 613 is a barred spiral, classified as SBbc in the CO index as a starburst dominated by low metallicity su- Third Reference Catalogue of Bright Galaxies (de Vau- pergiants. However, their data (see Table 2 of Oliva et al. couleurs et al. 1991; henceforth RC3). It has a major axis ′ 1995) do show significantly deeper CO absorptions in both diameter of 5.5, a total blue magnitude of BT =10.73, −1 the centres of HII galaxies and young (6–17 Myr) LMC a recession velocity of 1475 kms , and a distance of clusters than in 4 comparison ellipticals or the bulges of 17.9 Mpc (Jungwiert, Combes & Axon 1996), assuming −1 −1 3 spiral galaxies. Thus, whilst there is clearly scope for H0 =75 km s Mpc (throughout the present paper) ′′ ambiguity of interpretation, and the possibility of metal- and correcting for a Virgo infall model. Thus 1 corre- licity effects must be considered when interpreting results, sponds to 86 pc at NGC 613. It shows clear sign of an empirically it appears that CO strength does contain in- active galactic nucleus, and V´eron-Cetty & V´eron (1986) formation on star formation history. classified it as a ‘Composite’ galaxy, with central spectral The organisation of the present paper is as follows. features showing both Seyfert activity and vigorous star Section 2 contains a description of the galaxies observed, formation. and of the spectroscopic observations obtained. The data NGC 628 (M74) is classified as an Sc (RC3), and has ′ reduction procedures are described in section 3, and sec- a major axis diameter of 10.5 and a total blue magnitude tion 4 contains the interpretation of the observed spectral of BT =9.95. Sharina, Karachentsev & Tikhonov (1996) indices in terms of the fraction of light from old and young calculate a distance of 7.3 Mpc for NGC 628, such that ′′ stars, and from hot dust, in the different galaxy regions. 1 corresponds to 35 pc. It has a recession velocity of −1 Section 5 summarises the conclusions reached, and con- 657 kms . There is no evidence of any nuclear activity in tains suggestions for future work to be undertaken in this NGC 628 (AGN or vigorous star formation), and Wakker area.
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