The giant HII region NGC 604 constitutes a complex and rich population to study in detail many aspects of massive star formation, such as their environments and physical conditions, the evolutionary processes involved, the initial mass function for massive stars and starformation rates, among many others. Here, we present our first results of a near-infrared study of NGC 604 performed with NIRI images obtained with Gemini North. Based on deep JHK photometry, 164 sources showing infrared excess were detected, pointing to the places where we should look for star-formation processes currently taking place. In addition, the color-color diagram reveals a great number of objects that could be giant/supergiant stars or unresolved, small, tight clusters. An extinction map obtained based on narrow-band images is also shown.Facultad de Ciencias Astronómicas y Geofísica

Barbá, Rodolfo Héctor

Bosch, Guillermo Luis

Fariña, Cecilia

Proceedings of the International Astronomical Union

English

SEDICI - Repositorio de la UNLP

Star clusters: basic galactic building blocks
Proceedings IAU Symposium No. 266, 2009
R. de Grijs & J. R. D. Le´pine, eds.
c° International Astronomical Union 2010
doi:10.1017/S1743921309991451
A deep dive into NGC604 with Gemini/NIRI
imaging
Cecilia Farin˜a,1,2 Guillermo L. Bosch1,2 and Rodolfo R. Barba´3,4
1Facultad de Ciencias Astrono´micas y Geof´ısicas, Universidad Nacional de La Plata,
Paseo de Bosque s/n, 1900 La Plata, Argentina
email: ceciliaf@fcaglp.unlp.edu.ar
2 IALP, CONICET, Argentina
3 ICATE, CONICET, Argentina
4Departamento de Fı´sica, Universidad de La Serena, Benavente 980, La Serena, Chile
Abstract. The giant Hii region NGC 604 constitutes a complex and rich population to study in
detail many aspects of massive star formation, such as their environments and physical condi-
tions, the evolutionary processes involved, the initial mass function for massive stars and star-
formation rates, among many others. Here, we present our first results of a near-infrared study
of NGC 604 performed with NIRI images obtained with Gemini North. Based on deep JHK
photometry, 164 sources showing infrared excess were detected, pointing to the places where
we should look for star-formation processes currently taking place. In addition, the color–color
diagram reveals a great number of objects that could be giant/supergiant stars or unresolved,
small, tight clusters. An extinction map obtained based on narrow-band images is also shown.
Keywords. ISM: individual (NGC 604), Hii regions, galaxies: individual (M 33), techniques:
photometric, stars: formation, stars: early-type, infrared: stars
1. Introduction
NGC604 is a giant Hii region (GHR) located in an outer spiral arm of M33, at a
distance of 840 kpc. It is the second most luminous Hii region in the Local Group, after
30 Doradus in the LMC. Both are nearby examples of giant star-forming regions whose
individual objects can be spatially resolved for further study. NGC604 has been the
target of many studies during the past few decades. A brief summary of known facts
about NGC 604 includes the following. This GHR is ionized by a massive, young cluster,
with at least 200 O stars (some as early as O3–O4). The cluster does not exhibit a
central core distribution. Instead, the stars are widely spread over its projected area in
a structure called ‘scaled OB association’ (SOBA; Hunter et al. 1996; Ma´ız Apella´niz
et al. 2004; Bruhweiler et al. 2003). Wolf–Rayet stars, a confirmed and many candidate
red supergiant stars, a luminous blue variable and a supernova remnant are all part of
NGC 604’s stellar population (Conti & Massey 1981; D’Odorico & Rosa 1981; Drissen
et al. 1993; Dı´az et al. 1996; Churchwell & Goss 1999; Terlevich et al. 1996; Barba´ et al.
2009). The age of the central ionizing cluster has been determined by different authors
as between 3 and 5 Myr (Gonza´lez–Delgado & Pe´rez 2000; Bruhweiler et al. 2003; Dı´az
et al. 1996; Hunter et al. 1996; Relan˜o & Kennicut 2009).
The interstellar medium reveals a complex structure with a high-excitation central
region (made up of multiple two-dimensional structures), asymmetrically surrounded
by a low-excitation halo. The whole region shows a very complex geometry of cavities,
expanding shells and filaments, as well as dense molecular regions. All of these structures
391
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392 C. Farin˜a et al.
Table 1. Main characteristics of broad-band and narrow-band filters used in our Gemini/NIRI
observations.
Broad bands Narrow bands
Filter Central λ (μm) Coverage (μm) Filter Central λ (μm) Coverage (μm)
J 1.25 0.97–1.07 Paβ 1.282 ∼ 0.1
H 1.65 1.49–1.78 Brγ 2.16 ∼ 0.1
Ks 2.15 1.99–2.30 H2 (2–1) 2.24 ∼ 0.1
show different kinematic behaviour (Ma´ız Apella´niz et al. 2004; Tenorio–Tagle et al. 2000;
Sabalisck et al. 1995; Relan˜o & Kennicut 2009).
Aiming to characterize the youngest stellar population and its environment, we per-
formed near-infrared (NIR) photometry (JHK) and analysed narrow-band images in
Paβ, Brγ and H2(2–1). Taking into account that NIR observations are less affected by
dust extinction characteristic in star-fomation environments, we can take a deep dive into
NGC604 to study those very young objects which are still immersed in their parental
clouds at the sites of current star formation.
2. Images and data processing
The images were obtained with the Near InfraRed Imager and Spectrometer (NIRI)
at Gemini North. The resulting plate scale is 0.117 arcsec pixel−1 , with a field of view of
120× 120 arcsec2. The filters used and their main characteristics are listed in Table 1.
The images were taken under excellent seeing conditions, on average ∼ 0.3500 in the
J,H and Ks images. A set of approximately 10 individual exposures was taken in each
band to combine into the final image. Data reduction and processing were performed with
specific tasks using the gemini-niri iraf package. Images were sky subtracted and flat
fielded and short dark exposures were used to identify bad pixels. Stellar magnitudes were
obtained by point-spread-function (PSF) fitting in crowded fields using the daophot
software (Stetson 1987) in iraf. Although a standard procedure, PSF construction and
fitting involves an iterative and careful process in which several tries were made to get
the best results. The effective area covered by our photometry is ∼ 107 × 107 arcsec2
(∼ 430 × 430 pc2 at the distance of M33). The average photometric errors are 0.09,
0.11 and 0.21 mags in the H,J and Ks filters, respectively, and the completeness limits
are 22 mag in J and 21 mag in H and Ks . Magnitudes in the individual filters were
matched in a unique list containing 5566 objects in the field in which all three J,H and
Ks magnitudes were measured. Astrometry was derived using 35 objects in common in
our field of NGC604 and the GSC–II Catalog, Version 2.3.2 (2006) in the ICRS, Equinox
J2000.0.
3. Results and discussion
The resulting color–magnitude (CM) and color–color (CC) diagrams are shown in
Figures 1 and 2, respectively. We have included ∼ 2000 selected objects, located within a
radius of 48 arcsec (∼ 200 pc) and centered on NGC604, meeting a certain photometric
quality level [magnitude error 6 median(error) + 1.0× standard deviation(error)]. Red
symbols in both plots are objects that lie on the right side of the reddening line for a
O6–O8 V star. For each of these objects, the error in (H −Ks) color is smaller than its
distance to the reddening line, so that we can ensure that they undoubtedly show an IR
excess.
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IAUS266. NGC604: near-infrared Gemini/NIRI imaging 393
Figure 1. Color–color (left) and color–magnitude (right) diagrams for objects observed in a
circular area centered on NGC 604. Red squares are objects that show an IR excess.
Among the objects with intrinsic IR excesses in GHRs we expect to find Wolf–Rayet
stars, early supergiants and Of stars, and massive young stellar objects (MYSOs). Many
of these types of objects (or candidates) have already been found in NGC 604. Based
on our deep JHK photometry we found a total of 164 objects that show IR excesses.
As can be expected, ∼ 70% of these lie in a small area near the region’s center and
a large proportion are tightly grouped in regions coincident with the radio-continuum
peaks at 8.4 GHz from Churchwell & Goss (1999), as shown in Figure 3 (left panel).
These results are in complete agreement with the analysis of Barba´ et al. (2009) based
on HST/NICMOS NIR images, where MYSO candidates also appeared aligned with the
radio peak structures.
What we found supports the idea mentioned by many authors, that those areas may
be embedded star-forming regions, also taking into account that regions coincident with
the radio knots show conditions of high temperature and density (Tosaki et al. 2007),
and in two of them Ma´ız Apella´niz et al. (2004) identified compact Hii regions. Most
objects with IR excesses will be the targets for further observing programs (making use
of the integral-field spectroscopic facilities at Gemini North) to elucidate their nature
and derive accurate properties by means of spectroscopic study. Also, those objects that
Figure 2. (left) Three–color (J, H and Ks ) composite image of NGC 604 with 8.4 GHz radio–
continuum countours overlaid (adapted from Churchwell & Goss 1999). Red circles are objects
that show IR excess in our photometry. (right) Extinction map from Brγ/Hα, regions with
higher extinction are red/white.
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394 C. Farin˜a et al.
are located at the bright, massive end on the CM diagram deserve further observation
and study.
On the basis of our narrow-band images we generated an extinction map, derived from
the observed variations of the Brγ to Hα ratio (with the Hα image taken from Bosch
et al. 2002). The color scale used in the map shows regions with higher extinction in
red/white. The present and future results of our NIR study will be placed in the context
of previous studies of NGC604 to complete the picture of the overall formation and
evolution scenario of this GHR.
Acknowledgements
RB acknowledges partial support from the Universidad de La Serena, project DIULS
CD08102.
References
Barba´, R. H., Ma´ız Apella´niz, J., Pe´rez, E., Rubio, M., Bolatto, A., Farin˜a, C., Bosch, G., &
Walborn, N. R. 2009, in: E. Pe´rez, R. de Grijs & R. M. Gonza´lez–Delgado (eds.), Young
massive star clusters: initial conditions and environments, ApSS, 324, 309
Bosch, G., Terlevich, E., & Terlevich, R. 2002, MNRAS, 329, 481
Bruhweiler, F. C., Miskey, C. L., & Smith Neubig, M. 2003, AJ, 125, 3082
Churchwell, E. & Goss, W. M. 1999, ApJ, 514, 188
Conti, P. S. & Massey, P. 1999, ApJ, 249, 471
Dı´az, A. I., Terlevich, E., Terlevich, R., Gonza´lez–Delgado, R. M., Pe´rez, E., & Garc´ıa–Vargas,
M. L. 1996, in: C. Leitherer, U. Fritze–von Alvensleben & J. Huchra (eds.), From Stars to
Galaxies: The Impact of Stellar Physics on Galaxy Evolution, ASP Conf. Ser., 98, 399
D’Odorico, S. & Rosa, M. 1981, ApJ, 248, 1015
Drissen, L., Moffat, A. F. J., & Shara, M. M. 1993, AJ, 105, 1400
Gonza´lez Delgado, R. M. & Pe´rez, E. 2000, MNRAS, 317, 64
Hunter, D. A., Baum, W. A., O’Neil Jr., E. J., & Lynds, R. 1996, ApJ, 456, 174
Ma´ı Apella´niz, J., Pe´rez, E., & Mas–Hesse, J. M. 2004, AJ, 128, 1196
Relan˜o, M. & Kennicutt, R. C. 2009, ApJ, 699, 1125
Sabalisck, N. S. P., Tenorio–Tagle, G., Castaneda, H. O., & Mun˜oz–Tun˜o´n, C. 1995, ApJ, 444,
200
Stetson, P. B. 1987, PASP, 99, 191
Tenorio–Tagle, G., Mun˜oz–Tun˜o´n, C., Pe´rez, E., Ma´ız Apella´niz, J., & Medina–Tanco, G. 2000,
ApJ, 541, 720
Terlevich, E., Dı´az, A. I., Terlevich, R., Gonza´lez–Delgado, R. M., Pr´ez, E., & Garc´ıa–Vargas,
M. L. 1996, MNRAS, 279, 1219
Tosaki, T., Miura, R., Sawada, T., Kuno, N., Nakanishi, K., Kohno, K., Okumura, S. K., &
Kawabe, R. 2007, ApJ, 664, 27
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A deep dive into NGC604 with Gemini/NIRI imaging

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A deep dive into NGC604 with Gemini/NIRI imaging

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