A multi-wavelength view of star clusters in the circumnuclear ring of NGC7469
T. Díaz-Santos1, A. Alonso-Herrero1, L. Colina1, S. Ryder2 and J. Knapen3
1
Departamento de Astrofísica Molecular en Infrarroja, IEM, CSIC, Serrano 113b, 28006 Madrid, Spain
2
Anglo Australian Observatory, Epping, NSW 1710, Australia
3
University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
ABSTRACT
We present results from a multi-wavelength imaging and near-infrared spectroscopic study of several star formation (SF) knots located in the ring-like circumnuclear region of the Seyfert1 galaxy
NGC7469. This galaxy is a nearby (distance of 65Mpc) example where two of the most energetic phenomena of the universe, SF and accretion onto a black hole (AGN activity), coexist within the
central kpc. The ultimate goal of this project is to estimate the bolometric contribution of both phenomena in the central region of this galaxy. Because separating both contributions is so delicate, and
because of the increasing contamination from the UV to the near-infrared from the underlying (older) stellar population of the galaxy, we require high spatial resolution. To this end we have used UV,
optical, and near-infrared images from the HST archive, and mid-infrared images from the literature, together with our own UKIRT near-infrared spectroscopy. We have developed an IDL routine
designed to simultaneously fit and perform accurate photometry on unresolved star clusters, and model the underlying (galaxy) emission. We have produced and modeled the UV to mid-infrared
spectral energy distributions of these knots to derive their sizes, ages, stellar masses, and extinctions.
1 Introduction
Recent mid-infrared (IR) cosmological surveys have shown that luminous and ultraluminous IR galaxies (LIRGs and ULIRGs, with L(IR)=1011-1012L and L(IR)>1012L ,
respectively) dominate the galaxy population out to a redshift of at least z=1.5. In the
local Universe LIRGs and ULIRGs are powered by both intense star formation (SF) and
AGN, with the AGN contribution increasing towards higher IR luminosities (Sanders &
Mirabel 1996, ARA&A, 34, 749S). Recent ground-based high resolution images of
LIRGs have shown most of the mid-IR emission originates from compact regions located
within a few hundred parsecs from the nucleus of the host galaxy, and that the most recent
SF is associated with super-star clusters (SSC) and giant HII regions as detected with
NICMOS on the HST (Alonso-Herrero et al. 2000, ApJ, 532, 845A; 2002, 2002AJ,
124,166A; also Scoville et al. 1999, Ap&SS, 266, 149S). These regions of young SF are
generally in or near dusty environments where a significant fraction of the UV-optical
light is absorbed and reprocesed into the mid and far IR. Thus, multi-wavelength studies
are essential to quantify the physical processes taking place in this class of galaxies.
We have awarded HST/NICMOS time in Cycle 13 to observe a sample of 25 Local
Universe LIRGs. All these galaxies are located at distances of less than 100Mpc, so the
the spatial resolution (~10-30pc) allow us to study SSC and giant HII regions. These
LIRGs cover a morphology range going from isolated galaxies to interacting systems and
mergers. As a preliminary study we have obtained archival HST data, and our own
UKIRT near-IR spectroscopy to study the SSCs and HII regions of the LIRG NGC7469.
3 UKIRT near-IR spectroscopy of
the circumnuclear ring of SF
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The high spatial resolution of CGS4 on the UKIRT has
allowed us to obtain K-band spectra of four diferent regions
(figure to the right) of the circumnuclear ring of SF
containing most of its clusters as well as of the nucleus of
NGC7469. The slit width was 0.6”. The spectra are shown
below. We mark the positions of the hydrogen
recombination lines (Paα, Brδ, and Brγ), and other features
such as the CO bands and molecular hydrogen emission
lines. We first measured the spectroscopic index of the CO
bands with the SBAND task in IRAF, as explained by Ivanov et
al. (2004, ApJS, 151, 387I), and then transformed it into the
photometric CO as described by Doyon et al. (1994, ApJ,
421, 101D). We have also measured equivalent widths (EW)
of the emission lines.
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2 NGC7469: a Seyfert 1 galaxy
with a ring of star formation
NGC7469 is a nearby LIRG that contains a
Seyfert 1 nucleus (the bright central source) and a
ring of star formation (Wilson et al. 1991, ApJ, 381,
79) of about 1kpc in size. The ring is detected in the
UV, optical, near and mid-IR, and radio, as can be
seen from the figure to the left. Genzel et al. (1995,
ApJ, 444, 129) established that approximately two
thirds of the bolometric luminosity of this galaxy is
produced by the ring of star formation. We are
currently using HST and ground based UV to midIR observations to characterize the properties of
different regions of the ring of star formation in this
galaxy.
Below is a 1.1µm contour map showing the
location of the four slits used to obtain K-band
spectroscopy with the UKIRT, and the star clusters
identified in the ring of SF and their associated
labels we use in the photometric analysis.
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4 Accurate photometry of individual clusters
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*
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We have developed an IDL routine to obtain accurate photometry from individual clusters observed
in HST images. Basically, this program is composed of two main procedures: finding & fitting. The
first one employs an iterative growing method to roughly locate all those clusters with a maximum
intensity of n standard deviations above a mean background interactively selected by the user. The
next step is to fit each individual cluster to a Gaussian plus a background plane by a least-square
method. The photometry of the unresolved sources is performed by first modeling in a similar
fashion synthetic PSFs generated by the Tinytim software (Krist et al. 1998, PASP, 110, 1046K) to
estimate accurate aperture corrections since we find that Gaussians do not accurately reproduce the
shape of the unresolved clusters.
For NGC7469 we used HST observations taken with the F218W, F547M, F606W, F814W filters of
WFPC2, and the F110W, F160W and F222M filters of NICMOS. We have selected star clusters at
1.1µm. As can be seen from the figure to the left, within each of the UKIRT slits we detect a number
of clusters that we model individually. A similar procudure was followed with the other filters.
Finally we cross-correlated the positions of the 1.1mm clusters with those detected at other
wavelengths to produce the spectral energy distributions (SEDs) shown below.
CO index & EW(Brγ) as age indicators
Slit 4
Slit 3
Slit 2
Slit 1
Nucleus
The EW of Brγ (2.16µm) and the photometric CO index (2.36µm)
are both good indicators of the age of the star formation. The EW
of hydrogen emission lines measures the ionizing photons from
young stars relative to the supergiants. The CO bands are
prominent when the first red supergiants appear. This occurs at
about 3Myr after the peak of SF -in our case we are assuming
Gaussian bursts of SF with FWMH of 1, 5, and 100Myr (Rieke et
al. 1993, ApJ, 412, 99R) and a Salpeter IMF. Thus the EW of Brγ
and the CO indices can be used to estimate the age of the clusters.
The diagram below shows the clusters in the ring of NGC7469 as
well as clusters of the SF ring of NGC2903 (Alonso-Herrero et al.
2001, MNRAS, 322, 757A) and the younger SSC in the nucleus of
NGC5253 (Alonso-Herrero et al. 2004, astro-ph/0405093). The
ages of the clusters of NGC7469 (~4Myr after the peak of SF, or
7Myr from the beginning of the 1Myr Gaussian burst) appear to be
all very similar and intermediate between those of NGC5253 and
those of NGC2903.
NGC7469
NGC5253
We compare the SEDs of the individual and the sum of the 1.1µm clusters detected within each
of the UKIRT slits with the integrated SEDs obtained by simulating the slit coverage at each HST
filter. In black we show Starburst99 (Leitherer et al. 1999, ApJS, 123, 3) SEDs for an instantaneous
burst of SF with ages of 5Myr, 8Myr and 15Myr. In most cases the slit SEDs do not characterize
the properties of the individual clusters or the sum of them. The slit SEDs include a large
contribution from the underlying (unresolved) stellar population which appear to dominate at
near-IR wavelengths.