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Almudena Alonso-Herrero, Marcia J. Rieke, and George H. Rieke Steward Observatory, the University of Arizona, Tucson, AZ 85721, E-mail: aalonso@as.arizona.edu, mrieke@as.arizona.edu, grieke@as.arizona.edu
Arp 299 (IC 694 + NGC 3690) is one of the nearest examples of interacting
galaxies with a high infrared (IR) luminosity (
,
for the assumed distance
Mpc),
close to the limiting value for ultraluminous IR galaxies.
We present HST/NICMOS observations taken with all three cameras. We
concentrate on the data reduction and analysis of the images taken
with camera 3 through the narrow-band filters F164N and F166N.
The [Fe II]
m morphology and properties are described
in detail. The relative ages of the burst of star formation together
with the supernova rate for the bright sources in the interacting
system are derived.
galaxies, interacting, stellar content, star formation, infrared, Arp299
The existence of luminous and ultraluminous IR galaxies
has long been known (Rieke & Low 1972), but it was with the launch of the
IR satellite IRAS that this class of galaxies was
detected in large numbers. Since then an intense debate started as to
whether there is an evolutionary link between luminous IR
galaxies, ultraluminous galaxies and optically selected quasars
(see Sanders & Mirabel 1996 for a recent review). Among the ultraluminous
IR class (
L
,
IR
luminosity between 8 and
m), a large
percentage is found to be
interacting/merging systems containing active galactic
nuclei. Sanders et al. (1988) proposed that the IR luminous phase
is the initial stage for the appearance of a quasar.
In this context the interacting system Arp 299 (NGC 3690+IC 694 or
Mrk 171) is an interesting study case by itself, not only because of its
high IR luminosity (
L
),
close to the limiting value for ultraluminous
IR galaxies, but also because it is one of the nearest
examples of interacting starburst galaxies (distance
Mpc for
km s-1 Mpc-1).
A number of bright IR and radio sources have been detected from
ground-based observations (see Gehrz et al. 1983, Wynn-Williams et al. 1991
and references therein). Following the notation introduced by
Gehrz et al. (1983) for the interacting
pair of galaxies Arp 299, the nucleus of IC 694 (eastern component) is referred
to as
source A, and the sources in NGC 3690 (western component) are called B1,
B2, C and C
(see Figure 1).
One of the most remarkable characteristics of this
interacting system (and other luminous IR galaxies) is the
high concentration of molecular hydrogen within relatively small
regions. From CO maps, Sargent & Scoville (1991) estimated the density
of molecular gas
pc-2 in
IC 694,
pc-2 in components
B1 and B2 of NGC 3690 and
pc-2
at the interface of both galaxies, region C+C'.
Numerical simulations of collisions between gas-rich galaxies
(see Barnes & Hernquist 1996 and references therein) show
that collisions are very efficient at transporting large quantities of
molecular gas into the centers of galaxies; such large
quantities are similar to those
observed in IC 694 and other ultraluminous IR galaxies.
HST NICMOS images of the system together with MMT optical and IR spectroscopy are analyzed to derive the star formation properties of the system.
NICMOS on the HST observations of the interacting galaxy Arp 299 were obtained on November 4 1997 using all three cameras. Images were taken with the following filters and cameras: NIC1 F110M; NIC2 F160W, F222M, F237M, F187N, F190N, F212N and F215N; NIC3 F164N and F166N. The pixel sizes for NIC1, NIC2 and NIC3 are 0.045arcsec pixel-1, 0.076arcsec pixel-1 and 0.20arcsec pixel-1 respectively. In this paper we will concentrate on the data reduction and data analysis of the camera 3 narrow-band filter images. The observational strategy consisted of taking a spiral dither with a 5.5 pixel spacing, with two, three or four positions. The orientation of the images is PA = 90degree. The FWMH of the point sources in the fully-reduced NIC3 F164N image is 0.29. It is important to note that the NIC3 images were not taken during the NIC3 observing campaign. Nevertheless the quality of the images is remarkable and most suitable for scientific purposes.
Part of the reduction of the NICMOS images was performed with routines of the package NicRed (McLeod 1997). This data reduction package works within the IRAF environment. Darks with exposure times corresponding to those of our observations were obtained from other proposals close in time. Usually between 10 and 20 darks were averaged together for a given sample sequence after the subtraction of the first readout. The flatfields for the filters NIC3 F164N and F166N are in-flight flats kindly reduced by Dr. Rodger Thompson. The first steps in the data reduction (done with the task nicfast within Nicred) involve subtraction of the first readout, dark current subtraction on a readout basis, correction for linearity and cosmic ray rejection (using fullfit), and flatfielding. Since our NICMOS images were obtained after August 1997, no correction for the pedestal effect was necessary. The background was measured on blank regions of the flatfielded images and subtracted from each image.
As a first try to produce the final [Fe II]
m
line emission image, the dithered
galaxy images for a given filter
were registered to a common position using fractional pixel
offsets and cubic spline interpolation, and combined to produce the
final images through filters F164N and F166N. Once the images were
combined, the flux calibration was performed using the conversion factors
based on measurements of the standard star P330-E during SMOV
(Marcia Rieke 1997 private communication), which are:
Jy ADU-1 and
Jy ADU-1 for NIC3 F164N and F166N
respectively. The combined continuum and line+continuum images were
again shifted to a common position and finally the F164N was subtracted
from the F166N to produce the [Fe II]
m line emission
image (note that at the redshift of Arp 299 the [Fe II]
m
line gets shifted into the F166N filter). The resulting image
showed some residuals
produced by both the under-sampling of the point spread function
(PSF) and the variation of the
shape of PSF with wavelength. In addition it appears that for point-like
sources and camera 3 there is a significant variation of the shape of the PSF
depending on the position along the pixel.
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To solve some of these problems we used a different strategy when subtracting the continuum images from the line+continuum images. Instead of combining all the images together prior to the continuum subtraction, we realigned both the F164N and F166N individual images for a given position (usually by less than a few tenths of a pixel), and subtracted the F164N individual images from the F166N individual images. The resulting individual continuum-subtracted F166N images were then shifted to a common position (using the offsets computed with the F166N images before continuum subtraction) and combined to the final line emission image. Even though we first dithered the images for a given filter and then changed filters, this method yielded better results. This is because the pointing of the HST is very accurate, and therefore after changing filter, the telescope goes back to the initial point of the dither sequence with a precision of less than one tenth of a pixel. The method described here would produce even better results for those cases in which for a given position both the continuum and the continuum+line images were taken before moving the telescope to the next position.
Finally we would like to point out that we performed a straight
subtraction using the continuum at
m, i.e., the continuum has not been converted to
m, wavelength of the underlying continuum of the
[Fe II] line in Arp 299.
For sources in which obscuration is patchy or/and
very high, a better approach would be to fit the continuum between two
different wavelengths, and construct an image in which the continuum
has been interpolated to the line wavelength.
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In Figure 1 we show a contour plot of the interacting galaxy Arp 299
through the NIC3 F164N filter which contains the continuum adjacent
to the [Fe II]
m emission line. Figure 2 is a grey
scale map of the [Fe II]
m line emission
(continuum-subtracted NIC3 F166N filter), and in Figure 3 we overlay
the line emission contours on the grey scale map of the
continuum emission (NIC3 F164N) for clarity.
The NIC3 F164N continuum image (Figure 1) of the system clearly shows not only the emission from the brightest sources (i.e., A, B1, B2, C and C') already known from ground-based images, but also reveals the spiral nature of IC 694 with a number of H II regions along the spiral arms, along with some compact sources in NGC 3690 surrounding B1, B2, and located south-west of C. In the large scales (better seen in Figure 3) Arp 299 shows the characteristic tidal tails common to interacting pairs extending south and west of NGC 3690.
Most of the [Fe II]
m line emission (Figure 2)
originates from
three regions, that is, source A (nucleus of IC 694), B1 in NGC 3690
and C+C' at the interface of both galaxies. These three regions are not
compact but quite extended with linear projected sizes of about 1kpc.
In addition sources B1 and C show structure. It is remarkable that
very little or no emission seems to be coming from source B2
(the brightest source at visible wavelengths). The
same result was found by Fisher, Smith & Glaccum (1991) from their
ground-based
Br
images. In addition to the emission from the bright sources
the [Fe II] image traces the emission from H II regions
located in the spiral arms south-east of IC 694. There is a number of
H II regions northwest of B1, and east of C extending all the
way to source C', and east of C. The resemblance in morphology
with the our NIC2 Pa
(
m)
images (not shown here, see Alonso-Herrero
et al. 1998) is remarkable, although some differences are found. The
[Fe II] emission is more extended in A, B1 and C
than the Pa
emission.
If most of the [Fe II] emission is excited by shocks
in supernova remnants
(SNR) in the star-forming regions, then it is expected that the
[Fe II] emission would be more extended as the shocks propagate
outwards the H II regions, whereas the Pa
emission
will tend to be more concentrated toward the center where the young
ionizing stars are located. The [Fe II]
m to
Pa
line ratio is found to vary radially within the
star-forming regions, increasing by at least a factor of five
for increasing radial distances. In addition,
age effects will also contribute to differing line ratios
(see next section).
In contrast the H2 morphology of Arp 299 is
quite different with point-like emission originating from B1 and C
in NGC 3690, whereas the nucleus of IC 694 is very bright in H2
and shows some diffuse
emission with a beautiful butterfly-like shape (see Alonso-Herrero
et al. 1998). The lack of resemblance between the [Fe II]
and the H2 emission suggests that the scenario in which both emissions
have a common origin may not be that simple, perhaps indicating that the
nature of the shocks producing both emissions is different.
So far we have not discussed the effects of extinction. From the
optical and near-infrared spectroscopy obtained for all the bright sources
in the system, we have estimated the extinction using hydrogen recombination
line ratios. However, the values of the extinction to the gas are
quite dependent on the wavelength of the lines involved, the highest
values obtained when using the
line ratio (indicating that some of the hydrogen lines may be still
optically thick in the near-infrared). The observed
f([Fe II]
m)/
line ratios have to be corrected for
extinction by a factor (assuming a simple model of foreground
dust screen):
where the
term 0.04 accounts for the differential extinction between
m and
m. Note that we analytically fit the extinction
curve of Rieke & Lebofsky (1985) for near-infrared wavelengths.
The values of the extinction derived for the bright components are
between
mag and at least
mag (assuming a foreground
dust screen model).
In Alonso-Herrero et al. (1997) we showed that the
f([Fe II]
m)/
line ratio in starburst galaxies
is understood as transition from pure H II region
(such as the Orion Nebula) to an
increasing rôle of shock excitation by supernova remnants.
The extinction-corrected values of this ratio are well apart from the typical
value for pure ionization for all the sources indicating
that an important fraction of the [Fe II] emission is produced
by supernova remnants. Moreover, as the starburst ages the
f([Fe II]
m)/
ratio
will increase as the number of SNR
grows and the ionization from very young stars decreases, making this
line ratio a good age indicator. From the predictions of the models
presented in Vanzi, Alonso-Herrero & Rieke (1998) we can estimate
that the relative difference in ages between the bursts in
both components A and B1, and components C and C' is approximately
3 million years assuming a Gaussian burst with FWMH
Myr. From
these models we derive an age (measured from the peak of star-formation)
as young as 4Myr for components C and C' in NGC 3690, indicating
that the most recent star-formation is occurring at the interface of
the two galaxies. A more detailed
discussion of the star-forming properties of this system will be presented in
Alonso-Herrero et al. (1998).
Finally the total [Fe II]
m fluxes can used to derive the
supernova rate (SNr) using the calibration for M82 derived in Vanzi &
Rieke (1996). For the (SNr) we measure the [Fe II]
m
fluxes through a 4arcsec-diameter aperture, except for the H II
regions north-west of B1 for which we use the flux through the
1arcsec-diameter aperture. The values of the
supernova rate are presented in the last column of Table 1. The two
values correspond to the supernova rates obtained from the
[Fe II]
m fluxes
not-corrected and corrected for extinction. Gehrz et al. (1983) estimated
the supernova rate from their 20cm radio measurements with a
5arcsec beam. Their values are 4.2, 2.5, 1.9 and 0.9yr-1 for
components A, B1+B2, C and C'. Taking into account the uncertainty in
the calibration for M82 for the (SNr) in terms of
the [Fe II]
m
flux (a factor of 2, Vanzi & Rieke 1996), the agreement between the two
independent estimates is quite good for components A and C. The largest
discrepancy occurs for B1. Given the fact that source B2
shows no [Fe II] emission it is possible that the radio emission
from B2 is not related to SNR, but still included in Gehrz et al.
(1983) calculations.
Most of the [Fe II]
m
line emission is found to be originating from three bright sources, A (nucleus
of IC 694) and B1 and C+C' in NGC 3690. Little or no emission is
coming from B2 (the brightest source at visible wavelengths). The
resemblance between the [Fe II]
m and Pa
emission is remarkable, although the [Fe II]
m
emission is extended to larger scales. The [Fe II]
m
to Pa
line ratio is a good age indicator, since as the
starburst ages the number of SNR grows whereas the flux from young
ionizing stars decreases. We find that the region C+C' at the
interface of the two galaxies is undergoing
the youngest star-formation process. Finally the
[Fe II]
m fluxes are used to compute the
supernova rate for each component.
During the course of this work AA-H was supported by the National Aeronautics and Space Administration on grant NAG 5-3042 through the University of Arizona. The work was also partially supported by the National Science Foundation under grant AST-95-29190.