AKARI infrared observations of edge-on spiral galaxy NGC 3079

Abstract

We present AKARI near- to far-infrared images of the nearby edge-on spiral galaxy NGC 3079 in 10 photometric bands. The spectral energy distribution consists of continuum emission from dust with a single temperature of 28-33 K together with strong mid-infrared emission features from polycyclic aromatic hydrocarbons (PAHs). We derive the dust masses of 5.6x10^6 M_sun and 1.4x10^7 M_sun for the central 4 kpc region and the whole galaxy, respectively, and find that a gas-to-dust mass ratio is unusually high in the central region (\sim 1100) and even for the whole galaxy (\sim 860). The ratio of the surface brightness distribution at the wavelength of 7 um to that at 11 um suggests that the properties of PAHs have spatial variations. Emission from ionized and neutral PAHs is relatively strong in the center and the disk regions, respectively, suggesting stronger radiation field and thus relatively active star formation in the center. Yet the total infrared luminosities of the galaxy indicate rather low star formation rates. These results suggest that NGC 3079 is in an early-phase starburst stage. Comment: 15 pages, 7 figures, accepted for publication in PASJ

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arXiv:1005.5251v1 [astro-ph.GA] 28 May 2010
AKARI infrared observations of edge-on spiral galaxy
NGC 3079
Mitsuyoshi Yamagishi, Hidehiro Kaneda, Daisuke Ishihara
Graduate School of Science, Nagoya University, Chikusa-ku, Nagoya 464-8602
yamagishi@u.phys.nagoya-u.ac.jp, kaneda@u.phys.nagoya-u.ac.jp, ishihara@u.phys.nagoya-u.ac.jp,
Shinya Komugi
Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai,
Sagamihara, Kanagawa 229-8510
skomugi@ir.isas.jaxa.jp
Takashi Onaka
Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-0033
onaka@astron.s.u-tokyo.ac.jp
and
Toyoaki Suzuki
Advanced Technology Center, National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588
suzuki@ir.isas.jaxa.jp
(Received ; accepted )
Abstract
We present AKARI near- to far-infrared images of the nearby edge-on spiral galaxy
NGC 3079 in 10 photometric bands. The spectral energy distribution consists of
continuum emission from dust with a single temperature of 28–33 K together with
strong mid-infrared emission features from polycyclic aromatic hydrocarbons (PAHs).
We derive the dust masses of 5.6×106M⊙and 1.4×107M⊙for the central 4 kpc region
and the whole galaxy, respectively, and find that a gas-to-dust mass ratio is unusually
high in the central region (∼ 1100) and even for the whole galaxy (∼ 860). The ratio
of the surface brightness distribution at the wavelength of 7 µm to that at 11 µm
suggests that the properties of PAHs have spatial variations. Emission from ionized
and neutral PAHs is relatively strong in the center and the disk regions, respectively,
suggesting stronger radiation field and thus relatively active star formation in the
center. Yet the total infrared luminosities of the galaxy indicate rather low star
formation rates. These results suggest that NGC 3079 is in an early-phase starburst
stage.
Key words: galaxies: individual(NGC 3079) — galaxies: ISM — galaxies: spiral
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— infrared: galaxies — ISM: dust,extinction
1.Introduction
NGC 3079 is a highly-inclined (i= 84◦) edge-on spiral galaxy at a distance of 15.6 Mpc,
the optical disk of which has a position angle of 166◦(Irwin & Seaquist 1991; Sofue & Irwin
1992). The nucleus of NGC 3079 is classified as a low-ionization nuclear emission-line region
(LINER; Heckman 1980) or Seyfert 2 (Ford et al. 1986). The galaxy is known to have an
ultra-high-density molecular core (3×108M⊙) in the center (Sofue et al. 2001). In addition,
molecular gas with a total mass of 5×109M⊙is contained within the central 2 kpc radius (Koda
et al. 2002). The nucleus shows very bright H2O emission, which is one of the most luminous
H2O maser sources (Henkel et al. 1984). Near the central region, kilo-parsec-scale outflows
along the minor axis are observed at various wavelengths (radio continuum: Duric & Seaquist
1988; Hα: Ford et al. 1986; Cecil et al. 2001; X-ray: Fabbiano et al. 1992). The superbubbles
may have been produced by either a central active galactic nucleus (AGN, i.e. acretion of dense
gas into the core; Irwin & Seaquist 1988; Hawarden et al. 1995) or a nuclear starburst (Sofue
& Vogler 2001), but the exact origin is yet to be understood.
Infrared (IR) imaging observations of NGC 3079 are relatively few, in contrast to other
wavelengths, mainly due to low spatial resolution especially in the far-IR. IR observations
however provide very important tools to study dust components that trace star formation
activities effectively. Young et al. (1989) and Klaas & Walker (2002) discussed the physical
states of far-IR dust in NGC 3079 from the observations of IRAS and ISO, respectively. However
the angular resolutions of IRAS and ISO are so large as compared to the apparent size of
NGC 3079 (7.′9 × 1.′4; de Vaucouleurs et al. 1991) and therefore IRAS and ISO could not
observe the galaxy as a spatially resolved far-IR source. Klaas & Walker (2002) derived the
dust-to-gas mass ratio of 200 for the whole galaxy of NGC 3079, which is close to the accepted
value of 100–200 for our Galaxy (Sodroski et al. 1997). Braine et al. (1997) and Stevens &
Gear (2000) discussed the properties of dust in NGC 3079 on the basis of the spatially resolved
submillimeter dust emission with IRAM and SCUBA, respectively.
Spitzer observed NGC 3079 with the IRAC, IRS, and MIPS. From the archived data,
however, we find that the MIPS 160 µm and the IRAC 8 µm band images are heavily affected by
saturation effects in the central region of NGC 3079. Weedman et al. (2005) and Bernard-Salas
et al. (2009) presented mid-IR spectra of the nuclear region between 5 and 37 µm obtained by
the IRS. These spectra showed that emissions from polycyclic aromatic hydrocarbons (PAHs)
are dominant in the mid-IR. The strong PAH emission suggests that the central activity of
NGC 3079 is better characterized by a nuclear starburst rather than an AGN.
In this paper, we present multi-band images of NGC 3079 in the near- to far-IR obtained
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Table 1. Observation log.
Date Observation IDObservation mode Details
2007 Apr 301401006 FIS01Scan speed: 8′′s−1, Reset interval: 0.5 s
2007 Apr 30 1401005IRC02N3, N4, S7, and S11 band imaging
2007 May 11401007IRC02 L15 and L24 band imaging
by the AKARI satellite (Murakami et al. 2007). AKARI has two instruments, the Infrared
Camera (IRC; Onaka et al. 2007) and the Far-Infrared Surveyor (FIS; Kawada et al. 2007),
with which we obtain 10 photometric band images at central wavelengths of 3, 4, 7, 11, 15, 24,
65, 90, 140, and 160 µm by pointed observations. The 7 and 11 µm bands enable us to discuss
the spatial distribution of the PAH emission. The 4 far-IR bands are efficient to determine the
properties of interstellar dust. We perform spatially-resolved studies of the interstellar dust in
the whole galaxy of NGC 3079. Here we present non-saturated images as well as new maps at
11 µm and 15 µm, although the spatial resolutions are slightly worse than Spitzer due to the
smaller diameter of the AKARI telescope (68.5 cm; Kaneda et al. 2005; 2007).
2.Observations and Data Reductions
The IRC and FIS imaging observations of NGC 3079 were carried out in April and May,
2007. They were performed in part of the AKARI mission program ”ISM in our galaxy and
Nearby Galaxies” (ISMGN; Kaneda et al. 2009). A summary of the observation log is listed
in table 1. With the IRC, we obtain N3 (reference wavelength of 3.2 µm), N4 (4.1 µm), S7
(7.0 µm), S11 (11.0 µm), L15 (15.0 µm), and L24 (24.0 µm) band images using a standard
observation mode, IRC02; each image has a size of about 10′×10′. The FWHMs of the Point
Spread Functions (PSFs) are 4.′′0, 4.′′2, 5.′′1, 4.′′8, 5.′′7, and 6.′′8 for the N3, N4, S7, S11, L15,
and L24 bands, respectively (Onaka et al. 2007). The near- and mid-IR images are created by
using the IRC imaging pipeline software version 20071017 (Lorente et al. 2007).
For the IRC, we obtained both short and long exposure image data in a single observa-
tion. In this paper, we mainly use long exposure images but the image in the N3, N4, S7, and
S11 bands are affected by saturation near the nuclear region of the galaxy. The region affected
by the saturation in the long exposure image is determined by visual inspection in comparison
with the PSFs. Then the pixels in the saturation-affected region plus 1-pixel (2.′′3) periphery
are replaced by those unaffected in the short exposure image. As a result, we have replaced a
region of about 12′′× 12′′near the center in the N3, N4, S7, and S11 bands.
With the FIS, we obtain N60 (centered at a wavelength of 65 µm), WIDE-S (90 µm),
WIDE-L (140 µm), and N160 (160 µm) band images using one of the FIS observation modes,
FIS01. It consists of two sets of round-trip slow scans with the user-defined parameters that
are the scan speed of 8′′s−1, the cross-scan shift length of 70′′, and the reset time interval of
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0.5 s. A region of approximately 10′×20′around the galaxy was covered with a scan map. The
FWHMs of the PSFs are ∼ 40′′for the N60 and WIDE-S bands and ∼ 60′′for the WIDE-L
and N160 bands (Kawada et al. 2007) . The FIS images are processed with the AKARI official
pipeline software version 20070914 (Verdugo et al. 2007).
3.Results
The IR 10 band images obtained for NGC 3079 are shown in figure 1. The bin size is
set to 0.′′5 for the near- to mid-IR bands, 15′′for the N60 and WIDE-S bands and 30′′for
the WIDE-L and N160 bands. Smoothing by a gaussian kernel of each FWHM (Kawada et al.
2007) has been performed for the far-IR maps. For every map, the image size is 6.′6 × 3.′5, and
the contours are drawn at 0.1, 0.3, 1, 3, and 10 % of the background-subtracted peak surface
brightness for the near- to mid-IR images, and 10, 30, 50, and 70 % for the far-IR images.
The background levels are estimated by selecting several blank sky areas located around the
galaxy, and taking median values of the surface brightness in these areas. The IR emission from
NGC 3079 is spatially resolved in all the bands including the far-IR bands. The original images
in the N3, N4, S7, and S11 bands show ghost signals at about 1′in the northeast direction
from the galactic center. We have removed the ghost signals by subtracting scaled images of
the galaxy from the original images, where a scaling factor is set to 3 % for the S11 band and
0.5 % for the others.
As seen in figure 1, the structure of the galaxy is different from band to band. The
near-IR images show more elongated distributions along the major axis of the optical disk than
the mid- and far-IR images. In the near-IR images, emission from evolved stars is prominent
along the arms, while in the mid-IR images, the center and the arm regions are bright, among
which relative intensities are different. The mid-IR emission is attributed mostly to PAHs in
the S7 and S11 bands (Weedman et al. 2005) and hot dust emission in the L15 and L24 bands.
In the far-IR images, cold dust emission from the galaxy is significantly extended along the
edge-on disk.
Table 2 shows the flux densities of NGC 3079. We set three circular aperture regions of
1′in diameter. They are named disk1, disk2, and center as shown in figure 1, along the disk
of NGC 3079, where 1′corresponds to ∼ 4 kpc for the distance to NGC 3079. In addition, we
obtain the total flux densities from the whole galaxy by integrating surface brightness within
an elliptical aperture of 3′and 5′in minor and major axis, respectively, for the near- and the
mid-IR images. We use a circular aperture of 5′in diameter for the far-IR images. For the
1′aperture that is not large enough as compared to the PSFs for the far-IR bands, we apply
aperture corrections by using the correction table provided by Shirahata et al. (2009). Color
corrections are also applied with factors of 0.89–1.05 for the obtained flux densities of the far-IR
bands, once we obtain a dust temperature.
The fractional errors of the flux densities are estimated to be 2.3–4.7 % for the near-
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Fig. 1. AKARI 10 band images of NGC 3079 in a size of 6.′6 × 3.′5 for each. North is up and east is to the
left. The contours are drawn at 0.1, 0.3, 1, 3, and 10 % levels of the background-subtracted peak surface
brightness for the near- to mid-IR images, and 10, 30, 50, and 70 % for the far-IR images. The peak
surface brightness is 91.4 MJy str−1, 86.3 MJy str−1, 631 MJy str−1, 256 MJy str−1, 296 MJy str−1, 371
MJy str−1, 194 MJy str−1, 270 MJy str−1, 166 MJy str−1, and 85 MJy str−1for the N3, N4, S7, S11,
L15, L24, N60, WIDE-S, WIDE-L, and N160 bands, respectively.
and mid-IR bands by considering photon noise, fluctuation of the background levels, and un-
certainties in the conversion factor of ADU to Jy (Tanab´ e et al. 2008). The flux uncertainties
including both systematic effects associated with the far-IR detectors and absolute uncertain-
ties are estimated to be no more than 20 % for N60 and WIDE-S and 30 % for WIDE-L
and N160 (Kawada et al. 2007; Verdugo et al. 2007). In table 3, we summarize the total flux
densities of NGC 3079 obtained from the previous mid-, far-IR, and submillimeter observations.
By comparing the flux densities in table 2 with those in table 3, we find that the total flux
densities obtained by AKARI are consistent with those previously measured by IRAS and ISO
within the above errors.
Figure 2 shows the spectral energy distributions (SEDs) of the four regions constructed
from the flux densities in table 2. Although these SEDs are quite similar from region to region
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