Uncovering the kiloparsec-scale stellar ring of NGC5128

Abstract

We reveal the stellar light emerging from the kiloparsec-scale, ring-like structure of the NGC5128 (Centaurus A) galaxy in unprecedented detail. We use arcsecond-scale resolution near infrared images to create a "dust-free" view of the central region of the galaxy, which we then use to quantify the shape of the revealed structure. At the resolution of the data, the structure contains several hundreds of discreet, point-like or slightly elongated sources. Typical extinction corrected surface brightness of the structure is K_S = 16.5 mag/arcsec^2, and we estimate the total near infrared luminosity of the structure to be M = -21 mag. We use diffraction limited (FWHM resolution of ~ 0.1", or 1.6 pc) near infrared data taken with the NACO instrument on VLT to show that the structure decomposes into thousands of separate, mostly point-like sources. According to the tentative photometry, the most luminous sources have M_K = -12 mag, naming them red supergiants or relatively low-mass star clusters. We also discuss the large-scale geometry implied by the reddening signatures of dust in our near infrared images. Comment: 5 pages, 4 figures, accepted for publication in A&A Letters. A version with high resolution images can be downloaded from http://www.helsinki.fi/~jtkainul/CenALetter

Full text

Page 1

arXiv:0906.3521v1 [astro-ph.GA] 19 Jun 2009
Astronomy & Astrophysics manuscript no. aa
June 19, 2009
c ? ESO 2009
Letter to the Editor
Uncovering the kiloparsec-scale stellar ring of NGC5128⋆
J. T. Kainulainen1,2, J. F. Alves3, Y. Beletsky4, J. Ascenso5, J. M. Kainulainen6, A. Amorim7, J. Lima7, R. Marques8, J.
Pinh˜ ao8, J. Rebord˜ ao9, and F. D. Santos7
(Affiliations can be found after the references)
Received ¡¿; accepted ¡¿
ABSTRACT
We reveal the stellar light emerging from the kiloparsec-scale, ring-like structure of the NGC5128 (Centaurus A) galaxy in unprecedented detail.
We use arcsecond-scale resolution near infrared images to create a “dust-free” view of the central region of the galaxy, which we then use to
quantify the shape of the revealed structure. At the resolution of the data, the structure contains several hundreds of discreet, point-like or slightly
elongated sources. The typical extinction-corrected surface brightness of the structure is KS≈ 16.5 mag/arcsec2, and we estimate the total near
infrared luminosity of the structure to be M ≈ −21 mag. We use diffraction limited (FWHM resolution of ≈ 0.1′′, or 1.6 pc) near infrared
data taken with the NACO instrument on the VLT to show that the structure decomposes into thousands of separate, mostly point-like sources.
According to the tentative photometry, the most luminous sources have MK≈ −12 mag, making them red supergiants or relatively low-mass star
clusters. We also discuss the large-scale geometry implied by the reddening signatures of dust in our near infrared images.
Key words. dust, extinction — Galaxies: individual: NGC5128 — Galaxies: ISM — Galaxies: structure — Infrared: galaxies
1. Introduction
The Centaurus A galaxy (NGC5128, Cen A hereafter) is one
of the most widely-recognized extragalactic objects in the sky.
Beingthe mostnearbygiantelliptical containinganactivegalac-
tic nucleus (AGN) and an associated jet, it has been the target of
a wide range of AGN studies in the past (see Israel 1998 for a
review).On a largescale, Cen A featuresa giantelliptical galaxy
that has a several kiloparsec wide disk of gas and dust in its cen-
tralregion.Thediskis suggestedtobearemnantofamergerofa
small,gas-richgalaxyanda giantelliptical(Baade&Minkowski
1954) some 2 − 7 × 108years ago (e.g. Quinn 1984; Quillen et
al. 1993; Sparke 1996). The merger is generally believed to be
responsible for the AGN activity in Cen A.
The central disk ofgas and dust in Cen A is manifestedby an
opaque and rather chaotic dust absorption lane at optical wave-
lengths (see Marconi et al. 2000 for high resolution optical im-
ages). Because of the obscuration, the detailed stellar content
and thereby the role of the merger for the star formation in Cen
A has remained quite elusive. Observations of dust and molecu-
lar line emission have revealed a kiloparsec-scale, S- or bar-like
structure that extends to about one kiloparsec radius, coincid-
ing with the dust lane (e.g. Mirabel et al. 1999; Leeuw et al.
2002; Espada et al. 2009). The structure is known to exhibit star
formation, as evidenced e.g. by Paα emission (Marconi et al.
2000). The velocity field of the gas in the structure has been
quite successfully modeled as an inner part of a rotating, warped
disk viewed from a high inclination angle (Bland et al. 1987;
Nicholson et al. 1992; Quillen et al. 1992). The warped disk
model can also qualitatively describe the obscuration at visual
wavelengths (Quillen et al. 1993). Most recently, Quillen et al.
Send offprint requests to: J. Kainulainen
⋆Based on observations made with ESO Telescopes at the La Silla
or Paranal Observatories under programme IDs 066.C-0310 and 081.C-
0477
(2006, Q06 hereafter) used Spitzer mid-infrared images to show
that the dust emission originatesprimarilyfroma parallelogram-
shaped structure, and that there is a void in the dust distribution
between 6 − 50′′(100− 800 pc).
In this Letter, we report near infrared observations revealing
the stellar light from the kiloparsec-scale, ring-like structure in
the central region of Cen A. These observations unveil the ge-
ometry of the structure in unprecedented detail and for the first
time using ground-based data. We describe the geometry of the
observed structure in the context of the warped disk model de-
veloped for Cen A by Quillen et al. (1993). Furthermore, we
present tentative, adaptive optics assisted near infrared data con-
firming the stellar nature of the sources in the structure. In §2 of
this Letter we describe the observations, and in §3 we present
the results and discuss them briefly. Throughout the paper, we
adopt the distance of 3.42 Mpc to Cen A (Ferrarese et al. 2007).
2. Observations
2.1. NTT/SOFI observations
We use the photometrically calibrated JHKS band surface
brightness images, presented in Beletsky et al. (2009), as the
starting point for our analysis. These images, taken with the
SOFI instrument on 3.5-m New Technology Telescope during
5. − 8. March 2003, cover ∼ 4′× 4′region from the central part
of Cen A. For the details of the data and data reduction we refer
to Beletsky et al. 2009). The J band image is shown in Fig. 1.
2.2. VLT/NACO observations
We obtained J (1.265 ± 0.25 µm), H (1.66 ± 0.33 µm), and
KS(2.18 ± 0.35 µm) band, high-resolution images of the Cen
A galaxy on June 30th 2008, using the adaptive-optics as-
sisted imager NACO at the Nasmyth-B focus of the ESO/VLT
8-m telescope, “Yepun”, located at Cerro Paranal, Chile. The

Page 2

2J. T. Kainulainen et al.: Uncovering the kiloparsec-scale stellar ring of NGC5128
Fig.1. The observed SOFI data and the revealed ring-like stellar structure of the Cen A galaxy. The intensity scale is inverted for
visibility. Left: The observed J band image. Right: The same after the first-order extinction correction in which EJ−Himage has
been used as a template for dust (see §3.1 in text). The inset shows a blowup of the SE part of the structure.
NACO (NAOS-CONICA) instrument is equipped with an adap-
tive optics system, NAOS (Rousset et al. 2003), which provides
both visible and infrared wavefront sensing and illuminates the
CONICA camera (Lenzen et al. 2003; Hartung et al. 2003)
equipped with an Aladdin 1024 × 1024 pixel InSb array detec-
tor. The images were collected with the L27 camera (28′′×28′′,
27.12 mas/pix), which provided fully sampled images accord-
ing to the Nyquist sampling criterion. The bright star at the SE
cornerof the field served as the naturalguidestar. A full descrip-
tion of the data acquisition and detailed analysis will appear in
Ascenso et al. (in prep.).
3. Results and discussion
3.1. Kiloparsec-scale ring of Cen A in near infrared
We subtracted the observed SOFI surface brightness images
from each other, yielding J − H and H − K color maps. We pro-
cessed the color maps further by calculating the color excess of
each image pixel with respect to the color of the elliptical galaxy
in that pixel:
Ei−j= (mi− mj)observed− (mi− mj)elliptical.
(1)
To estimate the color of the elliptical galaxy, we calculated the
average observed color as a function of radius inside spherical
annuli of 2′′, centered at the galaxy nucleus. In the region where
the dust lane prevents the determination of the color profile, we
evaluated the profile from the fit of a second order polynomialto
the colorprofile outside the dust lane. The colorexcess maps de-
rived in this way are morphologicallysimilar to the color excess
maps shown in Beletsky et al. (2009), except for the large scale
gradientdueto the elliptical, whichis eliminatedfromourmaps.
The images are included in the online version of this Letter (Fig.
4).
It is possible to make a first-order absorption correction to
the observed surface brightness images using the color excess
maps as templates for dust. If the absorbing dust is located in
front of all stellar emission, the extinction in band i is related to
the color-excess Ei−jvia equation:
Ai= Xij× Ei−j,
(2)
where Aiis extinction in band i and Xij = (1 + τj/τi)−1. The
constant Xijcan be calculated by adoptingthe extinction law e.g.
by Rieke & Lebofsky (1985) which yields XJH≈ XHK= 2.6. We
note that Eq. (2) is not generally valid for dust clouds embedded
in galaxies, because it is usually unrealistic to assume that the
clouds are located in front of the stellar emission of the galaxy.
However, we argue that in the exceptional case of Cen A this
simplification is, in fact, not unrealistic. We justify the argument
and discuss it in Section 3.2.2.
We used the color excess maps to perform an extinction cor-
rection for the observed images, as facilitated by Eq. (2). To
illustrate the result, the extinction corrected J band image is
shown in Fig. 1. These images reveal a bright,ring-like structure
coinciding with the southern part of the dust lane. The structure
is clearly similar to the parallelogram-shapedfeature previously
detected in the Spitzer data at 3.6 − 8 µm (see Q06 for the 8 µm
image). Especially, there is an obvious similarity between the
extinction corrected images and the Spitzer 3.6 µm image that is
dominatedbyemissionofthesame stellarcomponentas theNIR
data. However, the sub-arcsecond resolution SOFI data (Fig. 1)
reveals the structure in unprecedentedly high detail and com-
pletely undisturbedby thermal dust emission. The structure con-
tains several hundreds of sources that are point-like or slightly
elongated at the resolution of the SOFI data. The nature of these
sources is further discussed in Section 3.3.
We comparedthe integratedintensityin the observedand ex-
tinctioncorrectedimagesto examinethe totalattenuationcaused

Page 3

J. T. Kainulainen et al.: Uncovering the kiloparsec-scale stellar ring of NGC51283
by the dust lane. This yielded attenuations of 0.19 mag, 0.26
mag, and 0.4 mag in KS, H, and J, respectively. We also esti-
mated the total NIR luminosity of the ring-like structure. To do
this, we subtracted the intensity profile of the elliptical galaxy
from the extinction corrected images using a simple spherical
model. Fig. 2 shows the resulting image in KSband. In addi-
tion to the ring structure, the image shows numerous additional
sources, especially in the NW part of the image. Typical intensi-
ties in the most prominent regions are KS≈ 16.5 mag / arcsec2.
Integration of the intensity over the ring structure yields total
NIR luminosities of MJ = −20.5 mag, MH = −21.6 mag, and
MK = −21.8 mag. We note that these estimates are subject to
several sources of uncertainty, especially including the extinc-
tion correction. Therefore, they should be considered merely as
first-order estimates.
3.2. Geometry inferred by the data
The warped disk model developed for gas and dust in Cen A is
describedby a series ofthin, co-centeredrings whoseinclination
and position angles vary as a function of radius (Quillen et al.
1992; see also Q06). These rings define the central plane of the
disk. When viewed from a high inclination angle, the changing
sign of the disk inclination angle with respect to the line of sight
causes twists in the disk, both in north and south sides of the
nucleus. The dust present in the disk can qualitatively explain
the geometry of the observed absorption and emission features
(we refer to Q06 andreferencestherein fora detaileddescription
of the model). In the following, we examine our NIR data in the
context of the warped disk model.
3.2.1. Shape of the ring-like structure
The sub-arcsecond resolution view of the uncovered structure
provides an excellent basis for quantifying its shape. Except for
the “twisted” SE and NW edges, the shape of the structure is
quite well described by a ring with major and minor axes of
a = 67′′and b = 8.5′′(1100 pc and 140 pc), respectively, and
a diameter of d = 4′′(70 pc). The position angle of the ring
is 25.5◦, and in order to best match it with the observed struc-
ture, an offset of 1.5′′north along the minor axis is required.
The region inside which there are no sources has a major axis of
a ≈ 50′′(≈ 800 pc), in unison with Q06.
We used the ellipsoid subtracted images to determine the
inclination and position angles, α(r) and ω(r), that define the
warped disk model. We constructed the warped disk model in-
side r < 100′′as explained in Q06 with the difference that in our
model the emission from the disk was constant as a function of
radius from the nucleus. In Q06, the angles α(r) and ω(r) were
defined by giving a set of points [α,r] and [ω,r] and by mak-
ing an interpolation between the points using a spline function.
We construct our model by minimizing the χ2value between the
observedand model images where α(r) and ω(r) are free param-
eters. Fig. 2 shows the best-fitting model and the angles α(r) and
ω(r) resulting from the fit. We note that the fit is not satisfactory
in describing the detailed intensity structure of the ring, but the
model reproduces the general shape of the ring seemingly well.
3.2.2. Dust geometry
The location of the dust clouds responsible for the absorption
features in Cen A can be examined in a model-dependent man-
ner by assuming that the dust is located in the warped disk as
Fig.2.Top: The extinctioncorrected KSbandimage fromwhich
the intensity of the elliptical has been subtracted. The inset
shows the fit to the image of a model composed of co-centric
rings with varying inclination and position angles (see text).
The small box shows the position of the NACO observations.
Bottom: The position and inclination angles of the best-fitting
model for the ring structure. The dashed lines show the angles
given by Q06.
described in Q06. For each line-of-sight in the observed images,
it is straightforward to calculate the position, or the range of
positions, where that particular line-of-sight is connected to the
warped disk structure. For example, it is evident that within the
warped disk framework, the dust features north of the nucleus
are associated with the fold at r ≈ 80 − 150′′.
It is interesting to consider the fraction of stellar light emit-
ted in frontof the dust structures at each positionof the observed
images. To estimate the fraction, a radial model for the stellar
emission is needed. We adopted the radial luminosity profile
given by Marconi et al. (2006) who fitted the intensity profile
using an oblate spheroidal function. Using this model, we in-
tegrated the emission of the radial emission model along each
line-of-sight towards the dust features to the point closest to the
observer where that line-of-sight connects with the warped disk
structure. The fractions of foreground emission resulting from
this calculation are between 1 − 20 %. In particular, the fraction
of the foreground light in the fold at the northern side of the nu-
cleus is less than 10 %. Typical attenuation caused by the clouds
in the field is less than a magnitude, i.e. less than ∼ 60 % in

Page 4

4J. T. Kainulainen et al.: Uncovering the kiloparsec-scale stellar ring of NGC5128
Fig.3. KSband, adaptiveoptics assisted image of the NW end of
the ring-like structure taken with the NACO instrument on the
VLT (see Fig. 2 for the frame position). The brightest sources in
the image are KS≈ 16 mag (discluding the saturated star in the
lower left corner). Most sources have PSFs consistent with point
sources.
flux. Therefore, the intensity observed in any on-cloud pixel is
unlikely to be dominated by the flux of the foregroundstars.
While the discussion above relies on a certain model for the
dust geometry, it is possible to use the wavelength dependency
of the observed attenuation to make a model independent, first-
orderapproximationofthe overalldustgeometry(e.g.Gallagher
& Hunter 1981; Howk & Savage 1997; Kainulainen et al. 2008).
To examine the wavelength dependency of the reddening, we
measured the ratio of color excesses, EJ−H/EH−K, at each pixel
in the fold north of the nucleus where EH−K> 0.2. This resulted
in a mean ratio of EJ−H/EH−K= 1.7, a value similar to that im-
plied by the standard Galactic reddening laws, e.g. 1.7 by Rieke
& Lebofsky (1985). If the dust extinction law in Cen A is simi-
lar to the Galactic one, such a high ratio is expected only if the
fraction of the foreground light is low. Thus, the color excess
ratio we calculated for the dust in Cen A supports a geometry
where the dust is dominantly in front of most stellar emission,
and thereby the warped disk model. Furthermore, the low frac-
tion of foregroundlight suggests that it may not be unreasonable
to assume that the color excess is a linear measure of the dust
column density in the particular case of the Cen A dust lane.
3.3. Tentative results from high resolution NIR data
While a more complete analysis of our NACO data will be pre-
sented in a forthcoming paper (Ascenso et al., in prep.), we use
the data in this Letter to show that the ring harbors a large num-
ber of sources that are point-like at parsec-scale resolution. Fig.
3 shows the observed NACO KSband image (see Fig. 3 for the
position of the frame). The image is filled with apparentlypoint-
like sources, distributed throughoutthe observedfield. However,
thebrightestsourcesareclearlyarrangedtofollowtheringstruc-
ture seen in SOFI images. Comparison of the image to the SOFI
datashows howthesourcesinSOFI images decomposeintosev-
eral or even tens of individual sources. In addition to the stellar
sources, the image contains extended obscuration features due
to dust. The features closely follow the morphology of the color
excess maps made from SOFI data, although the superior sensi-
tivity ofthe NACO data makesthe features moreprominent.Our
tentative PSF-photometryof the KSband image contains∼ 3000
sources above the 5-σ detection limit. The brightest sources are
mK ≈ 16 mag while the majority is mK ≈ 18.5 mag. Given
that the SOFI data suggests extinction of AK ≈ 0.3 mag in the
least obscured part of the field, the absolute magnitudes of the
brightest objects are MK≈ −12 mag. According to the tentative
photometry, almost all sources have PSFs consistent with point
sources. These properties suggest that the brightest sources are
red supergiants, or rather low-mass star clusters, located within
the ring.
In conclusion, the observations presented in this Letter pro-
vide a new view of the structure and the stellar content of the
Cen A galaxy. We have uncovered and quantified the shape of a
large-scale stellar structure in the interior of Cen A, previously
exposed only by space-based mid-infrared data. Such informa-
tion can be used to better constrain the dynamical models and
star-forming history of the Cen A galaxy.
Acknowledgements. The authors would like to thank A. Quillen who kindly pro-
vided the electronic Spitzer and HST images to be compared with our data.
References
Baade, W., & Minkowski, R. 1954, ApJ, 119, 215
Beletsky, Y. 2009, PhD Thesis, Max Planck Institute of Astrophysics, Garching,
Germany
Bland, J., Taylor, K., & Atherton, P. D. 1987, MNRAS, 228, 595
Espada, D., Matsushita, S., Peck, A. et al. 2009, ApJ, 695, 116
Ferrarese, L., Mould, J. R., Stetson, P. B. et al. 2007, ApJ, 654, 186
Gallagher, J. S., & Hunter, D. A. 1981, AJ, 86, 1312
Hartung, M., Lenzen, R., Hofmann, R., et al. 2003, Proc. SPIE, 4841, 425
Howk, J. C., & Savage, B. D. 1997, AJ, 114, 2463
Israel, F. P. 1998, A&A Rev., 8, 237
Kainulainen, J., Juvela, M., & Alves, J. 2008, A&A, 482, 229
Leeuw, L. L., Hawarden, T. G., Matthews, et al. 2002, ApJ, 565, 131
Lenzen, R., Hartung, M., Brander, W., et al. 2003, Proc. SPIE, 4841, 944
Marconi, A., Schreier, E. J., Koekemoer, et al. 2000, ApJ, 528, 276
Marconi, A., Pastorini, G., Pacini, F., et al. 2006, A&A, 448, 921
Mirabel, I. F., Laurent, O., Sanders, D. B., et al. 1999, A&A, 341, 667
Nicholson, R. A., Bland-Hawthorn, J., & Taylor, K. 1992, ApJ, 387, 503
Quillen, A. C., de Zeeuw, P. T., Phinney, E. S., & Phillips, T. G. 1992, ApJ, 391,
121
Quillen, A. C., Graham, J. R., & Frogel, J. A. 1993, ApJ, 412, 550
Quillen, A. C., Brookes, M. H., Keene, J., et al. 2006, ApJ, 645, 1092, Q06
Quinn, P. J. 1984, ApJ, 279, 596
Rieke, G. H., & Lebofsky, M. J. 1985, ApJ, 288, 618
Rousset, G., Lacombe, F., Puget, P., et al. 2003, Proc. SPIE, 4839, 140
Sparke, L. S. 1996, ApJ, 473, 810
1Observatory, P.O. Box 14, FIN-00014 Univ. of Helsinki, Finland
e-mail: jtkainul@mpia-hd.mpg.de
2TKK/Mets¨ ahovi Radio Observatory, Mets¨ ahovintie 114, FIN-02540
Kylm¨ al¨ a, Finland
3Calar Alto Observatory, Centro Astron´ omico Hispano, Alem´ an, C/q
Jes´ us Durb´ an Rem´ on 2-2, 04004 Almeria, Spain
4European Southern Observatory (ESO), Alonso de Cordova 3107,
Santiago, Chile
5Harvard-Smithsonian Center for Astrophysics, 60 Garden Street,
Cambridge, MA 02138, USA
6TKK/Department of Radio Science and Engineering, P.O. Box 3000,
FIN-02015 TKK, Finland
7SIM-IDL, Faculdade de Ciˆ encias da Universidade de Lisboa, Ed. C8.
Campo Grande 1749-016 Lisbon, Portugal

Page 5

J. T. Kainulainen et al.: Uncovering the kiloparsec-scale stellar ring of NGC51285
Fig.4. Near infrared color excess maps of the Centaurus A galaxy (see text for the derivation). Left: EH−K. Right: EJ−H.
8LIP-Coimbra, Department of Physics, University of Coimbra, 3004-
516 Coimbra, Portugal
9INETI, Estrada da Portela, Zambujal-Alfragide, Apartado 7586,
2720-866 Amadora, Portugal