---
abstract: 'Results from the Hubble Deep Field (HDF) North and South show a large percentage of high-redshift galaxies whose appearance falls outside traditional classification systems.  The nature of these objects is poorly understood, but sub-mm observations indicate that at least some of these systems are heavily obscured (Sanders [@sanders00]).  This raises the intriguing possibility that a physically meaningful classification system for high-redshift galaxies might be more easily devised at rest-frame infrared wavelengths, rather than in the optical regime.  Practical realization of this idea will become possible with the advent of the [*Next Generation Space Telescope*]{} (NGST).  In order to explore the capability of NGST for undertaking such science, we present NASA-IRTF and SCUBA observations of , a chaotic system in our local Universe which bears a striking resemblance to objects such as HDF 2-86 ($z=0.749$) in the HDF North.  If objects such as are common at high-redshifts, then this galaxy may serve as a local morphological ‘Rosetta stone’ bridging low and high-redshift populations.  In this paper we demonstrate that quantitative measures of galactic structure are recoverable in the rest-frame infrared for seen at high redshifts using NGST, by simulating the appearance of this galaxy at redshifts z=0.7 and z=1.2 in rest-frame K$''$. While this object cannot be classified within any optical Hubble bin, simulated NGST images at these redshifts can be readily classified using the dust penetrated z $\sim$ 0 template of Block and Puerari ([@blockpuerari99]) and Buta and Block ([@butablock01]).  The near-infrared disk of NGC 922 is not peculiar at all; rather, it is remarkably [*regular*]{}, even presenting spiral arm modulation, a characteristic signature of several grand design galaxies.  Our results suggest that the capability of [*efficiently*]{} exploring the rest-wavelength IR morphology of high-z galaxies should probably be a key factor in deciding the final choice of instruments for the NGST.' 
author:
- 'David L. Block , Ivânio Puerari , Marianne Takamiya , Roberto Abraham , Alan Stockton , Ian Robson'
- Wayne Holland
date: 'Received .................. / Accepted ..................'
title: 'Dust-penetrated morphology in the high-redshift universe: clues from '
---

Introduction
============

Considerable progress has recently been made in establishing the morphological mix of field galaxy populations as a function of redshift (eg.  Glazebrook et al. [ @glazebrook95], Driver et al. [ @driver98], Abraham et al. [ @abraham96a]).  As a by-product of this work, a number of investigations probing the systematic changes in galaxy morphology as a function of rest-wavelength have been undertaken (Abraham et al. [ @abraham96b], Bouwens et al. [ @bouwensetal98], Giavalisco et al. [ @giavaliscoetal96]).  These studies have focused primarily on changes which occur as the rest-wavelength of observation moves from the optical regime into the near-ultraviolet (so-called “morphological K-corrections”).  Remarkably little quantitative work has explored the systematic variations in morphology which occur as one moves from the optical in the [*other*]{} wavelength direction, namely into the near-IR.  This is rather surprising, since the nearly constant mass-to-light ratio as a function of age for simple stellar populations suggests that near-IR morphology probes underlying stellar mass much more fairly than does optical morphology (Kauffmann and Charlot [@kauffmann98]; fig.  4 in Charlot [@charlot98]; Charlot [@charlot96]; Frogel et al. [ @frogel96]). 

The morphology of local galaxies can completely change once Population I disks are dust penetrated.  One of many examples is , an optical irregular which presents an SBa morphology in the near-infrared.  The extinction at K$'$ (2.1$\mu
 m$) is only 10% that in the V (0.55$\mu m$) band.  For local field galaxies, there is no correlation between dust penetrated classes and optical Hubble types; the Hubble tuning fork does not constrain the morphology of the old stellar Population II disks (Block and Puerari [@blockpuerari99]).  Less is known at higher redshifts, since the evolved stellar disks of high-z galaxies in the HDF have never been explored at restframe K$'$ band.  Images of galaxies with redshifts ${\rm z}\sim 0.5 - 1$ or higher secured using the Hubble Space Telescope and NICMOS never penetrate the dusty, gaseous Population I mask.  At $z>3$, even H band (1.6$\mu
 m$) observed flux stems from emission shortward of 4000 Å. Early results from SCUBA suggest that a substantial proportion of high-redshift star-formation is occurring in heavily obscured galaxies (Sanders [@sanders00]; see also fig.  5 in Hughes [@hughes96]). 

Fortunately, upcoming advances in technology will soon allow the rest-frame IR morphology of galaxies to be probed at both low and high-redshifts.  Near-infrared imaging surveys of local galaxies (such as the IR component of the VISTA survey) will be entirely comparable to those currently achievable at optical wavelengths.  Similarly, the [*Next Generation Space Telescope*]{} (NGST) will allow studies of rest-wavelength IR morphology to be extended to high-redshifts.  In this regard, the synergy between VISTA and NGST may turn out to be rather similar to that currently existing between ground-based optical facilities and HST. 

Motivated by this possibility, we have begun an investigation of the capability of NGST for extending local morphological studies to high redshifts.  In this preliminary investigation, our strategy will be to focus on a single extreme case (NGC 922), which highlights the potential benefits of extending quantitative analyses of rest-wavelength near-IR morphology to high redshifts.  This object is particularly well-suited for our purposes, since (in common with numerous high-redshift galaxies) “The morphology of is so peculiar as to be outside the classification system.  It would be called a sport by nineteenth-century animal breeders” (see Sandage and Bedke [@sandagebedke94], Panel 313).  This galaxy bears a striking optical resemblance to chaotic objects seen at high redshifts such as HDF 2-86 (van den Bergh [@vandenbergh98]) at $z=0.749$ in the Hubble Deep Field (HDF), which van den Bergh et al. ([ @vandenbergh96]) suggest might be a spiral in the process of being assembled.  Both and HDF 2-86 contain a prominent arc-like feature in the one-half of the galaxy in which star formation is apparently proceeding at a vigorous rate (see Table 1 in Cohen [@cohen76] and Devereux [@devereux89]). 

Simulations of with NGST at Restframe K$'$
==========================================

$K'$ images of were obtained at the NASA Infrared Telescope Facility with NSFCam, operating in the 0.3 arcsec mode, giving a field of 76.  NGC922 was placed successively in each quadrant of the detector in order to have good coverage of nearly uncontaminated blank sky, from which a median averaged sky flat could be obtained.  Small dithers superposed on this basic 4-point pattern ensured that stars in the field fell at different points of the detector each time.  In all, 17 integrations of 1 minute each were obtained.  Our K$'$ mosaic appears in fig.  1.  The galaxy was also observed at 850$\mu m$ using the Submillimetre Common-User Bolometer Array (SCUBA; see Holland et al. [ @holland99]) at the James Clerk Maxwell Telescope (JCMT) at Mauna Kea.  The observing mode used was jiggle-mapping in which the secondary mirror of the telescope compensates for the instantaneous under-sampled images by offsetting the array position to produce a Nyquist-sampled image.  Some 300 separate ‘jiggle-maps’ (or integrations) were secured over a three night period, with a total on-source integration time of 2.7 hours.  The data was reduced using the SURF software package (Jenness and Lightfoot [@jenness98]), correcting for noisy pixels, flatfield variations, atmospheric opacity, and short-time sky variations.  The data were flux calibrated using beam-maps of Uranus and CRL618.  The peak flux in the map is 25.0$\pm$1.7 mJy/beam (following smoothing by a 7$''$ gaussian) and the integrated flux over a 100$''$ diameter aperture, centered on the peak, is estimated to be $\sim$ 36mJy.  Absolute calibration accuracy is estimated to be about 6%. 

Emission at 850$\mu m$ is not confined to only one-half of the disk in which star-formation is occurring.  The dust morphology is [*radically different*]{} to the near-infrared NASA-IRTF morphology (see fig.  2); cold dust grains, seen in emission at 850$\mu m$, are not restricted to the bright one-half of the almost ’sliced-in-two’ disk. 

Assuming a mass absorption coefficient of 0.007 at 850$\mu m$ and a grain temperature of 10-20K (consistent with cold dust grain temperatures inferred using radiative transfer codes e.g. Block et al. [ @blocketal94a]), we compute an atomic and molecular hydrogen gas mass of $\sim$ 2-4 10$^{9}$ M$_{\odot}$. The dust mass for of $\sim$ 10$^{7}$ M$_{\odot}$, one order of magnitude greater than that of the ‘Evil Eye’ Galaxy (often cited as a prototype of a very dusty galaxy). 

Our artificially redshifted images of the dusty are based on the preliminary description of the NGST facility given in Gillett and Mountain ([@gillettmountain98]).  For concreteness, we assumed an 8m telescope.  Our images assume an imager corresponding to the design of an 8192 $\times$ 8192 pixel format near-infrared camera operating on NGST from 0.6 to 5.3$\mu m$ (which includes the L and M bands), as described in Bally and Morse ([@ballymorse99]).  It is of course emphasized that at the present time the NGST telescope and camera designs have not been finalized, and the orbit placement of NGST not yet been decided.  Indeed the present investigation of what can be achieved with the currently planned facility may provide some useful guidance to instrument designers. 

In order to probe galaxies using NGST at redshifts 0.7 and 1.2 in their K$'$ restframes, imaging detectors in the broadband L (3.7$\mu m$; bandpass $\delta$L=0.65$\mu m$) and M (4.7$\mu m$; $\delta$M=0.45$\mu m$) regimes are required.  At M, for example, the sky background at an excellent groundbased site such as Mauna Kea is 10 million times brighter than it will be for NGST (see fig.  1 in Gillett and Mountain [@gillettmountain98], where a sky background at Mauna Kea of $\sim$ 305 Jansky arcsec$^{-2}$ is adopted, but only 1.9$\times$10$^{-5}$ Jansky arcsec$^{-2}$ for NGST). 

We follow the simulation methodology of Takamiya ([@takamiya99]).  The simulations recreate the images of when moved out to higher redshifts, always in a pre-selected restframe.  In this paper, the images of are simulated at redshifts z=0.7 and z=1.2 in the dust penetrated K$'$ rest-frame.  Since the rest-frames are matched, no pixel $k$ corrections are applied (Takamiya [@takamiya99]).  If this was not the case, then $k$ corrections in simulations are necessary (e.g., Bouwens et al. [ @bouwensetal98], Giavalisco et al. [ @giavaliscoetal96]).  Furthermore, no spectral energy distribution templates are needed: these are only required for rest-frames which are not matched, such as when simulating HDF rest-frame UV images from local optical ones (see for example, Abraham et al. [ @abraham96b]). 

We assume that the NGST sky background is about twice the minimum background observed by COBE (Hauser [@hauser94]), corresponding to a sky surface brightness at L and M of 19.55 and 17.31 magnitudes per square arcsecond, respectively. ( From good near-infrared groundbased sites, the corresponding sky surface backgrounds at L and M are mag arcsec$^{-2}$ and –0.24 mag arcsec$^{-2}$, respectively.)  We furthermore assume pixel sizes of 50 milliarcseconds, a gain of 4 electrons per ADU (analog digital unit), a readout noise of 4 electrons and an output point spread function of gaussian distribution, with a full width at half maximum (FWHM) of 0$\farcs$12.  Assuming eight optical surfaces with 95% transmission (these include the primary and secondary mirrors as well as lenses and filters in the detector) and a 40% detector quantum efficiency, the system throughput is taken to be 26%.  An on-source integration time of one hour was adopted.  Results from these simulations are shown in the left hand column of fig.  3.  A morphological decomposition based on these images is shown in the right hand column of this figure, and will be described in the next section.  As a comparison, we ran a set of simulations on a galaxy which [*can*]{} readily be classified in the optical regime: .  NGC 2997 shows a magnificent grand design structure in the optical regime; it is of Hubble type Sc and belongs to van den Bergh luminosity class I bin.  The results of the NGC 2997 simulations are presented in fig.  4. 

Morphological classification
============================

In the present paper we adopt the dust penetrated classification scheme for local spirals proposed by Block and Puerari ([@blockpuerari99]) and Buta and Block ([@butablock01]).  In the near-infrared, the morphology of the older star dominated disk indicates a simple classification scheme (see figs.  5 and 7): H$m$ (where $m$ is the dominant Fourier harmonic), three pitch angle classes $\alpha$, $\beta$ and $\gamma$ and thirdly, a bar strength parameter.  Bar strengths are derived from the gravitational potential or torque of the bar, and not from bar ellipticity.  A bar which might be highly elongated (and therefore ‘strong’ in optical images) may sometimes have a negligible bar strength, as inferred from its mass distribution and potential in near-infrared images (Buta and Block [@butablock01]).  We will show that the dust-penetrated tuning fork template for local galaxies shown in fig.  7 readily accommodates . 

The 2-D Fast Fourier decomposition of images uses a program developed by I. Puerari (Schröder et al. [ @schroder94]).  Following Danver ([@danver42]) and Kalnajs ([@kalnajs75]), logarithmic spirals of the form $r=r_o {\rm
 exp} (-m \theta /p_{\rm max})$ are employed in the decomposition. 

The amplitude of each Fourier component is given by (Schröder et al. [ @schroder94])

$$\displaystyle A(m,p) = \frac{\Sigma_{i=1}^I \Sigma_{j=1}^J
 I_{ij} ({\rm ln}\;r,\theta)\; {\rm exp}\;(-i(m \theta + p\; {\rm
 ln}\;r))}{\Sigma_{i=1}^I \Sigma_{j=1}^J I_{ij} ({\rm
 ln}\;r,\theta)}$$

where $r$ and $\theta$ are polar coordinates, $I({\rm
 ln}\;r,\theta)$ is the intensity at position $({\rm ln}\;r,
 \theta)$, $m$ represents the number of arms or modes, and $p$ is the variable associated with the pitch angle $P$, defined by $\displaystyle \tan P = -\frac{m}{p_{\rm max}}$.

After having deprojected the K$'$ images and identifying the dominant modes, we calculate the inverse Fourier transform, as follows:

We define the variable $\displaystyle u \equiv {\rm ln}\;r$. Then

$$\displaystyle S(u,\theta) = \sum_m S_m (u) {\rm e}^{im \theta}$$

where

$$\displaystyle S_m(u) = \frac{D}{{\rm e}^{2u} 4 \pi^2}
 \int_{-\infty}^ {+\infty}\;G_m (p) A(p,m) {\rm e}^{i p u}\;dp$$

and

$$\displaystyle D=\Sigma_{i=1}^I \Sigma_{j=1}^J I_{ij} (u,
 \theta)$$

$G_m(p)$ is a high frequency filter used by Puerari and Dottori ([@pueraridottori92]).  For the spiral with $\displaystyle \tan
 P = -\frac{m}{p_{max}^m}$ it has the form

$$\displaystyle G_m(p) = {\rm exp} \left[ -\frac{1}{2} \left(
 \frac{p - p_{max}^m}{25} \right)^2 \right]$$

where $p_{\rm max}^m$ is the value of $p$ for which the amplitude of the Fourier coefficients for a given $m$ is maximum.  This filter is also used to smooth the $A(p,m)$ spectra at the interval ends (see Puerari and Dottori [@pueraridottori92]). 

is close to face-on; deep optical CCD images present an almost circular arc/disk.  No deprojection corrections were therefore necessitated.  Fourier spectra were determined for at z=0.7 and z=1.2 (see fig.  8), and inverse Fourier transform contours were then overlayed on the simulated restframe K$'$ mosaics, following the prescription above.  The right hand columns of figs.  3 and 4 show the results of the inverse Fourier contour overlays on our one-hour NGST simulations. 

The Fourier spectra of NGC 922 (see fig.  8) shows a strong lopsided m=1 component, with a pitch angle corresponding to 39$^{\circ}$ (ie.  class $\gamma$ in fig.  5).  The m=1 component is associated with the arc so clearly seen in our NASA-IRTF image.  The m=2 component is almost as strong in amplitude as the m=1 component (fig.  8), and corresponds to the bisymmetric spiral seen at K$'$ which lies within the arc (fig.  1).  What is intriguing about the m=2 component in fig.  8 is that it is doubly peaked at opposite sides of the origin of the $p$-axis, indicative of a set of leading and trailing wave modes which are propagating inward and outward, being reflected off a central bulge (see Puerari et al. [ @puerarietal00]). [* In other words, shows the unmistakable signature of spiral arm modulation in its stellar disk*]{}.  Spiral arm modulation has hitherto only been detected in grand design disks (such as Messier 81).  With these beating mode considerations, could well be a grand design spiral in the process of assembly.  In our template of figs.  5 and 7, the bisymmetric spiral in would carry the classification H2$\gamma$.

has a gravitational bar strength or torque (Buta and Block [@butablock01]) of 2 (on a scale of 0 to 6).  The complete quantitative classification of (focussing on the inner spiral seen at K$'$) is therefore H2$\gamma$2. 

The bar strength component of this classification scheme is particularly interesting, given the evidence for a rapid decline in the proportion of [*optically*]{} barred galaxies at high redshifts (Abraham et al. [ @abraham99]; van den Bergh et al. [ @sidney00]).  The corresponding behaviour in the rest-frame infrared, where bars are much more prominent in local data (Eskridge et al. [ @eskridge00]), is unknown. 

It must be stressed that Fourier spectra of are being generated on square “postage stamp” FITS images only 3$''$ on a side (64$\times$64 pixels, where 1 pixel corresponds to 50 mas), yet the appearance of the modes are strikingly similar to those of the local $z\sim 0$ spectra.  The decrease of spatial resolution in higher redshift space does not wash out the dominant modes of seen locally (fig.  8).  In fact, given two sets of Fourier spectra, one for the galaxy at its original distance, and another, generated from the simulated image at z=0.7 or z=1.2, it is a challenge to find any noticeable difference in the dominant modes at all (independent of q$_0$).  Identical conclusions pertain to the preservation of pitch angle in NGC 2997 (see fig.  9).  The class $\beta$ bin is retained throughout. 

An interesting point to note is that is that the first and second order harmonics in figs.  8 and 9 are not mutually exclusive.  A galaxy often presents both H1 and H2 components.  Modal theories of spiral structure predict that m=1 modes should generally be accompanied by m=2 modes when available, since the latter are more efficient in transporting angular momentum outwards.  For example, the inner leading arm in NGC 4622 (labeled in fig.  6a of Block et al. [ @blocketal94b]) co-exists with an outer set of m=2 spirals.  Block et al. ([ @blocketal00]) classify the lopsided arm in NGC 4622 as H1$\alpha$ and the outer set of tightly wrapped spirals as H2$\alpha$. What fig.  8 and 9 do show is part of a general trend already noted by Block et al. ([ @blocketal94b]) that modes higher than 2 (ie.  multi-arm features) are generally suppressed in stellar disks imaged at restframe K$'$, whereas the dusty, gaseous components seen in optical images are dynamically very active, fueling Jeans instability and readily supporting multi-arm features. 

A note on deprojections at high-z is in order.  By the time NGST is launched, IFU-based spectroscopy ought to be the norm (in fact, some form of resolved spectroscopic capability is likely to be a component of the NGST instrument load).  Kinematical information can then be used, as in our local Universe, to secure accurate deprojection parameters.  However, we do wish to point out that lessons from z$\sim$0 galaxies show that deprojections based on axial ratios alone are excellent (kinematical studies simply finely-tune them).  In our high-z universe, axial ratios from even relatively low signal-noise NGST images could readily be determined from image moments; being based on calculations summed over many – rather than a few – pixels, they should be remarkably robust (see Frei [@frei00]), even without spectroscopic/kinematical NGST information. 

When high redshift galaxies are observed at the L and M bands with NGST, the expectations are that many will be spirals undergoing intense bursts of star formation (Ferguson and Babul [@fergusonbabul98]; Takamiya [@takamiya99]).  To classify two-armed spirals such as NGC 2997 (fig.  4) according to the templates in figs.  5 and 7 is straightforward.  According to the robustly determined Fourier spectra, its dust penetrated classification would be H2$\beta$. The point of this investigation is that even if a large percentage of these high z systems are optically as irregular as , only two low order modes (m=1 and m=2) are invariably required to describe the dust penetrated disk, in agreement with the ubiquity of low-order stellar modes found in our local Universe (Block and Wainscoat [@blockw91]; Block et al. [ @blocketal94b], [@blocketal00]). 

Conclusion
==========

The utility of an optical imaging capability for NGST is currently being debated (a list of potential instruments is listed on the NASA Webpage www701.gsfc.nasa.gov/isim/science.htm).  The performance of such an instrument can, to a limited degree, be extrapolated from the current capability of HST.  Owing to the paucity of work in this area, it has been less clear what one hopes to learn from morphological studies undertaken in the $L$ and $M$ near-infrared bands that are the focus of much of the NGST science case. 

Our goal in this paper has been to argue (from a single illustrative example) that important morphological work will be achieved at these longer wavelengths.  We have identified as a reasonably representative “archetypal” candidate similar in appearance to high redshift morphologically peculiar counterparts (see fig.  10).  By simulating dust-penetrated images of this object at z $\sim$ 1, and undertaking a detailed analysis of this synthetic data, we have shown that quantitative morphological investigation of such systems is well within the capabilities of the current NGST design. 

Perhaps more importantly, we have argued for the necessity of such observations in order to interpret the nature of optically peculiar systems at high redshifts.  Optical observations of the high redshift Universe may not effectively trace underlying baryonic mass distributions or ‘galactic backbones’, but rather focus for example, on the degree of symmetry/asymmetry of the dusty Population I component.  This is especially true in systems undergoing large bursts of star formation, where the focus in the optical or UV is on OB associations and HII regions.  A crucial lesson from our local Universe is that galaxies on opposite ends of the Hubble tuning fork may present almost identical [*mass*]{} distributions, and belong to the same dust-penetrated class.  This result is clearly seen in figs.  11 and 12.  It is not, a priori, possible to predict what galactic backbones might look like when the disks are dust penetrated at low, and at high, redshift.  At redshifts greater than 1.5, substantial mass could be locked into ‘old’ (ages greater than one or two Gyr) stellar populations at surface brightness levels that are too faint to be visible on deep HST images (Abraham [@abraham98]).  Near-IR imaging with NGST may well provide the most effective means for probing both evolved and dust-obscured stellar components in such systems. 

It is a pleasure to thank the referee, Dr A. Moorwood, as well as Garth Illingworth and Stephane Charlot for some very helpful comments.  AS is a Visiting Astronomer at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract from the National Aeronautics and Space Administration.  DLB, IP and AS are indebted to the Anglo-American Chairman’s Fund Educational Trust.  A note of deep appreciation is expressed to Mrs M. Keeton and the Board of Trustees.  Dr Rolf Chini is also warmly thanked for his input.  This research is partially supported by the Mexican Foundation CONACYT under the grant No.  28507-E.

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