ZEUS: The Redshift(z) and Early Universe Spectrometer

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1 From Z-Machines to ALMA: (Sub)millimeter Spectroscopy of Galaxies ASP Conference Series, Vol. TBD, 2006 A. J. Baker, J. Glenn, A. I. Harris, J. G. Mangum, and M. S. Yun, eds. ZEUS: The Redshift(z) and Early Universe Spectrometer Gordon J. Stacey 1, Steven Hailey-Dunsheath 1, Thomas Nikola 1, Thomas E. Oberst 1, Stephen C. Parshley 1, Dominic J. Benford 2, Johanes G. Staguhn 2, S. Harvey Moseley 2, Carole Tucker 3 1 Deparment of Astronomy, Cornell University, Ithaca, NY 14853,USA 2 Observational Cosmology Laboratory, Code 665, NASA/GSFC, Greenbelt, MD USA, 3 School of Physics and Astronomy, Cardiff University, 5, The Parade, Cardiff, CF24 3YB, Wales, UK Abstract. The redshift (z) and early Universe spectrometer (ZEUS) is an echelle grating spectrometer optimized for studies of starformation in the Universe from about 1 to 2 billion years after the Big Bang to the present epoch by observing spectral lines in the submillimeter bands. ZEUS has a resolving power, R 1000, optimized for extragalactic point source sensitivity. At present, ZEUS employs a 1 32 pixel thermister sensed bolometer array configured to deliver simultaneous 16 element spectra in the 350 and 450 µm windows for a point source. When completed, ZEUS will have a pixel TES sensed bolometer array, delivering an instantaneous 64 element (6.4% bandwidth spectrum) at 12 spatial positions on the sky. ZEUS can be used on most large aperture submillimeter telescopes including the JCMT, CSO, SMT, and APEX. We obtained our first light on the CSO in early April Our primary science goals are to: (1) trace starformation in the early Universe by observing redshifted far-ir fine structure lines from distant (z 0.7 to 6) galaxies, (2) measure the redshifts of optically obscurred submillimeter galaxies by detecting their bright 158 µm [CII] line emission, and (3) study star formation in starbursts and ULIG galaxes by observing their [CI] and mid-j CO rotational line emission. 1. Introduction The redshift (z) and Early Universe Specrometer, ZEUS was designed to study the starformation history of the Universe from early times until the present epoch using far-ir and submillimeter spectral lines as probes. In particular, ZEUS will observe the ground state fine structure lines of C 0, C +, O 0, and the mid-j rotational lines of CO. These lines cool cloud interiors permitting clouds to collapse and form stars, but are often strongest from the photodissociated surfaces of molecular clouds that are exposed to the far-uv (6 to 13.6 ev) radiation of nearby early type stars. The gas in these photodissociation regions, (PDRs) is primarily heated by the photo-ejection of energetic electrons from grains, and cooled by these same spectral lines (cf. Tielens and Hollenbach, 1985). PDRs are an important component of the ISM in galaxies, amounting to about 10% of the ISM in normal galaxies, and as much as half of the ISM in starburst nuclei and low metalicity dwarfs (cf. Stacey et al. 1985, 1991; Wright et al. 1991; Lord et al. 1996; Smith & Madden 1997). 1

2 2 Stacey 2. Spectral Lines Observable with ZEUS 2.1. Mid-J CO and [CI] The mid-j CO rotational lines probe the warm dense gas associated with molecular shocks or PDRs. Since much ( 30%) of the molecular ISM in starburst nuclei is warm and dense (cf. Harris et al. 1991) it is important to study this gas to understand the interplay between the nascent molecular ISM and the newly formed stars. For some starburst nuclei, supernovae blasts, or molecular outflows might compress the ISM, so that the starburst is self sustaining, while for others, these processes plus UV and cosmic ray heating may energize or disrupt the ISM making the starburst self limiting. For example, we mapped the starburst nucleus of NGC 253 in the CO(7 6) line using SPIFI on the JCMT (Bradford et al. 2003). From the molecular gas excitation we deduced that the entire molecular ISM was quite warm and dense, so that further starformation was inhibited there. The mid-j lines are important as the lower J (J 4) lines have much smaller excitation requirements, so that their intensities are much less sensitive to the physical conditions of the gas. The [CI] lines are important coolants of the neutral ISM. Observations in the lower J (609 µm line) showed that C 0 is well mixed in molecular cloud interiors, and the line is a good molecular cloud mass tracer (Gerin & Phillips 1999). Both the 609 µm and the upper J (370 µm) lines are detectable with ZEUS. Since the two lines are easily thermalized, and usually optically thin, the line ratio yields T gas. The combined [CI] cooling is similar to the total CO line cooling for most galaxies including our own (Gerin and Phillips). Most ( 85%) of the C 0 cooling is from the 370 µm line, so that this single line can be as much as 40% of the molecular gas cooling! Studies of the [CI] lines are therefore critical to understand the structure and energy balance of molecular clouds. Figure 1. (left) The CO(7 6)/[CI] 370 µm line intensity ratio as a function of gas density for various G (right)the [CII] to far-ir luminosity ratio, R, as a function of gas density for various G (from Kaufmann et al. 1999). It is particularly valuable to observe both the mid-j CO (pressure sensitive) and the [CI] (temperature sensitive) lines. The [CI] 370 µm line iss just 1000 km

3 ZEUS 3 s 1 away from the CO(7 6) line - easily contained within a ZEUS spectrum, resulting in excellent relative calibration and spatial registration. The line ratio is very density sensitive (Figure 1, left), as illustrated by our mapping of the pair in the Antennae interacting galaxy pair (Nikola et al. 2006). Over a 3 kpc footprint covering the system, the two lines are equally bright, but in the inner 700 pc of the interaction zone, the CO(7 6) line becomes three times as bright as the [CI] line. The strong CI(7 6) line signals the highly excited molecular ISM as a result of the starburst there [CII] The far-ir fine structure lines from abundant species are the primary coolants for much of the ISM, and sensitive extinction free probes of the gas physical conditions and ambient radiation fields. The brightest of these is the [CII] line at 158 µm, which is the dominant coolant for PDRs, atomic clouds, and an important coolant for diffuse ionized and neutral media. The [CII] emission from most galaxies is dominated by the PDR component, for which the [CII]/far-IR continuum luminosity ratio, R, is strongly inversely proportional to the strength of the ambient far-uv radiation field, parameterized in units of the Habing field by G (Figure 1, right). R maximizes at about 1% for G = 10, n 10 3 cm 3. The inverse relationship holds since the efficiency of photoelectric heating is reduced due to build up of grain charge at high G, and due to increased cooling in the [OI] 63 µm line. Measuring R determines G, and comparing G to the observed far-ir continuum intensity, yields the source beam filling factor, hence the physical size of the emitting regions (Wolfire, Tielens, & Hollenbach 1990; Stacey et al. 1991). For distant galaxies we therefore determine both the strength of the starburst radiation fields, G, and its spatial extent by observing the [CII] line and far-ir continuum. If other PDR lines are detectable (e.g. [OI], [CI], or mid-j), then we further constrain PDR parameters including cloud density, temperature, column density, and clumping factors. 3. Science Objectives 3.1. ULIG and Starburst Galaxies The IRAS mission discovered the ULIRG galaxies, galaxies with luminosities in excess L, most of which emerges in the far-ir. ISO/SWS and Spitzer studies suggest that 3 / 4 of these galaxies are powered by starbursts, with the remainder powered by buried AGN (Genzel et al. 1998, Armus et al. 2004). Surprisingly, Luhman et al (1998, 2003) find that 12 of the 15 ULIRGs studied with ISO/LWS have weak [CII] with R 2 to 6 times smaller than the value for nearby starforming galaxies (R 0.1% to 1%, Stacey et al. 1991). It could be that the far-ir emission arises from non-pdr sources, either from dusty HII regions, or from dust heated by an AGN, which emits little [CII] due to the hardness of the radiation field. However, a more natural explanation, that is consistent with the ISO/SWS and Spitzer studies, is that ULIRGs starbursts are very young and localized (large G), so that the [CII] emission is weak (Figure 1). Strong UV fields suppress [CII], but not the mid-j CO line emission. For example for the Orion interface region G 10 5, and R 0.03%, but the CO(7 6)/far-IR

4 4 Stacey Figure 2. Starformation rate per unit comoving volume as a function of 1+z (Blain et al.1998). Superposed are the redshift intervals probed by ZEUS in the [CII] line luminosity ratio is the same in Orion (Stacey et al. 1993) as it is in the starburst nucleus of (the strong [CII] emitter) NCG 253. Strong mid-j CO line emission is expected from ULIRGs that are powered by starbursts even if their [CII] line emission is weak. To test these ideas, we plan to use ZEUS to observe nearby ULIRG galaxies in their mid-j CO and [CI] line emission. Since distant [CII] emitters are very IR luminous, it is critical to understand the mechanism behind the variable [CII] in the ULIRGs, to validate the conclusions drawn from observing [CII] at high z Distant Galaxies The COBE satellite detected a cosmic far-ir background with integrated energy greater than that of the cosmic optical background (Fixen et al. 1998), implying that most of the optical/uv energy from AGN and stars in the Universe is reprocessed by dust. The SCUBA surveys resolved this background (cf. Smail et al. 1997), revealing a population of distant (z 2 to 3), extremely luminous systems commonly referred to as submillimeter galaxies (SMGs). SMGs account for much of the submillimeter background, and may be the starforming progenitors of modern day giant elliptical galaxies. Submillimeter continuum surveys provide the means of discovery, but spectroscopy is required to investigate the physical conditions of the gas and the nature of the ambient UV radiation fields. On large aperture submm telescopes ZEUS can detect several of the brightest far-ir fine structure lines (especially [CII]) from SMGs as they fall into the submm telluric windows. ZEUS can survey [CII] lines from distant galaxies the redshift range between 1 and 5 (Table 1). This interval is of particular interest, as at these times the greatest change in starformation per unit co-moving volume occurred (Figure 2). The [CII] to far-ir luminosity ratio, R constrains the strength of the ambient interstellar radiation fields, G, hence the concentration of the starbursts.

5 ZEUS 5 Very high G ( 10 4, for R 10 4 ) indicates a very compact source with UV fields similar to those found in Orion (0.4 pc from an O6 star). Extended starbursts, such as in M82 will have G 10 3 (R 10 3 ), while normal starforming galaxies like the Milky Way will have G 1 to 100 (R 10 3 to 10 2 ). The very intense and hard radiation fields associated with AGN will result in very small R. By comparing G with the far-ir continuum (e.g. from IRAS, SCUBA, SHARC-2, or Spitzer) we derive the source beam filling factor hence its physical size. In a colliding galaxy induced starburst paradigm, G gets smaller with the age of the galactic merger, or of the starburst itself. In the early merger stage, the starburst will be very intense and localized resulting in very high G. As the merger progresses, other regions are stimulated to form stars either by collision dynamics or gas compression by supernovae in the original burst, so that as the burst progresses, it will tend to lower the overall G, increasing R. In many sources, we expect other redshifted fine-structure lines will be observable. The [OI] 63 µm line is often nearly as bright as [CII], and is a signpost of warm, dense PDRs associated with an intense starburst. The [NII] lines both count the numbers of Lyman continuum photons, and yield the fraction of the observed [CII] that arises from the ionized medium. The [OIII] lines can be as bright as [CII]. Their presence signals a young starburst headed by early type ( O6) stars. The [OIII] line flux, together with the free-free continuum can be used to constrain the hardness of the UV radiation fields ZEUS as a Redshift Machine It is important to obtain redshifts for the SMG population to constrain starformation as a function of redshift. SMGs are optically very faint, so it is challenging to obtain optical redshifts. However, by clever use of the radio-infrared correlation, Chapman et al have been successful by using VLA positions for follow-up optical spectroscopy using the Keck telescope. Still, only about half of the known SMGs have redshifts obtained in this manner. ZEUS, however provides an alternative means by which redshifts can be obtained using the

6 6 Stacey several bright far-ir fine structure lines as they are redshifted into the telluric submm windows. ZEUS is a particularly interesting redshift tool for objects whose redshifts lie in the optical desert between z 1.2 to 1.8. The best far-ir line is the 158 µm [CII] line. It is bright enough that one can argue that since most SMGs are in the redshift range from 1 to 3, any line detected from an SMG in the 350, 450, 610, and 900 µm windows is likely the [CII] line. Redshifts can be verified, with a gain in the physical understanding by detecting other bright far-ir lines ([OI], [NII], [OIII]). Observations with [CII] are arguably better for redshift determination than those using CO lines. For starforming galaxies, the brightest CO lines are 200 times fainter than [CII]. However, the photon background is much lower in the millimeter bands (where the CO lines will be observed) than in the submm bands (where the [CII] line will be observed), resulting in 10 to 40 times better sensitivity in the mm bands. Therefore, given the same telescope, and that both receivers are background limited the [CII] line is still 5 to 20 times easier to detect than the brightest CO lines. Off course, the CO lines have a distinct advantage over [CII] in that with a sufficiently broad band receiver, two CO lines are observed simultaneously, and the spacing between the lines provides a unique determination of the source redshift as is done with the Z-Spec spectrometer in this volume (Glenn et al.). Naturally, the most exciting physics is achieved by obtaining both a set of CO lines and the [CII] line from these galaxies. 4. ZEUS Design We have detailed our scientific goals above. Below we justify the choice to construct a direct detection echelle grating spectrometer with a large format bolometer array to pursue these goals. We follow with a detailed discussion of the ZEUS design, and finish with its current status and sensitivity Design Considerations Our scientific goals require detection of very weak and broad spectral lines from distant galaxies. To date, most spectroscopy in the submm bands has been done using heterodyne (coherent) receivers. These receivers will remain the instruments of choice at the highest resolving powers. However, it can be shown that if both systems have the same throughput-quantum efficiency product, a direct detection system is more sensitive in principle than a heterodyne system as long as the direct detection system is background limited (Harris, 1990). This is due to the quantum noise associated with the phase sensitive coherent detection process. Modern silicon bolometers are sensitive enough to achieve background limited performance at resolving powers in excess of 10 4 in the submm bands. In addition, it is straightforward to achieve large spectral bandwidths with a direct detection spectrometer. ZEUS currently has an instantaneous bandwidth of 23 GHz at 860 GHz, in its final incarnation this will improve to 46 GHz a bandwidth that is challenging for coherent work. Therefore, it is clearly desirable to build a direct detection spectrometer such as ZEUS. Distant galaxies are essentially point sources, so that a spectrally multiplexing spectrometer (e.g. grating spectrometer) is desired over a spatial multiplexers such as a Fabry Perot (e.g. SPIFI) or Fourier transform spectrometer that

7 ZEUS 7 Figure 3. ZEUS spectral orders (2 through 5) superposed on Mauna Kea windows on an excellent night. must scan spectrally. Since we desire only a moderate resolving power ( 1000), which is easily attainable with a modest sized ( 30 cm) grating, the grating spectrometer is the instrument of choice. Grating Design We wish to cover the submm spectral windows with high efficiency in order to observe redshifted [CII] at 1 < z < 5. We choose to use an R2 echelle (blazed at ) grating blazed at 359 µm in 5 th order (groove spacing 992 µm) so that it efficently covers the 350, 450, 610, and 900 µm windows in the 5 th, 4 th, 3 rd, and 2 nd orders respectively (Figure 3). ZEUS is designed for a filter wheel that holds up to 3 bandpass filters (λ/ λ 10, Cardiff University) centered on the appropriate wavelengths so that we can switch to three different telluric windows while cold, and on the telescope. With a 64 spectral element array (see below) ZEUS covers roughly a 5% spectral bandwidth at each grating setting. Each telluric window is fully covered with 2 to 3 such settings, achieved by tipping grating over its 57 to 73 range. The grating is used in Littrow mode and is 35 cm in length so as to ensure all the light is captured as it is tipped over its entire range of motion. The resolving power is 1200 at 370 µm. Optics Figure 4 shows the primary optical and cryogenic features within ZEUS. ZEUS accepts an f/12 beam that is imaged just inside of its polyethylene dewar entrance window. From there the beam passes through a scatter filter on the 78 K shield, and through a quartz filter on the outer 4 K shield. The beam then passes through a long wavelength (λ > 180 µm) filter (LP1) on the inner 4 K shield. M1 sends the beam to the collimating mirror M2 then on to flat mirror M3. A second long pass filter, LP2 is located at the pupil and Lyot stop between M3 and M4. M4 reimages at f/2.9 onto the entrance slit to the echelle section. The beam is them recollimated at 10 cm by an off-axis paraboloid mirror, M5, which illuminates the echelle. In near Littrow mode, the beam then returns to M5, and is sent by flat mirror M6 through the bandpass filter of choice on

8 8 Stacey Figure 4. ZEUS optical path the wheel, and is finally imaged onto the array. The total transmission of the optical system including the various filters and mirrors and the grating efficiency is 27% at 350 µm. The offsets of the entrance and exit beams with respect to M5 open up space for the entrance slit and detector, and cancels coma in the echelle section. The Zeemax ray-trace program shows beam sizes < 0.3fλ (rms) at 350 µm even at the corners of a pixel array. Bolometers In its final configuration, ZEUS will contain up to a transition edge sensed (TES) bolometer array, yielding a 64 element spectrum for 12 spatial positions on the sky. The pixels are 1 mm square, with near unit filling factor. The final f/# delivers a plate scale fλ at 350 µm, or 6.8 per pixel on the 10.4 m CSO, so that the footprint will be on the sky. The instantaneous bandwidth will be 5.35%, or 46 GHz at 350 µm. The slit size is presently fixed at 1.2 mm, appropriate for work at 350 and 450 µm, but in the future will be adjustable to match the diffraction limit at 610 and 900 µm. At present, we are using a 1 32 pixel thermister sensed array from GSFC. The array has an electrical NEP W/Hz 1/2 at 250 mk, so that it promises performance within a factor of two of optimum most most ZEUS wavelengths (see Table 2 below). We are currently testing our first 1 32 pixel TES array, and plan to install it in ZEUS this fall. The TES arrays are more sensitive at a given bath temperature than the thermister sensed arrays, and should deliver background limited performance in all of our bands (c.f. Benford et al. 2004). Furthermore, the TES sensors are naturally integrated into SQUID readouts so that the large format (12 64 pixel) arrays we desire for ZEUS become possible.

9 ZEUS Sensitivity Table 2 lists the expected sensitivity for ZEUS on the CSO as a function of wavelength. The noise equivalent flux (NEF) accounts for all loses including chopping (factor of 2), telescope efficiency, η tel, sky transmission η sky, and point source coupling to the detectors, η point. We have assumed a very good submillimeter night for these calculations (zenith H 2 O = 0.7 mm). For the 12 m APEX telescope on the Chajnantor site, in the shorter submillimeter bands, the expected sensitivity is about a factor of 2 better due to the larger size of the dish, and the better transmission at the Atacama site (zenith H 2 O = 0.5 mm). It is instructive to compare the sensitivity of ZEUS to heterodyne receivers. A simple method is to compare the noise temperatures measured at the front end of the receiver. The noise equivalent flux at the front end (not including the losses of the telescope and sky, but including chopping losses) is related to the noise temperature by: NEF=2k T νω/λ 2, where Ω is the beam solid angle, and ν is the frequency bandwidth. The noise temperature, in turn, is related to the single side band receiver temperature, T rec (SSB) by: T = 2κ(T rec + T bac) /( νt) 1/2, where T bac is the background temperature, κ is the backend degradation factor (= 1.15 for a 2 bit correlator), and chopping losses introduce the factor of two. Solving for the receiver temperature: T rec = NEF λ 2 (t/ ν) 1/2 /(4kκΩ) - T bac. The background temperature is a function of wavelength and observing conditions, but for very good (0.7 mm H 2 O) conditions our calculated NEF corresponds to a single side band receiver temperature of 30 to 50 K in the 350 and 450 µm windows. We have measured an noise equivalent temperature of K and K at 370 and 444 µm respectively in front of the coupling optics on the CSO. The noise figures will be somewhat (10%) better at the front end of the receiver. These values correspond to single side band receiver temperatures of 240 and

10 10 Stacey 160 K at 370 and 444 µm respectively. The measured ZEUS values compare favorably with the best reported double side band receiver temperatures 205 K, and 150 K at 370 and 444 µm respectively (cf. Kooi et al. 2000). This comparison strictly holds only for sources that are well matched to the resolving power of ZEUS, i.e. most galaxies Current Status In very poor weather, we had our first light on CSO in early April At the telescope, ZEUS delivered sensitivity at 370 and 444 µm roughly a factor of 2 worse than the fundamental limits in Table 2. For this run, we chose to split this array into two bands by placing the 350 and 450 µm bandpass filters in the focal plan directly in front of the detector. Although this sacrifices bandwidth, it enables simultaneous detection of the 12 CO(6 5) (434 µm) and 13 CO(8 7) (340 µm) lines. We have our second run on CSO with our thermister sensed array in late May 2006, and plan to have our TES sensed array, which should bring us closer to the fundamental limits installed for a winter 2006 run on CSO. Acknowledgments. This work was supported by NSF grants AST , and AST , and NASA grants NGT and NNG05GK70H. References Armus, L., et al ApJS 154, 178 Benford, D.J. et al. 2004, SPIE Blain, A.W., Smail, I., Ivison, R.J., & Kneib, J.P MNRAS 302, 632 Bradford, C.M., Nikola, T., Stacey, G.J., Bolatto, A.D., Jackson, J.M., Savage, M.L., Davidson, J.A., & Higdon, S.J ApJ 586, 89 Chapman, S.C., Blain, A.W., Ivison, R.J., &Smail, I.R Nature 422, 695 Fixen, D. L., Bennett, C.L., & Mather, J.C. 1999, ApJ, 526, 207 Genzel, et al. 1998, ApJ 498, 579 Gerin, M., & Phillips, T.G. 1999, ApJ, 509, L17 Harris, A.I. 1990, ESA SP-314, 165 Harris, A.I., Hills, R. Stutki, J., Graf, U., Russell, A., & Genzel, R ApJ, 382, L75 Kaufman, M.J., Wolfire, M.G., Hollenbach, D.J., & Luhman, M.L., 1999, ApJ, 527, 795 Kooi, J.W. et al. 2000, Int. J. Inf. Millim. Waves 21, 1357 Lord, S.D., Hollenbach, D.J., Haas, M.R., Rubin, R.H., Colgan, S.W.J., & Erickson, E.F. 1996, ApJ 465, 703 Luhman, M.L, Satyapal, S., Fischer, J., Wolfire, M.G., Cox, P., Lord, S.D., Smith, H.A., Stacey, G.J., & Unger, S. J. 1998, Ap.J. 504, L11 Luhman, M.L, Satyapal, S., Fischer, J., Wolfire, Sturm, E., Dudley, C.C., Lutz, D., & Genzel, R. 2003, Ap.J. 594, 758 Nikola, T., Isaak, K., Bradford, C.M., Bolatto, A., & Stacey, G.J in prep. Smail, I., Ivison, R.J., & Blain, A.W ApJ 490, L5 Smith, Beverly J., Madden, Suzanne C. 1997, AJ, 114, 138 Stacey, G.J., Viscuso, P.J., Fuller, C.E., & Kurtz, N.T ApJ, 289, 803 Stacey, G.J., Geis, N. Genzel, R., Lugten, J., Poglitsch, A. Sternberg, A., & Townes, C.H. 1991, ApJ, 373, 423. Stacey, G.J., Jaffe, D.T., Geis, N. Genzel, R., Harris, A.I., Poglitsch, A., Stutzki, J., & Townes, C.H. 1993, ApJ, 404, 219 Tielens, A.G.G.M., & Hollenbach, D. 1985, ApJ, 291, 722. Wolfire, M.G, Tielens, A.G.G.M., & Hollenbach, D.J. 1991, ApJ 383, 205 Wright, N. et al. 1991, ApJ, 381, 200