We have conducted a sensitive search for the redshifted HCN(1−0) line (v_(rest) = 88.6304 GHz) towards three luminous submillimeter galaxies at z ∼ 2.5 − 2.8 using the K-band receiver (17.6-27.1GHz) on the 110m Robert C. Byrd Telescope (also known as the Green Bank Telescope). Aided by gravitational lensing, we place strong upper limits on the intrinsic line luminosity of all three sources (L′_(HCN(1−0)) ≤ 2×10^(10) K kms^(−1) pc^2). We conclude that all four submillimeter galaxies that have been observed in HCN to date are consistent with the tight relation between FIR and HCN luminosity observed in a variety of local star-forming systems, ranging from Galactic dense cores to (ultra)luminous infrared galaxies at z ≤ 0.1

Greve, T. R.

Caltech Authors - Main

From Z-Machines to ALMA: (Sub)millimeter Spectroscopy of Galaxies
ASP Conference Series, Vol. 375, 2007
A. J. Baker, J. Glenn, A. I. Harris, J. G. Mangum and M. S. Yun, eds.
Dense Gas at High Redshifts: A Search with the 110m
Robert C. Byrd Telescope
T. R. Greve
California Institute of Technology, 1200 E. California Blvd., Pasadena,
CA 91125, USA
Abstract. We have conducted a sensitive search for the redshifted HCN(1−0)
line (νrest = 88.6304GHz) towards three luminous submillimeter galaxies at
z ∼ 2.5 − 2.8 using the K-band receiver (17.6–27.1GHz) on the 110m Robert
C. Byrd Telescope (also known as the Green Bank Telescope). Aided by gravi-
tational lensing, we place strong upper limits on the intrinsic line luminosity of
all three sources (L′HCN(1−0) <∼ 2×1010 Kkms−1 pc2). We conclude that all four
submillimeter galaxies that have been observed in HCN to date are consistent
with the tight relation between FIR and HCN luminosity observed in a variety of
local star-forming systems, ranging from Galactic dense cores to (ultra)luminous
infrared galaxies at z <∼ 0.1.
1. Introduction
The search for metal-enriched molecular gas (H2) at high redshifts (z >∼ 1) has so
far largely been pursued using the rotational lines of CO. The reasons for this are
primarily (1) the much larger abundance of CO compared to other molecular or
atomic species (except of course H I and H2), (2) the low excitation requirements
of low-J CO rotational transitions (n(H2) ∼ 102−3 cm−3 and Tkin ∼ 5 − 15K),
and (3) the observability of a number of CO lines given the constraints imposed
by the atmosphere and current instrumentation. All of this makes CO ideally
suited to probe the bulk of the diffuse molecular gas, and as such CO is an
excellent tracer of the “fuel” potentially available for star formation and black
hole accretion.
It is well known from our own Galaxy that the nurseries of star formation
reside in the dense (n(H2) >∼ 10
4 cm−3) cores of molecular clouds– regions that
are best traced by molecular species such as HCN and CS with high critical
densities. Studies of star-forming systems ranging from Galactic dense cores
(LIR = 10
4.5−6 L⊙) to (ultra)luminous infrared galaxies (ULIRGs/LIRGs: LIR ≥
1011−12 L⊙) have shown that the IR-to-HCN(1–0) luminosity ratio is remarkably
constant over 7–8 orders of magnitude (Gao & Solomon 2004a,b; Wu et al. 2005).
This has been interpreted as showing that the true star formation efficiency
(SFE)– i.e., the star formation rate (∝ LIR) per unit dense gas (∝ L′HCN)– is
the same in these widely different systems. Extending this relation to higher
IR luminosities (LIR ∼ 1013 L⊙) as well as higher redshifts (z >∼ 1) could imply
that the fundamental mechanisms governing star formation are the same in all
environments across cosmic time (cf. Gracia´-Carpio et al. 2006).
Throughout this text we adopt a flat cosmology, with Ωm = 0.27, ΩΛ = 0.73,
and H0 = 71 km s
−1Mpc−1 (Spergel et al. 2003).
229
230 Greve
2. Observations and Data Reduction
Three submillimeter-selected galaxies (SMGs) were targeted for HCN(1−0) ob-
servations with the 110m Robert C. Byrd Green Bank Telescope (GBT) in West
Virginia1: SMMJ02399−0136 (z = 2.808), SMMJ16359+6612 (z = 2.517), and
SMMJ04431+0210 (z = 2.509)2. The three sources are amongst the SMGs best
suited for HCN detection since they (a) are very bright submillimeter sources
(Smail, Ivison, & Blain 1997; Kneib et al. 2004), and (b) have strong CO de-
tections (Frayer et al. 1998; Genzel et al. 2003; Sheth et al. 2004; Kneib et al.
2005).
The observations were carried out in the period between March 21st and
April 5th, 2005 using the GBT’s K-band receiver, which covers the redshifted
HCN(1–0) lines from the three SMGs. We adopted the dual-beam observing
mode in which the telescope nods between integrations, thus ensuring that two
of the K-band receiver’s four feeds are alternating between being ON and OFF
source. The GBT Spectrometer in its wide-bandwidth, low-resolution submode
was used as backend, giving 200MHz of total instantaneous bandwidth divided
into 8192 channels each of width 24.41 kHz. This corresponded to a velocity
coverage of ∼ 9800 km s−1 and a resolution of ∼ 1.2 km s−1 per channel.
The data were reduced in two different ways. The first method used the
standard GETNOD routine in GBTIDL, which takes the OFF signal from one
scan and subtracts it from the ON signal of the neighboring scan, and vice
versa. In a second reduction scheme (devised by us), a template baseline for a
single scan was derived from the best-fit linear combination of its neighboring
OFF scans, and then subtracted (see Hainline et al. 2006 and Greve et al. 2006
for details). The difference spectra produced in these two different ways were
then averaged separately using their system temperatures as weights.
3. Result
Comparing the final HCN(1–0) spectra of J02399, J16359, and J04431 reduced
in the two ways described above (see left panel of Fig. 1), it is clear that the
former reduction method results in lower channel-to-channel noise in the spec-
tra. Although we fail to detect any significant line emission above the noise, the
sensitivity of the observations allow us to put tight upper limits on the HCN(1–
0) line emission of SHCN(1−0) ≤ 0.08, ≤ 0.13 and ≤ 0.20 Jy km s−1 for J02399,
J16359, and J04431, respectively. These translate into apparent line luminosities
of L′HCN(1−0) ≤ 5.0× 1010, ≤ 6.6× 1010, and ≤ 9.9× 1010Kkms−1 pc2, respec-
tively. Correcting for gravitational lensing, we find intrinsic line luminosities of
L′HCN(1−0) ≤ 2.0× 1010, ≤ 0.3× 1010, and ≤ 2.3× 1010Kkms−1 pc2.
It is seen from Figure 1 (right panel), where we have plotted LFIR versus
L′HCN(1−0) for our three SMGs along with all other high-z observations of HCN,
that the upper limits on the four SMGs observed to date all lie to the right
1The GBT is operated by the National Radio Astronomy Observatory under cooperative agree-
ment with the NSF.
2Hereafter we will refer to these three sources as J02399, J16359, and J04431, respectively.
Dense Gas at High Redshifts 231
Table 1. Observations of HCN at high redshift to date. The three sources
studied here are at the top of the table.
HCN L′
HCN
Source Type z line (109 Kkms−1 pc−1) µ Reference
SMMJ02399−0136 SMG 2.808 1− 0 ≤ 20 2.5 this work
SMMJ16359+6612 SMG 2.517 1− 0 ≤ 3 22 this work
SMMJ04431+0210 SMG 2.509 1− 0 ≤ 2.3 4.4 this work
SMMJ14011+0252 SMG 2.565 1− 0 ≤ 1.6 5 Carilli et al. (2005)
SDSS J1148+5251 QSO 6.419 1− 0 ≤ 10 1 Carilli et al. (2005)
BR1202−0725 QSO 4.693 1− 0 ≤ 45 1 Isaak et al. (2004)
APM08279+5255 QSO 3.911 5− 4 1.4± 0.2 1 Wagg et al. (2005)
MG0751+2716 QSO 3.200 1− 0 ≤ 1.0 17 Carilli et al. (2005)
J1409+5628 QSO 2.583 1− 0 6.7± 2.2 1 Carilli et al. (2005)
Cloverleaf QSO 2.558 1− 0 3.2± 0.5 11 Solomon et al. (2003)
. . . . . . 4− 3 7.8± 1.5 . . . Barvainis et al. (1997)
IRASF10214+4724 QSO 2.286 1− 0 1.3± 0.3 13 Vanden Bout et al. (2004)
of the solid line, thus consistent with the extrapolation of the local IR-HCN
correlation to higher luminosities. In contrast, the other high-z objects, which
are all QSOs, lie to the left of the solid line. Clearly, this is not statistically
significant. However, it is at least consistent with the expected scenario in which
the FIR luminosity of SMGs is predominantly powered by a starburst (Alexander
et al. 2003, 2005), while in QSOs it is dominated by an accreting supermassive
black hole. However, the data are also consistent with a steepening of the
correlation at higher luminosities, meaning that in both QSOs and SMGs the
contribution from active galactic nuclei (AGN) is higher than in local (U)LIRGs–
something that may be supported by the tentative increase in AGN dominance
with IR luminosity in local ULIRGs (Veilleux, Kim, & Sanders 1999; Tran et al.
2001).
4. Summary
In summary, we conclude that based on the four observations of SMGs to date–
of which three were made as part of this study– there is no evidence to suggest
that SMGs are not consistent with the local FIR/HCN relation. Although the
capability of the GBT to detect HCN in extremely luminous high-z QSOs has
been demonstrated (Vanden Bout, Solomon, & Maddalena 2004), it seems ap-
parent that more sensitive observations from future facilities such as the EVLA,
say, are needed to detect HCN firmly in a large enough sample of SMGs to
reliably address issues such as the IR-HCN relation at high redshifts.
Acknowledgments. I’m grateful to NRAO for financial and scientific sup-
port for this project. I’m also indebted to my trusted collaborators: Laura
Hainline, Andrew Blain, Rob Ivison, Padelis Papadopoulos, and Ian Smail.
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Karen O’Neil


Dense Gas at High Redshifts: A Search with the 110m Robert C. Byrd Telescope

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Dense Gas at High Redshifts: A Search with the 110m Robert C. Byrd Telescope

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