Origins of Scatter in the Relationship Between HCN 1-0 and Dense Gas Mass in the Galactic Center

3.3 MB·

by  E. A. C. Mills

Draft version January 19, 2017
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
ORIGINS OF SCATTER IN THE RELATIONSHIP BETWEEN HCN 1-0 AND DENSE GAS MASS IN THE
GALACTIC CENTER
Elisabeth A.C. Mills
SanJoseStateUniversity,1WashingtonSquare,SanJose,CA95192,USA
Cara Battersby
Harvard-SmithsonianCenterforAstrophysics,60GardenSt.,Cambridge,MA02138,USA
Draft version January 19, 2017
7
1 ABSTRACT
0 WeinvestigatethecorrelationofHCN1-0withgasmassinthecentral300pcoftheGalaxy. Wefind
2 thatonthe 10pcsizescaleofindividualcloudcores,HCN1-0iswellcorrelatedwithdensegasmass
n when plotte∼d as a log-log relationship. There is 0.75 dex of scatter in this relationship from clouds
a like Sgr B2, which has an integrated HCN 1-0 in∼tensity of a cloud less than half its mass, and others
J thathaveHCN1-0enhancedbyafactorof2-3relativetocloudsofcomparablemass. Weidentifythe
7 two primary sources of scatter to be self-absorption and variations in HCN abundance. We also find
1 that the extended HCN 1-0 emission is more intense per unit mass than in individual cloud cores. In
fact the majority (80%) of HCN 1-0 emission comes from extended gas with column densities below

] 7 1022 cm−2, accounting for 68% of the total mass. We find variations in the brightness of HCN 1-0
A w×ould only yield a 10% error in the dense gas mass inferred from this line in the Galactic center.
G However, the observ∼ed order of magnitude HCN abundance variations, and the systematic nature of
thesevariations, warnofpotentialbiases intheuseof HCNasdensegas masstracerin moreextreme
.
h environments such as AGN and shock-dominated regions. We also investigate other 3 mm tracers,
p finding that HNCO is better correlated with mass than HCN, and might be a better tracer of cloud
- mass in this environment.
o
r
t
s 1. INTRODUCTION luminous galaxies, CO 1-0 is systematically weak com-
a
pared to the far-infrared luminosity, and the Schmidt-
[ While the history of the cosmic star formation rate
is well known, peaking around a redshift of 1.9 (Madau Kennicutt relationship is not linear (Gao & Solomon
1 et al. 1998; Madau & Dickinson 2014), the physics that 2004a). Incontrast,HCN1-0retainsalinearrelationship
v withfar-infraredluminosityover7-8ordersofmagnitude
drives the variation of the star formation rate with red-
2 ininfraredluminosity(TheGao-SolomonrelationGao&
shift, including its steep decline since a redshift of 2
2 Solomon2004b;Wuetal.2005). Thedifferenceinbehav-
(Behroozietal.2013;Mosteretal.2013),arestilluncer-
8 ior between HCN 1-0 and CO 1-0 in these systems can
tain. In particular, there has been a lack of direct mea-
4 be explained if the HCN:CO ratio represents the frac-
surements of molecular gas mass and density over the
0 tion of dense (n (cid:38) 104 cm−3) gas, which is suggested to
sametimescalestoconnectthestarformationproperties
1. with the conditions in the gas reservoir. This lack of bemorefundamentallyrelevantforstarformation(Lada
0 data is largely due to the difficulties of observing molec- et al. 2012).
7 ular gas, especially at large distances, which has previ- In recent years the universal applicability of the Gao-
1 Solomon relation to all extragalactic environments has
ously limited observations to bright tracers of molecu-
: been called into question. HCN 1-0 is observed to be
v lar gas (e.g., single lines of CO or HCN) that serve as
anomalouslybrightinactivegalacticnuclei(AGN),com-
i proxiesfor(dense)gasmass. Thebulkofthesemeasure-
X pared to CO and HCO+ (Davies et al. 2012; Imanishi
ments havebeen limited to nearby systems (z <0.1; Gao
et al. 2009; Kohno et al. 2003). Graci´a-Carpio et al.
r &Solomon2004a;Graci´a-Carpioetal.2008;Leroyetal.
a (2006) also find that HCN 1-0 may be overluminous
2009; Saintonge et al. 2011; Garc´ıa-Burillo et al. 2012),
compared to HCO+ in ultraluminous infrared galaxies
though recently larger samples have begun to push out
(ULIRGs), and Privon et al. (2015) also find that over-
beyondaredshiftof1(e.g.,Gaoetal.2007;Tacconietal.
luminous HCN 1-0 is not limited to galaxies contain-
2010, 2013; Saintonge et al. 2013; Carilli & Walter 2013;
ing AGN. In addition to these indications that HCN 1-0
Genzel et al. 2015).
is overluminous in some environments, there are obser-
In the local universe, the use of CO as a gas tracer
vations that a single power-law spectrum is insufficient
reveals a fundamental relationship between star forma-
to describe both normal galaxies and ULIRGs. Other
tion (as traced by far-infrared luminosity) and gas sur-
studies have found that the Gao-Solomon relation be-
facedensity(Kennicutt1998),althoughvariationsinthe
comessuperlinear,withULIRGshavingasystematically
abundance and excitation of CO can change the CO
higher(byafactorof2-3)ratiooffar-infraredluminosity
to gas mass conversion factor (Sandstrom et al. 2013;
compared to HCN 1-0, which is interpreted as increased
Mashian et al. 2015). However, in massive, infrared-
starformationefficiencyinthesesystems(Graci´a-Carpio
etal.2008;Garc´ıa-Burilloetal.2012). Gaoetal.(2007)
[email protected]
2
similarlyfoundthattheirsampleofhigh-redshiftgalaxies The first source of data for this analysis is a publicly-
hadsystematicallybrighterL comparedtoHCN1-0. available3mmsurveyoftheGalacticcenterwiththe12
FIR
It is standard to explain the variations in the HCN:IR m Mopra telescope (Jones et al. 2012). These observa-
ratio as differences in the star formation efficiency (or tions covered frequencies from 85.3 to 93.3 GHz, which
gas depletion time) as a function of environment, rather in addition to HCN 1-0 includes observations of C H 1-
2
than a failure of HCN 1-0 to trace the total dense gas 0, c-C H 2 -1 , CH CN 5 -4 , CH CCH 5 -4 ,
3 2 12 01 3 K K 3 K K
mass (e.g., Juneau et al. 2009; Leroy et al. 2015; Usero HC N 10-9, HOCO+ 4 -3 , HNCO 4 -3 , HCO+
3 04 03 04 03
et al. 2015). 1-0, N H+ 1-0, HNC 1-0, HN13C 1-0, H13CO+ 1-0,
2
However,thereareseveralreasonsthatHCN1-0could H13CN 1-0,13CS 2-1,SO2 -1 ,andSiO2-1. Theveloc-
2 1
fail to be a reliable tracer of the total dense gas mass ity resolution of these data is 2 km/s. The typical an-
in all environments. Although the modeling of Gao & gularresolutionofthesedatais∼36(cid:48)(cid:48)which,atanassumed
Solomon(2004b)indicatethatHCN1-0intensityshould distanceof8kpctotheGalacticcenter(Ghezetal.2008;
be proportional to dense gas mass across a wide range Boehle et al. 2016), corresponds to a spatial resolution
of physical conditions, this assumes both a constant of 1.4 pc. The total extent of the mapped region spans
abundance and purely collisional excitation. X-ray and 70 pc in Galactic latitude, and 370 pc in Galactic lon-
photon-dominated regions are suggested to change the gitude. For the purposes of this analysis, we integrate
gaschemistry,increasingtheabundanceofHCNrelative the data cubes for each line over a velocity range [-230
to other species (Lepp & Dalgarno 1996; Meijerink et al. km s−1,+230km s−1] sufficient to include all CMZ gas,
2007; Harada et al. 2013), while a strong 14 µm radia- discarding emission below a conservative 5-sigma cut off
tionfieldcaninduceradiativepumpinginHCN,altering ( 0.34Kkms−1)forthelineswithmoreprominentspu-
its level populations (Ziurys & Turner 1986; Aalto et al. r∼ious features (HCN, HCO+, HNC, H13CN, HNCO, and
2007b; Sakamoto et al. 2010; Mills et al. 2013). Infrared HC N; see discussion in Jones et al.), and a smaller cut
3
pumping has also been suggested to induce weak mas- offof3-sigma(0.2Kkms−1)fortheremaining,primarily
ing in the 1-0 line (Matsushita et al. 2015), analogous
weaker lines. This is sufficient to eliminate the effects of
to stronger masers observed in the envelopes of asymp-
artifactssuchasanomalousbaselineshapesinallbutthe
totic giant branch stars (Olofsson et al. 1993; Izumiura
weakest lines (e.g., C H, c-C H ). Following Jones et al.
2 3 2
et al. 1995). Although the optical thickness of HCN 1-0
(2012),wedonotcorrectforthemainbeamefficiencyof
(or CO 1-0) is suggested not to matter significantly, as
Mopra, which for the extended emission in these maps
extragalactic scaling relationships are often explained as
will be 0.65 (as measured at 86 GHz by Ladd et al.
countingopticallythick(orthin)clouds,whichasanen- 2005) an∼d variable with frequency. Line luminosities in
semble are then proportional to the total mass (e.g. Wu
thispaperarethengiveninintrinsicunitsofuncorrected
et al. 2005), this assumption can break down: for exam-
antennatemperature(T *)ratherthanmainbeamtem-
A
ple, when HCN is subject to self-absorption in very high
perature. TheresultingintegratedintensitymapofHCN
column density environments like those in the nuclei of
1-0 is shown in Figure 1.
ULIRGs (Aalto et al. 2015b).
To understand the relationship between the molecular
gas supply and the peak and decline of the cosmic star 2.2. Dust column density data
formationrate,weneedtobeabletomakeaccuratemea- ThesecondsourceofdataisanH columndensitymap
2
surements of the gas mass out to high redshifts. To un- (Figure1,Battersbyetal. inprep),whichisderivedfrom
derstandthescatterthatlimitstheusedofHCN1-0asa measurements of the submillimeter dust emission made
proxy, it necessary to understand any environmental de- with Herschel Hi-Gal in the Galactic center (Molinari
pendenciesonthefidelityofthistracer. Inthispaperwe et al. 2011). At the wavelengths observed with Herschel,
presentthefirstresolvedcomparisonofHCN1-0withan there is a significant contribution from Galactic cirrus
independenttracerofdensegasmassinanextremeenvi- emission. In Battersby et al. (2011), the background
ronment: theGalacticcenter(thecentral 300pcofthe wasdefined(andlatersubtracted)through16iterations,
MilkyWay),aregionwithphysicalproper∼ties(highden- however, in the current version, the process iterates dy-
sities, temperatures, and turbulence) that are analogous namically until the fluctuations in the final cutoff stabi-
tothoseinhigh-redshiftgalaxies(Kruijssen&Longmore lize, i.e. the standard deviation over the last 3 iterations
2013). We compare the HCN 1-0 intensity in individual is less than 1 MJy/Sr. This variable iteration improves
giantmolecularcloudswiththemoleculargasmassfrom theoverallcontiguityofthemethodoverdifferentregions
Herschel column density maps, and report on variations intheGalaxy. Whilethebackground/foregroundcontri-
in the ratio of these two quantities: the dense gas con- bution is uncertain, this new variable iteration method
version factor. We also compare the behavior of HCN allowsagreatdegreeofflexibilityinbackgroundremoval
1-0 to that of other 3 mm tracers, including H13CN 1-0, based on Galactic environment. We do not expect that
HCO+ 1-0,HNCO4 -3 ,andHC N 10-9. Finally,we alloftheHerschelemissionatthislongituderangeisfrom
04 03 3
discuss the implications of this study for the interpreta- theGalacticCenter, but, inthisregion, thecontribution
tionofpastandfuturestudiesusingHCN1-0asatracer from dense sources is much greater than the contribu-
of dense gas in the extreme environments of luminous tion from Galactic cirrus, so we expect uncertainties in
infrared galaxies and high-redshift systems. the cirrus removal to be minor. Moreover, any system-
aticdifferencesintheGalacticcirrustowardtheGalactic
Center from our model would affect all of our targets, so
the relative column densities would remain unchanged.
2. DATAANDMETHODS
The resolution of the resulting column density map is
2.1. Molecular line data 36”, identical to the resolution of the Mopra data. Note
3
thatdataislackingfromthecentralpixelsofthebright- tiguous Sgr B2 complex, but excluding the two cores
est submillimeter source, Sgr B2, because the emission (Sgr B2 and GCM0.50+0.00) where the column density
in these pixels is saturated. As a result, these pixels are is larger than 1.5 1023 cm−2. This threshold is chosen
masked, and the total mass of Sgr B2 will be underesti- sothatthemasso×ftheSgrB2coreandhaloareroughly
mated. The method assumes a gas to dust ratio of 100 equivalent. Total masses are then calculated from the
and dust with a β of 1.75, as described in more detail in integrated H column density according to the formula:
2
Battersby et al. (2011). The median of the formal errors
(cid:90)
in the column density is 3 1021 cm −2, however, the
M =µ m N dA (1)
error is a strong function of×parameter value, with typi- H2 H2 H H2
cal percentage errors of about 70%. Uncertainties in the
where µ is the mean molecular weight of the gas,
background/foreground determination are not known a H2
which we take to be 2.8, assuming abundances of 71%
priori, however since the background/foreground model
H,27%He,and2%metals(e.g.,Kauffmannetal.2008),
is subtracted from the entire region, any changes would (cid:82)
m is the mass of a Hydrogen atom, and N dA is
likely affect the absolute, not relative column densities. H H2
the column density of H (in cm−2 integrated over the
2
area of the cloud (in cm2). Note that we are assuming
2.3. Analysis Methods
in this calculation that all of the observed clouds are at
As the molecular line and column density maps have the same distance, 8 kpc.
identicalresolution,thefirststepistoidenticallypixelize Second, we analyze the pixel-by-pixel variation of the
the maps. Ratio maps are then constructed, represent- HCN 1-0 intensity (shown in Figure 4) in which this
ingthecorrespondencebetweentheintensityofeachline quantity is plotted against N(H ). To aid in the inter-
2
and the dense gas mass, which is proportional to the H2 pretation of these plots we additionally colorize points if
column density at each position. Only those pixels in all they correspond to one of the thirteen selected clouds.
maps that have an H2 column density above a chosen Note that pixels in the core of Sgr B2 for which the col-
threshold (N(H2) = 7 1022 cm −2) are retained in the umn density value is saturated are not included in this
main analysis; this thr×eshold is selected in order to iso- plot.
latetheemissionfromthemostmassivecloudcores,and
theresultingthresholdedratiomapsareshowninFigure 3. RESULTS
2. Comparing log (HCN 1-0) with log (M ) in Fig-
10 10 H2
We employ two methods for comparing the variations ure 3 we find a largely linear relationship over 2 orders
in the ratio of HCN 1-0 intensity and N(H2): first, we of magnitude in mass. However, we also see a (maxi-
plot the base-10 logarithm of the total HCN 1-0 inten- mal) scatter of 0.75 dex in the HCN 1-0 line intensity
sity and the base-10 logarithm of the total mass of H2 at a given mass. Some notable sources contributing to
in Figure 3 for a sample of 13 giant molecular cloud this scatter include the core of Sgr B2, GCM-0.02-0.07,
cores: GCM1.6-0.03, Sgr D, GCM0.83-0.18 (a ring of and GCM0.11-0.08. The core of Sgr B2 appears under-
gas and dust to the southwest of Sgr B2), Sgr B2 – luminous, having an integrated HCN 1-0 line intensity
both its dense core (R 15 pc) and its ‘halo’ (R 50 equivalent to that of clouds up to five times less mas-
pc), GCM0.50+0.00 (the∼easternmost cloud in the d∼ust sive. It also has five times weaker HCN 1-0 than the
ridge, near Sgr B1), the remaining dust ridge clouds, halooftheSgrB2cloudwhichcontainsaroughlyequiv-
GCM0.25+0.01 (the ‘Brick’), GCM0.11-0.08 (together alent mass of dense gas. Additionally, GCM-0.02-0.07
with two other neighboring clouds to the east of the 50 and GCM0.11-0.08 have integrated HCN 1-0 line inten-
km s−1 cloud), GCM0.07+0.04 (A cloud on the edge of sities 3 times brighter than other clouds of comparable
the Arched filaments), GCM-0.02-0.07 (the 50 km s−1 mass.
cloud), GCM-0.13-0.08 (the 20 km s−1 cloud), and Sgr Examining other 3 mm tracers, we find that HNCO
C. The locations of these clouds are shown in the first 4 -3 appears to have an even tighter linear relation-
04 03
subplot of Figure 2. In choosing to focus this analy- ship than HCN 1-0 (maximal scatter of only 0.5 dex ).
sis on the cores of these clouds above a given column In contrast, HCO+ 1-0 exhibits at most nearly 1 dex
density, the goal is to ensure that HCN 1-0 intensity is of scatter. Interestingly, the 13C isotopologues of HCN
primarily being compared just to the mass of dense gas exhibits much more scatter than the main 12C isotopo-
in the Galactic center. While the column density should logue: we see a maximum of around 1.1 dex of scatter
inprinciplebesensitivetothemassofgasatanydensity, in the line intensity at a given mass (which may be an
thecoresofthesecloudsarebelievedtohaveaverageden- underestimate of the true scatter, as a number of the
sities > 104 cm−3 (Gu¨sten & Henkel 1983; Zylka et al. moremassivecloudsdetectedinH12CNarenotdetected
1992; Serabyn et al. 1992; Longmore et al. 2012, 2013a). inH13CN).WealsonotethatGCM-0.02-0.7isoverlumi-
Note that the line intensity and the column density nousinthemajorityoftracerscomparedtootherclouds
are integrated over a region in the cloud above a given having a similar mass. Some of the largest variation in
column density threshold, so the masses given here are lineintensityatagivencloudmassisseeninHC N and
3
equivalent to the mass of a subset of the cloud core, and CH CN, with both having a maximum of 1.2-1.4 dex of
3
arenotequivalenttothetotalcloudmasses. Thecolumn scatter in these plots.
density thresholds used are 2 1023 cm−2 for the core of In order to better quantify how well a linear fit de-
Sgr B2 and GCM0.50+0.00,×in order to separate them scribes the relationship between log ( integrated inten-
10
from the surrounding cloud ‘halo’, and 7 1022 cm−2 for sity ) of each molecule and log ( mass ), we determine
10
all other clouds. The ‘halo’ of Sgr B2 is×defined by gas the linear correlation coefficient (r) and the two-sided p-
above this column density threshold over the entire con- value for a hypothesis test whose null hypothesis is that
4
the slope is zero. The r- and p-values for each molecule theoriginofthescatterintherelationshipbetweenHCN
are given in Table 1. The strongest linear correlation is 1-0 and gas mass in this environment.
seenforHNCO(r=0.95),withHNC,HCN,HCO+,SiO
and N2H+ being similarly strongly correlated (r 0.82-
0.88). The remaining molecules having a linear c∼orrela- 4.1. Sgr B2: Underluminous HCN 1-0 in the most
massive Galactic center cloud
tion coefficient r > 0.5 also have p > 0.003 and cannot
be distinguished with a 3 sigma certainty from the null Oneofthemostsurprisingresultsofthisanalysisisthe
hypothesis. Of these, H13CN, H13CO+, 13CS, and c- relative weakness of 3 mm line emission from the core of
C H have p-values greater than 0.1 (so the null hypoth- SgrB2,themostmassivemolecularcloudintheGalactic
3 2
esis can only be ruled out with a 1 sigma certainty). For center.
manyoftheselines,thislargescattermayinpartbedue Examining the spectra of all detected transitions to-
tothesmallernumberofcloudsthataredetectedinthese ward the core of Sgr B2 (Figure 6), we note that the
lines. CH CCH, HOCO+, and SO are not detected in a shape of many transitions have a two-peaked structure,
3
sufficientnumberofcloudsforthecorrelationtobeexam- withapronounceddipatthecentralvelocityofthebulk
ined. These results suggest that HNCO 4 -3 (despite of the gas in Sgr B2 ( 65 km s−1, as traditionally de-
not being the fundamental rotational tran04siti0o3n of that terminedfromobserva∼tionsofabsorptiontowardtheHII
molecule) could be a comparable if not better tracer of regions in Sgr B2 using tracers such as NH3, e.g. Wilson
mass than HCN 1-0 in the Galactic center environment. et al. 1982). While there is a second velocity compo-
We also look at the pixel-by-pixel distribution of the nent also observed in absorption toward Sgr B2 at 80
HCN 1-0 intensity versus column density (shown on km s−1(Winnewisser et al. 1979), the observed two-∼peak
a log-log plot in Figure 4) in these Galactic center structure is not consistent with the detection of these
clouds.We find that pixels in individual clouds also ap- two kinematic components. Instead, the symmetry of
peartoshowalinearrelationshipbetweenthelogarithm the two peaks around a central velocity at which more
oftheintensityofHCN1-0andthelogarithmofthecor- optically-thintracers(e.g.,lessabundantandmorecom-
responding molecular gas mass (where the plotted col- plex molecules) have a single peak is a clear signature
umn density for a uniform-sized pixel is proportional to of self absorption. In self absorption, cooler or lower-
this mass). However individual clouds are offset by 0.7- excitation gas is present along the line of sight and at
0.8 dex in their HCN 1-0 intensity. We also see that the the same velocity as warmer or higher-excitation gas.
scatterinHCN1-0intensityforanindividualcloudisnot The foreground column of cooler molecules then absorbs
constant: it varies from 0.1 dex in GCM-0.02-0.07 up theemissionsignaturefromthemoreexcitedbackground
to 0.5 dex in GCM0.25+∼0.01 and higher column-density molecules. As would be expected for self absorption,
regions of GCM1.6-0.03. This indicates that variations the transitions which are most affected are the lowest-J
intheratioofHCN1-0intensitytogasmassarepresent transitions: All species whose 1-0 transition is observed
not just on the scales of entire Galactic center clouds ( (HCN,HCO+,HNC, including the 13C isotopologues of
10 pc) but also on scales down to a few parsecs. ∼ these species, C2H and N2H+) exhibit this dip. The dip
Whilewehavefocusedouranalysisonthecoresofwell- is also seen in the 2-1 transitions of 13CS, SiO, and c-
knowncloudswhichshouldberepresentativeofthedens- C3H2 though not in the 22-11 line of SO. However, it
est gas in the Galactic center, only 32% of the total gas is not present in HOCO+, CH3CN, CH3CCH, HC3N, or
massintheGalacticcenterastracedbythecolumnden- HNCO, all of which are observed in transitions having
sity map lies above the chosen column density threshold Jup 4. Wethusfavorselfabsorptionasthecauseofthe
of N(H ) = 7 1022 cm −2. We therefore also investigate appa≥rently weak emission of HCN 1-0 in Sgr B2. Less
2
the behavior×of the more diffuse or extended component prominent self-absorption is also seen in the spectra of
in HCN 1-0. The ratio of HCN 1-0 intensity to the col- HCN 1-0 toward GCM1.6-0.03, GCM-0.13-0.08, and Sgr
umndensityaboveathresholdof1 1022 cm−2 isshown C,thoughnoneofthesesourcesalsohaveself-absorption
in Figure 5. The ratio varies by mo×re than two orders of in H13CN.
magnitude: it reaches a minimum of 0.5 (the units on Self-absorption in higher-J lines of HCN and HCO+
this ratio are K km s−1 per 1022 cm−∼2) in Sgr B2, and has also been observed in the Galactic center ‘Circum-
maxima of 60 are reached in the gas infrared bubble nuclear Disk’ surrounding the supermassive black hole,
(Rodr´ıguez-∼Fern´andez et al. 2001) and north of Sgr D. however here only the 12C lines are self-absorbed (Mills
Theseextendedregionshaveratiosmorethan3 greater et al. 2013). The observed self absorption of even the
than those found in any of the cloud cores. O×verall, a 13C isotopologues toward the Sgr B2 core then likely re-
HCN 1-0 is slightly brighter in the lower column density quires a higher column of self-absorbing gas toward this
gas: 80% of the HCN 1-0 emission comes from gas with source. Given that the self-absorption happens at the
column density less than 7 1022 cm −2, accounting for same velocity as the peak emission in the Sgr B2 core,
68% of the total mass, whi×le only 20% of the HCN 1-0 it is likely that this is a fairly localized effect (i.e., this
comes from gas with column density above 7 1022 cm is not a resulting of intervening, foreground gas in the
−2, accounting for 32% of the total gas mass. × Galacticdisk). Wenotethatindependentoftheobserved
self-absorption there is also likely absorption against the
4. DISCUSSION embedded HII regions in Sgr B2 – however, as these HII
Although the brightness of HCN 1-0 appears to corre- regions are more centrally concentrated than the self-
late well with the gas mass over several orders of magni- absorption signature, and there are molecules for which
tude in Galactic center clouds, we also see a few individ- dip is not seen, we do not believe the observed signature
ual clouds that are outliers from this relationship. We can be due to absorption against HII regions alone.
thus focus on these clouds in an attempt to characterize Higher-excitation tracers of more common molecules
5
are also likely to be less affected by self-absorption. For thickness of HCN 1-0 in a number of clouds is actually
example,inobservationsoftheGalacticcentercircumnu- maskingagreaterunderlyingvariationintheintensityof
cleardiskwhileself-absorptionisseenintheJ=3-2and HCN lines. We suggest then that although GCM-0.13-
4-3 lines of HCN, self absorption is no longer present in 0.08 does not appear overluminous in HCN 1-0 for its
the8-7line(Millsetal.2013). Thiscouldmakediatomic mass,thisisonlyduetotheopticalthicknessoftheHCN
tracers of dense gas such as CS a more optimal tracer in 1-0 line in this source, which is comparable to that in
a given band, as they will have higher-J transitions than GCM-0.02-0.07 (HCN 1-0 / H13CN 5-6). This would
HCN, HNC, or HCO+at similar frequencies. In the ob- then mean that all of the clouds pro∼jected to lie within
servations analysed here, the 2-1 transition of 13CS can 30 pc of the supermassive black hole Sgr A* have en-
be seen in Figure 6 to have less self absorption than the ∼hanced HCN 1-0 emission for their mass.
1-0 transitions of H13CN, HN13C, and H13CO+. How-
ever,careshouldstillbetakeninavoidingtracerssubject 4.2.2. Variation in excitation conditions
to self-absorption to also avoid tracers such as SiO that
We next ask whether this difference could be due to
are known to be enhanced by shocks.
varying excitation conditions for HCN 1-0. Other stud-
ThefactthatthecoreofSgrB2,themostmassiveand
ies have shown that there is no significant difference in
dense cloud in the Galactic center is underluminous in
the kinetic temperature (Hu¨ttemeister et al. 1993; Ao
HCN 1-0 has important implications, as it is often taken
et al. 2013; Mills & Morris 2013; Ginsburg et al. 2016)
to be a template for conditions that are widespread in
or line width (Jones et al. 2012) between the clouds in
more extreme starbursts or high-redshift systems. This
ourstudywithrelativelyweakandbrightHCN1-0. Our
resultindicatesthatcautionshouldbetakenwheninter-
RADEX analysis then indicates that for a constant col-
pretingvariationsoftheratioofHCN1-0tofar-infrared
umn density, temperature, and linewidth, variations in
luminosity in extreme systems, as they could instead be
volume density from n=105 107 should change the line
due in part to HCN 1-0 underestimating the total dense brightnessbylessthanafac−torof2. Inthiscase,theob-
gas mass. We discuss this further in Section 4.3.
served overluminosity can only be explained if the three
4.2. Overluminous HCN in Galactic center clouds bright clouds are the only clouds (apart from the Sgr B2
core)withn>104.5 cm−3. Independentstudiesofcloud
While self-absorption appears to be the dominant ef-
densities (Serabyn et al. 1992; Longmore et al. 2013a;
fect contributing to the weakness of integrated HCN 1-0
Gu¨sten & Henkel 1983; Zylka et al. 1992) are inconsis-
emission from Sgr B2, we also see variation in the rela-
tent with this scenario, and in fact even higher ratios of
tionship between the HCN 1-0 line strength and the gas
HCN 1-0 to N(H ) are seen in the more extended gas
massforotherGalacticcentercloudsinoursurvey. Out- 2
which is likely to be at yet lower densities. We note that
side of the Sgr B2 core, three dense clouds: GCM-0.02-
there could still be excitation differences due to radia-
0.07, GCM0.07+0.04, and GCM0.11-0.08, have brighter
tive excitation or masing, however for either of these to
HCN1-0emissionbyafactorof 3thanothercloudsof
comparable mass, including clou∼ds that show no signa- be applicable, HCN 1-0 should be well correlated with
IR luminosity.
ture of self-absorption (e.g., GCM0.25+0.01).
Our RADEX estimates could however be changed if
While the observed overluminosity of HCN 1-0 in sev-
thefillingfactorofHCN1-0(hereassumedtobe1inall
eral Galactic center clouds is small (on the order of a
of the observed clouds) varies significantly from cloud to
factor of 3), determining the mechanism that is respon-
cloud. In the case of a smaller filling fraction, the col-
sible is important for assessing whether this effect might
umn density of HCN 1-0 would be higher for the same
become more pronounced in other environments. Identi-
observed line brightness. However, observations of sev-
fying the mechanism which is leading to enhanced HCN
eral Galactic center clouds in lines of NH , which should
1-0canalsogiveinsightintoadditionaldiagnosticswhich 3
trace a similar dense gas component as HCN 1-0, show
shouldbeperformedinordertoassesswhetherHCN1-0
that for all clouds observed so far, the bulk of the gas
is accurately representing the total dense gas mass in a
emission is in an extended component (The fraction of
given environment.
emission on scales larger than a few tens of arcseconds
4.2.1. Variation in HCN 1-0 optical depth thatisresolvedoutbyinterferometricobservationsis75-
95% Armstrong et al. 1985; Mills & Morris 2013).This
WefirstexaminewhethertherelativeweaknessofHCN
is consistent with the assumed filling fraction of 1. As
1-0 in individual clouds could be due to a high opti-
the fraction of emission resolved out by interferometric
cal depth of this line. In addition to HCN 1-0, we
also observed H13CN, which should be 25 times less observations has not been measured for all clouds in our
abundant than H12CN (Wilson & Roo∼d 1994; Wilson study,andthismeasurementhasnotbeendoneforHCN,
it is possible that this effect could contribute to the ob-
1999; Riquelme et al. 2010). We find that the ratios
served variation in the relative brightness of HCN 1-0.
of HCN 1-0 and H13CN 1-0 do not show clouds with
However, we do not believe that it is likely that this is
relatively weaker HCN 1-0 to be preferentially optically
the dominant source of the observed variation.
thick. Thus,opticaldepthvariationsarenotaviableex-
planation for the observed variations in HCN 1-0 inten-
4.2.3. Variation in conditions in the local environment
sity as a function of cloud mass. In fact, the clouds with
relatively bright HCN 1-0 for their mass are some of the We then examine whether the variations in the HCN
most optically thick: GCM-0.02-0.07, which has some of 1-0 intensity could be consistent with a dependency of
the brightest HCN 1-0 emission is 10 times brighter in this quantity on the environment. It has been suggested
the optically-thin H13CN1-0 line than the comparably- thatoverluminousHCNcouldbeduetoX-raychemistry
massive GCM0.25+0.01. This indicates that the optical (Lepp & Dalgarno 1996; Meijerink et al. 2007; Harada
6
et al. 2013). While several regions of enhanced emission Sgr D is also coincident with SiO emission, which is of-
are associated with X-ray sources (GCM-0.02-0.07 bor- ten indicative of shock chemistry (Martin-Pintado et al.
ders a supernova remnant, and clouds in the GCM0.11- 1997). The other large area of extended enhanced HCN
0.08 complex are observed to have a propagating X-ray 1-0 emission visible in Figure 5 is largely centered on
light echo Ponti et al. 2010) that may also be related the nucleus. A direct link to shock activity is not im-
to enhanced SiO in these regions (Mart´ın-Pintado et al. mediately apparent, although it is possible to hypothe-
2000), there is no X-ray emission associated with the size several shock-related scenarios: it could be related
brightextendedHCN1-0emissionnorthofSgrD(Ponti to shocks driven by stellar winds, given that it corre-
et al. 2015). We also investigate whether the enhanced lates will with the IR emission tracing the influence of
HCN 1-0 is correlated with infrared emission. Clouds centrally-concentrated young stars and clusters. Or, it
thatareactivelyformingstarshavebothweakandbright could be linked to shocks from a dissipation of turbu-
HCN 1-0, so an internal IR radiation field is likely not lent energy fed by the change in shear as gas migrates
an important factor. The three clouds with brightest inward from the 100 pc ring (Molinari et al. 2011; Krui-
HCN 1-0 (G0.07+0.04, GCM0.11-0.08, and GCM-0.02- jssen et al. 2015) toward the central supermassive black
0.07) are projected against the infrared bubble. While hole (Wilson et al. 1982).
this might suggest a connection between HCN 1-0 and As Sgr B2 has been suggested to have undergone a
the presence of a strong external IR field, the region cloud-cloud collision, this cloud might also be expected
north of Sgr D has extremely elevated HCN 1-0 in Fig- to host enhanced shock activity– and indeed, it is possi-
ure 5 without significant IR emission. We also note that ble that in the Sgr B2-halo, which is largely free of self-
althoughSgrB2hasbothastronginternalinfraredfield absorption,thattheHCN1-0lineisoverbright(SiOand
from embedded formation and a recent X-ray light echo HNCO, another shock tracer, both appear to be strong
observed to be propagating through the cloud (Terrier and perhaps overluminous in this region as well). Ad-
etal.2010),selfabsorptionpreventsusfromdetermining ditional work is needed to confirm this hypothesis: for
whether HCN 1-0 is enhanced in the Sgr B2 core. Based example, shocks might be expected to correspond with
on these data we then find that the HCN 1-0 intensity regionsofincreasedturbulencethatcouldmanifestasin-
does not appear to depend upon the local IR radiation creasedlinewidths. Enhancedemissionfromothershock
field or nearby X-ray sources. However, given that all of tracers could also be searched for, such as the 36 GHz
the clouds with enhanced HCN 1-0 emission are located methanol maser, which has been recently observed to
neareachother,andprojectedwithin30pcofthesuper- be widespread in the Galactic center (Yusef-Zadeh et al.
massive black hole, we cannot rule out an importance of 2013; Mills et al. 2015).
other environmental factors.
4.2.4. Variation in HCN abundance 4.2.5. Variation in dust properties
Given that the observed differences in HCN 1-0 in- Finally, we also investigate whether the observed vari-
tensity for a given cloud mass do not appear likely to ations in the ratio of HCN 1-0 intensity to cloud mass
be a function of optical depth or excitation variations couldbeafunctionoferrorsinthederivationofthecloud
(including radiative excitation or masering, as the HCN massesthemselves. Theresultsofthisstudy(attributing
1-0 emission is not uniformly correlated with strong IR these variations to differences in the intensity of HCN 1-
emission), we suggest that they are a function of vary- 0)arebasedupontheassumptionthatthesubmillimeter
ing HCN abundance. Although the scale of variations in dust emission traced by Herschel is a high-fidelity tracer
the brightness of HCN 1-0 for a given cloud mass (apart of the total gas column density (and thus mass) in the
from Sgr B2 and regions affected by self-absorption) is Galactic center. We cannot, at this time, rule out that
only a factor of 3, we will assume that the true scale of variationsinthegastodustratio(weassume100inthis
variations in the abundance of HCN in Galactic center work) or dust emissivity (we assume β of 1.75) could be
clouds is at least an order of magnitude, as is measured partly responsible for some of the discrepancies we see.
in the optically-thin H13CN line. However, we note that these values would need to vary
One possibility for explaining overluminous emission on a cloud-to-cloud scale to explain our results, as the
from HCN 1-0 is that HCN is preferentially enhanced relative column densities of the clouds do not change if
in some clouds and regions of the Galactic center due the global Galactic Center value of the gas to dust ratio
to elevated shock activity. HCN abundances have been is revised.
observed to be enhanced by 1-2 orders of magnitude in The assumption that the dust is a good tracer for the
shockedenvironmentsrangingfromprotostellaroutflows gas column density appears to be valid in less-extreme
to extragalactic outflows (Jørgensen et al. 2004; Tafalla environments in our Galaxy (e.g. Battersby et al. 2014).
et al. 2010; Aalto et al. 2015a). GCM-0.02-0.07 borders Bulk dust properties have also been shown to be ex-
a supernova remnant and shows multiple signs that it tremelysimilarinenvironmentsrangingfromourGalaxy
is experiencing shocks from this interactions (Sjouwer- to nearby normal star forming galaxies and submillime-
man et al. 2010; Yusef-Zadeh et al. 1996), and while the ter galaxies (Scoville et al. 2016). The strongest argu-
GCM0.11-0.08 complex is not suggested to be interact- mentfortheGalacticcenterdustcontinuumbeingagood
ing with a supernova remnant or to have undergone a tracer of mass in the Galactic center that dust-derived
cloud collision, it could be possible to explain enhanced cloud masses of this study are consistent with cloud
shockactivityinthesecloudsduetointeractionwiththe masses previously inferred using C18O (Dahmen et al.
expanding IR bubble, or as a result of their recent peri- 1998). For example, the approximate mass of Sgr B2
center passage in the model of Kruijssen et al. (2015). complex estimated from the dust continuum (a few 106
Importantly, the enhanced HCN 1-0 emission north of M ) is comparable to that determined using optically-
(cid:12)
7
thin measurements of C18O (Dahmen et al. 1998). We formation, where HCN 1-0 is a proxy for the amount of
thus do not believe that the use of dust emission to de- this gasand the IRluminosity is aproxyfor theamount
termine masses affects the main results of this paper, of star formation. Assuming a fundamental relationship
although the assumption that the dust to gas ratio does between the amount of dense gas and the star forma-
not significantly vary as a function of environment has tion rate exists, then scatter in the relationship between
yet to be carefully tested in the Galactic center. the HCN 1-0 intensity and the actual amount of dense
gas would contribute to the observed scatter in the Gao-
4.3. Implications of a varying HCN 1-0 to dense gas Solomon relationship. Our identification of HCN abun-
mass conversion factor dance variations and self absorption as sources of this
scatter could help explain the increased scatter and sys-
We have found that the logarithm of the HCN 1-0 in-
tematic deviations seen when testing the Gao Solomon
tensity is linearly correlated with the logarithm of the
relationship for larger samples of more extreme sources,
densemoleculargasmassovertwoordersofmagnitudein
includingULIRGs(Privonetal.2015)andAGN(Davies
the gas mass in Galactic center molecular clouds. How-
et al. 2012; Imanishi et al. 2009; Kohno et al. 2003).
ever individual clouds in this environment show devia-
Wu et al. (2005) interpret the Gao-Solomon relation-
tions from this relationship that contribute to around
shipasindicatingthatindividual,sub-parsecscaledense
0.75 dex of scatter. The two primary sources of scat-
cores are the fundamental unit of dense gas mass associ-
ter that we have identified are self-absorption (which re-
ated with star formation, and that the Gao Solomon re-
duces the observed HCN 1-0 intensity of Sgr B2) and
lation on larger scales is essentially counting the number
variations in HCN abundance (possibly due to shock en-
of these structures. However, our observations that only
hancements), which appear to increase the brightness of
20% of the HCN emission in the Galactic center comes
HCN 1-0 in GCM-0.02-0.07 and other clouds projected
from the cores of the Giant molecular clouds (above a
to lie within the central 30 pc of the Galactic center.
column density threshold of N(H ) = 7 1022 cm −2)
Here, we discuss the implications of our observations for 2
challenge this interpretation. Instead, th×e bulk of the
interpreting the Gao-Solomon relation and for generally
molecular gas traced by HCN 1-0 in this environment
deriving accurate total gas masses from 3 mm tracers.
liesoutsideofthesedenseclouds,thestructuresinwhich
all observed Galactic center star formation is occurring.
4.3.1. The interpretation of the Gao-Solomon relation
This indicates that in the environment of the Galactic
The scatter that we measure in the relationship be-
center, the Gao-Solomon relation cannot be entirely a
tween HCN 1-0 intensity and dense gas mass for 13
relationship built up from the star-forming properties of
Galactic center clouds ( 0.75 dex) is actually less than
individual dense, high-mass clumps. This is consistent
the scatter measured by∼Wu et al. (2010) in their Fig-
witharecentstudyoftheclump-scaleGao-Solomonrela-
ure 43 ( 1.5 dex), for a sample of a few dozen Galactic
tionusingtheMALT90surveydataoftheGalacticplane,
clumps.∼There are several differences between our sam-
which found that if the Galactic-scale Gao-Solomon re-
ple and that of Wu et al. (2010). In our sample, the
lation were due just to individual star forming clumps,
clouds have typical radii of 3-5 pc and typical masses
it would require far more high-mass star forming clumps
of 105 M , with masses spanning two orders of magni-
(cid:12) than are actually observed. (Stephens et al. 2016). Any
tude (from a few 104 - 106 M ). The clump sample of
(cid:12) correlation between HCN 1-0 and IR emission in this re-
Wu et al. (2010) in contrast has typical radii of 1 pc
gion must be explained in the context of the dominant
and typical masses around 1000 M (spanning 4 orders
(cid:12) extended molecular gas component that is not currently
of magnitude from 10 to 105 M(cid:12)). The masses mea- associated with the most massive star forming clouds,
suredbyWuetal.(2∼010)arealsovirialmasses,measured and the extended IR emission from past generations of
from the linewidth of C34S 5-4. Studies of the relation- star formation (as indeed, the bulk of even the dense
ship between the virial mass and the actual mass (the clouds are not actively star forming here, e.g., Lis et al.
virialparameter)overarangeofenvironmentsandcloud 2001; Longmore et al. 2013b).
masses show that there is significant scatter in this re-
lationship (e.g., Kauffmann et al. 2013; Battersby et al. 4.3.2. Deriving accurate total gas masses from 3 mm tracers
2014). In particular, Kauffmann et al. (2013) find 2 In the center of our Galaxy, scatter in the relationship
ordersofmagnitudeofscatterinthevirialparameter∼for between HCN 1-0 and total cloud mass would not con-
masses comparable to those in the sample of Wu et al. tribute to a large inaccuracy of the mass inferred just
(2010), which may contribute to the larger scatter that using HCN 1-0 intensity. Although the Sgr B2 core
theyobserveintheirrelationbetweenHCN1-0anddense has HCN 1-0 emission consistent with a cloud 5 times
gas mass. We conclude then that the results from these less massive, this missing luminosity would on∼ly result
samples are not directly comparable, but we note that in missing (cid:46) 10% of the total mass of dense gas in the
the scatter we have identified for Galactic center clouds Galactic center. Some of this could also be balanced out
is not unprecedented, and may indeed be less than that by the “extra mass” inferred from overbright HCN 1-0
already observed on smaller scales. in other clouds. However, in more globally-extreme en-
Identifying the origin of this scatter is important for vironmentsthantheGalacticcenter,whatweobserveas
the related Gao-Solomon relation, which shows that the scattercouldleadtosystematicerrorswheninterpreting
HCN1-0integratedintensityispositivelycorrelatedwith HCN 1-0 as proportional to the total mass of dense gas.
the far infrared luminosity over 10 orders of magnitude Certain environments in particular should be more
inIRbrightness(Gao&Solomon2004b;Wuetal.2005). pronetotheself-absorptioninHCN1-0observedtoward
Thisrelationistypicallyinterpretedasarelationshipbe- the Sgr B2 core. In clouds having an excitation gradient
tweenthetotalamountof(dense)moleculargasandstar that increases radially toward the center, one might ex-
8
pecttoseeself-absorptioninthelowerJlevelsofagiven prior observations of SiO and CH OH Martin-Pintado
3
common gas tracer. Such excitation gradients would be et al. 1997; Yusef-Zadeh et al. 2013) Interestingly, Hen-
typical of clouds with embedded active star formation shaw et al. (2016) also found HNCO 4 -3 to be one of
04 03
(as in Sgr B2), or embedded AGN, as is has recently the best kinematic tracers of Galactic center clouds.
been noted in a number of compact sources of nuclear Finally, we also caution that some systematic devia-
emission (Aalto et al. 2015b). Self-absorption could also tions in molecular abundance may not be accounted for
occurincaseswheretherearestrongglobalsignaturesof byemployingmultiplegastracers,aswefindthatGCM-
outflow or infall. Self absorption also might be expected 0.02-0.07 is overbright in nearly every 3 mm tracer (ex-
to depend on the relative orientation of a warm back- ceptperhapsHN13C).Thismayberelatedtotheunique
ground source and cool foreground emission, but as it chemistry or excitation in this cloud, which is by far the
requires the foreground and background gas to be at the brightest source of C+ emission in the Galactic center,
samevelocity,itshouldnotbeageneralfeatureofgalaxy likely due to its unique proximity to a nearby supernova
disks observed edge-on (for example, we note that self- remnant (Tanaka et al. 2011). In such extreme cases, it
absorptionisonlyseenforasmallnumberofthesurveyed maybethatmeasurementsofthemassusingthesubmil-
Galactic center clouds, though all are observed through limeterdustcontinuumScovilleetal.(2016)providethe
edge-on disk of our own Galaxy). most accurate results.
If the enhanced HCN abundance is linked to nuclear
activity, as the concentration of clouds overluminous in 5. CONCLUSIONS
theHCN1-0lineprojectedwithin30pcofSgrA*might
Below, we summarize the main findings of this paper:
suggest,thennuclearstarburstsorAGNmightalsoexpe-
rience a similar effect. In this case, HCN could system-
atically overestimate the total dense gas mass present.
For our observed sample of 13 Galactic center
WethusrecommendthatobservationsofHCN1-0inex-
• clouds,log (HCN1-0)islinearlycorrelatedwith
tremeenvironmentsshouldbecheckedforsystematically 10
thelog (densemoleculargasmass)overtwoor-
weak emission due to self-absorption (e.g., via compari- 10
ders of magnitude in the gas mass. However indi-
son with tracers like HC N or CH CN) before seeking a
3 3
vidual clouds in this environment deviate, in some
physicalexplanationforalackofdensegassuchasvaria-
casessystematically,fromtherelationshipandcon-
tionsinthegasdepletiontime(e.g.,Garc´ıa-Burilloetal.
tribute to around 0.75 dex of scatter. The two pri-
2012; Juneau et al. 2009; Leroy et al. 2015; Usero et al.
mary sources of scatter that we have identified are
2015). Such studies will also be important for determin-
self-absorption and variations in HCN abundance.
ingwhethertheGalacticcenterconditionsresponsiblefor
variations in the HCN 1-0 to dense gas conversion factor
are actually relevant for other galaxies, or whether devi- ThecoreofSgrB2hasanHCN1-0intensityequiv-
ations from the Gao-Solomon relation in other systems • alent to clouds having five times less mass. We
are due to entirely separate effects that are a result of findthatthefaintnessofthislineisaresultofself-
environments that are not sampled in our own Galaxy. absorptioninthissource. Basedonthis,wecaution
In general, care should be taken in using any single against the interpretation of increased LFIR/HCN
transition to represent total mass: although HC N and 1-0 as indicative of a higher star formation effi-
3
CH CN are better tracers of the total mass of the Sgr ciencyorlowergasdepletiontimeinextremeenvi-
3
B2corebecauseoftheirimmunityfromself-absorptionin ronments that might be dominated by sources like
this source, the overall intensity of both tracers in other embedded starbursts or AGN having a similar ex-
Galactic center clouds are extremely poorly-correlated citationgradient,withmorehighly-excitedgassur-
with total mass (perhaps related to observations that roundedbyalower-excitationenvelope. The3mm
HC3N is enhanced in active nuclei Aalto et al. 2007a). lines of HC3N and CH3CN do not suffer from this
Our findings indicate that, in order to determine accu- self absorption in Sgr B2, and so may be used to
rate gas masses, one should primarily use a tracer like identify regions where HCN 1-0 is self-absorbed.
HCN 1-0, and supplement it with a tracer like HC N or
3
CH CN to guard against underestimation of the dense Several cloud cores, all projected to lie within the
gas3mass due to the effects of self-absorption. Further • central 30 parsecs of the Galactic center, have 3
study of the correspondence of HCN 1-0 with shocks in times brighter HCN 1-0 than other clouds of si∼m-
extremeextragalacticenvironmentsshouldalsobemade ilar mass. We find that this is likely a result of
to determine whether bright tracers that are similarly enhanced HCN abundance, and suggest that this
well-correlated with mass like HNCO 4 -3 , HNC 1- could be a result of enhanced shock activity.
04 03
0, HCO+1-0, or CS 2-1 are less affected and thus more
suitable for determination of accurate gas masses in en- Wealsofindseveralcontiguousregionsofmoreex-
vironments dominated by shocks. The especially good • tended gas emission that have 3-10 times more lu-
correspondence of HNCO with dense gas in this envi- minous HCN 1-0 per unit N(H ) than in any of
2
ronment is worthy of further investigation. HNCO is the cloud cores studied here. Overall HCN 1-0 is
suggested to behave like a shock tracer in some environ- brighter in this lower column density gas: 80% of
ments,basedonitscorrelationwithknownshocktracers theHCN1-0emissioncomesfromgaswithcolumn
like SiO (e.g., Zinchenko et al. 2000; Meier & Turner densitylessthan7 1022 cm−2,accountingfor68%
2005; Li et al. 2013). The observed correspondence may of the total mass,×while only 20% of the HCN 1-0
then simply be confirming that shocked gas conditions comesfromgaswithcolumndensityabove7 1022
are ubiquitous in Galactic center gas (as suggested by cm−2, accounting for 32% of the total gas m×ass.
9
We examine other 3mm tracers and find that ACKNOWLEDGEMENTS
• HNCO 4 -3 shows a tighter correlation with
04 03 We thank the anonymous referee for detailed and in-
cloudmass,suggestingthatHNCO4 -3 (despite
04 03 sightful comments that substantially improved the pre-
not being the fundamental rotational transition of
sentation of the results in this paper. The authors
that molecule) could be a comparable if not better
also thank Jonathan Henshaw, Steven Longmore, David
tracer of mass than HCN 1-0 in the Galactic cen-
Meier, Susanne Aalto, and George Privon for useful dis-
ter environment. HNC 1-0 and HCO+ 1-0 are less
cussion and suggestions that significantly improved the
well-correlated than HCN 1-0. HC N and CH CN
3 3 paper. This material is based upon work supported
show a poor correlation between their brightness
by the National Science Foundation under Award No.
and total mass, and should not be used to replace
1602583.
HCN 1-0, despite being free of self-absorption ef-
fects.
REFERENCES
Aalto,S.,Monje,R.,&Mart´ın,S.2007a,A&A,475,479 Lepp,S.,&Dalgarno,A.1996,A&A,306,L21
Aalto,S.,Spaans,M.,Wiedner,M.C.,&Hu¨ttemeister,S.2007b, Leroy,A.K.,Walter,F.,Bigiel,F.,etal.2009,AJ,137,4670
A&A,464,193 Leroy,A.K.,Bolatto,A.D.,Ostriker,E.C.,etal.2015,ApJ,
Aalto,S.,Garcia-Burillo,S.,Muller,S.,etal.2015a,A&A,574, 801,25
A85 Li,J.,Wang,J.Z.,Gu,Q.S.,&Zheng,X.W.2013,A&A,555,
Aalto,S.,Mart´ın,S.,Costagliola,F.,etal.2015b,A&A,584,A42 A18
Ao,Y.,Henkel,C.,Menten,K.M.,etal.2013,A&A,550,A135 Lis,D.C.,Serabyn,E.,Zylka,R.,&Li,Y.2001,ApJ,550,761
Armstrong,J.T.,Ho,P.T.P.,&Barrett,A.H.1985,ApJ,288, Longmore,S.N.,Rathborne,J.,Bastian,N.,etal.2012,ApJ,
159 746,117
Battersby,C.,Bally,J.,Dunham,M.,etal.2014,ApJ,786,116 Longmore,S.N.,Kruijssen,J.M.D.,Bally,J.,etal.2013a,
Battersby,C.,Bally,J.,Ginsburg,A.,etal.2011,A&A,535, MNRAS,433,L15
A128 Longmore,S.N.,Bally,J.,Testi,L.,etal.2013b,MNRAS,429,
Behroozi,P.S.,Wechsler,R.H.,&Conroy,C.2013,ApJ,770,57 987
Boehle,A.,Ghez,A.M.,Sch¨odel,R.,etal.2016,ApJ,830,17 Madau,P.,&Dickinson,M.2014,ARA&A,52,415
Carilli,C.L.,&Walter,F.2013,ARA&A,51,105 Madau,P.,Pozzetti,L.,&Dickinson,M.1998,ApJ,498,106
Dahmen,G.,Huttemeister,S.,Wilson,T.L.,&Mauersberger,R. Martin-Pintado,J.,deVicente,P.,Fuente,A.,&Planesas,P.
1998,A&A,331,959 1997,ApJ,482,L45+
Davies,R.,Mark,D.,&Sternberg,A.2012,A&A,537,A133 Mart´ın-Pintado,J.,deVicente,P.,Rodr´ıguez-Fern´andez,N.J.,
Gao,Y.,Carilli,C.L.,Solomon,P.M.,&VandenBout,P.A. Fuente,A.,&Planesas,P.2000,A&A,356,L5
2007,ApJ,660,L93 Mashian,N.,Sturm,E.,Sternberg,A.,etal.2015,ApJ,802,81
Gao,Y.,&Solomon,P.M.2004a,ApJS,152,63 Matsushita,S.,Trung,D.-V.,Boone,F.,etal.2015,ApJ,799,26
—.2004b,ApJ,606,271 Meier,D.S.,&Turner,J.L.2005,ApJ,618,259
Garc´ıa-Burillo,S.,Usero,A.,Alonso-Herrero,A.,etal.2012, Meijerink,R.,Spaans,M.,&Israel,F.P.2007,A&A,461,793
A&A,539,A8 Mills,E.A.C.,Butterfield,N.,Ludovici,D.A.,etal.2015,ApJ,
Genzel,R.,Tacconi,L.J.,Lutz,D.,etal.2015,ApJ,800,20 805,72
Ghez,A.M.,etal.2008,ApJ,689,1044 Mills,E.A.C.,Gu¨sten,R.,Requena-Torres,M.A.,&Morris,
Ginsburg,A.,Henkel,C.,Ao,Y.,etal.2016,A&A,586,A50 M.R.2013,ApJ,779,47
Graci´a-Carpio,J.,Garcia-Burillo,S.,Planesas,P.,&Colina,L. Mills,E.A.C.,&Morris,M.R.2013,ApJ,772,105
2006,ApJ,640,L135 Molinari,S.,Bally,J.,Noriega-Crespo,A.,etal.2011,ApJ,735,
Graci´a-Carpio,J.,Garc´ıa-Burillo,S.,Planesas,P.,Fuente,A.,& L33
Usero,A.2008,A&A,479,703 Moster,B.P.,Naab,T.,&White,S.D.M.2013,MNRAS,428,
Gu¨sten,R.,&Henkel,C.1983,A&A,125,136 3121
Harada,N.,Thompson,T.A.,&Herbst,E.2013,ApJ,765,108 Olofsson,H.,Eriksson,K.,Gustafsson,B.,&Carlstroem,U.
Henshaw,J.D.,Longmore,S.N.,Kruijssen,J.M.D.,etal.2016, 1993,ApJS,87,305
MNRAS,457,2675 Ponti,G.,Terrier,R.,Goldwurm,A.,Belanger,G.,&Trap,G.
Hu¨ttemeister,S.,Wilson,T.L.,Bania,T.M.,&Martin-Pintado, 2010,ApJ,714,732
J.1993,A&A,280,255 Ponti,G.,Morris,M.R.,Terrier,R.,etal.2015,MNRAS,453,
Imanishi,M.,Nakanishi,K.,Tamura,Y.,&Peng,C.-H.2009, 172
AJ,137,3581 Privon,G.C.,Herrero-Illana,R.,Evans,A.S.,etal.2015,ApJ,
Izumiura,H.,Ukita,N.,&Tsuji,T.1995,ApJ,440,728 814,39
Jones,P.A.,Burton,M.G.,Cunningham,M.R.,etal.2012, Riquelme,D.,Amo-Baladro´n,M.A.,Mart´ın-Pintado,J.,etal.
MNRAS,419,2961 2010,A&A,523,A51
Jørgensen,J.K.,Hogerheijde,M.R.,Blake,G.A.,etal.2004, Rodr´ıguez-Ferna´ndez,N.J.,Mart´ın-Pintado,J.,&deVicente,P.
A&A,415,1021 2001,A&A,377,631
Juneau,S.,Narayanan,D.T.,Moustakas,J.,etal.2009,ApJ, Saintonge,A.,Kauffmann,G.,Wang,J.,etal.2011,MNRAS,
707,1217 415,61
Kauffmann,J.,Bertoldi,F.,Bourke,T.L.,Evans,II,N.J.,& Saintonge,A.,Lutz,D.,Genzel,R.,etal.2013,ApJ,778,2
Lee,C.W.2008,A&A,487,993 Sakamoto,K.,Aalto,S.,Evans,A.S.,Wiedner,M.C.,&Wilner,
Kauffmann,J.,Pillai,T.,&Goldsmith,P.F.2013,ApJ,779,185 D.J.2010,ApJ,725,L228
Kennicutt,Jr.,R.C.1998,ApJ,498,541 Sandstrom,K.M.,Leroy,A.K.,Walter,F.,etal.2013,ApJ,777,
Kohno,K.,Ishizuki,S.,Matsushita,S.,Vila-Vilar´o,B.,& 5
Kawabe,R.2003,PASJ,55,L1 Scoville,N.,Sheth,K.,Aussel,H.,etal.2016,ApJ,820,83
Kruijssen,J.M.D.,Dale,J.E.,&Longmore,S.N.2015, Serabyn,E.,Lacy,J.H.,&Achtermann,J.M.1992,ApJ,395,
MNRAS,447,1059 166
Kruijssen,J.M.D.,&Longmore,S.N.2013,MNRAS,435,2598 Sjouwerman,L.O.,Pihlstr¨om,Y.M.,&Fish,V.L.2010,ApJ,
Lada,C.J.,Forbrich,J.,Lombardi,M.,&Alves,J.F.2012,ApJ, 710,L111
745,190 Stephens,I.W.,Jackson,J.M.,Whitaker,J.S.,etal.2016,ApJ,
Ladd,N.,Purcell,C.,Wong,T.,&Robertson,S.2005, 824,29
PublicationsoftheAstronomicalSocietyofAustralia,22,62 Tacconi,L.J.,Genzel,R.,Neri,R.,etal.2010,Nature,463,781
10
Tacconi,L.J.,Neri,R.,Genzel,R.,etal.2013,ApJ,768,74 Wu,J.,Evans,II,N.J.,Gao,Y.,etal.2005,ApJ,635,L173
Tafalla,M.,Santiago-Garc´ıa,J.,Hacar,A.,&Bachiller,R.2010, Wu,J.,Evans,II,N.J.,Shirley,Y.L.,&Knez,C.2010,ApJS,
A&A,522,A91 188,313
Tanaka,K.,Oka,T.,Matsumura,S.,Nagai,M.,&Kamegai,K. Yusef-Zadeh,F.,Cotton,W.,Viti,S.,Wardle,M.,&Royster,M.
2011,ApJ,743,L39 2013,ApJ,764,L19
Terrier,R.,Ponti,G.,B´elanger,G.,etal.2010,ApJ,719,143 Yusef-Zadeh,F.,Roberts,D.A.,Goss,W.M.,Frail,D.A.,&
Usero,A.,Leroy,A.K.,Walter,F.,etal.2015,AJ,150,115 Green,A.J.1996,ApJ,466,L25+
Wilson,T.L.1999,ReportsonProgressinPhysics,62,143 Zinchenko,I.,Henkel,C.,&Mao,R.Q.2000,A&A,361,1079
Wilson,T.L.,&Rood,R.1994,ARA&A,32,191 Ziurys,L.M.,&Turner,B.E.1986,ApJ,300,L19
Wilson,T.L.,Ruf,K.,Walmsley,C.M.,etal.1982,A&A,115, Zylka,R.,Guesten,R.,Henkel,C.,&Batrla,W.1992,A&AS,
185 96,525
Winnewisser,G.,Churchwell,E.,&Walmsley,C.M.1979,A&A,
72,215