Mapping the high ionization rate of the GC starburst Sgr B2 through low HCO + / N 2 H + J =1-0 intensity ratios

. We still do not understand which mechanisms dominate the heating and ionization of the extended molecular gas in galactic nuclei. The starburst Sgr B2, in the Galactic Center (GC), is an excellent template to spatially resolve the high-mass star-forming cores from the extended cloud environment, and to study the properties of the warm neutral gas in conditions likely prevalent in star-forming galaxies. We mapped ⇠ 1000 pc 2 of Sgr B2 complex, using the IRAM30m telescope, in the N 2 H + , HCO + J = 1-0 and SiO J = 2-1 line emission. The extended nature of the N 2 H + J = 1-0 emission is remarkable. Compared to molecular clouds in the disk of the galaxy, the N 2 H + J = 1-0 emission is not con-ﬁned to cold and dense cores and ﬁlaments. This can be explained by the high ionization rate ( ⇣ & 10 − 15 s − 1 ), leading to overabundant H + 3 , He + , and N 2 H + . The enhanced ionization rate is likely responsible of the much lower line intensity ratio R I = HCO + / N 2 H + J = 1-0 observed in Sgr B2 ( R I ⇡ 2 ± 2), Arp 220 ( R I ⇡ 2), and NGC 253 ( R I ⇡ 5), compared to disk clouds such as Orion B ( R I ⇡ 24) and starburst galaxies such as M82 ( R I ⇡ 21).


Introduction
The conditions prevailing in the interstellar medium (ISM) of galactic nuclei can be extreme compared to those in the more quiescent disk of galaxies.Such conditions are regulated by the feedback from OB-type stars: strong UV radiation fields, stellar winds, and supernovae explosions [1]; cloud-cloud collisions [2], enhanced cosmic ray (CR) fluxes [3], and X-ray emission (from accretion onto a super-massive black hole, SMBH) [4].To understand star formation in these environments, and galaxy evolution in general, it is critical to characterize the gas heating and ionization mechanisms in the ISM of galactic nuclei.
Sagittarius B2 is the most massive and far-infrared (FIR) luminous (⇠10 7  L � ) molecular cloud in the Galactic Center (GC).It shows ongoing high-mass star formation and line-ofsight column densities above N H 2 & 10 24 cm −2 [3].Sgr B2 is located at a projected distance of about 100 pc from the dynamical center of the Milky Way (Sgr A ⇤ ) [5].It contains three main high-mass star-forming cores (named N, M, and S), with more than 49 compact H ii regions [6] embedded in a moderately high density (n(H 2 ) & 10 5 cm −3 ) molecular cloud which is surrounded by a more extended and lower density envelope, hereafter "Sgr B2 envelope" [7].
It is a long-standing debate whether radiation, CRs, or shocks dominate the heating of the widespread warm molecular gas in the Sgr B2 complex.Plausible candidates are: cloudcloud collisions [2,8], UV-irradiated low-velocity shocks [9], X-rays from brief outburst periods of Sgr A ⇤ , or low-energy cosmic ray (LECRs) showers [4].
Variations in the abundances of particular molecular ions, such as the HCO + over N 2 H + abundance ratio, help to constrain the ionization processes that result from X-ray and LECRs irradiation [10].In this paper we briefly summarize the main results of our IRAM 30m mapping study of Sgr B2 complex, focusing on the characteristics of the N 2 H + and HCO + J=1-0 emission.

Mapping observations
We mapped Sgr B2 in the 3mm band (⇠90 GHz) using the IRAM 30m telescope.The size of our maps is 12 0 ⇥ 12 0 (see Fig. 1).The angular resolution at the target frequencies is ⇠ 27 00 (⇠ 1 pc at the distance to Sgr B2).We complement these observations with NuSTAR hard X-rays (3-78 keV) continuum observations.However, these X-rays observations are not conclusive of whether Sgr B2 is (still) reprocessing X-rays from past flaring activity of Sgr A ⇤ or if (the observed) X-rays arise from LECRs [4,12].In this work we refer to the ionization rate (⇣), without distinction between X-rays and CRs ionization rate.

Widespread N 2 H + emission far from star-forming cores
The observed N 2 H + J=1-0 emission is extended at pc scales.This is an unexpected result as in nearby molecular clouds of the Galactic disk the N 2 H + emission is confined to dense filaments and cold prestellar cores inside them [13].The common interpretation is that N 2 H + traces very dense gas at high extinction depths, that is, cold gas (T k < 20 K) shielded from Triangles mark other H ii regions.Black dashed circle marks an X-ray irradiated region [4].Contours on the R I map show the hard X-ray emission published in 2015 [4].Dashed white lines delineate the regions with no X-ray observations.The X symbols and the numbers are NuSTAR X-ray point sources [11].Variations in the abundances of particular molecular ions, such as the HCO + over N 2 H + abundance ratio, help to constrain the ionization processes that result from X-ray and LECRs irradiation [10].In this paper we briefly summarize the main results of our IRAM 30m mapping study of Sgr B2 complex, focusing on the characteristics of the N 2 H + and HCO + J=1-0 emission.

Mapping observations
We mapped Sgr B2 in the 3mm band (⇠90 GHz) using the IRAM 30m telescope.The size of our maps is 12 0 ⇥ 12 0 (see Fig. 1).The angular resolution at the target frequencies is ⇠ 27 00 (⇠ 1 pc at the distance to Sgr B2).We complement these observations with NuSTAR hard X-rays (3-78 keV) continuum observations.However, these X-rays observations are not conclusive of whether Sgr B2 is (still) reprocessing X-rays from past flaring activity of Sgr A ⇤ or if (the observed) X-rays arise from LECRs [4,12].In this work we refer to the ionization rate (⇣), without distinction between X-rays and CRs ionization rate.

Widespread N 2 H + emission far from star-forming cores
The observed N 2 H + J=1-0 emission is extended at pc scales.This is an unexpected result as in nearby molecular clouds of the Galactic disk the N 2 H + emission is confined to dense filaments and cold prestellar cores inside them [13].The common interpretation is that N 2 H + traces very dense gas at high extinction depths, that is, cold gas (T k < 20 K) shielded from Triangles mark other H ii regions.Black dashed circle marks an X-ray irradiated region [4].Contours on the R I map show the hard X-ray emission published in 2015 [4].Dashed white lines delineate the regions with no X-ray observations.The X symbols and the numbers are NuSTAR X-ray point sources [11].
stellar FUV radiation where CO freezes-out on dust grains [14].In Sgr B2, however, the N 2 H + emission is remarkably widespread and extended.In addition, it spatially correlates with the SiO J=2-1 emission (see Fig. 1; ⇢ sp = 0.7 [3]), considered to be tracer of warm shocked gas.
The main formation route of N 2 H + is reaction of H + 3 with N 2 (the major gas-phase nitrogen reservoir inside molecular clouds).Its main destruction route is reacting with CO, N 2 H + + CO !HCO + + N 2 .
(2) H + 3 forms after H 2 ionization and drives the production of molecular ions in dense interstellar clouds [15].The ionization rate in Sgr B2 is high, ⇣ '(1-10)⇥10 −15 s −1 [4], about 10-100 times higher than that in typical molecular clouds of the Galactic disk.This enhanced rate increases the H + 3 and He + production rate, but also the destruction rate of CO through gasphase reactions with H + 3 and He + : (4) In addition, He + reacts with N 2 to form N + 2 , which then reacts with H 2 to form more N 2 H + .For high values of ⇣, the ionization fraction, x e = n e /n H , is also high.This implies that N 2 H + and HCO + start to be destroyed by dissociative recombinations with electrons.In consequence, the column density ratio R N =N(HCO + )/N(N 2 H + ) decreases as ⇣ increases (see also [10]).This reasoning applies as long as the destruction of H + 3 is dominated by reactions with CO (so-called "low ionization phase", LIP) and not by recombinations with electrons [16,17].In this phase, protonation reactions with H + 3 dominate the formation of abundant molecular ions such as N 2 H + .Our photochemical models of the Sgr B2 envelope (with G 0 = 10 3 and ⇣ &10 −15 s −1 ; Fig. 2 left) show that the N 2 H + emission must arise from moderately dense gas (n H &10 5 cm −3 ) and from moderate cloud depths (A V ) so that the CO abundance is higher than x e .
At low densities (n H < 10 4 cm −3 ) and high ionization rates (⇣ & 10 −15 s −1 ) the gas chemistry switches to a phase in which destruction of H + 3 via electron recombinations is comparable to destruction via reactions with CO.This phase is called "high ionization phase" (HIP).The abundances of molecular ions such as HCO + and N 2 H + are much lower in this phase.It is unlikely that the observed extended N 2 H + emission in Sgr B2 is in the HIP. Figure 2. Left: Model predictions for di↵erent ionization rates, ⇣, (constant G 0 =10 3 , n H =10 5 and 10 6 cm −3 , and A V, tot =15 mag).Green curves show the column density ratio R N .Colored curves (blue, pink, gray, and cyan) show the column density enhancement, N/N 0 , with respect to models with ⇣ 0 =2⇥10 −17 s −1 (average ionization rate in Galactic disk molecular clouds) [3].Right: Histogram of all observed R I = I(HCO + )/I(N 2 H + ) J=1-0 line intensity ratios in our map of Sgr B2 (green).The black arrow indicates the direction of increasing ionization rate.The red line marks the median value of the intensity ratio in Sgr B2 complex.Dashed blue lines mark the line intensity ratios R I observed in Orion B at large spatial scales [13], in M 82, NGC 1068, NGC 253, Arp 220 [18], and in Sgr C [19].Recall that, in general, the value R N (left) is di↵erent than R I (right).

HCO + /N 2 H + intensity ratio in Sgr B2 envelope and in other environments
At sufficiently high densities, n H & 10 5 cm −3 and in the LIP phase, the column density ratio R N (HCO + /N 2 H + ) inversely scales with the ionization rate ⇣, and it scales with gas density.In Sgr B2, the R I (HCO + /N 2 H + ) map (Fig. 1, bottom-right) shows low ratios (<1) in specific X-rays irradiated regions (e.g.,G0.66-0.13)but higher ratios in others (e.g.,western regions).The average value of the line intensity ratio R I is low, R I = 2 ± 2. This is much lower than that the ratio observed in Orion B averaged over large spatial scales, R I ⇠ 24 [13].However, the observed intensity ratio matches that of Arp 220 (a merger ULIRG galaxy).This match suggests similar environmental conditions (Arp 220 possibly hosts thousands of embedded Sgr B2-like starburst regions).NGC 253 (a spiral galaxy with widespread shocks) shows a ratio of R I ⇠ 5, whereas the starburst galaxy M 82 shows a much higher ratio, R I ⇠ 21, closer to the values observed in molecular clouds of the galactic disk (i.e., a↵ected by lower ionization rates).
In this study we find that low R I (HCO + /N 2 H + ) line intensity ratios trace relatively dense molecular gas and high ionization rates (produced by enhanced doses of cosmic rays and X-rays).In Sgr B2, the intensity ratio R I is low (<1) over large spatial scales.This result suggests that part of the rich and characteristic chemistry of Sgr B2 is produced by the high ionization rate in the GC.This also implies that the emissivity and behavior of certain molecules di↵er from that in typical molecular clouds of the disk (or in normal galaxies).
In order to determine the column densities of HCO + and N 2 H + , and to constrain R N accurately, one would need to map their higher frequency rotationally excited lines and study their excitation and radiative transfer.Much higher angular resolution maps will allow us to spatially separate the di↵erent environments in these complex regions and to dissect their dominant heating and ionization mechanisms.

Figure 1 .
Figure 1.IRAM 30m total line intensity maps (in K km s −1 ): N 2 H + , HCO + J=1-0 and SiO J=2-1 integrated in the LSR velocity range [-20, 120] km s −1 .Bottom-right: R I =I(HCO + )/I(N 2 H + ) J=1-0 intensity ratio.The three white stars mark the position of the main star-forming cores (N, M, S).Triangles mark other H ii regions.Black dashed circle marks an X-ray irradiated region[4].Contours on the R I map show the hard X-ray emission published in 2015[4].Dashed white lines delineate the regions with no X-ray observations.The X symbols and the numbers are NuSTAR X-ray point sources[11].

Figure 1 .
Figure 1.IRAM 30m total line intensity maps (in K km s −1 ): N 2 H + , HCO + J=1-0 and SiO J=2-1 integrated in the LSR velocity range [-20, 120] km s −1 .Bottom-right: R I =I(HCO + )/I(N 2 H + ) J=1-0 intensity ratio.The three white stars mark the position of the main star-forming cores (N, M, S).Triangles mark other H ii regions.Black dashed circle marks an X-ray irradiated region[4].Contours on the R I map show the hard X-ray emission published in 2015[4].Dashed white lines delineate the regions with no X-ray observations.The X symbols and the numbers are NuSTAR X-ray point sources[11].