J/MNRAS/536/3803 Line fluxes & abundances in M31 HII regions (Bosomworth+, 2025)
Cloud-scale elemental abundance variations and the CO-to-dust-mass conversion
factor in M31.
Bosomworth C., Forbrich J., Lada C.J., Caldwell N., Kobayashi C.,
Viaene S.
<Mon. Not. R. Astron. Soc., 536, 3803 (2025)>
=2025MNRAS.536.3803B 2025MNRAS.536.3803B
ADC_Keywords: H I data; H II regions; Abundances; Extinction; Galaxies;
Molecular clouds; Spectroscopy; Optical; Infrared;
Interstellar medium
Keywords: ISM: HII regions ; ISM: abundances ; galaxies: individual: M31 ;
galaxies: ISM
Abstract:
From a spectroscopic survey of candidate H ii regions in the Andromeda
Galaxy (M31) with MMT/Hectospec, we have identified 294 H ii regions
using emission line ratios and calculated elemental abundances from
strong-line diagnostics (values ranging from subsolar to supersolar)
producing both oxygen and nitrogen radial abundance gradients. The
oxygen gradient is relatively flat, while the nitrogen gradient is
significantly steeper, indicating a higher N/O ratio in M31's inner
regions, consistent with recent simulations of galaxy chemical
evolution. No strong evidence was found of systematic galaxy-scale
trends beyond the radial gradient. After subtracting the radial
gradient from abundance values, we find an apparently stochastic and
statistically significant scatter of standard deviation 0.06dex, which
exceeds measurement uncertainties. One explanation includes a possible
collision with M32 200-800Myr ago. Using the two-point correlation
function of the oxygen abundance, we find that, similar to other
spiral galaxies, M31 is well-mixed on sub-kpc scales but less so on
larger (kpc) scales, which could be a result of an exponential
decrease in mixing speed with spatial scale, and the aforementioned
recent merger. Finally, the MMT spectroscopy is complemented by a dust
continuum and CO survey of individual giant molecular clouds,
conducted with the Submillimeter Array (SMA). By combining the MMT and
SMA observations, we obtain a unique direct test of the oxygen
abundance dependence of the α'(12CO) factor which is crucial
to convert CO emission to dust mass. Our results suggest that within
our sample there is no trend of the α'(12CO) with oxygen
abundance.
Description:
Our targets were primarily selected from a catalogue of candidate
HII regions (Azimlu+2011, J/AJ/142/139), many of which are
associated with 1 of the 326 known Giant Molecular Associations (GMAs)
in M31 from Kirk+2015 (J/ApJ/798/58), from which the previously
mentioned Submillimeter Array (SMA) targets were selected. Some
additional sources from a Very Large Array (VLA) survey (Toomey+ in
preparation) were also observed.
Observations were performed using the Hectospec spectrometer at the
MMT telescope located in Arizona, USA, using the 270gpm grating
covering the wavelength range 3700-9150Å with a 5Å resolution.
Within this wavelength we have access to multiple strong emission
lines; Hα, Hβ, [OII]λ3727,
[OIII]λλ(4959,5007),
[NII]λλ(6548,6584), and
[SII]λλ(6717,6731). Observations were taken during two
separate observing runs; the first in October 2020 and the second in
November and December of 2021.
We have high-quality spectra for 294 HII regions in M31, and 300
other sources including 44 planetary nebulae. The majority of our
targets were observed multiple times, thus allowing us to confirm
measurements from multiple spectra.
Objects:
-----------------------------------------------------------
RA (2000) DE Designation(s)
-----------------------------------------------------------
00 42 44.33 +41 16 07.5 M31 = NAME Andromeda Galaxy
-----------------------------------------------------------
File Summary:
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FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
table1.dat 111 294 Emission line flux ratios (relative to Hβ=100)
for M31 HII regions
table2.dat 88 294 Chemical abundances for M31 HII regions
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See also:
V/143 : Revised Bologna Catalog of M31 clusters, V.5 (Galleti+ 2012)
V/156 : LAMOST DR7 catalogs (Luo+, 2019)
J/AJ/131/2478 : M31 and M33 UBVRI photometry (Massey+, 2006)
J/AJ/142/139 : A new catalog of HII regions in M31 (Azimlu+, 2011)
J/ApJ/758/133 : Metallicity profile of M31 HII regions and PNe (Sanders+, 2012)
J/ApJ/798/58 : HELGA VI. Giant mol. cloud associations in M31 (Kirk+, 2015)
J/AJ/152/62 : Revised LGGS UBVRI phot. of M31 and M33 stars (Massey+, 2016)
J/MNRAS/455/2627 : Oxygen abundance gradient in M81 (Arellano-Cordova+, 2016)
J/MNRAS/465/4180 : Hα stars in M31 HST photometry (Prichard+, 2017)
J/ApJ/868/55 : Large-scale struct. of M31 halo. II. PAndAS (McConnachie+, 2018)
J/ApJ/852/2 : CO emission from 78 WISE HII region in the OSC (Wenger+, 2018)
J/ApJ/887/114 : VLA ∼8-10GHz obs. of WISE Galactic HII regions (Wenger+, 2019)
J/ApJ/887/80 : Gas phase oxygen abundances for HII regions (Kreckel+, 2019)
J/ApJS/254/36 : Southern HII Region Discov. Surv. II. Full cat (Wenger+, 2021)
J/ApJ/925/76 : M33 with LAMOST surv. sp. I. PNe & HII regions (Alexeeva+, 2022)
J/A+A/658/A51 : New M31 star cluster candidates (Wang+, 2022)
J/ApJ/954/206 : A GCs search in M31 with Gaia, PS1, LAMOST,PAndAS (Wang+, 2023)
J/ApJ/943/137 : Deep NIR Subaru phot. in Sh 2-209 HII region (Yasui+, 2023)
J/A+A/691/A173 : HII in galaxies abundances (Brazzini+, 2024)
J/A+A/694/A61 : Sulphur abundance distribution of HII galaxies (Gavilan+, 2025)
J/AJ/169/174 : M31 nebulae, clusters, & supergiants with LAMOST (Chen+, 2025)
Byte-by-byte Description of file:table1.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 3 I3 --- IDt1 Sequence number identifier from Table 1 (1)
5- 6 I2 h RAh Hour of right ascension (J2000)
8- 9 I2 min RAm Minute of right ascension (J2000)
11- 15 F5.2 s RAs Second of right ascension (J2000)
17 A1 --- DE- Sign of declination (J2000)
18- 19 I2 deg DEd Degree of declination (J2000)
21- 22 I2 arcmin DEm Arcminute of declination (J2000)
24- 28 F5.2 arcsec DEs Arcsecond of declination (J2000)
30- 35 F6.1 --- OII-3727 [84.5/2123.3] The [O II] 3727A line flux
ratio (2)
37- 41 F5.1 --- e_OII-3727 [0.3/216.2] Uncertainty in [O II] 3727A (3)
43- 47 F5.1 --- OIII-4959 [3.3/148.9] The [O III] 4959A line flux
ratio (2)
49- 52 F4.1 --- e_OIII-4959 [0.1/18.3] Uncertainty in [O III] 4959A (3)
54- 58 F5.1 --- OIII-5007 [9.4/452.1] The [O III] 5007A line flux
ratio (2)
60- 63 F4.1 --- e_OIII-5007 [0.1/34.9] Uncertainty in [O III] 5007A (3)
65- 68 F4.1 --- NII-6548 [5.9/85.3] The [N II] 6548A line flux ratio
(2)
70- 72 F3.1 --- e_NII-6548 [0.1/8.1] Uncertainty in [N II] 6548A (3)
74- 78 F5.1 --- Ha [264.7/301.2] The Hα line flux ratio
(2)
80- 83 F4.1 --- e_Ha [0.1/15.2] Uncertainty in Hα (3)
85- 89 F5.1 --- NII-6584 [18.1/255.6] The [N II] 6584A line flux
ratio (2)
91- 93 F3.1 --- e_NII-6584 [0.1/9.9] Uncertainty in [N II] 6584A (3)
95- 98 F4.1 --- SII-6717 [2.4/67.4] The [S II] 6717A line flux ratio
(2)
100- 102 F3.1 --- e_SII-6717 [0.1/6.2] Uncertainty in [S II] 6717A (3)
104- 107 F4.1 --- SII-6731 [4.2/52.1] The [S II] 6731A line flux ratio
(2)
109- 111 F3.1 --- e_SII-6731 [0/4.8] Uncertainty in [S II] 6731A (3)
--------------------------------------------------------------------------------
Note (1): The IDs, in the form of sequential numbers, do not match between the
tables in the article.
Note (2): Line fluxes are reported relative to Hβ=100 as described in
Section 3.1. Flux calibration and extinction correction have been
applied as described in Sections 2.3. and 3.2. respectively.
Note (3): Uncertainty is from the standard deviation of values from repeat
observations of the same source. We introduce a lower bound of 0.1
to uncertainties. For cases where we have only one observation for a
source, we calculate the mean percentage uncertainty of all sources
for which we have multiple observations, and use this.
--------------------------------------------------------------------------------
Byte-by-byte Description of file:table2.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 3 I3 --- IDt1 Sequence number identifier from Table 1
(column added by CDS) (1)
5- 7 I3 --- IDt2 Sequence number identifier from Table 2
9- 10 I2 h RAh Hour of right ascension (J2000)
12- 13 I2 min RAm Minute of right ascension (J2000)
15- 19 F5.2 s RAs Second of right ascension (J2000)
21 A1 --- DE- Sign of declination (J2000)
22- 23 I2 deg DEd Degree of declination (J2000)
25- 26 I2 arcmin DEm Arcminute of declination (J2000)
28- 32 F5.2 arcsec DEs Arcsecond of declination (J2000)
34- 38 F5.2 kpc Rad [4.07/20.71] Galactocentric Radius (2)
40- 43 F4.2 mag Av [0.01/4.7] The V band extinction (3)
45- 48 F4.2 mag e_Av [0.01/0.89] Uncertainty in Av (4)
50- 53 F4.2 --- (O/H)Rcal [8.21/8.84] Pilyugin+2016 R-calibration
calculation of log(O/H)+12 (5)
55- 58 F4.2 --- e_(O/H)Rcal [0.01/0.07] Uncertainty in (O/H)Rcal (4)
60- 63 F4.2 --- (O/H)Scal [8.37/8.83] Pilyugin+2016 S-calibration
calculation of log(O/H)+12 (5)
65- 68 F4.2 --- e_(O/H)Scal [0.01/0.04] Uncertainty in (O/H)Scal (4)
70- 73 F4.2 --- (O/H)Z94 [6.79/9.18] Zaritsky+1994 calculation of
log(O/H)+12 (5)
75- 78 F4.2 --- e_(O/H)Z94 [0.01/0.29] Uncertainty in (O/H)Z94 (4)
80- 83 F4.2 --- (N/H)Rcal [6.64/8.61] Pilyugin+2016 R-calibration
calculation of log(N/H)+12 (5)
85- 88 F4.2 --- e_(N/H)Rcal [0.01/0.21] Uncertainty in (N/H)Rcal (4)
--------------------------------------------------------------------------------
Note (1): The IDs, in the form of sequential numbers, do not match between the
tables in the article.
Note (2): Calculated following Haud (1981Ap&SS..76..477H 1981Ap&SS..76..477H) as described in
Section 4.
Note (3): Calculated by applying the extinction curve from
Cardelli+ (1989ApJ...345..245C 1989ApJ...345..245C) as described in Section 3.2.
Note (4): Uncertainty is from the standard deviation of values from repeat
observations of the same source. We introduce a lower bound of 0.01
to uncertainties. For cases where we have only one observation for a
source, we calculate the mean percentage uncertainty of all sources
for which we have multiple observations, and use this.
Note (5): From Pilyugin+ (2016MNRAS.457.3678P 2016MNRAS.457.3678P) and
Zaritsky+ (1994ApJ...420...87Z 1994ApJ...420...87Z). The abundance calibrations applied
are described in Section 3.4.
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History:
From electronic version of the journal
(End) Prepared by Robin Leichtnam [CDS] 08-Jan-2026