J/A+A/509/A61 Long slit spectroscopy in M31 (Saglia+, 2010)
The old and heavy bulge of M31. I. Kinematics and stellar populations.
Saglia R.P., Fabricius M., Bender R., Montalto M., Lee C.-H., Riffeser A.,
Seitz S., Morganti L., Gerhard O., Hopp U.
<Astron. Astrophys. 509, A61 (2010)>
=2010A&A...509A..61S 2010A&A...509A..61S
ADC_Keywords: Galaxies, nearby ; Radial velocities ; Equivalent widths
Keywords: galaxies: kinematics and dynamics - galaxies: individual: M31 -
galaxies: stellar content
Abstract:
We present new optical long-slit data along six position angles of the
bulge region of M31. We derive accurate stellar and gas kinematics
reaching 5-arcmin from the center, where the disk light contribution
is always less than 30%, and out to 8-arcmin along the major axis,
where the disk provides 55% of the total light. We show that the
velocity dispersions of McElroy (1983ApJ...270..485M 1983ApJ...270..485M) are severely
underestimated (by up to 50km/s). As a consequence, previous
dynamical models have underestimated the stellar mass of M31's bulge
by a factor of 2. As a further consequence, the light-weighted
velocity dispersion of the galaxy grows to 166km/s and to 170km/s if
rotation is also taken into account, thus reducing the discrepancy
between the predicted and measured mass of the black hole at the
center of M31 from a factor of 3 to a factor of 2. The kinematic
position angle varies with distance, pointing to triaxiality, but a
quantitative conclusion can be reached only after simultaneous proper
dynamical modeling of the bulge and disk components is performed. We
detect gas counter-rotation near the bulge minor axis. We measure eight
emission-corrected Lick indices. They are approximately constant on
circles. Using simple stellar population models we derive the age,
metallicity and α-element overabundance profiles. Except for the
region in the inner arcsecs of the galaxy, the bulge of M31 is
uniformly old (≥12Gyr, with many best-fit ages at the model grid
limit of 15Gyr), slightly α-elements overabundant
([α/Fe]∼0.2) and of solar metallicity, in agreement with studies
of the resolved stellar components. The predicted u-g, g-r and r-i
Sloan color profiles match the dust-corrected observations reasonably
well, within the known limitations of current simple stellar
population models. The stellar populations have approximately radially
constant mass-to-light ratios (M/LR∼4-4.5M☉/L☉_ for a
Kroupa IMF), which is in agreement with the stellar dynamical
estimates based on our new velocity dispersions. In the inner arcsecs
the luminosity-weighted age drops to 4-8Gyr, while the metallicity
increases to above three times the solar value. Starting from 6-arcmin
from the center along the major axis, the mean age drops to ≥8Gyr
with slight supersolar metallicity (~+0.1dex) and α-element
overabundance (~+0.2dex) for a mass-to-light ratio
M/LR<=3M☉/L☉_. Diagnostic diagrams based on the
[OIII]/Hβ and [NI]Hβ emission line equivalent widths (EWs)
ratios indicate that the gas is ionized by shocks outside 10-arcsec,
but an AGN-like ionizing source could be present near the center. We
speculate that a gas-rich minor merger happened some 100Myr ago,
causing the observed minor axis gas counter-rotation, the recent star
formation event and possibly some nuclear activity.
Description:
The stellar and gas kinematics (Table 2), the equivalent widths of the
Hβ gas emission line, plus the EW ratios [OIII]/Hβ and [NI]/NI
(Table 3), and the Lick absorption line strength indices (Table 4) of
the bulge of M31 presented here have been measured from spectra taken
with the Hobby-Eberly Telescope and the Low Resolution Spectrograph.
We observed at six position angles, with a slit width of 1.5-arcsec, a
spectral resolution of sigma=57km/s and covering the spectral range
4787-5857Å, with seeing values between 1.4 and 2.5-arcsec.
Objects:
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RA (2000) DE Designation(s)
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00 42 44.3 +41 16 09 M31 = Andromeda galaxy
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File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
table1.dat 55 24 Log of the observations
table2.dat 53 478 Stellar kinematics along 6 position angles
table3.dat 70 478 Gas kinematics and emission equivalent width
and ratios along 6 position angles
table4.dat 128 478 Lick indices along 6 position angles
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Byte-by-byte Description of file: table1.dat
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Bytes Format Units Label Explanations
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1- 10 A10 "YYYY/MM/DD" Date Observation date
12- 18 A7 --- PAc Position angle code
20- 22 I3 deg PA Corresponding position angle
24- 27 F4.2 arcsec Seeing ? FWHM of seeing
29- 55 A27 --- Com Comments
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Byte-by-byte Description of file: table2.dat
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Bytes Format Units Label Explanations
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1- 6 F6.1 arcsec R Distance from the center, positive East (G1)
8- 10 A3 --- PAc Position angle code (see Table 1) (G2)
12- 17 F6.1 km/s RV Stellar radial velocity, positive receiding (G3)
19- 21 F3.1 km/s e_RV Error on stellar radial velocity
23- 27 F5.1 km/s sigma Stellar velocity dispersion
29- 31 F3.1 km/s e_sigma Error on stellar velocity dispersion
33- 37 F5.2 --- h3 3rd order Hermite coefficient
39- 42 F4.2 --- e_h3 Error on 3rd order Hermite coefficient
44- 48 F5.2 --- h4 4th order Hermite coefficient
50- 53 F4.2 --- e_h4 Error on 4th order Hermite coefficient
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Byte-by-byte Description of file: table3.dat
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Bytes Format Units Label Explanations
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1- 6 F6.1 arcsec R Distance from the center, positive East (G1)
8- 10 A3 --- PAc Position angle (see Table 1) (G2)
12- 17 F6.1 km/s Vgas Gas radial velocity, positive receiding (G3)
19- 23 F5.1 km/s e_Vgas ?=999.9 Error on gas radial velocity (1)
25- 29 F5.1 km/s sgas Gas velocity dispersion
31- 35 F5.1 km/s e_sgas ?=999.9 Error on gas velocity dispersion (1)
38- 41 F4.2 0.1nm Hbe Equivalent width (EW) of Hβ gas emission
43- 46 F4.2 0.1nm e_Hbe Error on EW of the Hβ gas emission
48- 52 F5.1 --- O/H ?=999.9 Ratio [OIII]/Hβ of the gas
emission equivalent widths (2)
54- 58 F5.1 --- e_O/H ?=999.9 Error on O/H (1)
60- 64 F5.1 --- N/H ?=999.9 Ratio [NI]/Hβ of the gas
emission equivalent widths (2)
66- 70 F5.1 --- e_N/H ?=999.9 Error on N/H (1)
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Note (1): data with errors 999.9 are not reliable
Note (2): ratios larger than 999.9 are given the value 999.9
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Byte-by-byte Description of file: table4.dat
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Bytes Format Units Label Explanations
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1- 6 F6.1 arcsec R Distance from the center, positive East (G1)
8- 10 A3 --- PAc Position angle (see Table 1) (G2)
12- 15 F4.2 0.1nm Hbeta Hβ line strength
18- 21 F4.2 0.1nm e_Hbeta Error on the Hβ line strength
26- 29 F4.2 0.1nm Mgb Mgb line strength
32- 35 F4.2 0.1nm e_Mgb Error on the Mgb line strength
38- 41 F4.2 0.1nm Fe5015 Fe5015 line strength
45- 48 F4.2 0.1nm e_Fe5015 Error on the Fe5015 line strength
54- 57 F4.2 0.1nm Fe5270 Fe5270 line strength
61- 64 F4.2 0.1nm e_Fe5270 Error on the Fe5270 line strength
70- 73 F4.2 0.1nm Fe5335 Fe5335 line strength
77- 80 F4.2 0.1nm e_Fe5335 Error on the Fe5335 line strength
86- 89 F4.2 0.1nm Fe5406 Fe5406 line strength
93- 96 F4.2 0.1nm e_Fe5406 Error on the Fe5406 line strength
102-105 F4.2 0.1nm Fe5709 Fe5709 line strength
109-112 F4.2 0.1nm e_Fe5709 Error on the Fe5709 line strength
118-121 F4.2 0.1nm Fe5782 Fe5782 line strength
125-128 F4.2 0.1nm e_Fe5782 Error on the Fe5782 line strength
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Global notes:
Note (G1): the MN slit data (PA=138) with |R|>100arcsec are not aligned
with the center (see paper), but R is given as if no shift was present.
Note (G2): Position angles codes are given in table1;
these orientations are also illustrated in Fig.1 of the paper
Note (G3): relative to the center and with symmetrization shifts applied
(see paper).
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Acknowledgements:
Roberto Saglia, saglia(at)mpe.mpg.de
History:
* 12-Nov-2009: Original version
* 28-Mar-2010: 'R' parameter could vary of ±0.1arcsec in table3 and
table4 compared to table2; R values were homogenized.
(End) Roberto Saglia [MPE, Germany], Patricia Vannier [CDS] 12-Nov-2009