J/ApJ/853/87 MEGaSaURA. II. Stacked spectra (Rigby+, 2018)
The Magellan Evolution of Galaxies Spectroscopic and Ultraviolet Reference Atlas
(MEGaSaURA).
II. Stacked spectra.
Rigby J.R., Bayliss M.B., Chisholm J., Bordoloi R., Sharon K.,
Gladders M.D., Johnson T., Paterno-Mahler R., Wuyts E., Dahle H.,
Acharyya A.
<Astrophys. J., 853, 87 (2018)>
=2018ApJ...853...87R 2018ApJ...853...87R
ADC_Keywords: Galaxies; Gravitational lensing; Spectra, optical; Ultraviolet
Keywords: galaxies: evolution; galaxies: high-redshift; stars: winds, outflows;
gravitational lensing: strong; intergalactic medium
Abstract:
We stack the rest-frame ultraviolet spectra of N=14 highly magnified
gravitationally lensed galaxies at redshifts 1.6<z<3.6. The resulting
new composite spans 900<λrest<3000Å, with a peak
signal-to-noise ratio (S/N) of 103 per spectral resolution element
(∼100km/s). It is the highest S/N, highest spectral resolution
composite spectrum of z∼2-3 galaxies yet published. The composite
reveals numerous weak nebular emission lines and stellar photospheric
absorption lines that can serve as new physical diagnostics,
particularly at high redshift with the James Webb Space Telescope
(JWST). We report equivalent widths to aid in proposing for and
interpreting JWST spectra. We examine the velocity profiles of strong
absorption features in the composite, and in a matched composite of
z∼0 COS/HST galaxy spectra. We find remarkable similarity in the
velocity profiles at z∼0 and z∼2, suggesting that similar physical
processes control the outflows across cosmic time. While the maximum
outflow velocity depends strongly on ionization potential, the
absorption-weighted mean velocity does not. As such, the bulk of the
high- ionization absorption traces the low-ionization gas, with an
additional blueshifted absorption tail extending to at least
-2000km/s. We interpret this tail as arising from the stellar wind and
photospheres of massive stars. Starburst99 models are able to
replicate this high-velocity absorption tail. However, these
theoretical models poorly reproduce several of the photospheric
absorption features, indicating that improvements are needed to match
observational constraints on the massive stellar content of
star-forming galaxies at z∼2.
Description:
In this paper, we stack the spectra of N=14 gravitationally lensed
galaxies from Project MEGaSaURA: the Magellan Evolution of Galaxies
Spectroscopic and Ultraviolet Reference Atlas (Rigby+ 2018, J/AJ/155/104
hereafter Paper I). The resulting stacked spectra represent a new
high S/N and moderate spectral resolution, rest-frame ultraviolet
composite for star-forming galaxies at z∼2.
In brief, the MEGaSaURA spectra were obtained with the MagE instrument
on the Magellan telescopes. The spectra cover the rest-frame
ultraviolet (900≲λr≲3000Å). The spectral resolving
powers range from R=2500 to R=4700. The median-quality spectrum has
S/N=21 per resolution element at λobs=5000Å.
To enable an apples-to-apples comparison of the MagE MEGaSaURA stack
to the spectra of galaxies in the nearby universe, we stack the
spectra of z∼0 galaxies with COS/HST spectra using the G130M and
G160M gratings from Chisholm+ (2016MNRAS.457.3133C 2016MNRAS.457.3133C). See Section 2.5.
File Summary:
--------------------------------------------------------------------------------
FileName Lrecl Records Explanations
--------------------------------------------------------------------------------
ReadMe 80 . This file
megas99.dat 88 20872 *MEGaSaURA MagE/Magellan spectra of z=1.6-3.6
gravitationally-lensed galaxies; S99 models
megapivot.dat 88 20872 MEGaSaURA MagE/Magellan spectra of z=1.6-3.6
gravitationally-lensed galaxies;
lambda pivot normalized stack
megashape.dat 88 20875 MEGaSaURA MagE/Magellan spectra of z=1.6-3.6
gravitationally-lensed galaxies;
shape-normalized stack
cosr3300.dat 84 3090 R=3300 stacks of COS/HST spectra of z∼0 galaxies
cosmaxr.dat 84 13106 Native spectral resolution stacks of COS/HST
spectra of z∼0 galaxies
--------------------------------------------------------------------------------
Note on megas99.dat: S99 = Starburst99 (Leitherer+ 1999ApJS..123....3L 1999ApJS..123....3L,
2010ApJS..189..309L 2010ApJS..189..309L) models.
--------------------------------------------------------------------------------
See also:
B/hst : HST Archived Exposures Catalog (STScI, 2007)
J/A+AS/103/97 : Grids of stellar models V. (Meynet+ 1994)
J/ApJ/794/156 : MgII/FeII absorption profile for 0.3<z<1.4 gal. (Rubin+, 2014)
J/AJ/155/104 : MEGaSaURA. I. The sample and the spectra (Rigby+, 2018)
Byte-by-byte Description of file: cos*.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 13 F13.8 0.1nm lambda [1145.4/1753] Rest wavelength, vacuum
Angstroms (1)
15- 30 F16.14 --- Fweightavg [0.004/1.6] Weighted average of
normalized flux (2)
32- 46 F15.13 --- e_Fweightavg [0.01/0.2] Uncertainty on fweightavg
48- 64 F17.14 --- Fmed [-0.07/8.1] Median of normalized flux (2)
66- 67 I2 --- Ngal [2/40] Number of galaxies in the stack at
this wavelength
69- 84 F16.14 --- stdFjack [0.001/3.1] Standard deviation
from jackknife variance
--------------------------------------------------------------------------------
Note (1): The input spectra using the G130M and G160M gratings from Chisholm+
(2016MNRAS.457.3133C 2016MNRAS.457.3133C) are at original spectral resolution.
Note (2): Normalized by automatic continuum fitting, so both value and shape
are normalized.
--------------------------------------------------------------------------------
Byte-by-byte Description of file: mega*.dat
--------------------------------------------------------------------------------
Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 6 F6.1 0.1nm lambda [912.1/2999.5] Rest wavelength, vacuum
Angstroms (1)
8- 27 F20.14 --- Fweightavg [-24.3/247.1]?=0 Weighted average of
normalized flux (2)
29- 43 F15.13 --- e_Fweightavg [0/0.9] Uncertainty on fweightavg
45- 63 F19.15 --- Fmed [-0.4/110.2]?=0 Median of normalized flux (2)
65- 66 I2 --- Ngal [0/13] Number of galaxies in the stack at
this wavelength
68- 88 F21.15 --- stdFjack [0.0001/255]? Standard deviation
from jackknife variance
--------------------------------------------------------------------------------
Note (1): The input spectra include ['rcs0327-E', 'S0004-0103',
'S0108+0624', 'S0033+0242', 'S0900+2234', 'S0957+0509', 'Horseshoe',
'S1226+2152', 'S1429+1202', 'S1458-0023', 'S1527+0652', 'S2111-0114',
'Cosmic∼Eye'] -- see Paper I (Rigby+, 2018, J/AJ/155/104) for the complete
names.
Note (2): For megaS99 table, normalized by best-fit linear combination of
John Chisholms Starburst 99 fits to each spectrum,
so both value and shape are normalized.
For megapivot table, flux density normalized to median in spectral
region (1267.0, 1276.0) but spectral shape not flattened.
For megashape table, normalized by Janes hand-fit spline continuua,
so both value and shape are normalized.
--------------------------------------------------------------------------------
History:
From electronic version of the journal
References:
Rigby et al. Paper I. 2018AJ....155..104R 2018AJ....155..104R Cat. J/AJ/155/104
Rigby et al. Paper II. 2018ApJ...853...87R 2018ApJ...853...87R This catalog
(End) Prepared by [AAS], Emmanuelle Perret [CDS] 17-Oct-2018