Abstract
It is not yet settled how the combination of secular processes and merging gives rise to the bulges and pseudobulges of galaxies. The nearby (
D
∼ 4.2 Mpc) disk galaxy M94 (NGC 4736) has the ...largest pseudobulge in the local universe, and offers a unique opportunity for investigating the role of merging in the formation of its pseudobulge. We present a first ever look at M94's stellar halo, which we expect to contain a fossil record of M94's past mergers. Using Subaru's Hyper Suprime-Cam, we resolve and identify red giant branch (RGB) stars in M94's halo, finding two distinct populations. After correcting for completeness through artificial star tests, we can measure the radial profile of each RGB population. The metal-rich RGB stars show an unbroken exponential profile to a radius of 30 kpc that is a clear continuation of M94's outer disk. M94's metal-poor stellar halo is detectable over a wider area and clearly separates from its metal-rich disk. By integrating the halo density profile, we infer a total accreted stellar mass of ∼2.8 × 10
8
M
⊙
, with a median metallicity of M/H = −1.4. This indicates that M94's most-massive past merger was with a galaxy similar to, or less massive than, the Small Magellanic Cloud. Few nearby galaxies have had such a low-mass dominant merger; therefore we suggest that M94's pseudobulge was not significantly impacted by merging.
Abstract
In the third paper from the COOL–LAMPS Collaboration, we report the discovery of COOL J0542-2125, a gravitationally lensed quasar at
z
= 1.84, observed as three images due to an intervening ...massive galaxy cluster at
z
= 0.61. The lensed quasar images were identified in a search for lens systems in recent public optical imaging data and have separations on the sky up to 25.″9, wider than any previously known lensed quasar. The galaxy cluster acting as a strong lens appears to be in the process of merging, with two subclusters separated by ∼1 Mpc in the plane of the sky, and their central galaxies showing a radial velocity difference of ∼1000 km s
−1
. Both cluster cores show strongly lensed images of an assortment of background sources, as does the region between them. A preliminary strong lens model implies masses of
M
<
250
k
p
c
=
1.79
−
0.01
+
0.16
×
10
14
M
⊙
and
M
<
250
k
p
c
=
1.48
−
0.10
+
0.04
×
10
14
M
⊙
for the east and west subclusters, respectively. This line of sight is also coincident with an ROSAT ALL-sky Survey source, centered between the two confirmed cluster halos reminiscent of other major cluster-scale mergers. Archival and new follow-up imaging show flux variability in the quasar images of up to 0.4 mag within ∼1 yr, and new multicolor imaging data reveal a 2
σ
detection of the underlying quasar host. A lens system with this configuration offers rare opportunities for a range of future studies of both the lensed quasar and its host and the foreground cluster merger causing the lensing.
Abstract We present a strong lensing analysis of COOL J1241+2219, the brightest known gravitationally lensed galaxy at z ≥ 5, based on new multiband Hubble Space Telescope (HST) imaging data. The ...lensed galaxy has a redshift of z = 5.043, placing it shortly after the end of the “Epoch of Reionization,” and an AB magnitude z AB = 20.47 mag (Khullar et al.). As such, it serves as a touchstone for future research of that epoch. The high spatial resolution of HST reveals internal structure in the giant arc, from which we identify 15 constraints and construct a robust lens model. We use the lens model to extract the cluster mass and lensing magnification. We find that the mass enclosed within the Einstein radius of the z = 1.001 cluster lens is M ( < 5 .″ 77 ) = 1.079 − 0.007 + 0.023 × 10 13 M ☉ , significantly lower than other known strong lensing clusters at its redshift. The average magnification of the giant arc is 〈 μ arc 〉 = 76 − 20 + 40 , a factor of 2.4 − 0.7 + 1.4 greater than previously estimated from ground-based data; the flux-weighted average magnification is 〈 μ arc 〉 = 92 − 31 + 37 . We update the current measurements of the stellar mass and star formation rate (SFR) of the source for the revised magnification to log ( M ⋆ / M ⊙ ) = 9.7 ± 0.3 and SFR = 10.3 − 4.4 + 7.0 M ⊙ yr −1 , respectively. The powerful lensing magnification acting upon COOL J1241+2219 resolves the source and enables future studies of the properties of its star formation on a clump-by-clump basis. The lensing analysis presented here will support upcoming multiwavelength characterization with HST and JWST data of the stellar mass assembly and physical properties of this high-redshift lensed galaxy.
Abstract
We report the discovery of five bright, strong gravitationally lensed galaxies at 3 <
z
< 4: COOL J0101+2055 (
z
= 3.459), COOL J0104−0757 (
z
= 3.480), COOL J0145+1018 (
z
= 3.310), COOL ...J0516−2208 (
z
= 3.549), and COOL J1356+0339 (
z
= 3.753). These galaxies have magnitudes of
r
AB
,
z
AB
< 21.81 mag and are lensed by galaxy clusters at 0.26 <
z
< 1. This sample nearly doubles the number of known bright lensed galaxies with extended arcs at 3 <
z
< 4. We characterize the lensed galaxies using ground-based
grz
/
giy
imaging and optical spectroscopy. We report model-based magnitudes and derive stellar masses, dust content, and star formation rates via stellar population synthesis modeling. Building lens models based on ground-based imaging, we estimate source magnifications ranging from ∼29 to ∼180. Combining these analyses, we derive demagnified stellar masses in the range
log
10
(
M
*
/
M
⊙
)
∼
9.69
−
10.75
and star formation rates in the youngest age bin in the range
log
10
(
SFR
/
(
M
⊙
yr
−
1
)
)
∼
0.39
−
1.46
, placing the sample galaxies on the massive end of the star-forming main sequence in this redshift interval. In addition, three of the five galaxies have strong Ly
α
emissions, offering unique opportunities to study Ly
α
emitters at high redshift in future work.
Abstract
M64, often called the “Evil Eye” galaxy, is unique among local galaxies. Beyond its dramatic, dusty nucleus, it also hosts an outer gas disk that counter-rotates relative to its stars. The ...mass of this outer disk is comparable to the gas content of the Small Magellanic Cloud (SMC), prompting the idea that it was likely accreted in a recent minor merger. Yet, detailed follow-up studies of M64's outer disk have shown no evidence of such an event, leading to other interpretations, such as a “flyby” interaction with the distant diffuse satellite Coma P. We present Subaru Hyper Suprime-Cam observations of M64's stellar halo, which resolve its stellar populations and reveal a spectacular radial shell feature, oriented ∼30° relative to the major axis and along the rotation axis of the outer gas disk. The shell is ∼45 kpc southeast of M64, while a similar but more diffuse plume to the northwest extends to >100 kpc. We estimate a stellar mass and metallicity for the southern shell of
M
⋆
= 1.80 ± 0.54 × 10
8
M
⊙
and M/H = −1.0, respectively, and a similar mass of 1.42 ± 0.71 × 10
8
M
⊙
for the northern plume. Taking into account the accreted material in M64's inner disk, we estimate a total stellar mass for the progenitor satellite of
M
⋆,prog
≃ 5 × 10
8
M
⊙
. These results suggest that M64 is in the final stages of a minor merger with a gas-rich satellite strikingly similar to the SMC, in which M64's accreted counter-rotating gas originated, and which is responsible for the formation of its dusty inner star-forming disk.
Abstract
The faint and ultrafaint dwarf galaxies in the Local Group form the observational bedrock upon which our understanding of small-scale cosmology rests. In order to understand whether this ...insight generalizes, it is imperative to use resolved-star techniques to discover similarly faint satellites in nearby galaxy groups. We describe our search for ultrafaint galaxies in the M81 group using deep ground-based resolved-star data sets from Subaru’s Hyper Suprime-Cam. We present one new ultrafaint dwarf galaxy in the M81 group and identify five additional extremely low surface brightness candidate ultrafaint dwarfs that reach deep into the ultrafaint regime to
M
V
∼ − 6 (similar to current limits for Andromeda satellites). These candidates’ luminosities and sizes are similar to known Local Group dwarf galaxies Tucana B, Canes Venatici I, Hercules, and Boötes I. Most of these candidates are likely to be real, based on tests of our techniques on blank fields. Intriguingly, all of these candidates are spatially clustered around NGC 3077, which is itself an M81 group satellite in an advanced state of tidal disruption. This is somewhat surprising, as M81 itself and its largest satellite M82 are both substantially more massive than NGC 3077 and, by virtue of their greater masses, would have been expected to host as many or more ultrafaint candidates. These results lend considerable support to the idea that satellites of satellites are an important contribution to the growth of satellite populations around Milky Way–mass galaxies.
Abstract
Upcoming space-based integral field spectrographs will enable spatially resolved spectroscopy of distant galaxies, including at the scale of individual star-forming regions (i.e., down to ...just tens of parsecs) in galaxies that have been strongly gravitationally lensed. In the meantime, there is only a very small set of lensed galaxies where such spatial detail is possible at wavelengths containing important rest-optical emission lines, even with the Hubble Space Telescope’s Wide Field Camera 3 infrared channel grisms. Here, we examine two of these sources, SDSS J1723+3411 and SDSS J2340+2947, using HST WFC3/IR grism data and supporting spatially unresolved spectroscopy from several ground-based instruments to explore the size of spatial variations in observed strong emission-line ratios like O32 and R23, which are sensitive to ionization parameter and metallicity, and the Balmer decrement, which is an indicator of reddening. We find significant spatial variation in the reddening and in the reddening-corrected O32 and R23 values that correspond to spreads of a few tenths of a dex in ionization parameter and metallicity. We also find clear evidence of a negative radial gradient in star formation in SDSS J2340+2947 and tentative evidence of one in SDSS J1723+3411, though its star formation is quite asymmetric. Finally, we find that reddening can vary enough spatially to make spatially resolved reddening corrections necessary in order to characterize gradients in line ratios and the physical conditions inferred from them, necessitating the use of space-based integral field units for future work on larger, more statistically robust samples.
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