Abstract We present results from an optical search for Local Group dwarf galaxy candidates associated with the Ultra-Compact High Velocity Clouds (UCHVCs) discovered by the ALFALFA neutral hydrogen ...survey. The ALFALFA UCHVCs are isolated, compact H i clouds with projected sizes, velocities, and estimated H i masses that suggest they may be nearby dwarf galaxies, but that have no clear counterpart in existing optical survey data. We observed 26 UCHVCs with the WIYN 3.5 m telescope and One Degree Imager (ODI) in two broadband filters and searched the images for resolved stars with properties that match those of stars in typical dwarf galaxies at distances <2.5 Mpc. We identify one promising dwarf galaxy candidate at a distance of ∼570 kpc associated with the UCHVC AGC 268071, and five other candidates that may deserve additional follow-up. We carry out a detailed analysis of ODI imaging of a UCHVC that is close in both projected distance and radial velocity to the outer-halo Milky Way globular cluster Pal 3. We also use our improved detection methods to reanalyze images of five UCHVCs that were found to have possible optical counterparts during the first phase of the project, and confirm the detection of a possible stellar counterpart to the UCHVC AGC 249525 at an estimated distance of ∼2 Mpc. We compare the optical and H i properties of the dwarf galaxy candidates to the results from recent theoretical simulations that model satellite galaxy populations in group environments, as well as to the observed properties of galaxies in and around the Local Group.
SEGUE-2: Old Milky Way Stars Near and Far Rockosi, Constance M.; Sun Lee, Young; Morrison, Heather L. ...
The Astrophysical journal. Supplement series,
04/2022, Letnik:
259, Številka:
2
Journal Article
Recenzirano
Odprti dostop
Abstract
The Sloan Extension for Galactic Understanding and Exploration 2 (SEGUE-2) obtained 128,288 low-resolution spectra (
R
∼ 1800) of 118,958 unique stars in the first year of the Sloan Digital ...Sky Survey III (2008–2009). SEGUE-2 targeted prioritized distant halo tracers (blue horizontal-branch stars, K giants, and M giants) and metal-poor or kinematically hot populations. The main goal of SEGUE-2 was to target stars in the distant halo and measure their kinematics and chemical abundances to learn about the formation and evolution of the Milky Way. We present the SEGUE-2 field placement and target selection strategies. We discuss the success rate of the targeting based on the SEGUE-2 spectra and other spectroscopic and astrometric surveys. We describe the final SEGUE-2/SDSS-III improvements to the stellar parameter determinations based on the SEGUE Stellar Parameter Pipeline. We report a (
g
−
i
) color−effective temperature relation calibrated to the IRFM. We evaluate the accuracy and uncertainties associated with these stellar parameters by comparing with fundamental parameters, a sample of high-resolution spectra of SEGUE stars analyzed homogeneously, stars in well-studied clusters, and stars observed in common by the APOGEE survey. The final SEGUE spectra, calibration data, and derived parameters described here were released in SDSS-III Data Release 9 and continue to be included in all subsequent SDSS Data Releases. Because of its faint limiting magnitude and emphasis on the distant halo, the public SEGUE-2 data remain an important resource for the spectroscopy of stars in the Milky Way.
ABSTRACT We report the discovery and follow-up observations of a system of three objects identified by the ALFALFA extragalactic HI survey, cataloged as (almost) dark extragalactic sources, i.e., ...extragalactic HI detections with no discernible counterpart in publicly available, wide-field, imaging surveys. We have obtained deep optical imaging with WIYN pODI and HI synthesis maps with WSRT of the HI1232+20 system. The source with the highest HI flux has a newly discovered ultra-low surface brightness (LSB) optical counterpart associated with it, while the other two sources have no detected optical counterparts in our images. Our optical observations show that the detected LSB optical counterpart has a peak surface brightness of ∼26.4 mag arcsec−2 in g′, which is exceptionally faint. This source (AGC 229385) has the largest accurately measured HI mass-to-light ratio of an isolated object: = 46 , and has an HI mass of 7.2 × . The other two HI sources (with HI masses 2.0 × 108 and 1.2 × ) without optical counterparts have upper limit surface brightnesses of 27.9 and 27.8 mag arcsec−2 in g′, and lower limits on their gas mass-to-light ratio of > 57 and > 31 . This system lies relatively close in projection to the Virgo Cluster, but velocity flow models indicate that it is located at 25 Mpc, substantially beyond Virgo. The system appears to be quite isolated, with no known object closer than ∼500 kpc. These HI sources may represent both sides of the threshold between "dark" star-less galaxies and galaxies with stellar populations. We discuss a variety of possible formation scenarios for the HI1232+20 system.
Abstract
We present results from deep H
i
and optical imaging of AGC 229101, an unusual H
i
source detected at
v
helio
=7116 km s
−1
in the Arecibo Legacy Fast ALFA (ALFALFA) blind H
i
survey. ...Initially classified as a candidate “dark” source because it lacks a clear optical counterpart in Sloan Digital Sky Survey (SDSS) or Digitized Sky Survey 2 (DSS2) imaging, AGC 229101 has 10
9.31±0.05
M
⊙
of H
i
, but an H
i
line width of only 43 ± 9 km s
−1
. Low-resolution Westerbork Synthesis Radio Telescope (WSRT) imaging and higher-resolution Very Large Array (VLA) B-array imaging show that the source is significantly elongated, stretching over a projected length of ∼80 kpc. The H
i
imaging resolves the source into two parts of roughly equal mass. WIYN partially populated One Degree Imager (pODI) optical imaging reveals a faint, blue optical counterpart coincident with the northern portion of the H
i
. The peak surface brightness of the optical source is only
μ
g
∼ 26.6 mag arcsec
−2
, well below the typical cutoff that defines the isophotal edge of a galaxy, and its estimated stellar mass is only 10
7.32±0.33
M
⊙
, yielding an overall neutral gas-to-stellar mass ratio of
M
/
M
*
=
98
−
52
+
111
. We demonstrate the extreme nature of this object by comparing its properties with those of other H
i
-rich sources in ALFALFA and the literature. We also explore potential scenarios that might explain the existence of AGC 229101, including a tidal encounter with neighboring objects and a merger of two dark H
i
clouds.
We present a sample of 115 very low optical surface brightness, highly extended, H i-rich galaxies carefully selected from the ALFALFA survey that have similar optical absolute magnitudes, surface ...brightnesses, and radii to recently discovered "ultra-diffuse" galaxies (UDGs). However, these systems are bluer and have more irregular morphologies than other UDGs, are isolated, and contain significant reservoirs of H i. We find that while these sources have normal star formation rates for H i-selected galaxies of similar stellar mass, they have very low star formation efficiencies. We further present deep optical and H i-synthesis follow-up imaging of three of these H i-bearing ultra-diffuse sources. We measure H i diameters extending to ∼40 kpc, but note that while all three sources have large H i diameters for their stellar mass, they are consistent with the H i mass-H i radius relation. We further analyze the H i velocity widths and rotation velocities for the unresolved and resolved sources, respectively, and find that the sources appear to inhabit halos of dwarf galaxies. We estimate spin parameters, and suggest that these sources may exist in high spin parameter halos, and as such may be potential H i-rich progenitors to the ultra-diffuse galaxies observed in cluster environments.
ABSTRACT We characterize the radial density, metallicity, and flattening profile of the Milky Way's stellar halo, based on the large sample of spectroscopically confirmed giant stars from ...SDSS/SEGUE-2, spanning galactocentric radii 10 kpc ≤ rGC ≤ 80 kpc. After excising stars that were algorithmically attributed to apparent halo substructure (including the Sagittarius stream), the sample has 1757 K giants, with a typical metallicity precision of 0.2 dex and a mean distance accuracy of 16%. Compared to blue horizontal branch stars or RR Lyrae variables, giants are more readily understood tracers of the overall halo star population, with less bias in age or metallicity. The well-characterized selection function of the sample enables forward modeling of those data, based on ellipsoidal stellar density models, *(R, z), with Einasto profiles and (broken) power laws for their radial dependence, combined with a model for the metallicity gradient and the flattening profile. Among models with constant flattening, these data are reasonably well fit by an Einasto profile of n = 3.1 0.5 with an effective radius and a flattening of q = 0.7 0.02, or comparably well by an equally flattened broken power law, with radial slopes of in = 2.1 0.3 and out = 3.8 0.1, with a break radius of rbreak = 18 1 kpc; this is largely consistent with earlier work. We find a modest but significant metallicity gradient within the "outer" stellar halo, Fe/H decreasing outward. If we allow for a variable flattening , we find the distribution of halo giants to be considerably more flattened at small radii, q(10 kpc) = 0.55 0.02, compared to q(>30 kpc) = 0.8 0.03. Remarkably, the data are then very well fit by a single power law with index of 4.2 0.1 on the variable . In this simple and better-fitting model, there is a break in flattening at ∼20 kpc, instead of a break in the radial density function. While different parameterizations of the radial profile vary in their parameters, their implied density gradient, , is stable along a direction intermediate between major and minor axis; this gradient is crucial in any dynamical modeling that uses halo stars as tracers.
We present results from the analysis of WIYN pODI imaging of 23 ultracompact high-velocity clouds (UCHVCs), which were identified in the ALFALFA H i survey as possible dwarf galaxies in or near the ...Local Group. To search for a resolved stellar population associated with the H i gas in these objects, we carried out a series of steps designed to identify stellar overdensities in our optical images. We identify five objects that are likely stellar counterparts to the UCHVCs, at distances of ∼350 kpc to ∼1.6 Mpc. Two of the counterparts were already described in Janesh et al.; the estimated distance and detection significance for one of them changed in the final analysis of the full pODI data set. At their estimated distances, the detected objects have H i masses from 2 × 104 to 3 × 106 M , MV from −1.4 to −7.1, and stellar masses from 4 × 102 to 4 × 105 M . None of the objects shows evidence of a young stellar population. Their properties would make the UCHVCs some of the most extreme objects in and around the Local Group, comparable to ultrafaint dwarf galaxies in their stellar populations, but with significant gas content. Such objects probe the extreme end of the galaxy mass function and provide a test bed for theories regarding the baryonic feedback processes that impact star formation and galaxy evolution in this low-mass regime.
ABSTRACT We statistically quantify the amount of substructure in the Milky Way stellar halo using a sample of 4568 halo K giant stars at Galactocentric distances ranging over 5-125 kpc. These stars ...have been selected photometrically and confirmed spectroscopically as K giants from the Sloan Digital Sky Survey's Sloan Extension for Galactic Understanding and Exploration project. Using a position-velocity clustering estimator (the 4distance) and a model of a smooth stellar halo, we quantify the amount of substructure in the halo, divided by distance and metallicity. Overall, we find that the halo as a whole is highly structured. We also confirm earlier work using blue horizontal branch (BHB) stars which showed that there is an increasing amount of substructure with increasing Galactocentric radius, and additionally find that the amount of substructure in the halo increases with increasing metallicity. Comparing to resampled BHB stars, we find that K giants and BHBs have similar amounts of substructure over equivalent ranges of Galactocentric radius. Using a friends-of-friends algorithm to identify members of individual groups, we find that a large fraction (∼33%) of grouped stars are associated with Sgr, and identify stars belonging to other halo star streams: the Orphan Stream, the Cetus Polar Stream, and others, including previously unknown substructures. A large fraction of sample K giants (more than 50%) are not grouped into any substructure. We find also that the Sgr stream strongly dominates groups in the outer halo for all except the most metal-poor stars, and suggest that this is the source of the increase of substructure with Galactocentric radius and metallicity.
We present new high-resolution H i spectral line imaging of Coma P, the brightest H i source in the system HI 1232+20. This galaxy with extremely low surface brightness was first identified in the ...ALFALFA survey as an "(Almost) Dark" object: a clearly extragalactic H i source with no obvious optical counterpart in existing optical survey data (although faint ultraviolet emission was detected in archival GALEX imaging). Using a combination of data from the Westerbork Synthesis Radio Telescope and the Karl G. Jansky Very Large Array, we investigate the H i morphology and kinematics at a variety of physical scales. The H i morphology is irregular, reaching only moderate maxima in mass surface density (peak pc−2). Gas of lower surface brightness extends to large radial distances, with the H i diameter measured at 4.0 0.2 kpc inside the pc−2 level. We quantify the relationships between mass surface density of H i gas and star formation on timescales of ∼100-200 Myr as traced by GALEX far-ultraviolet emission. While Coma P has regions of dense H i gas reaching the cm−2 level typically associated with ongoing star formation, it lacks massive star formation as traced by H emission. The H i kinematics are extremely complex: a simple model of a rotating disk cannot describe the H i gas in Coma P. Using spatially resolved position-velocity analysis we identify two nearly perpendicular axes of projected rotation that we interpret as either the collision of two H i disks or a significant infall event. Similarly, three-dimensional modeling of the H i dynamics provides a best fit with two H i components. Coma P is just consistent (within 3 ) with the known scaling relation. It is either too large for its H i mass, has too low an H i mass for its H i size, or the two H i components artificially extend its H i size. Coma P lies within the empirical scatter at the faint end of the baryonic Tully-Fisher relation, although the complexity of the H i dynamics complicates the interpretation. Along with its large ratio of H i to stellar mass, the collective H i characteristics of Coma P make it unusual among known galaxies in the nearby universe.
We report on the detection at >98% confidence of an optical counterpart to AGC 249525, an ultra-compact high-velocity cloud (UCHVC) discovered by the Arecibo Legacy Fast ALFA survey blind neutral ...hydrogen survey. UCHVCs are compact, isolated H i clouds with properties consistent with their being nearby low-mass galaxies, but without identified counterparts in extant optical surveys. Analysis of the resolved stellar sources in deep g- and i-band imaging from the WIYN pODI camera reveals a clustering of possible red giant branch stars associated with AGC 249525 at a distance of 1.64 0.45 Mpc. Matching our optical detection with the H i synthesis map of AGC 249525 from Adams et al. shows that the stellar overdensity is exactly coincident with the highest-density H i contour from that study. Combining our optical photometry and the H i properties of this object yields an absolute magnitude of , a stellar mass between and , and an H i to stellar mass ratio between 9 and 144. This object has stellar properties within the observed range of gas-poor ultra-faint dwarfs in the Local Group, but is gas-dominated.
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