Virgo Overdensity Research Articles

Common origin for Hercules-Aquila and Virgo Clouds inGaiaDR2

We use a sample of ~350 RR Lyrae stars with radial velocities and Gaia DR2 proper motions to study orbital properties of the Hercules-Aquila Cloud (HAC) and Virgo Over-density (VOD). We demonstrate that both structures are dominated by stars on highly eccentric orbits, with peri-centres around ~1 kpc and apo-centres between 15 and 25 kpc from the Galactic centre. Given that the stars in the HAC and the VOD occupy very similar regions in the space spanned by integrals of motion, we conclude that these diffuse debris clouds are part of the same accretion event. More precisely, these inner halo sub-structures likely represent two complementary not-fully-mixed portions of an ancient massive merger, also known as the "sausage" event.

Fitting the Density Substructure of the Stellar Halo with MilkyWay@home

Abstract We propose and test a method for applying statistical photometric parallax to main-sequence turnoff stars in the Sloan Digital Sky Survey (SDSS). Using simulated data, we show that if our density model is similar to the actual density distribution of our data, we can reliably determine the density model parameters of three major substructures in the Milky Way halo using the computational resources available on MilkyWay@home (a 20-parameter fit). We fit the stellar density in SDSS stripe 19 with a smooth stellar spheroid component and three major streams. One of these streams is consistent with the Sagittarius tidal stream at 21.1 kpc away, one is consistent with the trailing tail of the Sagittarius tidal stream in the north Galactic cap at 48 kpc away, and one is possibly part of the Virgo Overdensity at 6 kpc away. We find the 1σ widths of these three streams to be 1.0 kpc, 17.6 kpc, and 6.1 kpc, respectively. The width of the trailing tail is extremely wide (41 kpc full width at half maximum). This large width could have implications for the shape of the Milky Way dark matter halo. The width of the Virgo Overdensity-like structure is consistent with what we might expect for a “cloudlike” structure; analysis of additional stripes of data are needed to outline the full extent of this structure and confirm its association with the Virgo Overdensity.

The Next Generation Virgo Cluster Survey (NGVS). XXXI. The Kinematics of Intracluster Globular Clusters in the Core of the Virgo Cluster

Abstract Intracluster (IC) populations are expected to be a natural result of the hierarchical assembly of clusters, yet their low space densities make them difficult to detect and study. We present the first definitive kinematic detection of an IC population of globular clusters (GCs) in the Virgo cluster, around the central galaxy M87. This study focuses on the Virgo core, for which the combination of Next Generation Virgo Cluster Survey photometry and follow-up spectroscopy allows us to reject foreground star contamination and explore GC kinematics over the full Virgo dynamical range. The GC kinematics changes gradually with galactocentric distance, decreasing in mean velocity and increasing in velocity dispersion, eventually becoming indistinguishable from the kinematics of Virgo dwarf galaxies at R > 320 kpc. By kinematically tagging M87 halo and intracluster GCs, we find that (1) the M87 halo has a smaller fraction (52 ± 3%) of blue clusters with respect to the IC counterpart (77 ± 10%), (2) the (g′−r′)0 versus (i′−z′)0 color–color diagrams reveal a galaxy population that is redder than the IC population, which may be due to a different composition in chemical abundance and progenitor mass, and (3) the ICGC distribution is shallower and more extended than the M87 GCs, yet still centrally concentrated. The ICGC specific frequency, S N,ICL = 10.2 ± 4.8, is consistent with what is observed for the population of quenched, low-mass galaxies within 1 Mpc from the cluster’s center. The IC population at Virgo’s center is thus consistent with being an accreted component from low-mass galaxies tidally stripped or disrupted through interactions, with a total mass of .

They Might Be Giants: An Efficient Color-based Selection of Red Giant Stars

Abstract We present a color-based method for identifying red giants based on the Pan-STARRS grz and WISE W1 and W2 photometry. We utilize a subsample of bright stars with precise parallaxes from Gaia’s second data release (DR2) to verify that the color-based selection reliably separates dwarfs from giants. The selection is conservative in the sense that contamination is small (≈30%) but not all giants are included (the selection primarily identifies K giants). The color-based selection can be applied to stars brighter than W1 ≈ 16, more than two magnitudes fainter than techniques relying on shallower 2MASS photometry. Many streams and clouds are visible in the resulting sky maps, especially when binned by Gaia DR2 proper motions, including the Sagittarius stream, the Hercules–Aquila Cloud, the Eastern Banded Structure, Monoceros, and the Virgo Overdensity. In addition to the characterization of new and known stellar streams, we expect that this method for selecting red giants will enable detailed analysis of the diffuse stellar halo to distances exceeding 100 kpc.

The >100 kpc Distant Spur of the Sagittarius Stream and the Outer Virgo Overdensity, as Seen in PS1 RR Lyrae Stars

Abstract We report the detection of spatially distinct stellar density features near the apocenters of the Sagittarius (Sgr) stream’s main leading and trailing arm. These features are clearly visible in a high-fidelity stellar halo map that is based on RR Lyrae from Pan-STARRS1: there is a plume of stars 10 kpc beyond the apocenter of the leading arm, and there is a “spur” extending to 130 kpc, almost 30 kpc beyond the previously detected apocenter of the trailing arm. Such an apocenter substructure is qualitatively expected in any Sgr stream model, as stars stripped from the progenitor at different pericenter passages become spatially separated there. The morphology of these new Sgr stream substructures could provide much-needed new clues and constraints for modeling the Sgr system, including the level of dynamical friction that Sgr has experienced. We also report the discovery of a new, presumably unrelated halo substructure at 80 kpc from the Sun and 10° from the Sgr orbital plane, which we dub the outer Virgo overdensity.

Hubble Space Telescope Imaging of the Ultra-compact High Velocity Cloud AGC 226067: A Stripped Remnant in the Virgo Cluster

Abstract We analyze the optical counterpart to the ultra-compact high velocity cloud AGC 226067, utilizing imaging taken with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope. The color–magnitude diagram of the main body of AGC 226067 reveals an exclusively young stellar population, with an age of ∼7–50 Myr, and is consistent with a metallicity of [Fe/H] ∼ −0.3 as previous work has measured via H ii region spectroscopy. Additionally, the color–magnitude diagram is consistent with a distance of D ≈ 17 Mpc, suggesting an association with the Virgo cluster. A secondary stellar system located ∼1.′6 (∼8 kpc) away in projection has a similar stellar population. The lack of an old red giant branch (≳5 Gyr) is contrasted with a serendipitously discovered Virgo dwarf in the ACS field of view (Dw J122147+132853), and the total diffuse light from AGC 226067 is consistent with the luminosity function of the resolved ∼7–50 Myr stellar population. The main body of AGC 226067 has a M V = −11.3 ± 0.3, or M stars = 5.4 ± 1.3 × 104 M ⊙ given the stellar population. We searched 20 deg2 of imaging data adjacent to AGC 226067 in the Virgo Cluster, and found two similar stellar systems dominated by a blue stellar population, far from any massive galaxy counterpart—if this population has star-formation properties that are similar to those of AGC 226067, it implies ∼0.1 M ⊙ yr−1 in Virgo intracluster star formation. Given its unusual stellar population, AGC 226067 is likely a stripped remnant and is plausibly the result of compressed gas from the ram pressure stripped M86 subgroup (∼350 kpc away in projection) as it falls into the Virgo Cluster.

Another look at the size of the low‐surface‐brightness galaxy VCC 1661 in the Virgo Cluster

The morphologies of individual cluster galaxies are an important path to studying galaxy formation and evolution. Here, we present new wide‐field images of the low‐surface‐brightness Virgo Cluster dwarf galaxy VCC 1661. The extant literature lists a broad range of radii for this object, covering a factor of more than four, depending on the filters used and the details of the analyses. While some studies find a radius typical of other Virgo dwarfs and note the normality of this object, any larger spatial extent, taken at face value, would render this galaxy the largest dwarf in the Virgo Cluster samples. Confirmation of a large extent of dwarf galaxies has often led to the discovery of tidal tails and would then, also in VCC 1661, indicate a severe state of tidal disruption. Given the importance of galactic sizes for assessing tidal interactions of the satellites with their hosts, we thus combine our surface brightness profile with data from the literature to investigate further the nature of this galaxy. However, our new characteristic radius for VCC 1661 of re = 24.1″ ± 7.7″ and the previously noted smooth appearance of its isophotes are fully consistent with the remainder of the Advanced Camera for Surveys Virgo Cluster Survey dwarf galaxy population without any need to invoke tidal perturbations.

DISENTANGLING THE VIRGO OVERDENSITY WITH RR LYRAE STARS

ABSTRACT We use a combination of spatial distribution and radial velocity to search for halo substructures in a sample of 412 RR Lyrae stars (RRLSs) that covers a region of ∼525 square degrees of the Virgo overdensity (VOD) and spans distances from the Sun from 4 to 75 kpc. With a friends-of-friends algorithm we identified six high-significance groups of RRLSs in phase space, which we associate mainly with the VOD and with the Sagittarius stream. Four other groups were also flagged as less significant overdensities. Three high-significance and three lower-significance groups have distances between ∼10 and 20 kpc, which places them in the distance range attributed by others to the VOD. The largest of these is the Virgo stellar stream at 19 kpc, which has 18 RRLSs, a factor of two increase over the number known previously. While these VOD groups are distinct according to our selection criteria, their overlap in position and distance and, in a few cases, similarity in radial velocity are suggestive that they may not all stem from separate accretion events. Even so, the VOD appears to be caused by more than one overdensity. The Sagittarius (Sgr) stream is a very obvious feature in the background of the VOD at a mean distance of 44 kpc. Two additional high-significance groups were detected at distances . Their radial velocities and locations differ from the expected path of the Sgr debris in this part of the sky, and they are likely to be remnants of other accretion events.

Constraining the Galactic structure parameters with the XSTPS-GAC and SDSS photometric surveys

Photometric data from the Xuyi Schmidt Telescope Photometric Survey of the Galactic Anticentre (XSTPS-GAC) and the Sloan Digital Sky Survey (SDSS) are used to derive the global structure parameters of the smooth components of the Milky Way. The data, which cover nearly 11,000 deg$^2$ sky area and the full range of Galactic latitude, allow us to construct a globally representative Galactic model. The number density distribution of Galactic halo stars is fitted with an oblate spheroid that decays by power law. The best-fit yields an axis ratio and a power law index $\kappa=0.65$ and $p=2.79$, respectively. The $r$-band differential star counts of three dwarf samples are then fitted with a Galactic model. The best-fit model yielded by a Markov Chain Monte Carlo analysis has thin and thick disk scale heights and lengths of $H_{1}=$ 322\,pc and $L_{1}=$2343\,pc, $H_{2}=$794\,pc and $L_{2}=$3638\,pc, a local thick-to-thin disk density ratio of $f_2=$11\,per\,cent, and a local density ratio of the oblate halo to the thin disk of $f_h=$0.16\,per\,cent. The measured star count distribution, which is in good agreement with the above model for most of the sky area, shows a number of statistically significant large scale overdensities, including some of the previously known substructures, such as the Virgo overdensity and the so-called "north near structure", and a new feature between 150\degr $< l < $ 240\degr~and $-1$5\degr $< b < $ $-$5\degr, at an estimated distance between 1.0 and 1.5\,kpc. The Galactic North-South asymmetry in the anticentre is even stronger than previously thought.

Star-forming dwarf galaxies in the Virgo cluster: the link between molecular gas, atomic gas, and dust

We present $^{12}$CO(1-0) and $^{12}$CO(2-1) observations of a sample of 20 star-forming dwarfs selected from the Herschel Virgo Cluster Survey, with oxygen abundances ranging from 12 + log(O/H) ~ 8.1 to 8.8. CO emission is observed in ten galaxies and marginally detected in another one. CO fluxes correlate with the FIR 250 $\mu$m emission, and the dwarfs follow the same linear relation that holds for more massive spiral galaxies extended to a wider dynamical range. We compare different methods to estimate H2 molecular masses, namely a metallicity-dependent CO-to-H2 conversion factor and one dependent on H-band luminosity. The molecular-to-stellar mass ratio remains nearly constant at stellar masses <~ 10$^9$ M$_{\odot}$, contrary to the atomic hydrogen fraction, M$_{HI}$/M$_*$, which increases inversely with M$_*$. The flattening of the M$_{H_2}$/M$_*$ ratio at low stellar masses does not seem to be related to the effects of the cluster environment because it occurs for both HI-deficient and HI-normal dwarfs. The molecular-to-atomic ratio is more tightly correlated with stellar surface density than metallicity, confirming that the interstellar gas pressure plays a key role in determining the balance between the two gaseous components of the interstellar medium. Virgo dwarfs follow the same linear trend between molecular gas mass and star formation rate as more massive spirals, but gas depletion timescales, $\tau_{dep}$, are not constant and range between 100 Myr and 6 Gyr. The interaction with the Virgo cluster environment is removing the atomic gas and dust components of the dwarfs, but the molecular gas appears to be less affected at the current stage of evolution within the cluster. However, the correlation between HI deficiency and the molecular gas depletion time suggests that the lack of gas replenishment from the outer regions of the disc is lowering the star formation activity.

THE NEXT GENERATION VIRGO CLUSTER SURVEY. XIX. TOMOGRAPHY OF MILKY WAY SUBSTRUCTURES IN THE NGVS FOOTPRINT

ABSTRACT The Next Generation Virgo Cluster Survey (NGVS) is a deep u*giz survey targeting the Virgo Cluster of galaxies at 16.5 Mpc. This survey provides high-quality photometry over an ∼100 deg2 region straddling the constellations of Virgo and Coma Berenices. This sightline through the Milky Way is noteworthy in that it intersects two of the most prominent substructures in the Galactic halo: the Virgo overdensity (VOD) and Sagittarius stellar stream (close to its bifurcation point). In this paper, we use deep u*gi imaging from the NGVS to perform tomography of the VOD and Sagittarius stream using main-sequence turnoff (MSTO) stars as a halo tracer population. The VOD, whose centroid is known to lie at somewhat lower declinations (α ∼ 190°, δ ∼ −5°) than is covered by the NGVS, is nevertheless clearly detected in the NGVS footprint at distances between ∼8 and 25 kpc. By contrast, the Sagittarius stream is found to slice directly across the NGVS field at distances between 25 and 40 kpc, with a density maximum at ≃35 kpc. No evidence is found for new substructures beyond the Sagittarius stream, at least out to a distance of ∼90 kpc—the largest distance to which we can reliably trace the halo using MSTO stars. We find clear evidence for a distance gradient in the Sagittarius stream across the ∼30° of sky covered by the NGVS and its flanking fields. We compare our distance measurements along the stream with those predicted by leading stream models.

DISCOVERY OF A STELLAR OVERDENSITY IN ERIDANUS–PHOENIX IN THE DARK ENERGY SURVEY

ABSTRACT We report the discovery of an excess of main-sequence turnoff stars in the direction of the constellations of Eridanus and Phoenix from the first-year data of the Dark Energy Survey (DES). The Eridanus–Phoenix (EriPhe) overdensity is centered around l &sim; 285 &deg; ?> and b &sim; &minus; 60 &deg; ?> and spans at least 30° in longitude and 10° in latitude. The Poisson significance of the detection is at least 9 &sgr; ?> . The stellar population in the overdense region is similar in brightness and color to that of the nearby globular cluster NGC 1261, indicating that the heliocentric distance of EriPhe is about d &sim; 16 kpc ?> . The extent of EriPhe in projection is therefore at least ∼4 kpc by ∼3 kpc. On the sky, this overdensity is located between NGC 1261 and a new stellar stream discovered by DES at a similar heliocentric distance, the so-called Phoenix Stream. Given their similar distance and proximity to each other, it is possible that these three structures may be kinematically associated. Alternatively, the EriPhe overdensity is morphologically similar to the Virgo overdensity and the Hercules–Aquila cloud, which also lie at a similar Galactocentric distance. These three overdensities lie along a polar plane separated by ∼120° and may share a common origin. Spectroscopic follow-up observations of the stars in EriPhe are required to fully understand the nature of this overdensity.

Erratum: Dark matter and IMF normalization in Virgo dwarf early-type galaxies

Erratum: Dark matter and IMF normalization in Virgo dwarf early-type galaxies

Dark matter and IMF normalization in Virgo dwarf early-type galaxies

In this work we analyze the dark matter (DM) fraction, $f_{DM}$, and mass-to-light ratio mismatch parameter, $\delta_{IMF}$ (computed with respect to a Milky-Way-like IMF), for a sample of 39 dwarf early-type galaxies (dEs) in the Virgo cluster. Both $f_{DM}$ and $\delta_{IMF}$ are estimated within the central (one effective radius) galaxy regions, with a Jeans dynamical analysis that relies on galaxy velocity dispersions, structural parameters, and stellar M/L ratios from the SMAKCED survey. In this first attempt to constrain, simultaneously, the IMF normalization and the DM content, we explore the impact of different assumptions on the DM model profile. On average, for a NFW profile, the $\delta_{IMF}$ is consistent with a Chabrier-like normalization ($\delta_{IMF} \sim 1$), with $f_{DM} \sim 0.35$. One of the main results of the present work is that for at least a few systems the $\delta_{IMF}$ is heavier than the MW-like value (i.e. either top- or bottom-heavy). When introducing tangential anisotropy, larger $\delta_{IMF}$ and smaller $f_{DM}$ are derived. Adopting a steeper concentration-mass relation than that from simulations, we find lower $\delta_{IMF}$ ($< 1$) and larger $f_{DM}$. A constant M/L profile with null $f_{DM}$ gives the heaviest $\delta_{IMF}$ ($\sim 2$). In the MONDian framework, we find consistent results to those for our reference NFW model. If confirmed, the large scatter of $\delta_{IMF}$ for dEs would provide (further) evidence for a non-universal IMF in early-type systems. On average, our reference $f_{DM}$ estimates are consistent with those found for low-$\sigma_{e}$ ($\rm \sim 100 \, \rm km s^{-1}$) early-type galaxies (ETGs). Furthermore, we find $f_{DM}$ consistent with values from the SMAKCED survey, and find a double-value behavior of $f_{DM}$ with stellar mass, which mirrors the trend of dynamical M/L and global star formation efficiency with mass.

The scaling relations of early-type dwarf galaxies across a range of environments

We present the results of a Keck-ESI study of dwarf galaxies across a range of environment: the Perseus Cluster, the Virgo Cluster, the NGC 1407 group, and the NGC 1023 group. Eighteen dEs are targeted for spectroscopy, three for the first time. We confirm cluster membership for one Virgo dE, and group membership for one dE in the NGC 1023 group, and one dE in the NGC 1407 group for the first time. Regardless of environment, the dEs follow the same size-magnitude and $\sigma$-luminosity relation. Two of the Virgo dwarfs, VCC 1199 and VCC 1627, have among the highest central velocity dispersions ($\sigma_{0}$ = 58.4 km s$^{-1}$ and 49.2 km s$^{-1}$) measured for dwarfs of their luminosity ($M_{R}\approx -17$). Given their small sizes ($R_{e} < 300$ pc) and large central velocity dispersions, we classify these two dwarfs as compact ellipticals rather than dEs. Group dEs typically have higher mean dynamical-to-stellar mass ratios than the cluster dEs, with $M_{dyn}/M_{\star} = 5.1\pm0.6$ for the group dwarfs, vs. $M_{dyn}/M_{\star} = 2.2\pm0.5$ for the cluster sample, which includes two cEs. We also search for trends in $M_{dyn}/M_{\star}$ vs. distance from M87 for the Virgo Cluster population, and find no preference for dwarfs with high values of $M_{dyn}/M_{\star}$ to reside in the cluster outskirts vs. centre.

TheHerschelVirgo Cluster Survey

To assess the effects of the cluster environment on the different components of the interstellar medium, we analyse the far-infrared (FIR) and submillimetre (submm) properties of a sample of star-forming dwarf galaxies detected by the Herschel Virgo Cluster Survey (HeViCS). We determine dust masses and dust temperatures by fitting a modified black body function to the spectral energy distributions (SEDs). Stellar and gas masses, star formation rates (SFRs), and metallicities are obtained from the analysis of a set of ancillary data. Dust is detected in 49 out of a total 140 optically identified dwarfs covered by the HeViCS field; considering only dwarfs brighter than mB = 18 mag, this gives a detection rate of 43%. After evaluating different emissivity indices, we find that the FIR-submm SEDs are best-fit by β = 1.5, with a median dust temperature Td = 22.4 K. Assuming β = 1.5, 67% of the 23 galaxies detected in all five Herschel bands show emission at 500 μm in excess of the modified black-body model. The fraction of galaxies with a submillimetre excess decreases for lower values of β, while a similarly high fraction (54%) is found if a β-free SED modelling is applied. The excess is inversely correlated with SFR and stellar masses. To study the variations in the global properties of our sample that come from environmental effects, we compare the Virgo dwarfs to other Herschel surveys,such as the Key Insights into Nearby Galaxies: Far-Infrared Survey with Herschel (KINGFISH), the Dwarf Galaxy Survey (DGS), and the HeViCS Bright Galaxy Catalogue (BGC). We explore the relations between stellar mass and Hi fraction, specific star formation rate, dust fraction, gas-to-dust ratio over a wide range of stellar masses (from 107 to 1011 M⊙) for both dwarfs and spirals. Highly Hi-deficient Virgo dwarf galaxies are mostly characterised by quenched star formation activity and lower dust fractions giving hints for dust stripping in cluster dwarfs. However, to explain the large dust-to-gas mass ratios observed in these systems, we find that the fraction of dust removed has to be less than that of the Hi component. The cluster environment seems to mostly affect the gas component and star formation activity of the dwarfs. Since the Virgo star-forming dwarfs are likely to be crossing the cluster for the first time, a longer timescale might be necessary to strip the more centrally concentrated dust distribution.

A comprehensive view of the Virgo stellar stream

Radial velocities and metallicities have been measured for 82 RR Lyrae identified by the QUEST survey in the direction of the Virgo constellation. Distributed over 90 sq. deg. of the sky, they lie from 4 to 23 kpc from the Sun. Using an algorithm for finding groups in phase space and modeling the smooth halo component in the region, we identified the 5 most significant RRLS groups. We have examined the SEKBO and the Catalina catalog of RRLS (Prior et al. 2009, and Drake et al. 2013), as well as the bright QUEST RRLS sample (Vivas et al. in prep.), the catalogs of Blue Horizontal Branch stars compiled by Sirko et al (2004) and Brown et al (2008, 2010) and the catalog of Red Giant stars from the Spaghetti survey, for stars that may be related to the QUEST RRLS groups. The most significant group of RRLS is the Virgo Stellar Stream (VSS, Duffau et al 2006), group A, which is composed of at least 10 RRLS and 3 BHB stars. It has a mean distance of 19.6 kpc and a mean radial velocity Vgsr = 128 km/s, as estimated from its RRLS members. With the revised velocities reported here, there is no longer an offset in velocity between the RRLS in the VSS and the prominent peak in the velocities of main-sequence turnoff stars reported by Newberg et al (2007) in the same direction and at a similar distance (S297+63-20.5). The location in phase space of two other groups (F,H) suggests a possible connection with the VSS, which cannot be discarded at this point, although the turnoff colors of the VSS and group H, as identified from Newberg et al. (2007), suggest they might be composed of different populations. Two more groups (B,D), are found at mean distances of 19 and 5.7 kpc, and mean radial velocities of -94 and 32 km/s. None of our groups seems to relate to Sgr streams. The excess of stars observed in Virgo appears to be composed of several halo substructures along the same line of sight.

LA SILLA QUEST RR LYRAE STAR SURVEY: REGION I

A search for RR Lyrae stars (RRLS) in 840 sq.deg. of the sky in right ascension 150 - 210 deg and declination -10 - +10 deg yielded 1013 type ab and 359 type c RRLS. This sample is used to study the density profile of the Galactic halo, halo substructures, and the Oosterhoff type of the halo over distances from 5 to 80 kpc. The halo is flattened towards the Galactic plane, and its density profile steepens in slope at galactocentric distances greater than 25 kpc. The RRLS in the stellar stream from the Sagittarius dwarf spheroidal (dSph) galaxy match well the model of Law and Majewski for the stars that were stripped 1.3 to 3.2 Gyr ago, but not for the ones stripped 3.2 to 5.0 Gyr ago. Over densities are found at the locations of the Virgo Overdensity and the Virgo Stellar Stream. Within 1 deg of 1220-1, which Jerjen et al. identify as a halo substructure at a distance of 24 kpc, there are 4 RRLS that are possibly members. Away from substructures, the RRLS are a mixture of Oosterhoff types I and II, but mostly OoI (73%). The accretion of galaxies resembling in RRLS content the most massive Milky Way satellites (LMC, SMC, For,Sgr) may explain this preponderance of OoI. Six new RRLS and 3 new anomalous Cepheids were found in the Sextans dSph galaxy.

Search for cold and hot gas in the ram pressure stripped Virgo dwarf galaxy IC 3418

We present IRAM 30m sensitive upper limits on CO emission in the ram pressure stripped dwarf Virgo galaxy IC3418 and in a few positions covering HII regions in its prominent 17 kpc UV/Ha gas-stripped tail. In the central few arcseconds of the galaxy, we report a possible marginal detection of about 1x10^6 M_sun of molecular gas (assuming a Galactic CO-to-H_2 conversion factor) that could correspond to a surviving nuclear gas reservoir. We estimate that there is less molecular gas in the main body of IC3418, by at least a factor of 20, than would be expected from the pre-quenching UV-based star formation rate assuming the typical gas depletion timescale of 2 Gyr. Given the lack of star formation in the main body, we think the H_2-deficiency is real, although some of it may also arise from a higher CO-to-H_2 factor typical in low-metallicity, low-mass galaxies. The presence of HII regions in the tail of IC3418 suggests that there must be some dense gas; however, only upper limits of < 1x10^6 M_sun were found in the three observed points in the outer tail. This yields an upper limit on the molecular gas content of the whole tail < 1x10^7 M_sun, which is an amount similar to the estimates from the observed star formation rate over the tail. We also present strong upper limits on the X-ray emission of the stripped gas in IC3418 from a new Chandra observation. The measured X-ray luminosity of the IC3418 tail is about 280 times lower than that of ESO 137-001, a spiral galaxy in a more distant cluster with a prominent ram pressure stripped tail. Non-detection of any diffuse X-ray emission in the IC3418 tail may be due to a low gas content in the tail associated with its advanced evolutionary state and/or due to a rather low thermal pressure of the surrounding intra-cluster medium.

EXPLORING THE VARIABLE SKY WITH LINEAR. II. HALO STRUCTURE AND SUBSTRUCTURE TRACED BY RR LYRAE STARS TO 30 kpc

We present a sample of ~5,000 RR Lyrae stars selected from the recalibrated LINEAR dataset and detected at heliocentric distances between 5 kpc and 30 kpc over ~8,000 deg^2 of sky. The coordinates and light curve properties, such as period and Oosterhoff type, are made publicly available. We find evidence for the Oosterhoff dichotomy among field RR Lyrae stars, with the ratio of the type II and I subsamples of about 1:4. The number density distribution of halo RRab stars as a function of galactocentric distance can be described as an oblate ellipsoid with the axis ratio q=0.63 and with either a single or a double power law with a power-law index in the range -2 to -3. Using a group-finding algorithm EnLink, we detected seven candidate halo groups, only one of which is statistically spurious. Three of these groups are near globular clusters (M53/NGC 5053, M3, M13), and one is near a known halo substructure (Virgo Stellar Stream); the remaining three groups do not seem to be near any known halo substructures or globular clusters, and seem to have a higher ratio of Oosterhoff type II to Oosterhoff type I RRab stars than what is found in the halo. The extended morphology and the position (outside the tidal radius) of some of the groups near globular clusters is suggestive of tidal streams possibly originating from globular clusters. Spectroscopic followup of detected halo groups is encouraged.