arXiv subjects

Kim-Vy H. Tran

Publications and source records attributed to Kim-Vy H. Tran.

At least 73 records · Page 4Linked to original sources

arXiv2011

Multi-Wavelength Properties of Barred Galaxies in the Local Universe: Environment and evolution across the Hubble sequence

We investigate possible environmental and morphological trends in the $z\sim0$ bar fraction using two carefully selected samples representative of a low-density environment (the isolated galaxies from the AMIGA sample) and of a dense environment (galaxies in the Virgo cluster). Galaxies span a stellar mass range from $10^8$ to $10^{12}$M$_{\odot}$ and are visually classified using both high-resolution NIR (H-band) imaging and optical \texttt{rgb} images. We find that the bar fraction in disk galaxies is independent of environment suggesting that bar formation may occur prior to the formation of galaxy clusters. The bar fraction in early type spirals ($Sa-Sb$) is $\sim$50%, which is twice as high as the late type spirals ($Sbc-Sm$). The higher bar fraction in early type spirals may be due to the fact that a significant fraction of their bulges are pseudo-bulges which form via the buckling instability of a bar. i.e. a large part of the Hubble sequence is due to secular processes which move disc galaxies from late to early types. There is a hint of a higher bar fraction with higher stellar masses which may be due to the susceptibility to bar instabilities as the baryon fractions increase in halos of larger masses. Overall, the $S0$ population has a lower bar fraction than the $Sa-Sb$ galaxies and their barred fraction drops significantly with decreasing stellar mass. This supports the notion that $S0s$ form via the transformation of disk galaxies that enter the cluster environment. The gravitational harassment thickens the stellar disks, wiping out spiral patterns and eventually erasing the bar - a process that is more effective at lower galaxy masses.

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arXiv2011

A Search for Young Stars in the S0 Galaxies of a Super-Group at z=0.37

We analyze GALEX UV data for a system of four gravitationally-bound groups at z=0.37, SG1120, which is destined to merge into a Coma-mass cluster by z=0, to study how galaxy properties may change during cluster assembly. Of the 38 visually-classified S0 galaxies, with masses ranging from log(M_*)~10-11, we detect only one in the NUV channel, a strongly star-forming S0 that is the brightest UV source with a measured redshift placing it in SG1120. Stacking the undetected S0 galaxies (which generally lie on or near the optical red-sequence of SG1120) still results in no NUV/FUV detection (<2 sigma). Using our limit in the NUV band, we conclude that for a rapidly truncating star formation rate, star formation ceased *at least* ~0.1 to 0.7 Gyr ago, depending on the strength of the starburst prior to truncation. With an exponentially declining star-formation history over a range of time-scales, we rule out recent star-formation over a wide range of ages. We conclude that if S0 formation involves significant star formation, it occurred well before the groups were in this current pre-assembly phase. As such, it seems that S0 formation is even more likely to be predominantly occurring outside of the cluster environment.

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arXiv2011

A Census of Mid-Infrared Selected Active Galactic Nuclei in Massive Galaxy Clusters at 0 < z < 1.3

We conduct a deep mid-infrared census of nine massive galaxy clusters at (0 3 sigma level, we identify 12 that host mid-infrared selected active galactic nuclei (IR-AGN). To compare the IR-AGN across our redshift range, we define two complete samples of cluster galaxies: (1) optically-selected members with rest-frame V(AB) magnitude <-21.5 and (2) mid-IR selected members brighter than (M*+0.5), i.e. essentially a stellar mass cut. In both samples, we measure \agnfrac ~1% with a strong upper limit of ~3% at z<1. This uniformly low IR-AGN fraction at z<1 is surprising given the fraction of 24 micron sources in the same galaxy clusters is observed to increase by about a factor of four from z~0 to z~1; this indicates that most of the detected 24 micron flux is due to star formation. Only in our single galaxy cluster at z=1.24 is the IR-AGN fraction measurably higher at ~15% (all members; ~70% for late-types only). In agreement with recent studies, we find the cluster IR-AGN are predominantly hosted by late-type galaxies with blue optical colors, i.e. members with recent/ongoing star formation. The four brightest IR-AGN are also X-ray sources; these IR+X-ray AGN all lie outside the cluster core (Rproj>0.5 Mpc) and are hosted by highly morphologically disturbed members. Although our sample is limited, our results suggest that \agnfrac in massive galaxy clusters is not strongly correlated with star formation at z<1, and that IR-AGN have a more prominent role at z>1.

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arXiv2010

Reversal of Fortune: Confirmation of an Increasing Star Formation-Density Relation in a Cluster at z=1.62

We measure the rest-frame colors (dust-corrected), infrared luminosities, star formation rates, and stellar masses of 92 galaxies in a Spitzer-selected cluster at z=1.62. By fitting spectral energy distributions (SEDs) to 10-band photometry (0.4 micron < lambda(obs) <8 micron) and measuring 24 micron fluxes for the 12 spectroscopically confirmed and 80 photometrically selected members, we discover an exceptionally high level of star formation in the cluster core of ~1700 Msun/yr per Mpc^2. The cluster galaxies define a strong blue sequence in (U-V) color and span a range in color. We identify 17 members with L(IR)>10^(11) Lsun, and these IR luminous members follow the same trend of increasing star formation with stellar mass that is observed in the field at z~2. Using rates derived from both the 24 micron imaging and SED fitting, we find that the relative fraction of star-forming members triples from the lowest to highest galaxy density regions, e.g. the IR luminous fraction increases from ~8% at Sigma~10 gal per Mpc^2 to ~25% at Sigma>100 gal per Mpc^2. The observed increase is a reversal of the well-documented trend at z<1 and signals that we have reached the epoch when massive cluster galaxies are still forming a substantial fraction of their stars.

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arXiv2010

The Homogeneous Properties of Halpha-Selected Galaxies at (0.05<z<0.15)

We show that the Halpha line (6563 Angstrom) alone is an extremely effective criterion for identifying galaxies that are uniform in color (red), luminosity-weighted age (old), and morphology (bulge-dominated). By combining the Sloan Digital Sky Survey (Data Release 6) with the New York University Value-Added Galaxy Catalog, we have photometric and spectroscopic indices for over 180,000 galaxies at (0.05 2 (bulge-dominated); and 3) galaxies with n>2 that are also red in (g'-r'). We find that the Halpha-selected galaxies consistently have the smallest color scatter: for example, at z~0.05 the intrinsic scatter in apparent (g'-r') for the Halpha sample is only 0.0287+/-0.0007 compared to 0.0682+/-0.0014 for the Sersic sample. Applying a color-cut to the n>2 sample does decrease the color scatter to 0.0313+/-0.0007, but there remains a measurable fraction of star-forming and/or AGN galaxies (up to 9.3%). All of the EW(Halpha)<0 Angstrom galaxies have n>2, i.e. they are bulge-dominated systems. The spectra for the three samples confirm that the Halpha-selected galaxies have the highest D4000 values and are, on average, nearly twice as old as the Sersic-selected samples. With the advent of multi-object near-infrared spectrographs, Halpha alone can be used to reliably isolate truly quiescent galaxies dominated by evolved stellar populations at any epoch from z~0 up to z~2.

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arXiv2010

Multi-Wavelength Properties of Barred Galaxies in the Local Universe. I: Virgo Cluster

We study in detail how the barred galaxy fraction varies as a function of luminosity, HI gas mass, morphology and color in the Virgo cluster in order to provide a well defined, statistically robust measurement of the bar fraction in the local universe spanning a wide range in luminosity (factor of ~100) and HI gas mass. We combine multiple public data-sets (UKIDSS near-infrared imaging, ALFALFA HI gas masses, GOLDMine photometry). After excluding highly inclined systems, we define three samples where galaxies are selected by their B-band luminosity, H-band luminosity, and HI gas mass. We visually assign bars using the high resolution H-band imaging from UKIDSS. When all morphologies are included, the barred fraction is ~17-24% while for morphologically selected discs, we find that the barred fraction in Virgo is ~29-34%: it does not depend strongly on how the sample is defined and does not show variations with luminosity or HI gas mass. The barred fraction depends most strongly on the morphological composition of the sample: when the disc populations are separated into lenticulars (S0--S0/a), early-type spirals (Sa--Sb), and late-type spirals (Sbc--Sm), we find that the early-type spirals have a higher barred fraction (~45-50%) compared to the lenticulars and late-type spirals (~22-36%). This difference may be due to the higher baryon fraction of early-type discs which makes them more susceptible to bar instabilities. We do not find any evidence of barred galaxies being preferentially blue.

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arXiv2009

The Importance of AGN in an Assembling Galaxy Cluster

We present results from our multi-wavelength study of SG1120, a super galaxy group at z=0.37, that will merge to form a galaxy cluster comparable in mass to Coma. We have spectroscopically confirmed 174 members in the four X-ray luminous groups that make up SG1120, and these groups have velocity dispersions of sigma(1D)=303-580 km/s. We find that the supergroup has an excess of 24 micron members relative to CL1358+62, a rich galaxy cluster at z=0.33. SG1120 also has an increasing fraction of 24 micron members with decreasing local galaxy density, i.e. an infrared-density relation, that is not observed in the rich cluster. We detect nine of the group galaxies in VLA 1.4 Ghz imaging, and comparison of the radio to total infrared luminosities indicates that about 30% of these radio-detected members have AGN. The radio map also reveals that one of the brightest group galaxies has radio jets. We are currently analysing the 1.4 Ghz observations to determine if AGN can significantly heat the intrahalo medium and if AGN are related to the excess of 24 micron members.

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arXiv2009

A Spectroscopically Confirmed Excess of 24 micron Sources in a Super Galaxy Group at z=0.37: Enhanced Dusty Star Formation Relative to the Cluster and Field Environment

To trace how dust-obscured star formation varies with environment, we compare the fraction of 24 micron sources in a super galaxy group to the field and a rich galaxy cluster at z~0.35. We draw on multi-wavelength observations that combine Hubble, Chandra, and Spitzer imaging with extensive optical spectroscopy (>1800 redshifts) to isolate galaxies in each environment and thus ensure a uniform analysis. We focus on the four galaxy groups in supergroup 1120-12 that will merge to form a galaxy cluster comparable in mass to Coma. We find that 1) the fraction of supergroup galaxies with SFR(IR)>3 Msun/yr is four times higher than in the cluster (32% vs. 7%); 2) the supergroup's infrared luminosity function confirms that it has a higher density of IR members compared to the cluster and includes bright IR sources not found in galaxy clusters at z<0.35; and 3) there is a strong trend of decreasing IR fraction with increasing galaxy density, i.e. an IR-density relation, not observed in the cluster. These dramatic differences are surprising because the early-type fraction in the supergroup is already as high as in clusters, i.e. the timescales for morphological transformation cannot be strongly coupled to when the star formation is completely quenched. The supergroup has a significant fraction (~17%) of luminous, low-mass, IR members that are outside the group cores (R>0.5 Mpc); once their star formation is quenched, most will evolve into faint red galaxies. Our analysis indicates that the supergroup's 24 micron population also differs from that in the field: 1) despite the supergroup having twice the fraction of E/S0s as the field, the fraction of IR galaxies is comparable in both environments, and 2) the supergroup's IR luminosity function has a higher L(IR)* than that previously measured for the field.

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arXiv2008

Forming Early-type Galaxies in Groups Prior to Cluster Assembly

We study a unique proto-cluster of galaxies, the supergroup SG1120-1202. We quantify the degree to which morphological transformation of cluster galaxies occurs prior to cluster assembly in order to explain the observed early-type fractions in galaxy clusters at z=0. SG1120-1202 at z~0.37 is comprised of four gravitationally bound groups that are expected to coalesce into a single cluster by z=0. Using HST ACS observations, we compare the morphological fractions of the supergroup galaxies to those found in a range of environments. We find that the morphological fractions of early-type galaxies (~60 %) and the ratio of S0 to elliptical galaxies (0.5) in SG1120-1202 are very similar to clusters at comparable redshift, consistent with pre-processing in the group environment playing the dominant role in establishing the observed early-type fraction in galaxy clusters.

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arXiv2008

Spitzer/MIPS 24um Observations of Galaxy Clusters: An Increasing Fraction of Obscured Star-forming Members from z=0.02 to z=0.83

We study the mid-infrared properties of 1315 spectroscopically confirmed members in eight massive (M>5x10^14 Msun) galaxy clusters covering the redshift range from 0.02 to 0.83. The selected clusters all have deep Spitzer MIPS 24um observations, Hubble and ground-based photometry, and extensive redshift catalogs. We observe for the first time an increase in the fraction of cluster galaxies with mid-infrared star formation rates higher than 4 solar masses per year from 3% at z=0.02 to 13% at z=0.83. This increase is reproduced even when considering only the most massive members (Mstars >4x10^10 Msun). The 24 micron observations reveal stronger evolution in the fraction of blue/star-forming cluster galaxies than color-selected samples: the number of red but strongly star-forming cluster galaxies increases with redshift, and combining these with the optically-defined Butcher-Oemler members increases the total fraction of blue/star-forming cluster galaxies to ~30% at z=0.83. These results, the first of our Spitzer/MIPS Infra-Red Cluster Survey (SMIRCS), support earlier studies indicating the increase in star-forming members is driven by cluster assembly and galaxy infall, as is expected in the framework of hierarchical formation.

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arXiv2008

The Late Stellar Assembly of Massive Cluster Galaxies Via Major Merging

We present multi-wavelength observations of the brightest galaxies in four X-ray luminous groups at z~0.37 that will merge to form a cluster comparable in mass to Coma. Ordered by increasing stellar mass, the four brightest group galaxies (BGGs) present a time sequence where BGG-1, 2, and 3 are in merging systems and BGG-4 is a massive remnant [M(stars)=6.7x10^(11) Msun]. BGG-1 and 2 have bright, gravitationally bound companions and BGG-3 has two nuclei separated by only 2.5 kpc, thus merging at z<0.5 increases the BGG mass by >40% (merging timescale<2 Gyr) and V-band luminosity by ~0.4 mag. The BGGs' rest-frame (B-V) colors correspond to stellar ages of >3 Gyr, and their tight scatter in (B-V) color [sigma(BV)=0.032] confirms they formed the bulk of their stars at z>0.9. Optical spectroscopy shows no signs of recent (<1.5 Gyr) or ongoing star formation. Only two BGGs are weakly detected at 24 microns, and X-ray and optical data indicate the emission in BGG-2 is due to an AGN. All four BGGs and their companions are early-type (bulge-dominated) galaxies, and they are embedded in diffuse stellar envelopes up to ~140 kpc across. The four BGG systems must evolve into the massive, red, early-type galaxies dominating local clusters. Our results show that: 1) massive galaxies in groups and clusters form via dissipationless merging; and 2) the group environment is critical for this process.

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arXiv2007

IR observations of MS 1054-03: Star Formation and its Evolution in Rich Galaxy Clusters

We study the infrared (IR) properties of galaxies in the cluster MS 1054-03 at z=0.83 by combining MIPS 24 micron data with spectra of more than 400 galaxies and a very deep K-band selected catalog. 19 IR cluster members are selected spectroscopically, and an additional 15 are selected by their photometric redshifts. We derive the IR luminosity function of the cluster and find strong evolution compared to the similar-mass Coma cluster. The best fitting Schechter function gives L*_{IR}=11.49 +0.30/-0.29 L_sun with a fixed faint end slope, about one order of magnitude larger than that in Coma. The rate of evolution of the IR luminosity from Coma to MS 1054-03 is consistent with that found in field galaxies, and it suggests that some internal mechanism, e.g., the consumption of the gas fuel, is responsible for the general decline of the cosmic star formation rate (SFR) in different environments. The mass-normalized integrated SFR within 0.5R_200 in MS 1054-03 also shows evolution compared with other rich clusters at lower redshifts, but the trend is less conclusive if the mass selection effect is considered. A nonnegligible fraction (13%) of cluster members, are forming stars actively and the overdensity of IR galaxies is about 20 compared to the field. It is unlikely that clusters only passively accrete star forming galaxies from the surrounding fields and have their star formation quenched quickly afterward; instead, many cluster galaxies still have large amounts of gas, and their star formation may be enhanced by the interaction with the cluster.

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arXiv2007

A Keck Spectroscopic Survey of MS1054-03 (z=0.83): Forming the Red Sequence

Using a magnitude-limited, spectroscopic survey of the X-ray luminous galaxy cluster MS1054-03, we isolate 153 cluster galaxies and measure MS1054's redshift and velocity dispersion to be z=0.8307 and sigma_z=1156 km/s. The absorption-line, post-starburst (``E+A''), and emission-line galaxies respectively make up 63%, 15%, and 23% of the cluster population. With photometry from HST/ACS, we find that the absorption-line members define an exceptionally tight red sequence over a span of ~3.5 magnitudes in i775: their intrinsic scatter in (V-i) color is only 0.048, corresponding to a (U-B)_z scatter of 0.041. Their color scatter is comparable to that of the ellipticals (sigma_Vi=0.055), but measurably smaller than that of the combined E+S0 sample (sigma_Vi=0.072). The color scatter of MS1054's absorption-line population is approximately twice that of the ellipticals in Coma; this difference is consistent with passive evolution where most of the absorption-line members (>75%) formed by z~2, and all of them by z~1.2. For red members, we find a trend (>95% confidence) of weakening Hdelta absorption with redder colors that we conclude is due to age: in MS1054, the color scatter on the red sequence is driven by differences in mean stellar age of up to ~1.5 Gyr. We also generate composite spectra and estimate that the average S0 in MS1054 is ~0.5-1 Gyr younger than the average elliptical; this difference in mean stellar age is mainly due to a number of S0s that are blue (18%) and/or are post-starburst systems (21%).

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arXiv2005

Spectroscopic Confirmation of Multiple Red Galaxy-Galaxy Mergers in MS1054-03 (z=0.83)

We present follow-up spectroscopy of the galaxy cluster MS1054-03 (z=0.83) confirming that at least six of the nine merging galaxy pairs identified by van Dokkum et al. (1999) are indeed bound systems: they have projected separations of R_s<10 kpc and relative line-of sight velocities of dv<165 km/s. For the remaining three pairs, we were unable to obtain redshifts of both constituent galaxies. To identify a more objective sample of merging systems, we select bound red galaxy pairs (R_s<=30 kpc, dv<=300 km/s) from our sample of 121 confirmed cluster members: galaxies in bound red pairs make up 15.7+/-3.6% of the cluster population. The (B-K_s) color-magnitude diagram shows that the pair galaxies are as red as the E/S0 members and have a homogeneous stellar population. The red pair galaxies span a large range in luminosity and internal velocity dispersion to include some of the brightest, most massive members (L>L*, sigma>200 km/s); these bound galaxy pairs must evolve into E/S0 members by z~0.7. These results combined with MS1054's high merger fraction and reservoir of likely future mergers indicates that most, if not all, of its early-type members evolved from (passive) galaxy-galaxy mergers at z<~1.

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arXiv2005

Galaxy Cluster Assembly at z=0.37

We present X-ray and spectroscopic confirmation of a cluster assembling from multiple, distinct galaxy groups at z=0.371. Initially detected in the Las Campanas Distant Cluster Survey, the structure contains at least four X-ray detected groups that lie within a maximum projected separation of 4 Mpc and within dv=550 km/s of one another. Using Chandra imaging and wide-field optical spectroscopy, we show that the individual groups lie on the local sigma-T relation, and derive a total mass of M>=5e14 solar masses for the entire structure. We demonstrate that the groups are gravitationally bound to one another and will merge into a single cluster with >=1/3 the mass of Coma. We also find that although the cluster is in the process of forming, the individual groups already have a higher fraction of passive members than the field. This result indicates that galaxy evolution on group scales is key to developing the early-type galaxies that dominate the cluster population by z~0.

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arXiv2004

Infall, the Butcher-Oemler Effect, and the Descendants of Blue Cluster Galaxies at z~0.6

Using wide-field HST/WFPC2 imaging and extensive Keck/LRIS spectroscopy, we present a detailed study of the galaxy populations in MS2053--04, a massive, X-ray luminous cluster at z=0.5866. Analysis of 149 confirmed cluster members shows that MS2053 is composed of two structures that are gravitationally bound to each other; their respective velocity dispersions are 865 km/s (113 members) and 282 km/s (36 members). MS2053's total dynamical mass is 1.2x10^15 Msun. MS2053 is a classic Butcher-Oemler cluster with a high fraction of blue members (24%) and an even higher fraction of star-forming members (44%), as determined from their [OII] emission. The number fraction of blue/star-forming galaxies is much higher in the infalling structure than in the main cluster. This result is the most direct evidence to date that the Butcher-Oemler effect is linked to galaxy infall. In terms of their colors, luminosities, estimated internal velocity dispersions, and [OII] equivalent widths, the infalling galaxies are indistinguishable from the field population. MS2053's deficit of S0 galaxies combined with its overabundance of blue spirals implies that many of these late-types will evolve into S0 members. The properties of the blue cluster members in both the main cluster and infalling structure indicate they will evolve into low mass, L 3.

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arXiv2004

A VLT/FORS2 Multi-Slit Search for Lyman-alpha Emitting Galaxies at z~6.5

We present results from a deep spectroscopic search in the 9150A atmospheric window for z~6.5 Lyman-alpha emitting galaxies using the VLT/FORS2. Our multi-slit+narrow-band filter survey covers a total spatial area of 17.6 sq. arcmin in four different fields and reaches fluxes down to 5x10^(-18) erg/s/cm^2 (7 sigma detection). Our detection limit is significantly fainter than narrow-band searches at this redshift and fainter also than the unlensed brightness of Hu et al.'s HCM6A at z=6.56, and thus provides better overlap with surveys at much lower redshifts. Eighty secure emission line galaxies are detected. However, based on their clear continuum emission shortward of the line or the presence of multiple lines, none of these can be Ly-alpha emission at z~6.5. Our null result of finding no z~6.5 Ly-alpha emitters suggests that the number density of Ly-alpha emitters with L>2x10^(42) erg/s declines by ~2 between z~3 and z~6.5.

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arXiv2004

Field E+A Galaxies at Intermediate Redshifts (0.3<z<1)

We select E+A candidates from a spectroscopic dataset of ~800 field galaxies and measure the E+A fraction at 0.3<z<1 to be 2.7+/-1.1%, a value lower than that in galaxy clusters at comparable redshifts (11+/-3%). HST/WFPC2 imaging for five of our six E+A's shows they have a heterogeneous parent population: these E+A's span a range in half-light radius (0.8-8 kpc) and estimated internal velocity dispersion (50-220 km/s), and they include luminous systems (-21.6<M_Bz-5logh<-19.2). Despite their diversity in some aspects, the E+A's share several common characteristics that indicate the E+A phase is an important link in the evolution of star-forming galaxies into passive systems: the E+A's are uniformly redder than the blue, star-forming galaxies that make up the majority of the field, they are more likely to be bulge-dominated than the average field galaxy, and they tend to be morphologically irregular. We find E+A's make up ~9% of the absorption line systems in this redshift range, and estimate that ~25% of passive galaxies in the local field had an E+A phase at z<1.

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